EN1520

Examensarbete för civilingenjörsexamen i Energiteknik, 30 hp

High temperature corrosion on heat exchanger material exposed to alkali salt deposits

Högtemperaturkorrosion på värmeväxlarmaterial vid exponering för alkalisalt

Kajsa Persson

i

Abstract

Power generation through decentralized small scale CHP would facilitate the use of biomass

as an energy source, with the externally fired gas turbine (EFGT) being a promising

technology due to its high electrical efficiency. In an EFGT hot flue gases are heat-exchanged

with an air cycle, driving the turbine. The operation requires higher flue gas temperatures than

other technologies, for example steam turbines, to achieve optimal performance. The

operating conditions subjects the high temperature heat exchanger (HT-HE) to both physical

and chemical stress, with the corrosion related issues yet to be solved.

Problems concerning deposit formation and corrosion, on for example super heaters and heat

exchangers, when firing biomass are important issues even in commercially available

technologies, where the choice of fuel and fuel additives together with component design and

choice of material plays important roles in order to minimize the problems. The significantly

higher temperatures of the heat transferring surfaces for an EFGT entails combustion deposit

related problems less studied. The evaluation of turbine control, deposit formation and

corrosion as well as design of the HT-HE and system integration will enable the development

of the EFGT technology for applications with small- and medium-size biomass combustion.

In this work four potential HT-HE alloys of various grades have been evaluated with respect

to corrosion resistance, when exposed to alkali salts and salt mixtures in the KCl-K2CO3-

K2SO4 system. The exposures were done in a tube furnace during 24 h for each experiment at

four temperature levels between 700–1000oC. Morphological and elemental analysis of the

alloy surface and corrosion layers was performed with SEM-EDS.

The presence of KCl in the salt caused the most severe corrosion attacks while the corrosion

attacks of the pure sulfate and carbonate were more modest. Significant differences between

the four materials were observed. X20 experienced severe corrosion, with corrosion scale

formation in most cases. The KCl-containing salts caused 253MA to form corrosion scales at

all temperatures, while the corrosion resistance to other salts was fairly good. Inconel 600 had

the second best overall corrosion resistance. However, it should be pointed out that in some

cases the alloy was surpassed by 253MA. Kanthal showed the best overall performance, with

limited corrosion scale formation and surprisingly high corrosion resistance to the KCl-

containing ternary salt mixture at 900°C and 1000°C.

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Table of content

1 Introduction ......................................................................................................................... 1

1.1 Externally fired gas turbine (EFGT) ............................................................................ 1

1.2 Objective ...................................................................................................................... 2

2 Theoretical overview........................................................................................................... 3

2.1 The Brayton cycle and gas turbine .............................................................................. 3

2.2 Heat exchangers ........................................................................................................... 4

2.3 Alloys and stainless steels ........................................................................................... 4

2.4 Chemical composition of biomass and combustion related problems ........................ 7

2.5 Gibbs energy ................................................................................................................ 8

2.6 High temperature corrosion ....................................................................................... 10

2.6.1 Oxidation and formation of oxides ..................................................................... 10

2.6.2 Chloridation ........................................................................................................ 12

2.6.3 Sulfidation .......................................................................................................... 14

2.6.4 Carburization ...................................................................................................... 14

2.6.5 Nitridation .......................................................................................................... 15

2.6.6 Molten salt corrosion .......................................................................................... 15

2.7 Scanning electron microscopy with energy-dispersive X-ray spectroscopy ............. 16

3 Experimental ..................................................................................................................... 18

3.1 Materials .................................................................................................................... 18

3.1.1 Alloys ................................................................................................................. 18

3.1.2 Alkali salts and salt mixtures ............................................................................. 19

3.2 Exposure .................................................................................................................... 20

3.3 SEM-EDS analysis .................................................................................................... 21

4 Results ............................................................................................................................... 22

4.1 X20 ............................................................................................................................ 22

4.2 253MA ....................................................................................................................... 27

4.3 Inconel 600 ................................................................................................................ 34

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4.4 Kanthal ....................................................................................................................... 40

4.5 Result summary ......................................................................................................... 46

5 Discussion ......................................................................................................................... 50

5.1 Exposure to air ........................................................................................................... 50

5.2 Exposure to KCl-containing alkali salt ...................................................................... 51

5.3 Exposure to K2CO3 .................................................................................................... 52

5.4 Exposure to K2SO4 .................................................................................................... 52

5.5 SEM-EDS limitation .................................................................................................. 53

5.6 Experimantal method ................................................................................................. 53

5.7 Material recommendation .......................................................................................... 54

5.8 Suggestions for improvement of the method and future studies ............................... 54

6 Conclusions ....................................................................................................................... 54

7 Acknowledgements ........................................................................................................... 55

8 References ......................................................................................................................... 56

9 Appendix ........................................................................................................................... 61

1

1 Introduction

Combined heat and power (CHP), or cogeneration, gives an important contribution to the

European energy supply where biomass is considered to be the key source of sustainable

energy [1, 2]. In Sweden the use of biomass for heat and electricity generation is steadily

increasing and the technologies for large-scale CHP plants are well established [3]. Electricity

can be transported over large distances without greater distribution losses but the same does

not apply to heat, and thus the interest for decentralized small scale CHP plants is increasing

[4, 5].

1.1 Externally fired gas turbine (EFGT)

A distribution of the power generation through decentralized small scale CHP would facilitate

the use of biomass as an energy source. However, small size power units (up to some MWel)

such as steam turbines, organic rankine cycles (ORC), stirling engines, steam engines or

internal combustion technologies suffers from high electricity generation costs and low

electrical efficiency. The externally fired gas turbine (EFGT) has the potential for high α-

value [4] and electrical efficiency [6], in small-scale CHP production up to2 MWel. Together

with most components being commercially available and the relatively low electricity

production cost EFGTs seems to be an up-and-coming alternative for small scale biomass

power plants [4, 5]. Even though the concept of this technology is rather old there are still

critical issues remained unsolved to take it to a commercial level, mostly regarding the high

temperature heat exchanger (HT-HE).

An EFGT is based on a Brayton air cycle where the combustion chamber has been replaced

with a HT-HE and the combustion takes place outside air cycle. The air is compressed in the

compressor and then heated in the HT-HE before finally expanded in the turbine. The

expansion induces a mechanical work which is converted to electricity via a generator. The

hot air from the turbine outlet can further be used for pre-heating the combustion air while the

excess heat from flue gas can be used for local district heating. [5, 7]

The technology is advantageous as it enables combustion of solid fuel at atmospheric pressure

which promotes solid biofuels like wood chips or pellets, which are easier and cheaper to

produce than liquid or gaseous biofuels. Moreover, the turbine is only exposed to air and not

harmful combustion gases. Instead the ash/particle related flue gas problems, such as deposit

formation and corrosion, are potentially moved to the HT-HE. The desired turbine inlet

temperature (TIT) ranges between 800 – 1000oC, which requires an even higher flue gas

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temperature at the HT-HE inlet, and an operating temperature of the HT-HE which exceeds

the TIT by 100 – 150oC [8]. The flue gas temperature should be kept as high as possible, due

to the low gas-to-gas heat transfer coefficient, in order to reduce the surface area of the HT-

HE. Here the limiting factor is the condition of the HT-HE.

Problems concerning deposit formation and corrosion, on for example super heaters and heat

exchangers, when firing biomass are important issues even in commercially available

technologies, where the choice of fuel and fuel additives together with component design and

choice of material plays important roles in order to minimize the problems. Typically the

steam TIT in a CHP plants ranges between 500-550°C and during heating the heat

transferring surfaces of the superheaters exceeds the steam temperature by roughly 100°C [9].

For an EFGT the temperatures of the heat transferring surfaces are significantly higher and

thus the combustion deposit related problems are of different kind.

In 2013 the Technical Research Institute of Sweden, SP, together with Thermochemical

Energy Conversion Laboratory (TEC-Lab) at Umeå University, other universities and

industry partners launched a project on small-scale CHP with EFGT technology applied to

biomass combustion, in the range of 0.1 – 1 MWe. The project is financed by the Swedish

Energy Agency within the European Research Area Network (ERA-NET) Bioenergy

program. The goal is to evaluate the potential of EFGT by studying the turbine control,

deposit formation and corrosion as well as design of the HT-HE and system integration. This

will enable the development of the EFGT technology for applications with small- and

medium-size biomass combustion.

1.2 Objective

The aim of this work was to experimentally determine the corrosion resistance and interaction

between deposits and materials of four different alloys, when exposed to various synthetic

alkali salt deposits at temperatures relevant to the EFGT with biomass concept.

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2 Theoretical overview

2.1 The Brayton cycle and gas turbine

The Brayton-Joule cycle, developed by George Brayton around 1870, is nowadays only used

for gas turbines. Gas turbines can be operated as on an open or closed cycle, with the open

cycle being the most common. In the open cycle atmospheric air enters the compressor, where

its pressure and temperature is raised. The compressed air then enters the combustion

chamber where liquid or gaseous fuel is added for combustion, and the temperature is further

raised. The hot gas is finally expanded in the turbine before leaving the cycle, hence the name

open cycle. The turbine work is both used for driving the compressor and an electric

generator. In the closed cycle the combustion chamber is replaced by a heat exchanger (HE)

and the heat is recovered from an external source. After the turbine outlet another HE is

placed to reject the excess heat from the air. Figure 1 shows a simplified picture of the two

cycles.

Figure 1. Open (left) and closed (right) gas turbine [10].

The thermal efficiency of an ideal Brayton cycle under cold-air standard assumptions depends

on the pressure ration of the gas turbine and the specific heat ratio, 𝑐𝑝/𝑐𝑣, of the air, and

increases with both of these parameters. This also holds for real gas turbines even though the

process deviates from the ideal [10]. In the case of the EFGT the pressure ratio ranges

between 3.5 – 5 together with a TIT of 800 – 1000oC, resulting in an electrical efficiency of

about 20 – 30 %, Figure 2, and an overall efficiency in a CHP plant between 60 – 80 %,

depending on the heat demand [11, 8, 6].

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Figure 2. Electrical efficiency as a function of unit size for biomass fired CHP plants [6].

2.2 Heat exchangers

A heat exchanger (HE) is a widely used device which enables heat transfer from one fluid to

another without allowing them to mix. Typically the two fluids are separated by a solid wall

and heat transfer from the warmer fluid to the colder is carried out by convection (fluid to

wall or wall to fluid) and conduction (through the wall). The heat transfer due to radiation is

usually included in the convection heat transfer coefficient. Depending on the application

there are various designs of HEs, for example double-piped with parallel or counter flow,

compact, cross-flow, shell-and-tube, plate-and-frame and regenerative HE.

