17 high performance alloys

8/3/2019 17 High Performance Alloys

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HIGH PERFORMANCE ALLOYS IN THE PETROLEUM REFINING INDUSTRY

K. Ravindranath

Petroleum Research and Studies Center

Kuwait Institute for Scientific Research

P.O. Box 24885, 13109 Safat, Kuwaite-mail: [email protected]

Fax: (965) 398 0445

Abstract

Being inexpensive and readily available, carbon steel is considered as the material of

choice for majority of applications in the petroleum refining industry. However, carbon steel

is unsuitable for applications involving certain aggressive corrosive species such as sulfur

compounds, inorganic salts and inorganic and organic acids. Stainless steels and nickel-base

alloys perform well under such conditions and offer advantage in terms of reliability and life-cycle cost. They have high strength, good fabrication characteristics and can meet wide

range of design requirements. However, stainless steels and nickel alloys are also susceptible

to property degradations in specific corrosive environments. Hence, careful considerations

are to be given in the selection of a particular type of alloy for an application. The paper

discusses the corrosion problems encountered in various refinery units and applications of

stainless steels and nickel-base alloys to combat corrosion problems.

Key words: Refinery corrosion, naphthenic acids, sulfidation, stainless steels, nickel-basealloys.

Introduction

Economic considerations, increased competition and the need to meet regulatory

targets have made equipment reliability of refining units extremely important. These factors

have also led to effective and innovative uses of corrosion resistant alloys to reduce

equipment life cycle costs and to extend existing equipment life. Corrosion has always been

an unavoidable part of petroleum refining operations. Corrosion problems increase operating

and maintenance costs substantially. Scheduled and unscheduled shutdowns for repairing

corrosion damage in process equipment and piping can be extremely expensive and anything

that can be safely done to keep a process unit on-stream for long periods of time will be of

great benefit. Thus, the selection of materials of construction has a significant impact on theoperability, economics and reliability of refining units.

Carbon steel is the ideal material of choice for refining operations and is used for

about 80% of all components in refineries, because it is inexpensive, readily available and

easily fabricated. However, carbon steel is unsuitable for applications involving certain

aggressive corrosive species and high temperature. Stainless steels and nickel-base alloys

offer improved resistance under such conditions. They have high strength, good fabrication

characteristics and can meet wide range of design requirements such as load, service life, low

maintenance etc. In refineries, stainless steels have been primarily used for applications

involving high temperature sulfidic corrosion and some forms of aqueous corrosion attack.

Nickel alloys are especially resistant to various mineral acids and caustic solutions, all ofwhich can cause corrosion problems in certain refinery processes. Nickel also forms the


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basis for many high temperature alloys. Though, these alloys have good corrosion properties,

certain specific corrosive agents and conditions can make them unsuitable for particular

applications. Most stainless steels will pit in the presence of chlorides and are also

susceptible to stress corrosion cracking in certain specific media. Similarly, some nickel

alloys can be attacked and embrittled by sulfur bearing gases at elevated temperatures.

Hence, careful considerations are to be given in the selection of a particular type of alloy foran application.

Characteristics of stainless steels and nickel-base alloys, refinery corrosion problems,

and selected applications of stainless steels and nickel-base alloys to combat refinery

corrosion problem are discussed in the following sections.

Stainless Steels

Stainless steels are iron-base alloys containing at least 10.5% chromium. With

increasing chromium content and the presence of other alloying elements, stainless steels can

provide extraordinary corrosion resistance [1,2]. Stainless steels are categorized in fivedistinct families according to their crystal structure and strengthening precipitates. This paper

will focus only on the austenitic family of stainless steels as they are popularly used in the

petroleum refining industry. Austenitic stainless steels have a face centered cubic (fcc)

structure and the structure is accomplished by adding austenite stabilizers, most commonly

nickel but also manganese and nitrogen. Austenite is characterized as nonmagnetic with high

ductility and excellent toughness. Good mechanical properties and corrosion resistance,

combined with ease of fabrication, have made the austenitic grades the most common of the

stainless steels. A partial list of austenitic stainless steels and their compositions is given in

Table 1.

Table 1: Chemical compositions of Austenitic Stainless Steels

Alloy UNS

No.

C N Cr Ni Mo Cu Other PRE

No.

