High Temperature Corrosion

Question 1

What are the steps of high temp corrosion-oxidation?

Answer 1

> oxidation: 2Fe + 3/2O2 = Fe2O3
sulfidation: Fe + 1/2S2 = FeS
chlorination: Fe + Cl2 = FeCl2

Question 2

G =

Answer 2

Go + RTln(Q)

Question 3

How does scale formation occur during high temp oxidaiton?

Answer 3

> intial adsorption of reaction (O2)
chemical reaction (surface oxide formation)
oxide nucleation (patches)
lateral growth (continuous film)-formation of protective or non-protective film
further scale growth/oxidation

Question 4

What is G for metal oxide rxn?

Answer 4

> G = Go + RTln[(MxOy)/(M)^x(O2)^y/2]

Question 5

What is G at equilibrium?

Answer 5

> G = 0, Go = -RTln(Q)
Go = -RTln[1/(O2)^1/y] b/c most activities of solids = 1
Go = (y/2)RTln(Po2)

Question 6

What is an Ellingham diagram?

Answer 6

> oxidation reactions are written for pure M reacting with 1mol of reacting O2 to yield MO2
Go = -RTlnK = RTlnpO2

Question 7

What do we do in the case where two oxidants are present? (sulfur and oxygen, chlorine and oxygen)

Answer 7

we can describe the relative stabilities of various condensed phases with a Kellogg diagram (phase stability diagram)

Question 8

What are binary constitution diagrams? (T vs oxygen content)

Answer 8

> used for metal-oxide systems in which the metal has multiple oxidation states, and one or more of the oxides has appreciable nonstoichiometry

Question 9

What happens when the mass transport through iron oxides is slow relative to interfacial rxns?

Answer 9

the oxygen activity gradient is sustained through each oxide, and layered oxides start to form at the surface during the oxidation of iron (ex: a Fe2O3 layer on top of an Fe3O4 layer on top of Fe)

Question 10

–> What are the requirements for a protective scale?

Answer 10

> the scale should be adherent 
> high melting point 
> low vapor pressure 
> alloy and oxide should have similar thermal expansion coefficients
> high temperature plasticity
> low concentration of defects

Question 11

What is the Pilling Bedworth ratio?

Answer 11

= vol. 1 mol oxide produced / vol. 1 mol metal consumed

= Wd/nDw

Question 12

What is breakaway?

Answer 12

change in kinetics from a protective scale to a non-protective scale (microcracking or porosity, consumption of protective-scale forming constituents from the alloy surface)

Question 13

Linear

Answer 13

X = K1*t

Question 14

Logarithmic (low temp)

Answer 14

X = Kelog(at + 1)

Question 15

Parabolic (high temp)

Answer 15

X^2 = Kp*t
> Kp = CD∆c when t=0 and x=0
> C: proportionality constant
> D: diffusion constant 
> x: film thickness

Question 16

Effect of temperature on Kp

Answer 16

Kp = Ko*e^(-Q/RT)

Question 17

Effect of oxygen pressure on Kp

Answer 17

Kp = C*pO2^(1/2)