Due to the low thermal conductivities of gases the overall heat transfer coefficient for gas-to-

gas HEs only ranges between 10 – 40 W/m2o

C, compared to steam condensers, which ranges

between 1000 - 6000 W/m2o

C. Another performance reducing factor is the formation of

deposits on the heat transferring surfaces. This causes an additional thermal resistance which

lowers the heat transfer. The fouling tendency increases with increasing fluid temperature and

decreasing fluid velocity [12]. In conclusion, the rate of heat flow benefits from a high

temperature difference between the fluids and large and clean surface areas. Hence the desired

high flue gas temperature and prevention of deposit formation and corrosion on the HT-HE.

2.3 Alloys and stainless steels

An alloy is a mixture between metals or between a metal and a nonmetal. Together these

elements form a material with more desirable properties, such as corrosion or heat resistance,

than the pure elements. Often high temperature and deposit exposed components in biomass

and waste fired units are made from FeCr or FeCrNi alloys because these materials tend to

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form a protective oxide layer on the surface. This layer is usually a solid solution of iron and

chromium oxide and inhibits the corrosion attack from different alkali salts. Alloys with high

Cr/Fe ratios are more resilient against corrosion attacks, which imply that processes that break

down the chromium oxide, e.g. the formation of chromates, are harmful to the alloy [13, 14].

Stainless steels are iron based alloys containing >10.5 % chromium (Cr) and known for their

heat and corrosion resistance properties. Addition of >8 % nickel (Ni) will enhance the

corrosion resistance further. Depending on the microscopic structures, stainless steels can be

divided in different sub-groups: ferritic, martensitic, austenitic and duplex stainless steels, as

described in Table 1.

Table 1. Typical chemical composition, structure and propertiers of ferritic, martensitic,

austenitic and duplex stainless steels.

Steel sub-group Typical chemical composition

[wt-%]

Structure12

Ferritic Cr

Ni

C

Fe

12.5 – 17

0

<0.1

(balance)

Austenitic

Cr

Ni

C

Fe

16 – 26

6 – 12

<0.1

(balance)

Martensitic

Cr

C

Fe

10.5 – 18

0.2 – 1

(balance)

Duplex Cr

Ni

C

Mo

Fe

18 – 26

4 – 7

<0.1

0 – 4

(balance)

Mixture of

ferritic and

austenitic

stainless steel

1 http://en.wikipedia.org/wiki/Cubic_crystal_system

2 http://en.wikipedia.org/wiki/Tetragonal_crystal_system

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In a crystalline compound the crystals consists of a large number of atoms, grouped together

in a three dimensional space lattice. The lattice can, for example, be made up by tightly

packed cubes or cuboids, Table 1, where each of the eight corner atoms are shared with the

eight neighboring cubes or cuboids.

In the ferritic steel structure an atom is placed in the middle of each cube, a so called body-

centered cubic (bcc) crystal structure. Ferritic stainless steels consist of Fe and Cr together

with small amounts of C. They are not heat treatable but have adequate formability. The

corrosion resistance is higher than that of martensitic stainless steels and they also possess a

good resistance to oxidation. Regarding the economic aspects ferritic steels are relatively

cheap. [15, 16]

Austenitic steels also have a cubic crystal structure but with an atom centered on each side of

the cube, so called face-centered cubic structure (fcc). They consist of Fe, Cr and Ni together

with small amounts of C. This sub-group has more grades and is, due to its superior corrosion

resistance when compared to ferritic and martensitic stainless steels, more widely used.

Austenitic steels have excellent formability and higher thermal expansion and heat capacity

but lower thermal conductivity than other stainless or conventional steels. [15, 16]

During rapid cooling of heated austenite a martensitic structure is obtained. This procedure is

called hardening and results in, as the name purposes, an increased hardness of the steel. The

hardness, in its turn, increases with the C content up to 1 %. The crystal structure is equal to a

stretched bcc, and called body-centered tetragonal (bct). The hardness of the martensitic steels

causes poor formability. They have moderate corrosion resistance and similar thermal

expansion as conventional steel. [15, 16]

Duplex stainless steels have a structural mixture of ferritic and austenitic stainless steel,

resulting in the same excellent corrosion resistance as austenitic steel but with greater

strength. The steels possess reasonable formability and have a thermal expansion between that

of austenitic and ferritic stainless steels. [15, 16]

Other alloying elements than Cr and Ni are also used in stainless steel production. Aluminum

(Al) improves the oxidation and sulfidation resistance and other high temperature corrosion

attacks. Molybdenum (Mo) increases the resistance to both local and general corrosion and is

added to martensitic steels to improve the strength at high temperatures. Together with Ni,

Mo is added to increase the corrosion resistance in reducing environments. In stainless steels

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with operation at temperatures above 750oC Mo should be avoided as it can contribute to low

melting point temperatures. Nitrogen (N) increases the strength and improves the local

corrosion resistance, especially in combination with Mo. Copper (Cu) improves the general

corrosion resistance to acids and improves formability. Carbon (C) increases the strength but

may affect the corrosion resistance negatively. Titanium (Ti) protects the steel against grain

boundary corrosion (corrosion on the boundaries between crystalline grains in the steel

structure, Figure 3) and increases tempering resistance. Sulfur (S) improves the processability

(the preconditions of being processed, e.g. cutting) but reduces corrosion resistance. Cerium

(Ce) improves the strength and adhesion of the surface oxide layer at high temperatures.

Manganese (Mn) increases the solubility of nitrogen in the steel. Silicon (Si) improves the

oxidation resistance. [15, 17, 18, 19]

Stainless steels and alloys have applications in the chemical, processing and oil & gas

industries as well as in power generation.

Figure 3. Cross section of Inconel 600 attacked by grain boundary corrosion [19].

2.4 Chemical composition of biomass and combustion related problems

Biomass is a heterogeneous energy source with great variation in volatile matter, fixed

carbon, moisture content and ash yield, depending on for example biomass source and origin.

The major components in biomass and biomass fuels are C, O and H, but also N, although in

minor amounts [20]. The most commonly occurring ash-forming elements present in biomass

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are Si, Ca, Mg, K, Na, P, S, Cl, Al, Fe and Mn. Regarding the ash-related problems during

combustion Fe and Mn can be excluded, as they tend to form individual oxides with limited

tendencies to interact with the other ash-forming elements. Also, Na can be approximated by

K, as the concentration of Na in biomass, in general, is significantly lower than K and has

similar ash-forming behavior [21]. Concentration and total amount of the ash-forming

elements together with, for example kinetic properties and aggregation state of the matter,

play important roles in the ash-transformation processes. During biomass combustion

elements from the fuel, such as K, Na, P, S and Cl, are to varying degree released to the gas

phase. Through subsequent gas phase reactions alkali phosphates, sulfates, chlorides and

carbonates may form. Together with aerosols and fine fly ash particles the alkali compounds

may condense and form deposit on the cooler heat transferring surfaces causing reduced heat

transfer and corrosion. These problems are some of the main issues regarding the design and

operation of a combustion system [22].

Potassium chloride, KCl, is the most common potassium salt present during biomass

combustion, and is known, through extensive studies, to be highly corrosive even below its

melting point at 770°C [23]. Other potassium salts that can occur in the flue gas are potassium

carbonate, K2CO3, and potassium sulfate, K2SO4. These salts are not as studied as KCl but are

still of relevance since they also may cause deposit formation and corrosion. Particularly

K2CO3 and K2SO4 were found deposited on the HT-HE of an, with biomass fired, EFGT [7].

The three potassium salts have different melting points (892°C and 1069°C for K2CO3 and

K2SO4, respectively) and can therefore condensate on various locations in the combustion

system, depending on the surface temperatures.

2.5 Gibbs energy

Equilibrium is a state that all chemical reaction systems are striving for. When a chemical

equilibrium is reached the concentrations of the reactants and products will no longer change,

as long as the system remains unchanged.

To know in what direction a reaction is spontaneous or if the system is in equilibrium Gibbs

energy, 𝐺, can be used

𝐺 = 𝐻 − 𝑇𝑆 [J] (1)

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where 𝐻 is the enthalpy, 𝑇 is the temperature and 𝑆 is the entropy. For constant temperature

and pressure a chemical reaction is spontaneous in the way of decreasing Gibbs energy. If

there is no change in Gibbs energy the reaction is in chemical equilibrium.

The reaction Gibbs energy, ∆𝑟𝐺, is defined as the slope of the graph of Gibbs energy plotted

against the extent of reaction [24]

(𝜕𝐺

𝜕𝜉)

𝑝,𝑇

= ∆𝑟𝐺 = ∆𝑟𝐺0 + 𝑅𝑇 ln 𝑄 [J/mol] (2)

where the extent of reaction, 𝜉, refers to the amount (moles) of A that has turned into B in the

reaction 𝐴 ↔ 𝐵, 𝑅 is the gas constant, 𝑇 is the temperature and 𝑄 is the ratio between the

activities of the products and the activities of the reactants. ∆𝑟𝐺0 is the standard Gibbs energy

of reaction and is defined as the difference between the products and reactants standard Gibbs

energy of formation

∆𝑟𝐺0 = ∑ 𝑛∆𝑓𝐺0

𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

− ∑ 𝑛∆𝑓𝐺0

𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠

[J/mol] (3)

where ∆𝑓𝐺0 is found in tables. If ∆𝑟𝐺 < 0 the formation of B is spontaneous, if ∆𝑟𝐺 > 0 the

formation of A is spontaneous and if ∆𝑟𝐺 = 0 the reaction is at equilibrium, Figure 4.

Figure 4. Gibbs energy as a function of the extent of reaction. When ∆𝒓𝑮 < 𝟎 the formation of B is spontaneous and

when ∆𝒓𝑮 > 𝟎 the formation of A spontaneous. Equilibrium is reached at ∆𝒓𝑮 = 𝟎.

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2.6 High temperature corrosion

Corrosion is a wide term of a chemical reaction between a chemically unstable material

(usually a metal) and the surrounding environment, resulting in degradation of the physical

properties of the material. High temperature corrosion usually occurs at temperatures above

500oC but in many cases severe corrosion has been encountered well below that temperature

[19].

In this chapter some of the most relevant modes of high temperature corrosion are presented,

with oxidation being the most commonly occurring.

2.6.1 Oxidation and formation of oxides

Oxidation is a chemical half-reaction when a material, the reducing agent, loses electrons and

an ion is formed. For potassium (K) the oxidation becomes

2𝐾 → 2𝐾+ + 2𝑒− (i)

Another material, the oxidizing agent, gains the electron(s) and is reduced. For chlorine (Cl)

the reduction becomes

𝐶𝑙2 + 2𝑒− → 2𝐶𝑙− (ii)

Together reaction (i) and (ii) becomes

2𝐾 + 𝐶𝑙2 → 2𝐾𝐶𝑙 (iii)

which is called a redox reaction. In other words, oxidation does not necessarily have to do

with oxygen (O2), as the name indicates. But here oxidation is referred to as the formation of

oxides, i.e. a material (metal) is oxidized with O2 as the oxidizing agent as

𝑀 + 𝑂2 ↔ 𝑀𝑂2 (iv)

Oxidation is the most important chemical reaction concerning high temperature corrosion.