304 S30400 0.08 - 18-20 8-10.5 - - - 18

304L S30403 0.03 - 18-20 8-12 - - - 18316 S31600 0.08 - 18-20 10-14 2-3 - - 23316L S31603 0.03 0.10 16-18 10-14 2-3 - - 23317 S31703 0.08 0.10 18-20 11-15 3-4 - - 28321 S32100 0.08 - 17-19 9-12 - - Ti 17347 S34700 0.08 - 17-19 9-13 13 - Cb,Ta 17

310 S31000 0.25 - 24-26 19-22 - - Si 24Alloy 20 N08020 0.07 - 19-21 32-38 2-3 3-4 Cb,Ti 26Alloy 825 N08825 0.05 - 20-24 38-46 2.5-3.5 1.5-3.5 Al,Ti 28317LN S31753 0.03 0.1-0.2 18-20 11-15 3-4 - - 30317LM S31725 0.03 0.10 18-20 13.2-17.5 4-5 - - 31904L N08904 0.02 - 19-23 23-28 4-5 1-2 - 3220Mo-4 N08024 0.03 - 22.5-25 35-40 3.5-5 0.5-1.5 - 34

Alloy 28 N08028 0.02 - 26-28 29.5-32.5 3-4 0.6-1.4 - 36Alloy 926 N08926 0.02 0.18-0.2 20-21 24.5-25.5 6-6.8 0.8-1.0 - 40

254SMO S31254 0.02 0.18-0.22 19.5-20.5 17.5-18.5 6-6.5 0.5-1.0 - 42AL-6XN N08367 0.03 0.18-0.25 20-22 23.5-25.5 6-7 0.75 - 43Alloy 31 N08031 0.015 0.15-0.25 26-28 30-32 6-7 1.0-1.4 - 48

654SMO S32654 0.02 0.45-0.55 24-26 21-23 7-8 0.3-0.6 Mn 54


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Identification:

The most common designations of stainless steels are those of the American Iron and

Steel Institute (AISI). Austenitic stainless steels have a three digit designation in the 200 or

300 series and some have a one or two letter suffix that indicates a particular modification of

the composition. Austenitic stainless steels in the 300 category have better corrosionresistance and are popular in the industry.

The Unified Numbering System (UNS), introduced in 1970s provide a systematic

listing of alloys correlating many internationally used societies and producers. Although not

perfect, the UNS numbering system has been successful in maintaining a degree of order.

Most stainless steels, with more than 50% iron, have a UNS number that consists of the letter

S followed by five digits, while some stainless steels with higher nickel and chromium

contents have the letter N. For the AISI grades, the first three digits usually correspond to an

AISI number. The basic AISI grades have 00 as the last two digits, while the modifications of

the basic grades show some other two digit numbers. There are some exceptions in the UNS

system.

In addition to the standard grades, there are many special grades that represent

modifications, extensions or refinements of the basic standard grades. Introduction of modern

steel making process, Argon Oxygen Decarburization (AOD), facilitated production of newer

grades of stainless steels with controlled chemistry and improved properties. Many of the

special grades are identified by common trade names or trademarks.

Effects of Alloying Elements:

Carbon: Carbon is always present in stainless steel. In all categories, except

martensitic, the level is kept quite low. It is useful to the extent that it permits hardenability

by heat treatment. In all other applications, carbon is detrimental to corrosion resistance

through its reaction with chromium.

Chromium: Chromium is the element essential in forming the passive film and giving

stainless steels its stainless characteristics. Other alloying elements can influence the

effectiveness of chromium in forming or maintaining the passive film, but no other element

can, by itself, create the properties of stainless steel. Increasing the chromium content, as

typical of the new generation stainless steels, increases the stability of the passive film [3].

Chromium is a ferrite stabilizer. Chromium improves high temperature oxidation and

sulfidation resistance.

Nickel: Nickel is an austenite stabilizer and enhances mechanical properties andfabrication characteristics. Nickel promotes repassivation and is useful in resisting corrosion

in mineral acids [3]. Higher nickel content of about 30% improves stress corrosion cracking

(SCC) resistance. Nickel also improves high temperature oxidation resistance.

Molybdenum: In combination with chromium, molybdenum resists passivity

breakdown and imparts resistance to localized corrosion such as pitting and crevice

corrosion. Molybdenum is a ferrite stabilizer. Molybdenum favors precipitation of

intermetallic phases at high temperatures.

Nitrogen: Nitrogen is an austenite stabilizer and increases strength. Nitrogenimproves pitting resistance and retards the formation of intermetallic sigma phase.

Copper: Copper improves resistance of stainless steels to corrosion in sulfuric acid.


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Corrosion Characteristics:

The selection of a grade of stainless steel for a particular application involves the

consideration of many factors, but always begins with corrosion resistance. The types of

corrosion occurring in petroleum refining processes are numerous, ranging from general

corrosion to localized corrosion. Stainless steels are susceptible to several forms of localizedcorrosion attack. The focus of the effort involved in selecting stainless steels is to avoid such

localized corrosion attack. Various factors such as design, fabrication practices, surface

condition etc can strongly influence corrosion characteristics of stainless steels.

