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C8
Chemical Stability
26
Science
Undergraduate 3
05/31/2011

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Cards

Term
Pilling Bedworth Ratio
Definition

Voxide/Vmetal

Significant mainly for internal casting situations because it causes internal stresses and much damage

Term
Growth rates at various stages
Definition

Early stage: logarithmic and mainly based on adsorption to the surface of the metal where they then dissociate, then chemisorb.  as the scale gets larger, the potential difference is smaller

Later stages: parabolic and rate limited by diffusion (a potential difference that forms due to the creation of the oxide is a driving force)

-the thinner the film, the higher the oxidation rate

Asymptotic thickening: occurs in alloys as protective oxides of various metals grow

Term
Wagner theory of oxidation
Definition
provides a quantitative theory regarding the flux of cations, anions, and electrons based on the chemical gradient that exists in an oxide
Term
Sparring
Definition

upon oxide layers forming on the surface of a substrate, there are residual stresses forming which then cause cracking and ultimately bits of the MO layer break off resulting in a quasi-linear plot of thickness v. time.  Could also be known as re-oxidizing

 

Term
What is the significance of p<peq?  What happens if several oxides form on a metal?
Definition

if p<peq, the oxide will dissociate

 

if they all have different dissociation pressures, the oxide that is the richest in oxygen will dissociate to an oxide containing less

Term
Why is the study of oxide kinetics difficult
Definition
  1. sparring occurs often
  2. oxides are volatile
  3. weight gains are very small
Term
Corrosion prevention methods
Definition

protective coatings

corrosion resistant metals/alloys

corrosion inhibitors

anodic/cathodic protection

 

Term
high temperature environments
Definition

turbine

heat exchangers

combustion chambers

Term
Hauffe Effects
Definition

adding materials of different valences to create a solid solution.  After doing Kroger Vink analysis and can see if the main base metal is being added or removed and therefore if the diffusivity of that species is being increased or decreased. 

 

ie adding Li or Al to Zinc

Term
Predominance diagram
Definition
A predominance diagram purports to show the conditions of concentration and pH where a chemical species has the highest concentration in solutions.The lines on a predominance diagram indicate where adjacent species have the same concentration. Either side of such a line one species or the other predominates, that is, has higher concentration relative to the other species.
Term
Linear rate law
Definition

measures oxidation by thickness v. time > linear

x=kt

adsorption is rate limiting

Term
Origin of corrosion stresses
Definition

Epitaxial stresses (strained bonds)

Changes in composition (local stress fields)

Point defect stresses

thermal stresses

 

Recrystallization can reduce these stresses but it also causes grain sizes to growth>increases corrosion

Term

Oxidation of Alloys

Thermodynamic and Kinetic Considerations

Definition
  • Thermodynamic
    • components have different affinities for oxygen
    • ternary or higher oxides could be formed
    • different oxides may show mutual solid solubilities
  • Kinetic
    • different ions have different mobilities in oxide
    • metals have different diffusivities
    • dissolution of oxygen may lead to internal oxidation as oxygen has a high solubility
Term

hot corrosion

what is it and what effects it

Definition

interaction of a molten salt (sulphate typically) with a metal or metal oxide surface

 

it is affected by the deposit chemistry, alloy composition, microstructure, temperature, and erosion

Term

how do materials fail at these hgih temperatures

 

Definition

oxides deform, begin to creep and break away

there are stresses as a result of differeing lattice parameters

Term
Logarithmic growth
Definition
usually for consideration at hte eraly stages of a reaction
Term
what are the most protective oxides?
Definition

Cr2O3

Al2O3

 

they are good because they are insulators and also do not allow for any diffusion

Term
why are non stochiometric oxides not ideal?
Definition
ex. Fe3O4 has many vacancies
Term
how do spinels behave in corrosion situations
Definition
they have no vacancies therefore diffusion must occur through interstices
Term
what is a phase boundary reaction
Definition
what happens at the interface is rate limited
Term

what are the stages of hot corrosion (technical names - another slide asks for the 'story' of hot corrosion)

 

what is the determined parameter in the system?

Definition

1-initiation: involves chemical interaciton between sulphate and metal oxide.  end of initation when oxide is 'removed' and gas phases interact with alloy surface

2-propagation: acid or basic fluxing

Na2SO4 drops for and land on surface with a basic component Na2O and acidic SO3


SO42->>1/2O2+SO2+O2-  //aO2- is the only unknown

Term
hot corrosion story
Definition

alkali's from salt interact with S2 and O2 in the environment to form drops which lie on the MO surface

 

SO42->>SO2+1/2O2+O2-

if O2- at the boundary is the only unknown, it can be determined by system conditions

 

if aO2- is low, it will dissociate MO>>M2++O2- (acid fluxing)

if aO2- is high, oxide dissolves MO+O2->>MO22- (basic fluxing)

Term
What is the significance of materials high/low on the ellingham diagram
Definition

high: stable metal

low: stable oxide

Term
significance of the equilibrium partial pressure
Definition

pressure at which teh driving force is zero.  below this no oxidation will occur

 

well below this value, the oxide will dissociate

Term
how do you know if a material will reduce another
Definition
a chemical is a reducing agent if ΔG for its oxidation is more negative than that of the pure metal
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