Metals and alloys are oxidized when heated to extended temperatures in oxidizing

environments. Hot combustion atmospheres often contain excess air which enables high

temperature corrosion on for example super heaters, heat exchangers or turbines. However, in

some cases oxidation can be favorable due to the formation of a surface oxide layer protecting

the metal from other corrosion attacks.

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Whether an oxide is likely to form or not is determined by the partial pressure of O2 and the

temperature. It has been stated that if the oxygen partial pressure in the environment is

greater than the oxygen partial pressure in equilibrium with the oxide, the oxide is likely to

form on the metal surface. Conversely, the oxide is not likely to form [19]. From an Ellingham

diagram, as shown in Figure 5, this is visualized for Cr at 1000oC.

Figure 5. Ellingham diagram with the standard free energies of formation as a function of temperature for different

materials and theis oxides, with the equilibrium oxygen partial preassure for chromium at 1000oC marked in red.

12

A straight line is drawn from the O in the upper left corner through the intersecting point of

Cr at 1000oC. Where the straight line thereby intersects with the right y-axis the equilibrium

O2 partial pressure, 𝑝𝑂2, is shown, here just above 10

-22 atm. On the left y-axis, at the

intersection point with the horizontal red line, the standard Gibbs energy of formation, ∆𝑓𝐺0,

is approximated to -520 kJ/mole O2. This is the equilibrium conditions for the reaction, i.e.

where Eq. (2) equals zero. Here 𝑄 = 𝑎𝐶𝑟2𝑂3

2/3(𝑎𝐶𝑟

4/3∙ 𝑎𝑂2

)⁄ = 1/𝑝𝑂2, because for solids the

activities equals zero, leaving the reaction quotient 𝑄 = 1 𝑎𝑂2⁄ , where 𝑎𝑂2

=

𝑝𝑂2𝑝𝑎𝑡𝑚 = 𝑝𝑂2

⁄ , since 𝑝𝑎𝑡𝑚 = 1 𝑎𝑡𝑚. Thus, at 1000oC Cr2O3 is likely to form when the O2

partial pressure >10-22

atm and at O2 partial pressure <10-22

atm the oxide is not likely to form.

Generally, from Eq. 2 the equilibrium partial pressure of oxygen is given by

𝑝𝑂2= 𝑒∆𝑟𝐺0 𝑅𝑇⁄ [atm] (4)

At atmospheric pressure with 𝑝𝑂2= 0.21 𝑎𝑡𝑚 the formation of oxides is favored for a lot of

metals.

2.6.2 Chloridation

Chloridation is another type of corrosion mechanism and entails the formation of chlorides as

a corrosion product. A metal chloride may form by the reaction

𝑀 + 𝐶𝑙2 ↔ 𝑀𝐶𝑙2 (v)

Whether a chloride is likely to form or not on the metal surface, a phase stability diagram,

Figure 6, can be used. If for example Cr is exposed to an environment consisting of air with

10 ppm Cl2 and 21 % O2 at 600oC, then the partial pressures for Cl2 and O2 are 10

-5 and 10

-0.7

atm respectively. The horizontal and vertical lines of the two partial pressures intersect in the

Cr2O3 region, implying that Cr2O3 will form on the surface of Cr when exposed to the

environment. In order to form CrCl3 or CrCl2 the amount of Cl2 in the atmosphere must

increase and/or the amount of O2 must decrease. At low partial pressure of Cl2 and O2, 10-36

and 10-17

atm respectively, Cr is stable at 600oC. At this specific temperature both CrCl3 and

CrCl2 are solids, with melting point at 1150oC and 820

oC respectively. Some chlorides exhibit

low melting points, for example FeCl3 and FeCl2 at 303oC and 676

oC respectively, resulting

in a liquid corrosion product which enhances the corrosion attack [23].

13

Figure 6. Phase stability diagram for Cr-O-Cl system at 600oC [19]. The intersection between the two red lines shows

what product will form in an environment with 10 ppm Cl2 nd 21 % O2 (pCl2 = 10-5 & pO2 = 10-0.7).

Accordingly, a protective oxide layer lowers the partial pressure of Cl2 at the oxide/metal

interface, preventing chloridation. But if the oxide layer contains cracks or other damages Cl2

can pierce through the layer and reach the metal oxide/metal interface. Then, if the partial

pressure of Cl2 is high enough metal chlorides are formed, and a chloridation attack is

initiated [19]. Volatile metal chlorides can then diffuse to the oxide scale surface. The

increased partial pressure of O2 may lead to oxidation of the metal chlorides, resulting in

loose and non-protective metal oxide scale. The released Cl2 can then again penetrate through

the oxide scale and cause further chloridation. This Cl-cycle, where metals are transported

from the surface to form chlorides and then oxides, is called active oxidation [23] and

visualized in Figure 7.

Figure 7. Shematic picture of active oxidation caused by Cl2 [23].

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2.6.3 Sulfidation

Roughly, biomass contains about 0.01 – 2.33 mole-% sulfur (S) with woody biomass

commonly having the lower concentrations [25]. During combustion with excess air S reacts

with O2 to form SO2 or SO3. At elevated temperatures and together with Na and K these

oxides can form salt vapors. At the lower temperatures of, for example, super heaters or heat

exchangers these vapors may deposit causing sulfidation of the surface, sometimes referred to

as hot corrosion. The hot corrosion morphology is characterized by a thick porous layer of

oxides, followed by a chromium depleted alloy and internal chromium-rich sulfides.

Increasing content of Cr improves the resistance to hot corrosion [19].

As for chloridation, alloys rely on a protecting oxide scale, often Cr2O3, to resist sulfidation

attack. But even in the Cr2O3 region of a Cr-S-O stability diagram the oxide layer has a

limited life, i.e. in the upper right corner in Figure 8. When the protecting oxide scale

eventually breaks down, rapid formation of non-protective sulfides may occur referred to as

breakaway corrosion [19].

Figure 8. Phase stability diagram for Cr-O-S system showing the limited life area of the protecting oxide layer [19].

2.6.4 Carburization

Carburization can occur when a material is exposed to CO, CO2 or hydrocarbon gases. The

product is often internal carbides which results in an embrittlement and mechanical property

degradation of the material. An increase of Ni in a Fe-Ni-Cr or Fe-Ni alloy improves the

carburization resistance. Also Si and Al as alloying element can be beneficial. [19]

15

2.6.5 Nitridation

Nitridation may occur at high temperatures when the oxide scales no longer can provide

protection. Molecular nitrogen (N2) is less harmful than ammonia (NH3) but at temperatures

of 1000oC and higher even N2 can cause serious damage on the material. As the protective

oxide scale degrades pores and cracks are formed, leaving a porous and non-protective

surface. N2, along with other combustion gases, then can penetrate through the oxide scale

and reach the metal to form nitrides. Alloys containing Ti or Al are more likely to suffer from

a nitridation attack as those elements are strong nitride formers. Also Cr and Fe can form

nitrides while Ni does not. [19]

2.6.6 Molten salt corrosion

Molten salts increase the corrosion rate and contribute to removal of the protecting oxide

layer, by dissolving the metal oxides in the melt. The accelerated corrosion rate is partly due

to that liquid phase reactions are faster than solid-solid reactions. Also, the liquid phase can

serve as a pathway for ionic charge transfer for the electrochemical attack. The corrosion is

strongly dependent on temperature and can often be reduced by decreased temperature. Also,

the corrosivity varies between different salts with molten carbonates being less corrosive than

molten chlorides. [19]

Generally, pure salts have high melting points, but together with other salts the melting point

can be lowered. KCl, for example, has a melting point of 770°C and K2SO4 a melting point

of 1069°C, but together they have a, so called, eutectic temperature of 694°C. KCl can also

form eutectics with metal chlorides, as low as 202-220°C together with FeCl3. [23]

The breakdown of a Fe-oxide layer by molten KCl can be described by the reaction

𝐹𝑒2𝑂3(𝑠) + 𝐾𝐶𝑙(𝑠, 𝑙) + 3𝑆𝑂2(𝑔) +3

2𝑂2(𝑔)

→ 2𝐹𝑒𝐶𝑙3(𝑠, 𝑙, 𝑔) + 3𝐾2𝑆𝑂4(𝑠)

(vi)

FeCl3 and K2SO4 may then participate in further reactions

2𝐹𝑒𝐶𝑙3(𝑠, 𝑙, 𝑔) + 3𝑆𝑂2 + 3𝑂2(𝑔) → 𝐹𝑒2(𝑆𝑂4)3(𝑠) + 3𝐶𝑙2(𝑔) (vii)

3𝐾2𝑆𝑂4(𝑠) + 𝐹𝑒2𝑂3(𝑠) + 3𝑂2(𝑔) → 2𝐾3𝐹𝑒(𝑆𝑂4)3(𝑠, 𝑙) (viii)

releasing both Cl2, enabling active oxidation, and alkali-iron-trisulfate (K2Fe(SO4)3) which

also is known to be corrosive. [23]

16

Molten chlorides tend to cause grain boundary corrosion rather than metal loss. When

stainless steels are exposed to high temperatures, for example during heat treatment or

welding, Cr-carbides can precipitate to the grain boundaries. As a result the grain surfaces are

depleted in chromium, due to the formation of Cr-carbides, and thus less resistant to grain

boundary corrosion. Decreased C content increases the resistance to grain boundary corrosion.

Grain size also has a positive effect on the performance of the stainless steel. Another

common corrosion morphology is internal attack by void formation. The formation of a Cr

oxide layer on the steel surface causes outward migration of Cr, leading to internal void

formation, which appears as “spots" instead of “cracks”. [19, 26]

2.7 Scanning electron microscopy with energy-dispersive X-ray spectroscopy

The scanning electron microscope (SEM) is a widely used surface analytical technique, with a

magnification of 10 – 100 000 times. The advantage of electron microscopy compared to

optical microscopy is that the electron can have a much shorter wavelength than light. This

entails the possibility of significantly higher magnifications together with elementary

composition of the material.

The electrons are generated in an electron cannon. With electromagnetic lenses the electrons

are formed into a thin beam which scans a rectangular area on the sample surface. The

interaction between the electron beam and the sample causes electrons to emit from the

surface. If the incoming electron is reflected it is called a back-scattered electron (BSE). If, on

the other hand, the electron is absorbed by the sample another electron can be emitted, a so

called secondary electron (SE). The emitted electrons are detected by a BSE or SE detector

and an image is generated, where every pixel corresponds to a spot on the sample which has

been hit by the electron beam and electron have been detected.

With a SEM both the topography and the elemental composition of the sample surface can be

determined. When imaging, the details on the sample that are facing the detector appears

brighter on the screen, because the emitted electrons have a greater possibility of detection.

This enables a topographic contrast similar to that of a light source placed at the position of

the detector. Also, surfaces with high average atomic number, such as alloys, appear brighter

in the picture.