Pitting Corrosion: Pitting of stainless steels occurs by a local breakdown of the protectivepassive film, and then the localized development of an anodic corrosion site surrounded by a

cathodic area that remains passive. Once pits are initiated they may continue to grow by a

self-sustaining mechanism [4]. It can produce penetration of a stainless steel with almost

negligible weight loss to the total structure. Pitting initiation can be influenced by surface

condition and temperature. For a particular environment, a grade of stainless steel may be

characterized by a single temperature or a very narrow range of temperatures above which pitting will initiate and below which pitting will not initiate [2]. It is therefore possible to

select a grade that will not be subjected to pitting attack if the chemical environment and

temperature do not exceed the critical values.

Type 304 stainless steel, for example, is susceptible to pitting when exposed to acid

chloride salts, such as might occur in ammonium chloride formed in crude distillation

overhead systems. However, Types 316 and 317 contain molybdenum which reduces pitting

tendency in chloride environments [5]. Also, several of the proprietary alloys of the type 6%

Mo have excellent resistance to the severest of pitting environments [6,7]. Molybdenum is

the element mainly responsible for good pitting resistance. Tungsten and nitrogen also

impart good pitting resistance. Stainless steels can be ranked for pitting resistance based on

PRE (Pitting Resistance Equivalent) number. One definition for PRE is % Cr + 3.3% Mo +

16%N [6,8]. A higher PRE number relates semiquantitatively to a higher resistance to

localized corrosion in chloride containing environments. Figure 1 shows the relative

resistance of various commercial alloys to localized corrosion on the basis of chromium and

molybdenum contents [9].

Fig. 1 Effect of chromium and molybdenum on the localized corrosionResistance of selected commercial alloys [9].


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Crevice Corrosion: Crevice corrosion is a form of localized corrosion that can occur within

crevices or at shielded areas where a stagnant solution is present. Once attack begins in a

crevice, its progress is very rapid. It is most frequently associated with chloride environments

both neutral and low in pH. Stainless steels containing molybdenum are often used to

minimize the problem. Notwithstanding, the best solution to crevice corrosion is a design that

eliminates or minimizes crevices.

Intergranular Corrosion: When austenitic stainless steels are heated or cooled through the

temperature range of 400-850oC, carbon diffuses to the grain boundaries and combines with

chromium, forming chromium carbides [2,3]. The effect of this phenomenon termed

sensitization is depletion of chromium and the lowering of corrosion resistance in areas

adjacent to the grain boundaries (Fig. 2) [10]. A grain boundary precipitate is not the point of

attack; instead the low chromium region adjacent to the precipitate is susceptible. This is a

time-temperature dependant phenomenon as shown in Fig. 3.

Fig. 2 Intergranular corrosion of 321 SS tested as per

ASTM A-262 practice E [10].

Fig. 3 Time-temperature-sensitization curves for type 304 SS in a mixture of

CuSO4 and H2SO4 containing free copper. Carbides precipitate in theareas to the right of the carbon content curves [1].

Time to sensitization

Temperature


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Sensitization is not necessarily detrimental unless the grade is to be used in an

environment capable of attacking the region [1,5]. For example, elevated temperature

applications for stainless steel can operate with sensitized steel, but concern for intergranular

attack must be given to possible corrosion during downtime when condensation might

provide a corrosive medium.

Carbide precipitation and intergranular corrosion in austenitic stainless can be

minimized by the following methods:

Use of stainless steels in the annealed condition or annealing after fabrication. Selection of a low carbon grade for weld fabrication (such as 304L, 316L etc). Use of stabilized grades such as grade 321 (titanium stabilized) or 347 (niobium

stabilized). Titanium and niobium have a greater affinity for carbon than does

chromium, thus preventing precipitation of chromium carbide.

Type 304 gets sensitized rapidly in the temperature range 650-700oC. Sensitization

tests show that Type 304L shows evidence of sensitization after 100 hours at 550-600oC.

Type 321, which is stabilized with titanium may become sensitized in less than 1000 hours.

Type 347, on the other hand, sensitizes only in a relatively narrow temperature range, and

only for exposures greater than 1000 hours (Fig. 4).

Fig. 4 Time-temperature-sensitization diagrams for selected commercial stainless steels [5].

Stress Corrosion Cracking (SCC): SCC is a corrosion cracking process in which the

combination of susceptible alloy, sustained tensile stress and a particular environment leads

to cracking of the alloy. Stainless steel grades 304 and 316 are particularly susceptible to

SCC in chloride environments. SCC can occur in sensitized austenitic stainless steels exposed

to polythionic acids. Polythionic acid SCC is intergranular in nature, where as chloride stress

corrosion cracking is usually transgranular.

Polythionic acid solution can form in petroleum refining units during shutdown by the

reaction of sulfide scale with moisture and oxygen. Polythionic acid SCC is considered a


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possibility in almost any petroleum refining process using austenitic stainless steels.

