Hot CorrosionHot Corrosion

Focus on superalloys as examples of high temperature materialstemperature materials

•F. Petit, “Hot Corrosion of Metals and Alloys,”Oxidation of Metals 76 (2011) 1 21Oxidation of Metals 76 (2011) 1-21.

•Ref. Ch. 12 in Superalloys II

•Ch. 14 in Tien & Caulfield

Hot Corrosion

• Certain ions will cause corrosion at high temperatures:– S2-

– Cl-

– Na+

– V2-

– Others.Others.

• They may be present as a gas; they are frequently present as a liquid or solid depositfrequently present as a liquid or solid deposit(e.g., as salts or ash deposits).

Wh n th ph s th t tt ks th m t l is • When the phase that attacks the metal is present as a liquid or solid deposit, we refer to the process as “hot corrosion.”

Hot Corrosion

Temperature range over which hot corrosion occurs depends strongly on: (1) deposit chemistry

(2) gas constituents, and (3) alloy composition.

Hot Corrosion – cont’d

• If only Sulfur compounds are formed, the process is called “sulfidation.”

• In liquid deposits we can find Na, Li, Mg, Ca, K.q p f , , g, ,

These deposits can dissolve alloy constituents. For example, Co, Fe and Ni depress the melting point of Na2SO4point of Na2SO4.

• We need to consider this.

Types of Hot Corrosion

• Fluxing Processes – Type I

• Salt-Component Processes –Type II

• In general, hot corrosion is much more severe than simple oxidation or sulfidation alone.

• Hot corrosion is a two-stage process:

– Initiation – Reaction between the alloy and the gas to form a scale.

– Propagation – the actual corrosion that takes place.

Hot Corrosion

Type I: occurs above Tmp of salt

Type II: occurs near or below Tmp of salt

Stages of Hot Corrosion

• Hot corrosion is a two-stage process:g p

– Initiation –

Reaction between the alloy and the gas to form a scale.

– Propagation –

The actual corrosion that takes place.

Initiation in Fluxing Processes

• It is a mode of propagation. Result of reaction Type I

between the deposit and protective scale.

• Initiation may or may not take place, depending y y p , p gon the composition of the liquid phase that is present.

salt salt

Rxn. barrier

alloy alloy

Rxn. Product

salt

(Decreasing thickness as time )

N t tialloy

Non-protective layer

Propagation Modes

• Depend on the type of reaction that occurs between the molten deposits and the alloys.

• Ultimately reactions between the deposits and y pthe alloys (or protective scales on the alloys) result in the formation of non-protective reaction products.

• We can divide them into two groups:

– Non-protective reaction products form because of some “fluxing” action of the molten deposit.

– A component of the deposit (e.g., S or Cl) causes a non-protective reaction product to form.

Fluxing Processes

• Let’s consider an atmosphere containing O (from air), Na (from seawater), and S (from fuel).

• We can prepare an isothermal thermodynamic stability diagram (shown below).

• Na2SO4 forms as a flux.

2 4Na SO2Na O

2log OP

2Na S

log P3

log SOP

2log N Oa

Thermodynamic stability diagram for the Na-O-S system. Shows how composition of the salt can change due to reaction between the alloy and deposit.

2

2 24 3

andSO

SO SO O

K P a

23SO O

K P a

Use rxn. to define acidity or basicity of molten sulfate deposit

Basic Fluxing

• Oxide ions are produced from the Na2SO4 by reactions with the alloy to form oxides or sulfides:

2 22 4 4 2 2

1 32

2 2Na SO Na SO S O O

• Now:

2 2

REMEMBER that in this molten salt, this ion is always present.

Now:2 1 2

2 3 22 2

orAl O O AlO O

which destroys the protective qualities of the oxide.

2 1 22 3 4 2Cr O O CrO Cr O

• Because it takes time for these reactions to occur (for the O-2 to build up) there is an (f p)incubation time required.

• When O-2 builds up:

2 4 2 3 2 2 2

1

2or

Na SO Na O SO Na O SO O

2 12 3 2

2 2 3 2

2

2

Al O O AlO

Na O Al O NaAlO

• 2NaAlO2 is not protective!

NOTE th t th i ni f m (O 2) s s th is • NOTE that the ionic form (O-2) says there is dissolution taking place. When activities change, these phases can re-precipitate.

Acid Fluxing

• Now, the oxide ions which would have formed a protective scale, instead form a non-protective scale because the protective scale dissolves in the flux phase.