In addition to electrons, the sample also emits X-rays. When an electron in the beam hits the

specific electrons in an atom in the sample, the electron is ionized leaving a vacancy in one of

17

the shells. The empty space in the shell is filled with an electron of higher energy from an

outer shell and excess energy is emitted as an X-ray photon, Figure 9. The binding energy of

an electron depends on the element and in what shell the electron is situated, with higher

energy for heavy elements and shells close to the nucleus. The unique binding energies for

each element give a fingerprint which enables elemental discrimination when the X-ray

photons are detected. The detected photons create a spectrum and by the size of the peaks the

elemental composition can be determined. SEM-EDS is considered a semi-quantitative

method, as the accelerating voltage, morphology and actual chemical composition influence

the results.

Figure 9. Schematic picture of emission of en X-ray photon due to electron radiation.

Elemental analysis can be done by point analysis, surface analysis, mapping, or line scanning.

Point analysis gives information about a single point of the sample. During surface analysis

larger part of the sample is analyzed giving the average elemental composition in that area.

Mapping reveals the elemental distribution for the chosen area. With a linescan the elemental

distribution along a line is analyzed. [27]

18

3 Experimental

In this chapter the exposure and analytical methods used in this study are presented. Similar

experimental setup and approach have been used in previous research studies, with corrosion

patterns similar to those of real cases [28, 29, 30, 31]. However, these exposures have often

been performed at lower temperatures.

3.1 Materials

Four alloys and five salts and salt mixtures were chosen for the study.

3.1.1 Alloys

X20Cr13 (X20) is a martensitic stainless steel with, after hardening, good corrosion resistance

in chlorine free environments. The corrosion resistance is lower compared to common

austenitic stainless steels. X20 is resistant to scaling during continuous service at temperatures

≤675oC and ≤760

oC during periodical use. It is often used in knife blades, cutlery and surgical

instruments but also in high temperature (580oC) applications such as super heater tubes.

[32, 33, 34]

253MA is an austenitic stainless steel, suitable for high temperature applications between 750

– 1100oC. According to its manufacturer it has excellent oxidation resistance, good resistance

to embrittlement and superior strength at high temperatures with applications in oil industry,

power generation and energy conversion plants [35].

Inconel 600 is a NiCrFe superalloy with good resistance to corrosion caused by chloride,

alkaline environments and stress and can be used in applications in temperatures up to 1175oC

[36]. It is both used in muffles, furnace components and nuclear reactors.

Kanthal APMT (Kanthal) is a FeCrAl alloy with applications up to 1250oC. It is said to have

longer service life than for example NiCr alloys and provides excellent corrosion resistance in

most atmospheres [37]. The ability to form Al2O3 provides excellent protection against further

oxidation at high temperatures. At lower temperatures, when the grow rate of Al2O3 is slow,

the alloy depends on the formation of a protective Cr2O3 scale [19].

Chemical compositions of the alloying elements in the materials used in the corrosion study

are presented in Table 2, with own SEM-EDS analysis data shown in brackets. For a complete

elemental analysis see Appendix 1.

19

Table 2. Chemical composition of the alloys used in the corrosion study with own SEM-EDS analysis data shown in

brackets.

Chemical composition [wt-%]

X203 253MA

4 Inconel 6005 Kanthal

6

Fe 85 (85) 64 (66) 6-10 (10) 71 (70) Ni <1 (0) 11 (11) ≥72 (73) - (0) Cr 13 (14) 21 (21) 14-17 (16) 21 (22) Al <1 (0) - (0) - (0) 5 (5)

3.1.2 Alkali salts and salt mixtures

The five different potassium salts and

salt mixtures chosen were all relevant

for biomass combustion flue gas.

Chemical compositions of the alloys in

the corrosion study are presented in

Table 3.

Figure 10 shows the ternary phase diagram

of the K2Cl2-K2CO3-K2SO4 system, with the

salts and salt mixtures used in the exposures

marked in red. KCl (#1) has the lowest

melting point at 770oC while K2SO4 (#3) has

the highest at 1 069oC. If KCl is added to

K2SO4 the melting temperature decreases,

and by 20 mole-% KCl (that is 10 mole-%

K2Cl2) the melting temperature is a little over

1000oC (#4). If K2CO3 is added to the salt

mixture to 30 mole-% (#5), at the expense of

K2SO4, the melting point decreases further to

about 900oC. Pure K2CO3 (#2) has a melting

point at 892oC.

3 Fertiganalyse from Outokumpu

4 Test coupons dokumentation from Outokumpu

5 http://www.haraldpihl.se/attachments/007_Inconel%20alloy%20600.pdf

6 Certificate from Sandvik Technology AB

Figure 10. Ternary phase diagram of the K2Cl2-K2CO3-

K2SO4 system with the salt/salt mixtures used in the study

marked in red [28].

Table 3. Chemical composition of the alkali salts and salt

mixtures used in the corrosion study.

20

3.2 Exposure

The alloys were cut in 2 x 2 cm plates with a metal shear. After that, three replicates at the

time were placed on two ceramic cylinders on the sample holder and covered with a 3 mm

thick layer of salt or salt mixture, Figure 11.

Figure 11. Samples prior exposure.

No samples were exposed to KCl at 900oC and 1000

oC, nor to K2CO3 at 1000

oC, since the

melting temperatures of the salts were already exceeded. At those temperatures the salt would

either run off or evaporate, and the results would not be of relevance to the study.

The tube furnace, Figure 12, were heated to the desired temperature (700, 800, 900 or

1000oC) before the samples were inserted and exposed for 24 h. A thermocouple was attached

under the samples for measuring the temperature. For preventing any gaseous compounds

from dispersing into the lab a ventilation device was used.

After cooling, one of the samples were epoxy casted and ground with coarse paper until

reveling of the cross sectional area. With finer paper the area was further ground until a

smooth surface was achieved for analysis.

Figure 12. Tube furnace used in the study.

21

When the samples were casted in epoxy and grinded, the goal was to have a vertical sample

revealing the cross-section. Due to the rough procedures of casting and grinding, this goal was

hard to achieve. A 5 degree offset angle to the vertical was estimated to be the maximum error

giving a 0.38 % error.

3.3 SEM-EDS analysis

Chemical (elemental composition) and morphological (form and structure) analysis was done

using scanning electron microscopy equipped with energy dispersive X-ray analysis (SEM-

EDS). For the EDS used in this analysis the X-rays must first pass through a Be-window

before reaching the detector. This window may absorb elements lighter than Na, resulting in a

large experimental uncertainty in the detection of for example C, N and O.

The corrosion resistance and interaction between deposits and materials was determined by

studying the thickness and structure of the corrosion layer and the chemical composition of

the various layers that were formed when the alloys reacted with the different potassium salts.

As a reference samples of unexposed alloys were analyzed as well as samples exposed in air

at all temperatures. For each sample a linescan was carried out to determine the elemental

composition of the different layers of the samples. Figure 13 shows an arbitrary vertical line

applied for elemental analysis. The numbers represents typical layers/lines found along the

cross-section.

Figure 13. SEM picture of cross-sectional sample surface with a typical line used for alemental analysis together with

common layers; 1 – unaffected sample/sample substrate, 2 – corrosion front, 3 – internal corrosion (here grain

boundary corrosion), 4 – corrosion layer, 5 – scale, 6 – resin.

The total corrosion is defined as the sum of the thickness of the corrosion scale and corrosion

layer together with the depth of internal corrosion. If present, the alkali salt crust containing

significant amounts of alloying elements is included in the total corrosion. The location of the

linescan was chosen at an overall presentable place on the sample cross section.

22

4 Results

In this section the most relevant discoveries of the study are presented, one material at the

time. All chemical compositions are given exclusive of O, N and C, due to the large

experimental uncertainty of detection in de EDS. Pictures of all exposed samples are found in

Appendix 2 – Appendix 13.

4.1 X20

Figure 14 shows X20 exposed in air at 700oC and 800

oC. At both temperatures a Cr rich

corrosion layer was formed, being more visible at 800oC. Elemental analysis of the steel

reveals approximately 10 µm and 30 µm Cr depletion, respectively, under the corrosion layer.

Figure 14. Cross section of X20 exposed in air at 700oC (left) and 800oC (right).

At 900oC and 1000

oC X20 developed a thick Fe rich multi-layered corrosion scale on the

samples, Figure 15, with the outermost layers having the highest Fe content. The lower part of

the scales was Cr enriched but still dominated by Fe.

Figure 15. X20 exposed in air at 900oC (top) and 1000oC (bottom).

KCl caused the formation of an approximately 750 µm multi-layered corrosion scale on the

sample surface when exposed at 700oC, Figure 16. The upper and bottom part of the scale had

23

a Cr enriched layer but all layers were dominated by Fe. Also, alkali salt elements were found

throughout the whole scale. Under the corrosion scale the steel was Cr-depleted

approximately 50 µm into the substrate. No exposures were done at the higher temperatures.

Figure 16. X20 exposed to KCl at700oC.

When exposed to K2CO3 at 700oC and 800

oC, Figure 17, a corrosion layer was formed on the

sample surface, approximately 15 and 50 µm thick, respectively.

Figure 17. Cross section of X20 exposed to K2CO3 at 700oC (left) and 800oC (right).

Exposed to K2CO3 at 700oC the Cr content was more than twice as high at the corrosion front

as in the unaffected steel. When the temperature was raised to 800oC a slightly Fe enriched

corrosion front was detected. Both samples had corrosion layers containing K, with higher

amount at the higher temperature. At 900oC X20 developed a thick corrosion scale that

detached from the surface after cooling. At the bottom of the scale the Cr-content was

doubled while the middle zone almost entirely consisted of Fe. The upper part of the scale

contained a limited amount of Cr, lowered amount of Fe and about 30 wt-% K. Underneath

the scale the sample appeared unaffected, Figure 18.

Figure 18. X20 exposed to K2CO3 at 900oC.

24

Figure 19 shows X20 exposed to K2SO4 at 700 – 900oC.

Figure 19. Cross section of X20 exposed to K2SO4 at 700oC (left), 800oC (middle) and 900oC (right).

At 700oC an approximately 10 µm Cr depletion was detected at the surface. When the

temperature was raised to 800oC a 15 µm corrosion product formed on the surface, with the

outermost part waving almost a tripled Cr content compared to the pure steel. Under the

corrosion product the steel was Cr depleted approximately 15 µm into the substrate. At 900oC

a 150 µm two-layered corrosion product formed. The layers were dominated by Fe, with

increasing content towards the surface. At the corrosion scale/substrate interface the Cr

content was doubled compared to the unaffected steel. No alkali salt elements were detected

at the corrosion front at any of the temperatures. At 1000oC X20 experienced extensive

corrosion. At most, 0.2 mm of the original 1.0 mm thick sample was left unaffected, Figure

20. The corrosion scale was multi-layered and Fe enriched with S being the dominant alkali

salt element at the corrosion front and K most commonly occurring in the corrosion scale,

although traces of both elements were found throughout the entire corrosion product.