Austenitic stainless steels exposed to continuous service below 425oC may be susceptible to

polythionic acid stress corrosion cracking only if they are sensitized by welding or heat

treatment. In such cases, cracking can be avoided by using low carbon or stabilized grades.

For service above 425oC, equipment may become sensitized by long exposure times, so care

should be exercised in selecting a material that is more resistant to sensitization and SCC.

Proper shutdown procedures are also an effective method to control downtime

corrosion problems. Following NACE RP 01-70 can prevent polythionic acid SCC of

stainless steel equipment during shutdown periods [11].

Nickel-base Alloys

Nickel-base alloys are vitally important to modern industry, including the petroleum

refining industry because of their ability to withstand wide variety of severe operating

conditions involving corrosive environments, high temperatures, high stresses and

combinations of these factors. Nickel alloys have good resistance to corrosion and hightemperature and are also easily weldable and fabricable. Nickel maintains its fcc structure up

to melting point making them ductile and tough. Its atomic size and nearly complete 3d

electron shell enable it to receive large amounts of alloying additions before encountering

phase instabilities [12]. This allows a wide variety of alloys to be fashioned in a manner that

can adequately capitalize on the unique properties of specific alloying elements.

Nickel-base alloys are generally known by their common trade names or trade marks.

Their UNS number consists of the letter N followed by five digits. A partial list of nickel-

base alloys and their compositions is given in Table 2.

Table 2: Nominal Chemical Compositions of Some Typical Nickel-base Alloys

Alloy UNS No. C Nb Cr Cu Fe Mo Ni Ti W Others

Nickel-Copper

400 N04400 0.15 - - 31.5 1.25 - Bal - - -K-500 N05500 0.25 - - 29.0 2.0 - 63 0.6 - 2.7 Al

Nickel-Molybdenum

B-2 N10665 0.01 1.0 2.0 28 - - -

Nickel-Chromium-Iron

600 N06600 0.08 - 16.0 0.5 8.0 - Bal 0.3 - -

601 N06601 - - 23.0 - 14.1 - Bal - - 1.4 Al690 N06690 0.02 29 - 10 - 61 0.3 - -800 N08800 0.1 21 0.75 44.0 - 32.5 0.38 - -800H N08810 0.08 - 21 0.75 44.0 - 32.5 0.38 -

Nickel-Chromium-Iron-molybdenum

825 N08825 0.05 21.5 2.0 29.0 3.0 42 1.0 - -G-30 N06030 0.03 0.8 29.5 2.0 15.0 5.5 43 - 2.5 -625 N06625 0.1 4.0 21.5 - 5.0 9.0 62 - - -

Nickel-Chromium-Molybdenum-Tugsten

C-276 N10276 0.01 - 15.5 - 5.5 16 57 - 4.0 -C-22 N06022 0.01 - 22 - 3 13 56 - 3.0 -


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Effects of Alloying Elements:

Chromium: Chromium improves resistance to oxidizing media such as nitric acid and

chromic acid [13]. Chromium also improves resistance to high temperature oxidation and to

attack by sulfur bearing gases [14]. Chromium additions in nickel-base alloys are usually in

the range 15 to 30%.Molybdenum: Molybdenum in nickel substantially improves resistance to non-

oxidizing acids [13,14]. Molybdenum also improves the pitting and crevice corrosion

resistance of nickel base alloys [12-14]. In addition, it is an important alloying element for

imparting high temperature strength and creep resistance.

Iron: Iron is primarily used in nickel base alloys to reduce cost. However, iron also

increases the solubility of carbon in nickel and carburization resistance [15].

Tungsten: Tungsten, like molybdenum, provides improved resistance to non-oxidizing

acids and localized corrosion. Additions of tungsten of the order of 3-4% in combination with

13-16% molybdenum in a nickel-chromium base results in alloys with outstanding resistance

to localized corrosion [12].Silicon: Silicon promotes high temperature oxidation resistance. The level of silicon

is normally carefully controlled as it can stabilize carbides and harmful intermetallic phases.

Silicon also improves the resistance of nickel to hot concentrated sulfuric acid [15].

Cobalt: Cobalt imparts strength, resistance to carburization and sulfidation. Aqueouscorrosion resistance of cobalt is similar to that of nickel in most environments [12].

Niobium and Tantalum: Niobium and Tantalum are used to promote high temperaturestrength and to reduce tendency for hot cracking [16].

Aluminum and Titanium: Aluminum and Titanium additions improve strength.Aluminum also improves resistance to oxidation, carburization and chlorination [17].

Copper: Copper improves resistance of nickel to non-oxidizing acids [13].

Intermetallic Phases:

The occurrence of intermetallic phases in nickel alloys can be beneficial and also

detrimental [12]. On the positive side, the nickel-base system has been the most widely and

successfully exploited of any alloy base in the development of high strength high temperature

alloys because of the occurrence of unique intermetallic phases. On the negative side, the

precipitation of certain intermetallic phases can seriously degrade ductility and corrosion

resistance. This latter effect results from the fact that intermetallics, like carbides, can

deplete the matrix of elements vital to service performance.