3 22 2 3 33 2 3SO Al O Al SO

• There are two possibilities:

– Gas Phase Induced Acidic Fluxing

– Alloy Induced Acidic Fluxing.

G h i d d idi fl i• Gas phase induced acidic fluxing.

– Both S and V are impurities in fuels, which upon b i f SO d V Ocombustion form SO3 and V2O5.

2 23 4 2 7

2 12 5 4 3 32

SO SO S O

V O SO VO SO

– Now:

2 5 4 3 3V O SO VO SO

3 4NiO SO NiSO

Nickel (NiSO4) sulfate is non-protective. This reaction will continue until the sulfate ions are all used up

3 4

all used up.

• Alloy Induced Acidic Fluxing.

– Elements in the alloy such as Mo, W, and V are incorporated into the melts.

– E.g.,

3

3Mo O MoO 3

2 23 4 4 3

3 22 3 3 4

2

3 2 3

O O

MoO SO MoO SO

Al O MoO Al MoO

This process continues until all ions are used up.

– From all of it we can see that:

2 4Na SO2Na O

2S

IncreasingP

Thermodynamic stability diagram for the Na-O-S system showing h h2 4Na SO

2log OP

BASIC ACIDIC

how the composition of Na2SO4 may change due to reaction with the

2Na S

3log SOP

2log N Oa

BASIC ACIDICalloy with the deposit.

Effect of Temperature

1. Gas phase acidic fluxing occurs at 650 – 800°C (“low temperature” hot corrosion).

2. Alloy induced acidic fluxing occurs at 950°C.

3 B si fl xin nd s lfid ti n s t 950°C 3. Basic fluxing and sulfidation occurs at 950°C and above.

Effect of Cl-1 ions

1. Cl-1 embrittles the oxide layer causing it to crack more easily.

2. Cl-1 forms volatile compounds with Al and Cr so that the scale thins out in the area of the Cl-1ions.

Other Effects

1. Mechanical cracking when S-1 compounds form.

2. Oxide scale can dissolve in salt. When that happens, ionic mobilities change and reactions “short circuit.”

Type II Type II –– Salt Component Induced EffectsSalt Component Induced Effects

• Oxygen activity is low in hot corrosion.

In superalloys, this is because O2 reacts with Ti and Al in alloys, and oxygen in air must diffuse through the salt to reach the alloy surface.g y f

• Sulfur is bad because internal sulfates form and are oxidized (sulfur compounds are unstable) The are oxidized (sulfur compounds are unstable). The oxides that form are non-protective.

Chl id s ls p f nti ll t ith Al nd C • Chlorides also preferentially react with Al and Cr, eating out channels in the alloy and accelerating attack.

Type I

Type II

RECAP/SYNOPSIS

• Up to this point we have seen that:1. If the deposit covers the alloy, it separates it

from the gas phase.

2. Oxygen must diffuse through the layer to react i h h l l i diff with the metal or metal ions must diffuse to

react with O2 in the atmosphere.

3. SO4-2 ions are present in the salt layer (or Cl- or

SO3-2, etc., etc.)3 , , )

4. Because Ti and Al react with O2, there is an

oxygen gradient through the layer. P is lower at the surface than in the atmosphere.

-22OP

5. So, forming a protective oxide layer there is more difficult.

6. BUT, forming a non-protective layer is very easy (with SO4

-2 ions)(with SO4 ions).

RECAP/SYNOPSIS for Na2SO4 deposits

I. FLUXING MODES

A. Basic

1. Dissolution of reaction product barriers (i.e., oxide layers) as a result of removal ( , y ) fof S-2 and O-2 from Na:SO4 by alloy.

24 2

1( ) ( )

2sulfate deposit reacts w/ alloySO S

2

2

23

( )2

( )

reacts w/ alloy

reacts w/ oxide

O

MO

a. Continuous dissolution of Mo

1 2 22 2

1

2so (as seen with )

MO O O MO

Na SO Na MO Al O

This depends upon a continuous supply of Na2So4

2 4 2 2 2 3 (as seen with )Na SO Na MO Al O

supply of Na2So4.

RECAP/SYNOPSIS for Na2SO4 deposits

I. FLUXING MODES

B. Acidic

1. Gas Phase Induced2 21

M SO O M SO a.

needs both continuing supply of SO3and O2 from the gas phase.

2 23 2 42

M SO O M SO

b. Solution and re-precipitation2 2

3 3

2 2 1( )t

M SO M SO

M SO O M SO

c. Non-protective reaction layer with Ni or Co.