25

Figure 20. X20 exposed to K2SO4 at 1000oC.

The cross sections of X20 exposed to 0.2KCl + 0.8K2SO4 at 700oC and 800

oC are shown in

Figure 21. Both samples developed Fe enriched corrosion scales, with elevated Cr content at

the bottom, that detached after cooling. At 700oC traces of alkali salt elements were found

between the corrosion scale and the steel but not at the corrosion front for either of the

samples. Due to the extensive corrosion attack at 700oC and 800

oC temperatures, no

exposures were done at 900oC and 1000

oC.

Figure 21. Cross section of X20 exposed to 0.2KCl + 0.8K2SO4 at 700oC (top) and 800oC (bottom).

Exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 700oC a Fe enriched and unattached corrosion

scale formed. On top of that a framework containing about 90 wt-% K was found. Between

the corrosion scale and the steel traces of K, S and Cl were detected, but not at the corrosion

front. Because of the severe corrosion attack already at 700oC, this was the only exposure of

X20 to 0.2KCl + 0.3K2CO3 + 0.5K2SO4.

Figure 22. Cross section of X20 exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 700oC.

Total corrosion of X20 is shown in Figure 23. Exposed to air, K2CO3 and K2SO4 a distinct

increase in total corrosion is visible when the temperature was raised from 800°C to 900°C.

The most severe corrosion attack occurred when X20 was exposed to K2SO4 at 1000°C, but

the corrosion resistance at 700°C and 800°C is relatively good. When KCl was present in the

26

salt scale formation occurred at all temperatures. The missing bars are samples excluded from

the experimental matrix. The patterned bars show the samples that formed corrosion scales.

Figure 23. Total corrosion of X20 with the patterned bars showing the sample that formed corrosion scales.

The total corrosion of the samples that did not form corrosion scales are presented in Figure

24. The temperature is shown to be a corrosion accelerating factor regardless of exposure

environment. Both internal corrosion and corrosion layer grew thicker with increased

temperature.

Figure 24. Total corrosion of the samples of X20 that did not form corosion scales.

0

200

400

600

800

1000

1200

1400

Air KCl K2CO3 K2SO4 0.2KCl +0.8K2SO4

0.2KCl +0.3K2CO3 +0.5K2SO4

Tota

l co

rro

sio

n [

µm

]

700°C

800°C

900°C

1000°C

0

20

40

60

80

100

120

140

160

180

Air Air K2CO3 K2CO3 K2SO4 K2SO4 K2SO4

700°C 800°C 700°C 800°C 700°C 800°C 900°C

Tota

l co

rro

sio

n [

µm

]

Internal corrosion Corrosion layer

27

4.2 253MA

In Figure 25 the cross section of 253MA exposed in air at 700 – 1000oC is shown.

Figure 25. Cross section of 253MA exposed in air at 700 – 1000oC (left to right).

Exposed at 700oC no corrosion was visible. Elemental analysis showed a decrease of Cr

together with a corresponding increase of Fe at the surface and approximately 5 µm into the

sample. At 800oC the same tendencies were observed 10 µm into the sample, also, a thin Cr

rich layer formed on the sample surface. Exposed at 900oC a Cr-peak was detected 30 µm into

the sample. After that, a slightly Cr depleted zone followed before the 5 µm Cr rich corrosion

layer on the surface of the steel. When the temperature was raised to 1000oC the corrosion

attack got more severe with a total 115 µm depth of impact. The corrosion layer was Cr

enriched while the scales had elevated Fe content. One of the three replicates developed a

corrosion scale that detached after cooling.

When exposed to KCl at 700oC and 800

oC 253MA developed a loose corrosion scale that

detached after cooling, Figure 26. The outer layer of the corrosion product was rich in Fe

while the inner layer was rich in Cr.

28

Figure 26. Cross section of 253MA exposed to KCl at 700oC (top) and 800oC (bottom).

No Cr rich outermost layer was detected under the corrosion product on either of the samples,

instead the Ni content increased towards the sample surface. At 700oC traces of alkali salt

elements were found in the corrosion layer, and grain boundary corrosion was visible roughly

60 µm into the sample. At 800oC the steel was Cr depleted all the way to the corrosion front,

approximately 300 µm into sample where also traces of alkali salt elements were found. Here

the corrosion did not follow grain boundaries but appeared as dark zones or spots, Figure 27.

Figure 27. Corrosion attack of 253MA, exposed to KCl at 700oC (left) and 800oC (right).

253MA exposed to K2CO3 at 700oC and 800

oC developed an approximately 50 µm corrosion

layer on the surface. At 900oC the corrosion layer had grown to 100 µm. Figure 28 shows the

cross sections of the exposed steel. A decrease in Cr content was detected under the corrosion

29

layers, reaching approximately 6, 22 and 59 µm into the steel, respectively. The average

elemental composition of the corrosion layer changed as the temperature was raised. At 700oC

the corrosion layer was dominated by the alloying elements, Fe, Cr and Ni, while at 800oC

and 900oC the corrosion product was mainly composed by K.

Figure 28. Cross section of 253MA exposed to K2CO3 at 700 – 900oC (left to right).

SEM pictures of the corrosion layers formed when 253MA was exposed to K2SO4 are

presented in Figure 29. Exposed to K2SO4 at 700oC 253MA developed an approximately 15

µm corrosion layer. The bottom part of the corrosion layer was Cr enriched while the upper

part was Fe enriched, and from the alkali salt only K was detected. Under the corrosion layer

the sample was affected approximately 10 µm into the steel. At 800oC both K and S was

detected in the 5 µm Cr rich corrosion layer. Under the corrosion layer the depth of impact

was determined to about 10 µm with an increase in Fe and a decrease of Cr and Ni content.

The 10 µm corrosion layer formed at 900oC contained traces of K and S, and had increased Cr

content while the Fe and Ni content were decreased. The steel was affected 30 µm into the

30

substrate. At 1000oC a significant increase in the corrosion layer thickness was observed. The

20 µm corrosion layer had higher Fe content than the unaffected steel together with lower Cr

and Ni content, also, traces of alkali salt elements were found. At the bottom of the corrosion

layer a Cr-peak was detected followed by approximately 40 µm Cr depletion. Signs of grain

boundary corrosion were visible.

Figure 29. Cross section of 253MA exposed to K2SO4 at 700 – 1000oC (left to right).

Cross section pictures of the samples exposed to the binary alkali salt mixture of 0.2KCl +

0.8K2SO4 at 700 – 1000oC are shown in Figure 30. At 700

oC most of the scale consisted of

alkali salt melt while at the higher temperatures the scales were more multi layered. At

1000oC 253MA also experienced extensive pitting corrosion. K was the most common alkali

salt element throughout the scales and Fe were the most common alloying element. As for the

samples exposed to only KCl, the Ni content increased towards the sample surface. No Cr-

rich layer appeared under the scales, but on the underside of the scales the Cr content varied

from approximately 50 – 60 wt-% for all samples. Traces of K, S and Cl were detected in the

corrosion layers at al temperatures.

31

Figure 30. Cross section of 253MA exposed to 0.2KCl + 0.8K2SO4 at 700oC (top left), 800oC (top right), 900oC (bottom

left) and 1000oC (bottom right). At 1000oC pittin corrosion occured

The internal corrosion depth was about 50 µm at 700 – 900oC and 150 µm at 1000

oC. Signs of

grain boundary corrosion attacks is visible for all four samples, Figure 31.

Figure 31. Cross section of 253MA exposed to 0.2KCl + 0.8K2SO4 at 700 – 1000°C (left to right).

Figure 32 shows the cross sections of 253MA exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4. At

700oC a salt crust formed on top of the corrosion layer, at 800

oC an unattached corrosion scale

formed and at 900oC 253MA experienced pitting corrosion. Due to the severe corrosion attack

at 900oC, no exposure was done at 1000

oC.

32

Figure 32. 253MA exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at at700 - 900oC (top to bottom).

A Cr rich layer was detected on the underside of the corrosion products which increased in Cr

content with elevated temperature. No clear internal corrosion attack was found at 700oC,

only an approximately 60 µm corrosion layer under the alkali salt crust. At 800oC and 900

oC

a grain boundary corrosion attack was visible, with increased Ni content and decreased Cr

content, approximately 60 µm into the steel, Figure 33. At the corrosion front traces of K, S

and Cl were detected.

Figure 33. Grain boundary corrosion attack of 253MA exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 800oC (left) and

900oC (right).

33

Total corrosion of 253MA is shown in Figure 34, with the patterned bars showing the samples

that formed corrosion scales. The alkali salts containing KCl caused the most severe corrosion

attacks, with scale formation at all temperatures. The least harmful salt was K2SO4 which at

800oC and 1000

oC causes less total corrosion than air.

Figure 34. Total corrosion of 253MA with the patterned bars showing the sample that formed corrosion scales.

The total corrosion of the samples that did not form corrosion scales are shown in Figure 35.

Exposed to air the total corrosion is dominated by the internal corrosion while exposed to

K2CO3 the corrosion layer accounted for most of the total corrosion.

Figure 35. Total corrosion of the samples of 253MA that did not form corrosion scales.

No exposure to KCl was done at 900oC and 1000

oC. At 1000

oC exposures to K2CO3 and to

0.2KCl + 0.3K2CO3 + 0.5K2SO4 were also excluded.

0

200

400

600

800

1000

1200

1400

Air KCl K2CO3 K2SO4 0.2KCl +0.8K2SO4

0.2KCl +0.3K2CO3 +0.5K2SO4

Tota

l co

rro

sio

n [

µm

]

700°C

800°C

900°C

1000°C

0

20

40

60

80

100

120

140

160

180

Air Air Air Air K2CO3 K2CO3 K2CO3 K2SO4 K2SO4 K2SO4 K2SO4

700°C 800°C 900°C 1000°C 700°C 800°C 900°C 700°C 800°C 900°C 1000°C

Tota

l co

rro

sio

n [

µm

]

Internal corrosion Corrosion layer

34

4.3 Inconel 600

When exposed to air at 700oC and 800

oC, Inconel 600 showed no sign of corrosion, Figure

36. Elemental analysis showed a 2 µm Cr enriched layer at the surface followed by a slight Cr

depleted zone about 5 µm and 15 µm deep, respectively.

Figure 36. Cross section of Inconel 600 exposed to air at 700 oC (left) and 800oC (right).

At 900oC and 1000

oC a corrosion layer was visible, Figure 37. At the bottom of the corrosion

layer formed at 900oC a Cr peak was detected and the Cr content was elevated throughout the

10 µm corrosion layer. Under the corrosion layer the alloy had lowered Cr content another 10

µm into the substrate. At 1000oC cracks were visible 20 µm into the alloy, under the corrosion

layer, indicating that grain boundary corrosion had occurred. From Figure 37 it is also visible

that the corrosion layer formed at 1000oC is not completely attached to the rest of the sample.