Most high strength nickel alloys depend on the precipitation of gamma prime ().

Gamma prime phase is unique in a number of ways that it possesses the unusual

characteristic of having increased strength with increasing temperature. It is also ductile,

unlike most other intermetallic compounds. Another important intermetallic phase that can

be used to strengthen nickel base alloys is known as gamma double prime ().

The most common harmful intermetallic phases that occur in nickel base alloys are

the sigma, mu and Laves. These are complex phases that often nucleate at grain boundaries

[12]. Because of the detrimental effects on properties produced by the intermetallic phases

much effort has been devoted to finding ways to avoid their occurrence. Powerful tools arenow available for designing new alloy chemistries that are free of deleterious phases.


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Corrosion Characteristics:

Nickel and its alloys, like the stainless steels, offer a wide range of corrosion

resistance. However, nickel can accommodate larger amounts of alloying elements, chiefly

chromium, molybdenum and tungsten, in solid solution, than iron. Therefore, nickel basealloys, in general, can be used in more severe environments than the stainless steels [12]. A

distinction is usually made between those alloys that are primarily used for high temperature

strength, commonly referred to as superalloys and those that are primarily used for corrosion

resistance.

As with stainless steels, halide salts and particularly chlorides are recognized as well

known pit producers for nickel alloys. Molybdenum added to nickel alloys offers higher

nobility and resistance to localized corrosion. Figure 5 shows the effect of molybdenum and

nickel content on the corrosion resistance of selected alloys [18]. The nickel-chromium-

molybdenum alloys such as alloys C-276, 625 and C-22 exhibit very high resistance to pitting

in oxidizing chloride environments. The critical pitting temperatures of various nickel-chromium-molybdenum alloys in an oxidizing chloride solution are shown in Table 3.

Fig. 5 Effect of molybdenum and nickel contents

on the corrosion resistance of selected

commercial alloys [18] .

Nickel base alloys are generally used to combat SCC when austenitic stainless steelshave failed because of SCC. Nickel base alloys have been historically considered to be


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immune to SCC in all but a few environments. SCC of nickel-base alloys has been found to

occur mainly in three types of environments: high temperature halogen ion solutions, high

temperature waters and high temperature alkaline environments. In addition, SCC has been

reported in polythionic acid solutions and environments containing acids and hydrogen

sulfide [12].

Table 3: Critical Pitting Temperature of Selected Alloys [19](Test solution: 6% FeCl3, 24 h periods)

Alloy Critical Pitting TemperatureoC

Alloy 825 0, 0

904L SS 2.5, 5.0

317L SS 2.5, 2.5

Alloy 625 35.0, 40.0

Alloy C-276 60.0, 65.0

Alloy C-22 70.0, 70.0

Most of the corrosion resistant nickel-base alloys have very low carbon and

precipitation of carbides is not a serious problem in these alloys. When welded by qualified

procedures, most modern high nickel alloys are resistant to intergranular corrosion [12].

Nickel-base alloys, in general, have good high temperature environmental resistance

and stability. They have good oxidation and carburization resistance [12,20]. Table 4 shows

the results of oxidation tests carried out on some commercial alloys [21]. In intermediate

temperature environments that are oxidizing and carburizing, the nickel-iron-chromium

alloys perform well. The higher nickel versions are not recommended in sulfidizing

environments. The higher iron versions of the nickel-iron-chromium alloys offer bettersulfidation resistance.

Table 4: Oxidation of Nickel-base alloys and Stainless Steels in Air [21](Test temperature: 980oC, duration: 1008 hours)

Alloy Average Metal Affected m(mils)

Alloy 214 5 (0.2)

Alloy 625 18 (0.7)

Alloy 600 23 (0.9)

Type 310 SS 28 (1.1)

Alloy 601 33 (1.3)Alloy 800H 46 (1.8)

Type 304 SS 206 (8.1)

Type 316 SS 363 (14.3)

Petroleum Refining

Raw crude is separated by fractional distillation into petroleum gas, naphtha, middle

distillates, gas oils, lubricating oils and residual oils. All the streams from crude distillation

are further processed or treated to convert the many fractionated materials to saleable or

useful products. These processes involve the separation of gases and absorption or removalof unwanted polluting substances such as sulfur, hydrogen sulfide, carbon dioxide, ammonia


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etc. Gasoline and middle distillates are hydrogenated or hydrotreated to convert organic

sulfur and nitrogen to hydrogen sulfide and ammonia for easy removal. Gasoline fractions

are catalytically reformed to improve rating and combustion characteristics.