2 23 2 3( )

2pptM SO O Mo SO

RECAP/SYNOPSIS for Na2SO4 deposits

I. FLUXING MODES

B. Acidic – cont’d

2. Alloy phase induced

1 S l ti n f M in N SO m difi d 1. Solution of Mo in Na2SO4, modified by a second oxide phase such as AO3.

i. Modification of Na2SO4

ii. Solution reaction for Mo as

2 22 4 4 3

3( )

2alloyA O SO AO SO

fNa2SO4 is enriched in MAO4.

2

2 2

( ) ( ) 2alloy alloyM A O

M AO

iii. Solution and re-precipitation.4M AO

22 4( ) ( ) 2alloy alloyM A O SO

2 24 3M AO MO AO

Factors that influence hot corrosion

1. Resistance to hot corrosion is obtained by keeping the initiation phase as low as possible.

2. The S-2, Na+, V+, etc. are present in small , , , ppercentages in the fuel and air, but there is a large volume of fuel and air used to turn a gas turbine engine. Gas velocity is a factor.

3. NaCl significantly increases attack.

4. Basic fluxing requires a supply of oxide ions.Acid fluxing requires a supply of metal ions.

Corrosion-Erosion

• When we talked about solids causing corrosion (i.e., salts) we assumed that the solid was gently deposited on the surface of the component.

• But, what happens if the solid impacts the component at high velocity? What if it is:

– Sand (e.g., helicopter and jet engines in the gulf conflict or land-based turbines in the desert).

S lt i ( i t bi il – Sea salt grains (e.g., marine gas turbines, oil platforms, etc…).

– Pyrolytic carbon (from combustion products).

Powdered coal– Powdered coal.

– Rocks or gravel (foreign object damage – FOD).

• The theory is not well formed; but, let’s review erosion anyway.

Erosion in Ductile Materials

• Consider a Co-base superalloy rather than Ni-base (Ni-base superalloys only have elongations of up to 5% in tension).

• MECHANISMS:– Cratering – impact is normal to component

surface.

– Plowing – impact is at an angle to the surface.

Erosion in Ductile Materials

• MECHANISMS – cont’d:– Cutting – Particle has flat surfaces and is

rotating at impact.

Type IType I

Type II

• For “brittle” materials (not really well defined), Hertzian crack theory is used and there is surface and sub-surface fracture.

ductile brittle

Erosion rate

Angle of impact0° 90°

• This is independent of temperature.

• Of course as we add the influence of Of course, as we add the influence of temperature, a number of things happen to the material.– As T, YS, E, HK, erosion resistance .K

– Also, as E, energy required to cause erosion -but as HK, the energy absorbed from the particle - so as T, ductility , rate of recovery work hardening so erosion recovery , work hardening , so erosion resistance can also .

• All of this means that the effect of temperature on erosion rate depends on:– Alloy

– Particle

• And, note that as temperature increases, a “brittle” material may become “ductile”.

• Thus, there is NO WAY to correlate erosion ,behavior with alloy.

• Erosion:Erosion:– Breaks up oxide layer.

– Thins oxide layer.

D l h id l f i l – Depletes the oxide layer of protective elements by physically removing the Al+Cr-rich oxides –since the amount of Al and Cr is limited in the alloy and they diffuse slowly. Eventually they are no longer there to form the oxideare no longer there to form the oxide.

– The best way to eliminate eorsion corrosion is to remove the particles from the gas stream (screens, filters, etc. – reduce turbine efficiency).

• In some cases the attack is greater than an additive effect of corrosion attack and erosion attack alone.

• Effect of Hydrogen– So far we have considered air-breathing

environments. What about the space shuttle main engine, which runs on H2 and O2? What is the influence of hydrogen (also found in “sour the influence of hydrogen (also found in sour gas” wells – these are methane (CH4) wells containing high concentrations of H2S, CO2, and brine.

1. As stress increases, the resistance to H2embrittlement decreases in the same alloys because Phosphorous segregates to crack nucleation sitesnucleation sites.

2. Carbides and γ′ (Ni3Al) can become thermodynamically unstable under the influence of H2, and dissociate, weakening the alloys.

Review of Environmental Effects

Gas Phase: Oxides

Sulfides/Sulfates

Chlorides

Liquid (flux): Sulfates, chlorides

Solid Salts

Erosion

A p t ti l f ms b t nt ll is A protective layer forms, but eventually is degraded resulting in failure.