Figure 37. Cross section of Inconel 600 exposed to ait at 900 oC (left) and 1000oC (right).

35

When exposed to KCl, at 700oC and 800

oC, Inconel 600 developed a loose scale that peeled

of the sample surface after cooling. The underside of scales was dominated by Ni, but except

from that the two scales had different elemental composition. Under the scale of the sample

exposed at 700oC a thin Cr enriched corrosion layer covered the unaffected alloy. When the

temperature was raised to 800oC a distinct depletion of Cr and Fe was found together with an

enrichment of Ni, under the corrosion scale. The internal corrosion was approximated to 100

µm into the alloy. The cross section of Inconel 600 exposed to KCl, shown in Figure 38,

reveals the increased corrosion attack when the temperature is raised from 700oC to 800

oC.

Traces of alkali salt elements were found all the way down to the corrosion front for the

sample exposed at 800oC.

Figure 38. Cross section of Inconel 600 exposed to KCl at 700oC (left) and 800oC (right).

Exposed to K2CO3, Inconel 600 developed different types of corrosion layers at the different

temperatures, Figure 39.

Figure 39. Cross section of Inconel 600 exposed to K2CO3 at 700 - 900oC (left to right).

36

The corrosion layer thickness and the internal corrosion increased as the temperature was

raised. At all temperatures K was detected in the corrosion layers. At 700oC a 5 µm Fe

enriched corrosion layer formed and no distinct Cr gradient was detected in the substrate. At

800oC the 25 µm, Fe enriched, corrosion layer was covered by another corrosion product,

consisting mostly of K. At 900oC a multi-layered 60 µm corrosion product formed on the

surface and under that the alloy was affected another 30 µm. In all three cases a Cr-peak was

detected at the corrosion front.

The exposure to K2SO4 at 700oC resulted in an approximately 5 µm corrosion layer and a 10

µm Cr gradient. At 800oC a similar decrease in Cr was detected, but the corrosion layer

thickness had doubled to 10 µm. It also contained amounts of K and S. In both cases the

corrosion layers had elevated Cr and Fe content while the Ni content was lowered. When

exposed at 900oC and 1000

oC the corrosion impact increased, Figure 40.

Figure 40. Cross section of Inconel 600 exposed to K2SO4 at 900oC (left) and 1000oC (right).

At 900oC grain boundary corrosion was visible 30 µm into the sample, also, the corrosion

layer had grown. At 1000oC the grain boundary corrosion reached 64 µm into the sample. The

elemental composition of the corrosion layer had an elevated Fe and Cr content and a lowered

Ni content, together with traces of alkali salt elements.

37

The binary alkali salt mixture of 0.2KCl + 0.8K2SO4 caused scale formation and severe

corrosion at all temperatures, Figure 41. Together with salt residues, Ni rich compounds were

found in the scales while the bottom of the scales was Cr enriched.

Figure 41. Cross section of Inconel 600 exposed to 0.2KCl + 0.8K2SO4 at 700oC (top left), 800oC (top right), 900oC

(bottom left) and 1000oC (bottom right).

The internal corrosion of the alloy at 700 – 900

oC, shown in Figure 42, has a depth of 65, 210

and 75 µm respectively. All three samples experience some grade of Cr-depletion but only at

800oC a Cr rich surface was detected. Traces of alkali salt elements were found at the

corrosion front for all samples, aside from Cl that was only detected at the two lower

temperatures.

Figure 42. Cross section of Inconel 600 exposed to 0.2KCl + 0.8K2SO4 at 700 - 900oC (left to right).

38

At 1000oC the alloy experienced extensive corrosion, with a 200 µm corrosion layer and 750

µm internal corrosion, Figure 43. The Cr enriched corrosion layer is followed by a zone with

bright Ni-rich grains with a Cr-rich corrosion product in between. The 300 µm grain boundary

corrosion zone is almost completely Cr depleted.

Figure 43.Variations inte the elemantal composition of Inconel 600 exposed to 0.2KCl + 0.8K2SO4 at 1000oC.

Cross sections of Inconel 600 Exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 700 – 1000oC is

shown in Figure 44. Exposed to the salt mixture at 700oC Inconel 600 experienced a small

impact of corrosion, as a decrease in Cr content and increase in Ni content the outermost 15

µm of the sample. At 800oC and 900

oC the alloy developed a scale that detached after

cooling. The bottom of the corrosion scales had a Cr rich surface while the majority of the

scale consisted of alkali salt elements. Also, zones of Ni rich compounds were found in the

scales.

Figure 44. Cross section of Inconel 600 Exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at700oC (top left), 800oC (top

right), 900oC (bottom left) and 1000oC (bottom right).

39

Figure 45 shows the cross-sectional SEM pictures of the exposed alloy. At 800oC and 900

oC

the samples experienced Cr depleted and Ni enriched internal corrosion, 45 µm and 30 µm

into the sample respectively. Exposed at 1000oC, Inconel 600 developed a 15 µm piecewise

unattached corrosion layer on the surface, and under the Cr enriched corrosion layer 45 µm

internal corrosion was detected.

Figure 45. Cross section of Inconel 600 exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 700 – 1000oC (left to right).

Total corrosion of Inconel 600 is shown in Figure 46. As for 253MA the KCl containing salts

are the most corrosive. Relatively low total corrosion attacks are shown for K2CO3 and

K2SO4, although grain boundary corrosion was seen when exposed to K2SO4 at 900°C and

1000°C. The patterned bars show the samples that formed corrosion scales.

Figure 46. Total corrosion of Inconel 600 with the patterned bars showing the sample that formed corrosion scales.

0

500

1000

1500

2000

Air KCl K2CO3 K2SO4 0.2KCl +0.8K2SO4

0.2KCl +0.3K2CO3 +0.5K2SO4

Tota

l co

rro

sio

n [

µm

]

700°C

800°C

900°C

1000°C

40

Figure 47 shows the total corrosion of Inconel 600, with the samples that formed corrosion

scales excluded. Exposed to air the total corrosion rate is rather linear while the rate when

exposed to K2SO4 is more exponential. The total corrosion when exposed to K2CO3 is mainly

due to the thick corrosion layer. The ternary salt mixture did not cause any forming of loose

corrosion scale at 700°C and 1000°C, while at 800°C and 900°C flakes of corrosion scale

peeled of the samples.

Figure 47. Total corrosion of the samples of Inconel 600 that did not form corosion scales.

Exposures to KCl at 900°C and 1000°C together with K2CO3 at 1000°C were excluded from

the study.

4.4 Kanthal

Figure 48 shows a cross section of the pre-

oxidized steel. The pre-treatment resulted in

an approximately 10 µm Al enriched

corrosion layer. The Al gradient reached

from 5 wt-% at the bottom to 25 wt-% at the

surface. The bottom of the oxide-layer was

Cr enriched.

Figure 48. Cross section of pre-oxidized Kanthal.

0

20

40

60

80

100

120

Air

Air

Air

Air

K2

CO

3

K2

CO

3

K2

CO

3

K2

SO4

K2

SO4

K2

SO4

K2

SO4

0.2

KC

l + 0

.3K

2C

O3

+0

.5K

2SO

4

0.2

KC

l + 0

.3K

2C

O3

+0

.5K

2SO

4

700°C 800°C 900°C 1000°C 700°C 800°C 900°C 700°C 800°C 900°C 1000°C 700°C 1000°C

Tota

l co

rro

sio

n [

µm

]

Internal corrosion Corrosion layer

41

When Kanthal was exposed to air the corrosion layer grew thicker, Figure 49. At 700 - 900oC

the corrosion layer was determined to 70 – 80 µm while at 1000oC the thickness was

approximately 40 µm. No internal corrosion was distinguished for any of the samples.

Figure 49. Cross section of Kantal exposed in air at 700 – 1000oC (left to right).

Exposed to KCl Kanthal experienced severe corrosion at both 700oC and 800

oC. At 700

oC an

approximately 1000 µm unattached and multi layered corrosion scale formed, dominated by

Fe and Cr. The outermost corrosion layers had a higher Al content than the inner layers. At

800oC the 500 µm corrosion scale was dominated by Cr on the underside and by Fe on the

top, also, pitting corrosion occurred, Figure 50. K and Cl were found throughout the corrosion

scales.

Figure 50. Kanthal exposed to KCl at 700oC (top) and 800oC (bottom).

42

No distinct internal corrosion was distinguished at 700oC while at 800

oC an approximately 85

µm internal corrosion was detected, with traces of K and Cl all the way to the corrosion front.

The corrosion layer did not grow when exposed to K2CO3 at 700oC, and the same Al

depletion under the corrosion layer, as for the pre-oxidized steel, was detected. At 800oC the

lower corrosion layer had a deviating structure compared to the other samples and the upper

corrosion layer was dominated by K. When the temperature was raised to 900oC the corrosion

layer had almost doubled compared to the pre-oxidized steel. The outer part of the corrosion

layer was Cr enriched and the bottom was Al enriched. Under the corrosion layer the steel

appeared unaffected. In all corrosion layers of the samples exposed to K2CO3, Figure 51, 20 –

70 wt-% K was detected.

Figure 51. Cross section of Kanthal exposed to K2CO3 at 700oC (left) and 800oC (middle) and 900oC (right).

Kanthal exposed to K2SO4 is shown in Figure 52. From the picture a growth of the corrosion

layer is visible from 700 – 900

oC. When the temperature was raised to 1000

oC the corrosion

layer growth decreased. At all temperatures the corrosion layer was Al enriched and at 800oC

and 900oC an Al-peak was detected at the corrosion layer/substrate interface. The amount of

K and S detected in the corrosion layer increased with temperature. The samples did not

experience any internal corrosion.

43

Figure 52. Cross section of Kanthal exposed to K2SO4 at 700 – 1000oC (left to right).

The alkali salt mixture of 0.2KCl + 0.8K2SO4 caused Kanthal to form an unattached corrosion

scale at 700oC. At 800

oC an alkali salt dominated corrosion product covered the sample. At

900oC the alkali salt scale was unattached from the sample, except in the edges, forming a

void between the alloy and the salt. At 1000oC an approximately 120 µm corrosion layer

formed under the alkali salt scale. All alkali salt scales measured >500 µm and contained

about 15 – 25 wt-% alloying elements, but were dominated by K and S. Underneath the

corrosion products no internal corrosion was detected at any temperature. Figure 53 shows

Kanthal exposed to 0.2KCl + 0.8K2SO4 at 700 - 1000oC.

Figure 53. Cross section of Kanthal exposed to 0.2KCl + 0.8K2SO4 at 700 – 1000oC (left to right).

44

Figure 54 shows the cross section of Kanthal exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at

700oC and 800

oC. A >500 µm alkali salt scale, containing approximately 30 wt-% alloying

elements, covered the samples. At the bottom of the Fe dominated corrosion layers, closest to

the substrate, an Al-peak was detected. K and S were present in the corrosion layer for both

samples and at 700oC also Cl was detected.