Heavy oils, gas oils and vacuum residuum are thermally cracked, catalytically cracked

or hydrogenated to increase yields of valuable products. Olefins can be combined with anaromatic catalytically to produce a branch chain hydrocarbon of high octane number. Waste

water is treated and hydrogen sulfide is removed. A simplified refinery flow diagram is

shown in Fig 6.

Fig. 6 Simplified refinery process flow diagram [6]


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Corrosion in Refinery units and Applications of High Performance Alloys

Raw crude entering a refinery contains several forms of sulfur, salts, water, organic

acids and organic nitrogen, which split, combine or convert into numerous corrosive

compounds. Sulfur compounds, organic acids, hydrochloric acid, sulfuric acid, sulfur dioxide,carbon dioxide, cyanide, ammonia, phenol etc are the main corrosive species encountered in

refinery units. Corrosion problems encountered in selected refinery processes and material

selection guidelines are discussed in the following sections.

Crude Distillation:

Corrosives present in the refinery crude units are sulfur, naphthenic acid and

chlorides. The crude is first desalted to minimize corrosion problems due to inorganic salts

and water. As the crude is heated above 200 to 300oC, in exchangers and in the fired heater,

corrosion occurs as a result of breakdown of sulfur compounds. Condensing water containing

salts and hydrogen sulfide causes corrosion in the overhead system of the atmosphericcolumn. Corrosion caused by breakdown of sulfur compounds continues in the atmospheric

column.

Corrosion by various sulfur compounds at temperatures above 260oC is a common

problem in many petroleum refining processes. Various sulfur compounds present in crude

oil react with metal surfaces at elevated temperatures, forming metal sulfides, certain organic

molecules and hydrogen sulfide [22]. The relative corrosivity of sulfur compounds generally

increases with temperature. Sulfidation proceeds by converting metal to scale, which may be

protective, except that sulfide scales are friable and tend to exfoliate, exposing bare metal to

further sulfidation. Chromium imparts resistance to sulfidation, where as nickel alloys are

rapidly attacked by sulfur compounds. The presence of chromium in the steel helps to

stabilize the scale and slow the diffusion process.

In addition to time, temperature and concentration, sulfidation depends upon the form

in which the sulfur exists. An alloy possessing useful resistance to sulfur in one form may

actually experience accelerated corrosion when the sulfur is present in another form. Of

particular interest is hydrogen sulfide. The rate of corrosion in H2S depends on

concentration, temperature, pressure and permeability of the sulfide scale [5,22].

SS 321 is generally specified for heater tubes handling corrosive crude. Transfer lines

from the preheat furnaces are normally stainless steels, Types 304 or 316. Atmosphericfractionator is normally lined with stainless steel grade 316 up to the zone where

temperatures drop to 260oC to control sulfidic corrosion.

When naphthenic acids are present in sufficient quantities, carbon steel and chromium

steels are severely attacked. Naphthenic acids are organic acids that are present in many

crude oils especially those from California, Venezuela, Romania and Russia. The general

formula of naphthenic acid can be written as R(CH2)nCOOH, where R is a cyclopentane ring

[23-25]. Naphthenic acid is generally expressed in terms of the neutralization number which

is determined by titration with potassium hydroxide, as per ASTM D 664 [26]. Naphthenic

acid corrosion is of concern when the neutralization number of the feed stock is above 0.5

and the temperatures are 220-400o

C. Naphthenic acid corrosion is a problem when fluidvelocities are high, especially in transfer lines and return bends.


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Absence of scale and a characteristic grooving of the material, as if it were caused by

erosion, is often the sole evidence of naphthenic acid corrosion [5]. The problem is usually

most severe in vacuum units and atmospheric distillation units and it is found to certain

extent in thermal cracking units, especially in columns, piping, heaters, heat exchangers and

pumps. Naphthenic acid corrosion is most easily controlled by blending crude oils havinghigh neutralization numbers with other crude oils to reduce the neutralization number to less

than 0.5 to 1. However, this is not always successful to reduce corrosion rate in some parts of

the unit. Carbon steel is unsuitable when the TAN is above 1. Type 316 SS, 317 SS and

other molybdenum bearing steels are effective in naphthenic acid environments. As the

molybdenum content of an alloy increases, the resistance to naphthenic acid shows an

increase. Table 5 shows the corrosion rates for several materials exposed in a heavy gas oil

vacuum tower and illustrates the effect of composition [5].

Table 5: Corrosion Rates of Selected Alloys in Naphthenic Acid [5]

(Heavy Gas Oil, Vacuum Tower 271C, Acid Number-3.9)

Alloy Corrosion Rate (mpy)

Carbon Steel >25

Type 304 SS 13.7

Type 316 SS 0.046

Type 317 SS 0.024

Alloy 825 0.002

Alloy 20Cb-3 0.012

Alloy 625 0

Hastelloy C 0.002

When the salt content is high in the crude, the top of the tower is subjected to water

condensation that contains hydrogen chloride, ammonia and hydrogen sulfide. Corrosion rate

can be severe and carbon steel is unsuitable. Alloy 400, a nickel-copper alloy, is often used

as a tower lining material for combating corrosion due to salt and acid condensation.

