Voxide/Vmetal

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

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

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

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

1. sparring occurs often
2. oxides are volatile
3. weight gains are very small

protective coatings

corrosion resistant metals/alloys

corrosion inhibitors

anodic/cathodic protection

turbine

heat exchangers

combustion chambers

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

measures oxidation by thickness v. time > linear

x=kt

adsorption is rate limiting

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

Oxidation of Alloys

Thermodynamic and Kinetic Considerations

• 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

hot corrosion

what is it and what effects it

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

how do materials fail at these hgih temperatures

oxides deform, begin to creep and break away

there are stresses as a result of differeing lattice parameters

Cr2O3

Al2O3

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

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?

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

SO₄^2->>1/2O₂+SO₂+O^2-  //a_O^2- is the only unknown

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

SO₄^2->>SO₂+1/2O₂+O^2-

if O^2- at the boundary is the only unknown, it can be determined by system conditions

if aO^2- is low, it will dissociate MO>>M²⁺+O^2- (acid fluxing)

if aO^2- is high, oxide dissolves MO+O^2->>MO₂^2- (basic fluxing)

high: stable metal

low: stable oxide

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

well below this value, the oxide will dissociate