Figure 54. Cross section of Kanthal exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 700oC (left) and 800oC (right).

At 900oC and 1000

oC the alkali salt dominated scale had thinned, Figure 55. Like at the lower

temperatures the corrosion layers had an Al-peak in the bottom but were now dominated by

K. As for the binary alkali salt mixture, the alloy appeared rather unaffected under the

corrosion layers at all temperatures.

Figure 55. Cross section of Kanthal exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 at 900oC (left) and 1000oC (right).

Figure 56 shows the total corrosion of Kanthal exposed to air and the alkali salt/salt mixtures.

KCl and the binary salt mixture caused the most sever corrosion attacks. The ternary salt

mixture caused the formation of thick corrosion layers at the two lower temperatures but the

45

corrosion attack decreased distinctly at 900°C and 1000°C. The exposure to air caused greater

total corrosion than the exposure to K2CO3 and K2SO4. The patterned bars show the samples

that formed corrosion scales.

Figure 56. Total corrosion of Kanthal with the patterned bars showing the sample that formed corrosion scales.

Kanthal only formed corrosion scales during four exposures. The total corrosion of the

samples that did not form corrosion scales are presented in Figure 57 and Figure 58. For the

pure alkali salts the total corrosion is dominated by or consists entirely of the corrosion layer.

For the salt mixtures the total corrosion is dominated by the salt crust and the corrosion layer.

Internal corrosion was detected when exposed to air at 1000°C, K2CO3 at 700°C and 800°C

and the ternary salt mixture. The total corrosion does not peak at the highest temperature but

at 800°C or 900°C.

Figure 57. Total corrosion of the samples of Kanthal, exposed to the pure salts, that did not form corosion scales.

0

200

400

600

800

1000

Air KCl K2CO3 K2SO4 0.2KCl +0.8K2SO4

0.2KCl +0.3K2CO3 +0.5K2SO4

Tota

l co

rro

sio

n [

µm

]

700°C

800°C

900°C

1000°C

0

10

20

30

40

50

60

70

80

90

Air Air Air Air K2CO3 K2CO3 K2CO3 K2SO4 K2SO4 K2SO4 K2SO4

700°C 800°C 900°C 1000°C 700°C 800°C 900°C 700°C 800°C 900°C 1000°C

Tota

l co

rro

sio

n [

µm

]

Internal corrosion Corrosion layer

46

Figure 58. Total corrosion of the samples of Kanthal, exposed to the salt mixtures, that did not form corosion scales.

Exposures to KCl at 900°C and 1000°C together with K2CO3 at 1000°C were excluded from

the study.

4.5 Result summary

The total corrosion with respect to alkali salt or salt mixture is presented in Figure 59 – Figure

63.

Regardless of temperature or material, KCl caused severe corrosion, Figure 59.

Figure 59. Total corrosion of the alloys exposed to KCl with the patterned bars showing the sample that formed

corrosion scales.

0

100

200

300

400

500

600

700

800

0.8K2SO4 +0.2KCl

0.8K2SO4 +0.2KCl

0.3K2CO3 +0.2KCl +

0.5K2SO4

0.3K2CO3 +0.2KCl +

0.5K2SO4

0.3K2CO3 +0.2KCl +

0.5K2SO4

0.3K2CO3 +0.2KCl +

0.5K2SO4

900°C 1000°C 700°C 800°C 900°C 1000°C

Tota

l co

rro

sio

n [

µm

] Internal corrosion Corrosion layer Salt crust

0

200

400

600

800

1000

1200

1400

700°C 800°C

Tota

l co

rro

sio

n [

µm

]

X20 253MA Inconel 600 Kanthal

47

X20 corroded catastrophically when exposed to K2CO3 at 1000°C but withstood corrosion

attack as well as 253MA at the lower temperatures. Kanthal was the least affected by the pure

carbonate at all temperatures, followed by Inconel 600, Figure 60.

Figure 60. Total corrosion of the alloys exposed to K2CO3 with the patterned bars showing the sample that formed

corrosion scales.

Despite X20 at 1000°C, K2SO4 caused the lowest corrosion attack of the alkali salts or salt

mixtures, with 253MA having the best performance at 800°C and 900°C and Kanthal at

1000°C, Figure 61.

Figure 61. Total corrosion of the alloys exposed to K2SO4 with the patterned bars showing the sample that formed

corrosion scales.

0

100

200

300

400

500

600

700

800

700°C 800°C 900°C

Tota

l co

rro

sio

n [

µm

]

X20 253MA Inconel 600 Kanthal

0

200

400

600

800

1000

1200

1400

700°C 800°C 900°C 1000°C

Tota

l co

rro

sio

n [

µm

]

X20 253MA Inconel 600 Kanthal

48

The binary salt mixture caused scale formation to most of the samples, with exception for

Kanthal exposed at 900°C and 1000°C. Inconel 600 was the most affected alloy.

Figure 62. Total corrosion of the alloys exposed to 0.2KCl + 0.8K2SO4 with the patterned bars showing the sample

that formed corrosion scales.

The ternary alkali salt mixture was not as corrosive as the binary but still caused scale

formation in 7 of the 12 cases. Even though Kanthal shows extensive total corrosion at 700°C

and 800°C the samples did not form any corrosion scale, neither did Inconel 600 at 700°C.

Figure 63. Total corrosion of the alloys exposed to 0.2KCl + 0.3K2CO3 + 0.5K2SO4 with the patterned bars showing

the sample that formed corrosion scales.

0

500

1000

1500

2000

700°C 800°C 900°C 1000°C

Tota

l co

rro

sio

n [

µm

]

X20 253MA Inconel 600 Kanthal

0

100

200

300

400

500

600

700

800

900

1000

700°C 800°C 900°C 1000°C

Tota

l co

rro

sio

n [

µm

]

X20 253MA Inconel 600 Kanthal

49

To summarize the corrosion impact of the samples a 4-grade scale was established. The

results are presented in Table 4 - Table 7

<30 µm total corrosion

30 - 100 µm total corrosion

>100 µm total corrosion

Formation of unattached corrosion scale flakes scattered on the sample

Formation of unattached corrosion scale all over the sample

Table 4. Corrosion impact of X20.

Table 5. Corrosion impact of 253MA.

253MA

700oC 800

oC 900

oC 1000

oC

KCl

K2CO3

K2SO4

0.2KCl + 0.8K2SO4

0.2KCl + 0.3K2CO3 + 0.5K2SO4

Table 6. Corosion impact of Inconel 600.

Inconel 600

700oC 800

oC 900

oC 1000

oC

KCl

K2CO3

K2SO4

0.2KCl + 0.8K2SO4

0.2KCl + 0.3K2CO3 + 0.5K2SO4

Table 7. Corrosion impact of Kanthal.

X20

700oC 800

oC 900

oC 1000

oC

KCl

K2CO3

K2SO4

0.2KCl + 0.8K2SO4

0.2KCl + 0.3K2CO3 + 0.5K2SO4

Kanthal

700oC 800

oC 900

oC 1000

oC

KCl

K2CO3

K2SO4

0.2KCl + 0.8K2SO4

0.2KCl + 0.3K2CO3 + 0.5K2SO4

50

5 Discussion

The results in this study showed large corrosion resistance differences between the alloys. The

ability to form and retain a protective oxide layer to withstand a corrosion attack varied

considerably, from almost unaffected to 2 mm total corrosion attack. Both temperature and

alkali salt/salt mixture were proven to be important parameters regarding the alloys

performances.

X20, 253MA and Inconel 600 were not pre-oxidized before the exposures. Kanthal, on the

other hand, had been pre-oxidized by its manufacturer and had formed an approximately 10

µm protective oxide layer at the surface. This fact is likely to have influenced the comparison

between the materials, advantageous for Kanthal. It was also recommended by the

manufacturer to pre-oxidize the samples after cutting, at 1050°C for 8 h, in order to form

protective oxide layers at the edges. Unfortunately, this procedure was not performed in this

project due to time constraints and since only the center part of the samples was to be

analyzed. Considering this, it is worth mentioning that the edges of Kanthal did, during some

exposures, form unattached corrosion scales at the edges when the sample surface did not.

This may prove the theory that the relatively good performance of Kanthal is due to the pre-

oxidation. A pre-oxidizing procedure would most likely have improved the performance of

the other alloys as well, as a protective oxide layer will be able to form before the exposure to

the salts.

5.1 Exposure to air

All samples were exposed to air at the four temperatures. X20, which has a recommended

service temperature of 675 – 760oC, experienced severe corrosion at 900°C and 1000°C.

Inconel 600 had low linear corrosion layer growth up to 1000°C. 253MA followed a similar

corrosion layer growth at first but at 1000°C the growth soared, with an almost 90 µm

increase from 900°C. The fact that one of the three replicates of 253MA formed corrosion

scale that detached after cooling also implies that the steel is deteriorated also in air at such

high temperatures. To form a protective oxide layer with similar thickness as for the pre-

oxidized and unexposed Kanthal 253MA and Inconel 600 should be exposed to air at 700 –

800°C for 24 h. The corrosion layer of Kanthal, formed from pre-oxidizing procedures, grew

thicker when exposed to air, and exceeded the corrosion layer thickness of 253MA and

Inconel 600, at all temperatures.

51

5.2 Exposure to KCl-containing alkali salt

In accordance with what is known from previous studies [23, 29], the results show that alkali

chlorides cause high corrosion rates. Even at temperatures below the melting point of the pure

salt severe corrosion can occur and increase further with higher temperatures. This corrosion

is even more enhanced by the molten phase of the salt. As stated before KCl is known to form

low melting point eutectics with other alkali salts. But it can also form these eutectics with

metals, such as iron or chromium. The results in this study clearly show that salts containing

KCl increases the corrosion attack, compared to the other salts. Severe corrosion of X20

exposed to Cl-containing salts at similar temperature has been noticed in an earlier study [38].

The corrosion product formed during that study was a multi-layered porous and detached

corrosion scale dominated by Fe2O3. The authors explain the poor corrosion resistance as the

inability of the steel to form a retained protective oxide layer during these conditions. Instead

the Cl-containing deposits cause dissolution of the oxide. Regarding the Cr depletion and Ni

enrichment in the subscale region, this phenomenon has been addressed before, for both NiCr

alloys (e.g. Inconel 600) and stainless steels [39, 40]. The phenomenon is suggested to be due

to the higher solubility of Cr-oxide in molten chlorides than that of Ni-oxide [40]. Alloys with

high Ni content will therefore have better performance on corrosion resistance to molten

chlorides than FeCr alloys, which also holds true when exposed to KCl in this study. The

magnitude of the subscale attack on 253MA exposed to KCl at 700oC is also comparable with

previous work [41]. The same authors describe, in addition to Cr depletion and Ni enrichment

in the subscale region, similar scale morphology and behavior of austenitic steels (253MA,

353MA and 310SS) exposed to NaCl in 850oC for 36 h. In conclusion, 253MA had the worst

corrosion resistance of the three steels in that study. KCl-induced corrosion of Kanthal has

also been studied before. The results from a 24 h exposure of Kanthal to KCl at 600°C [42]

are comparable with the results in this work. The authors describe similar porous and

multilayered corrosion scales dominated by Fe and Cr. The same authors also conclude that

the presence of water causes rapid formation of K2CrO4 and HCl and thereby depletion of Cr

from the protective oxide layer. In dry O2 the formation of K2CrO4 was slower but the degree

of chlorination was higher since no formation of HCl occurred, causing KCl to stay on the

sample surface.