However, corrosion will occur if the ammonia content exceeds 3 wt% or if the pH becomes

high [6]. When the conditions are severe, Ni-Cr-Mo alloys such as alloys C-276 (UNS

N10276) and C-22 (UNS N06022) will give good corrosion performance [27].

Materials used in the overhead condensers vary with the source of the cooling water,

amount of chloride, pH control, wash water etc. Titanium, alloy 400, copper-nickel alloys etcare used for tubes. Copper-nickel alloys will undergo corrosion, if ammonia and hydrogen

sulfide contents are high. Stainless steels, UNS N08904, N08020 and nickel base alloy, alloy

C-276 have performed well [6,27].

Fluid Catalytic Cracking (FCC):

Oxidation resistance and creep strength, rather than sulfur attack, govern the material

selection in FCC units. High alloy materials used in FCC are Types 304, 304H, 321, 347 and

Alloy 625. Type 304H is the preferred material of choice for various high temperature areas

such as riser pipes, flue gas piping, plenum chamber, cyclones etc. Alloy 625 is commonlyused for bellows as they are subjected to condensation of acids.


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Hydrotreating and hydrocracking:

The corrosive species present in hydrodesulfurizers are hydrogen sulfide, ammonia,

ammonium bisulfide, ammonium chloride, and polythionic acids.

Hydrodesulfurizing and hydrocracking reactions takes place at temperatures of 350-

450oC at pressures anywhere from 1500 to 3000 psi. At high pressure and temperature, when

hydrogen is present, the character of sulfidation attack is considerably modified, to the extent

that low chromium steels are not adequate. Hydrogen converts organic sulfur compounds in

the feed stocks to hydrogen sulfide; corrosion becomes a function of hydrogen sulfide

concentration. Molecular hydrogen can also dissociate to produce atomic hydrogen, which is

more reactive and can diffuse through the alloy. The hydrogen combines with the sulfide

scale, reducing it and creating a porous structure through which the iron and sulfur ions can

maintain high diffusion rates [6,22]. Also, hydrogen suppresses the formation of coke

deposits that can act as effective corrosion barriers. Austenitic stainless steels with 18-20%

Cr and 8-20% Ni are the most desirable alloys for long term service. Type 321 SS isfrequently used for piping in services containing hydrogen sulfide and hydrogen.

High performance alloys used in the units are types 304, 321, 347, alloys 825 and

625. The choice of material, to a great extent depends on the amounts of hydrogen sulfide

and hydrogen and the temperature. 5% and 9% chromium steels are used for fired heater

tubes, but they experience higher corrosion rate resulting in scale, which frequently plugs

reactor inlet. SS Type 347 or 321 is a preferred choice for heater tubes. Reactors are normally

clad or weld overlaid with 347 SS. The overlay should not have more than 10% ferrite to

overcome the problem of sigma phase embrittlement. Reactor effluent piping and exchangers

are normally stainless steel Types 304, 321 or 347. Stainless steels are generally preferred for

sulfur containing environments over 260oC. When stainless steels are selected for equipment

in and around the reactor system, the stabilized Types 347 and 321 are preferred. They

provide better protection against polythionic acid stress corrosion cracking. Reactor effluent

air coolers and outlet piping are frequently subject to corrosion due to the presence of

ammonium bisulfide. Alloys 825 and 625 give satisfactory performance at the outlet piping.

Hydrogen Production:

Environments of concern in hydrogen production process are high temperature, high

velocity, steam condensates very rich in carbon dioxide etc. The high alloy materials

commonly use in hydrogen plants are SS Type 304, 310, 330, nickel-base alloy 800 and castgrade nickel-chromium alloy HK-40. Reformer tubes operate at temperature in the range

7001000oC and material concerns are creep resistance and high temperature strength.

Alkylation Plant:

Sulfuric acid alkylation plants are built basically of carbon steel, but stainless steels

and nickel alloys are required in some areas. Corrosion resistant alloys used primarily are

316, 316L, alloys 20, B-2 and C-276. Carbon steel is resistant to concentrated sulfuric acid,

but loses resistance under high velocity conditions above 40oC. When the temperature

exceeds 60oC, 316L SS also suffers high corrosion rate. Alloys 20, B-2 and C-276 perform

well in sulfuric acid media [5,22].


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Amine Plant:

Amine units experience aqueous corrosion due to hydrogen sulfide, carbon dioxide

and hydrogen sulfide. Stainless steel 316 L is used for piping and heat exchangers to control

corrosion problems.

Sour water strippers:

Corrosives in the unit are hydrogen sulfide, ammonia, carbon dioxide, cyanides,

phenols, salts and acids. Stainless steel types 304, 316 and alloy 20 are used for heat

exchanger tubes, trays, cladding etc.

Sea water:

Traditionally copper-nickel alloys, nickel-copper alloys and titanium have been used

for heat exchanger tubes. Copper-nickel alloys are susceptible to attack if sulfides are presentin water. Titanium is resistant to sulfides, but is subject to embrittlement from hydriding,

particularly in the presence of hydrogen, if the temperature exceeds 70oC [6]. In addition,

corrosion from salt plugs has occurred when the velocity in titanium tubes was below 3 ft/s.

Type 304 and 316 stainless steels suffer deep pitting if the sea water flow rate

decreases below about 1.5 m/s because of the crevices produced by fouling organisms. The

new super austenitic stainless steels, 6 Mo steels, that are high in molybdenum performs well

in sea water [6,7]. 6 Mo steels are usually used for the warm sea water applications or where

high reliability is imperative.

Summary

Increased emphasis on reliability and run length of refinery units and equipment along

with economic considerations and regulatory guidelines have led to increased consideration

of stainless steels and nickel-base alloys to solve corrosion problems. The stainless steels and

nickel-based alloys mentioned in this article have been used effectively to solve various

corrosion problems such as general corrosion, pitting, stress corrosion cracking, high

temperature attack etc in refinery units.

References

1. Corrosion of Stainless Steels, in ASM Handbook, Vol. 13, Corrosion, ASMInternational, 1996.

2. A. John Sedriks, Corrosion of Stainless Steels, John Wiley & Sons, 1979.3. M.A. Streicher, Austenitic and Ferritic Stainless Steels, in Uhligs Corrosion

Handbook, R. Winston Revie (Ed.), John Wiley & Sons, 2000.

4. M.G. Fontana and N.D. Greene, Corrosion Engineering, Mc Graw Hill, 1984.5. The Role of Stainless Steels in Petroleum Refining, Nickel Development Institute,

Publication No.9021, 1977.

6. R.A. White and E.F. Emhke, Materials Selection for Refineries and AssociatedFacilities, NACE International, 1991.

7. R.M. Davidson and J.D. Redmond, Materials Performance, Vol. 27 (12), 1988, p 1.


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8. High Performance Stainless Steels, Nickel Development Institute, PublicationNo.11021, 2000.

9. C.M. Schillmoller, World Oil, 1988, p 1.10.ASTM A262, Standard Practices for Detecting Susceptibility to Intergranular Attack

in Austenitic Stainless Steels, Annual Book of ASTM Standards, American Society

for Testing and Materials.11.NACE RP 0170, Protection of Austenitic Stainless Steels and other Austenitic Alloys

form Polythionic Acid Stress Corrosion Cracking during Shutdown of Refinery

Equipment.

12.Corrosion of Nickel-base Alloys, in ASM Handbook, Vol. 13, Corrosion, ASMInternational, 1996.

13.H.H. Uhlig, Corrosion and Control, John Wiley and Sons, 1963.14.W.Z. Friend, Corrosion of Nickel and Nickel-Base Alloys, John Wiley & Sons, 1980.15.W. Betteridge, Nickel and its Alloys, Ellis Horword Ltd. 1984.16.W. Betteridge and J. Heslop, in Nimonic Alloys, Crank Russak and Company, 1974.17.R.B. Herchenroeder, G.Y. Lai and K.V. Rao, J. Met, Vol. 35 (11), 1983, p 16.18.Nickel and Nickel Alloys, in Metals Handbook, Vol. 2, Properties and Selection: Non

Ferrous Alloys, ASM International, 1992.

19.E. Hibner, Paper 181, Presented at Corrosion/86, Houston, TX, NACE, 1986.20.Wrought and Cast Heat Resistant Stainless Steels and Nickel alloys for the Refining

and Petrochemical Industries, Nickel Development Institute Publication No. 10071,

1998.

21.Technical Literature H-0322D, Oxidation Resistance of High Temperature Alloys,Haynes International Inc.

22.Corrosion in Petroleum Refining and Petrochemical Operations, in ASM Handbook,Vol.13, Corrosion, ASM International, 1996.

23.J.J. Heller, Mat. Prot., Vol. 2 (9), 1963, p 90.24.D.B. Danilov, Anti.Corros., Vol. 22 (8), 1975, p 3.25.R.D. Kane and M.S. Cayard, Materials Performance, Vol. 38 (7), 1997, p 48.26.ASTM D664, Standard Test Method for Neutralization Number by Potentiometric

Titration, Annual Book of ASTM Standards, American Society for Testing and

Materials.

27.William J. Neill Jr., Materials Performance, Vol. 40 (5), 2001, p 50.

17 high performance alloys

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