The salt mixture of 0.2KCl + 0.8K2SO4 caused the most severe corrosion attack on Inconel

600. Ni is said to form low melting point compounds with sulfur resulting in non-protective

sulfide scales and increased corrosion rate [18]. This could explain why the corrosion

52

resistance of the Ni-based so called superalloy was surpassed by the stainless steels. Whether

the 21 wt-% Ni was the reason for the poor performance of 253MA, which also formed

corrosion scales at all temperatures, is left unsaid. However, it has been shown that low

amounts of Cl (0.3 – 1.3 wt-%) in a synthetic ash, consisting of 24.8 – 24.9 wt-% Na, 10.5 wt-

% K, 21.5 wt-% S and 42.2 – 42.9 wt-% O, increases the corrosion layer thickness of various

alloys, compared to the Cl-free salt [29].

The ternary salt mixture was the least corrosive of the salts containing KCl. From the results

the salt mixture was shown to be the most corrosive at 800°C and 900°C. The melting point of

KCl at 770°C and K2CO3 at 892°C could cause accelerated molten salt corrosion in these

cases while at 1000°C the molten salts have run off or evaporated before causing any severe

corrosion attack. Kanthal showed the best corrosion resistance, especially at 900°C and

1000°C where the corrosion attacks was decreased significantly. This, however, was not

surprising since the formation of a more protective Al-oxide layer is expected at these

temperatures.

5.3 Exposure to K2CO3

The corrosivity of K2CO3 has been less investigated than for KCl. In the temperature range of

500 - 600oC one study [28] showed that KCl was much more corrosive than K2CO3 on the

low-alloy steel 10CrMo and the superalloy Inconel 625 while another study [13] concluded

that the salts are equally corrosive on the stainless steel 304L, when water was present . Due

to the results in this study, where the air was dry, K2CO3 was less corrosive than KCl but

more corrosive than K2SO4. It was also, in most cases, less corrosive than the two alkali salt

mixtures. The salt had a relatively small but distinct corrosion impact on 253MA and Inconel

600, especially at temperatures >800oC Also here, Kanthal experienced lower total corrosion

than when exposed in air, and had the best corrosion resistance among the alloys.

5.4 Exposure to K2SO4

Pure K2SO4 was the least corrosive alkali salt in the study. The total corrosion was for 9 of the

16 samples lower than when the samples were exposed in air. For Kanthal this was the case at

all temperatures. The results could be explained by the Al content in the alloy, since Al in

known to improve sulfidation resistance [19]. Even though Inconel 600 showed grain

boundary corrosion at the two highest temperatures the total corrosion was still rather low.

This implies that the presence of KCl was required to induce a severe corrosion attack on

53

Inconel 600, not K2SO4 alone. The performance of 253MA, Inconel 600 and Kanthal was

rather similar, exposed to K2SO4 while X20 corroded dramatically at 900°C and 1000°C.

5.5 SEM-EDS limitation

The inability of the SEM to detect elements lighter than Na was a major setback, as oxygen

was left out in the analysis. The presence of oxides could thereby not be analytically

confirmed, but only theoretically discussed and assumed based on Ellingham and phase

stability diagrams together with results from previous studies within the area. Also, the

corrosion impact was based on one linescan only. As seen in the pictures in the result section

the corrosion products are rough and uneven, making it difficult to choose a representable line

for analysis. The total corrosion of the samples should thus be taken as an indication and a

comparison between the samples, and not a definite result.

5.6 Experimantal method

Even though the method of exposure is widely recognized and suggested to give comparable

results to those of actual boilers, there are reasons to suspect that there may be significant

differences between these tests in lab scale with synthetic salts, and the impact of the actual

flue gas. It is in this context important to consider that the salts in the tests are applied by hand

while deposits in a boiler environment are mainly made up of condensation of corrosive

substances, such as alkali chlorides. For some salts, especially chlorides, this condensation is

significantly decreased at surface temperatures >700°C. It is therefore reason to believe that

the concentrations of corrosive substances present at the surface of a HT-HE in a real

application is lower, and thus the corrosion rate will be significantly decreased. To assess the

long-term corrosion impact on the material in a full-scale facility it is necessary to find out

which deposits that are really formed. This information will partly be gained in the ongoing

ERA-NET project, through field measurements inside a real boiler in the location where a

HT-HE would be introduced. As a complement a theoretical review of the alkali salts

condensation processes should be made. In addition to this, studies should be made to identify

how the thermal and chemical conditions can be expected to vary at the location of the HT-

HE during operation, with respect to e.g. load, fuel and temperature. This will affect which

environment, and potential environmental variations, to expect and thus what choice of

material should be considered. This initial lab study shows that crucial parameters appear to

be the presence of KCl in the deposits and the material temperature. Within the ERA-NET

project suggestions have been made to set the operating temperature to 850°C, in order to

54

spare the condition of the HT-HE. This will lower the turbine efficiency but probably be

economically justifiable if the life of the HT-HE is increased.

5.7 Material recommendation

A specific recommendation of which alloy to be used for construction of the HT-HE is hard to

give from the lab results. But, with regard to the higher costs of Inconel 600 and Kanthal,

compared to 253MA, and together with the small variations in the corrosion resistance, a

reasonable recommendation is that 253MA constitute an appropriate choice of alloy for a first

assessment. Provided that preventive actions are taken to minimize (or eliminate) KCl in the

deposits on the HT-HE surfaces. It should also be taken into consideration that the further

studies will be able to change this recommendation.

5.8 Suggestions for improvement of the method and future studies

In order to use mass gain as a corrosion parameter the samples and salts should be weighed

before and after the exposure. The sample preparation method after the epoxy casting would

be less time consuming if a cutting device was at hand for revealing the cross sectional

surface of the samples. It is also suggested to do several linescans on each sample in order to

achieve an overview of the corrosion attack. This could preferably be done on the most

relevant samples.

6 Conclusions

Based on the performed experiments and analyzes of the exposed alloys, some conclusions

could be drawn:

The alloys experienced some grade of high-temperature corrosion at all exposures. The

KCl-containing salts were the most corrosive and caused scale formation in most cases.

The primary objective should therefore be to prevent the deposition of KCl on the HT-HE

surface, since none of the alloys showed sufficient corrosion resistance to the salt.

The alloys were susceptible to corrosion induced by pure K2CO3 deposits. However, pure

K2CO3 deposits are not likely to occur during combustion of most woody fuels and other

biofuels.

The corrosion impact was negligible or relatively low for the alloys when exposed to pure

K2SO4, except for X20, at the temperatures relevant for the application (700-900°C).

X20 should not be used at temperatures >800°C, regardless of environment.

55

Kanthal showed the overall best corrosion resistance, followed by Inconel 600, 253MA

and last X20.

A specific recommendation of which alloy to be used for construction of the HT-HE is

hard to give. Due to the relatively low cost and together with the reasonable corrosion

resistance 253MA constitutes an appropriate choice of alloy for a first assessment,

provided that KCl deposits are prevented from condensation on the HT-HE surface.

7 Acknowledgements

This study has been carried out at the Thermochemical Energy Conversion Laboratory, TEC-

Lab, at Umeå University. Many thanks are given to my office neighbors Anders Rebbling,

Markus Carlborg, Henrik Hagman, Nils Skoglund and Erik Steinvall for their patience,

enthusiasm and willingness to share their knowledge, lame humor and coffee breaks with me.

Thanks are also given to my supervisor Christoffer Boman for the opportunity to do my

master thesis at TEC-Lab and to Gustav Häggström for the experimental setup and

preparatory work. For material support I want to thank Mikael Fredriksson at TEC-Lab,

Anders Brickman at Sandvik and Gabriele Brückner at Outokumpu. For a huge contribution

of knowledge and good advices I want to thank Anders Hjörnhede at SP. Last but not least I

want to say thank you and I’m sorry to my mom, my sister, Björn Lidell and Madelene

Holmgren for you-know-what.

56

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61

9 Appendix Appendix 1. Chemical composition of the alloys used in the study.

Chemical composition [wt-%]

X207 253MA

8 Inconel 6009 Kanthal APMT

10

Fe 85.276 64.337 6-10 70.709 Ni 0.160 11.110 ≥72

Cr 13.210 20.750 14-17 20.810 C 0.280 0.089 ≤0.15 0.034 Si 0.280 1.610 ≤0.50 0.340 Mn 0.510 1.440 ≤1.00 0.150 P 0.020 0.021

0.010

S 0.040 0.001 ≤0.015

N 0.019 0.179

Mo 0.030 0.210 Cu 0.044 0.210 0.500 0.017

Ti 0.003 0.007 Ce

0.036

Al 0.003

4.940 Mo

2.990

Nd 0.010 As 0.005 W 0.010 B 0.001 V 0.080 Co 0.020

7 Fertiganalyse from Outokumpu

8 Test coupons documentation from Outokumpu

9 http://www.haraldpihl.se/attachments/007_Inconel%20alloy%20600.pdf

10 Certificate from Sandvik Technology AB

62

Appendix 2. Pictures of exposed X20.

X20

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

63

Appendix 3. Cross section of epoxy casted X20.

X20

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

64

Appendix 4. Cross section of exposed X20, 200x magnification.

X20

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

65

Appendix 5. Pictures of exposed 253MA.

253MA

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

66

Appendix 6. Cross section of epoxy casted 253MA.

253MA

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

67

Appendix 7. Cross section of exposed 253MA, 200x magnification.

253MA

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

68

Appendix 8. Pictures of exposed Inconel 600.

Inconel 600

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

69

Appendix 9. Cross section of epoxy casted Inconel 600.

Inconel 600

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

70

Appendix 10. Cross section of exposed Inconel 600, 200x magnification.

Inconel 600

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

71

Appendix 11. Pictures of exposed Kanthal.

Kanthal

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

72

Appendix 12. Cross section of epoxy casted Kanthal.

Kanthal

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4

73

Appendix 13. Cross section of exposed Kanthal, 200x magnification.

Kanthal

700oC 800

oC 900

oC 1000

oC

Air

KCl

K2CO3

K2SO4

0.2KCl

0.8K2SO4

0.2KCl

0.3K2CO3

0.5K2SO4