1. Surface Technology Part 4 Corrosion
Professor Kenneth W Miller
Office A108
Phone
Surface Technology

2. Outline
Mechanisms of Corrosions
Types
Causes

3. Fundamentals of Corrosion
It is an electro-chemical reaction
It happens in two parts
oxidation or loss of electrons
reduction or gain of electrons
Any electrical joint of dissimilar metals
batteries
thermocouples

4. Oxidation Anode – donates electrons General Reaction form is Examples
M → Mn+ + ne-
Examples
Fe → Fe2+ + 2e-
Al → Al3+ + 3e-
This results in a positive ion and free electron(s)

5. Reduction Cathode – receives electrons General form of reaction is
Mn+ + e- → M(n-1)+
Examples
O2 + 2 H2O + 4e- → 4 (OH-)
2 H+ + 2 e- → H2
Loose electrons join with other atoms resulting in a neutral atom or less positive ion

6. Galvanic Couple Oxidation is a half reaction
Reduction is a half reaction
They must happen together
galvanic couple

7. Complete Reactions
Combine one (or more) oxidation with one (or more) reduction
Rust
2 Fe + O2 + 2 H2O → 2 Fe (OH)-
→ 2 Fe (OH)2
then 4 Fe (OH)2 + O2 + 2 H2O → 4 Fe(OH)3
Result is an insoluble compound
Some reactions remain as ions in solution

8. Complete Reactions
A similar reaction is aluminum oxidation to form Al2O3, an insoluble compound
Another reaction lead-acid batteries
Use lead plates H2O, and H2SO4
Pb + SO H+ → PbSO4 + 2e- + 2H+
Pb + PbO2 + 2SO H+ → 2PbSO4 + 2H2O
PbO2 + SO H+ + 2e- → PbSO4 + 2 H2O

9. Reactions and Rate Reactions depend on “Standard Electrode Potential”
Reaction rate depends on temperature
Reaction rate depends on concentration

10. Electrical Potential Standard emf series shows half reactions
Two reactions are required
oxidation
reduction

11. Standard emf Series
Idealized reactions with solutions of the metal ions
Does not address effects of dilution, formation of protective layers, or secondary reactions

12. Reactions and Rates Standard Reaction
V2 is the cathode or reducing material
V1 is the anode or oxidizing material
Must be positive, or V1 and V2 are reversed

13. Potential Fe – Cu
Discuss cathode vs. anode in this example

14. Potential Fe – Cu / Fe - Zn
Discuss cathode vs. anode in this example

15. Reactions and Rates
Nernst Equation, addresses temperature and concentration
R – Universal gas constant
R = J / mole °K
F – Faraday constant
F = x C / electron
F = 96,485 C / (mole of electrons)

16. Reactions and Rates molar concentrations (a)
Numerator components are anode materials
Denominator components are cathode materials
Result still must be positive
Nernst Equation at 25°C

17. Very Base Metals emf < -0.4V
Corrode in neutral aqueous solutions, even without oxygen
includes Na, Mg, Be, Al, Ti, and Fe

18. Base Metals emf between -0.4V and 0.0 V
Corrodes in neutral aqueous solutions with oxygen
Corrodes in acids to produce hydrogen, even without oxygen
includes Cd, Co, Ni, Sn, and Pb

19. Semi-Noble Metals emf between 0.0 V and +0.7V
Corrodes in aqueous solutions only with the presence of oxygen
includes Cu, Hg, Ag

20. Noble Metals
emf between > +0.7V
includes Pd, Pt, Au

21. Types of Corrosion Group I Group II Group III
identifiable by visual inspection
Uniform, Pitting, Crevice, Galvanic, Rust
Group II
identifiable with special inspection tools
erosion, cavitation, fretting, intergranular
Group III
identifiable by microscopic examination
exfoliation, de-alloying, stress-corrosion cracking

22. Uniform or General Surface Corrosion
Evenly distributed loss of material over a surface
Allows corrosion evaluation through material thickness

23. Pitting Corrosion Local corrosion forming holes and pits
Depth is typically greater than diameter
Damage is localized and hard to measure
Damage is difficult to predict and model
typically requires a statistical model
May be covered with corrosive products to hide
Possible serious weakening with little material loss

24. Crevice Corrosion Attacks crevices in material
gaskets, fastener heads, disbonded coatings, clamps, and lap joints
Localized corrosion sensitive to micro-environment
May cause a localized anode condition at the base and cathode at the surface

25. Galvanic Corrosion Occurs around the junction of dissimilar metals
Typical of riveted and bolted joints
Corrosion products (reduction) can cause problems through volume increase

26. Rust Formation Formation of ferriferous oxide and hydroxide corrosion
Iron and Steel
Most common problem in steel bodies and frames

27. Erosion Corrosion
Corrosion accelerated by relative motion of electrolyte
Not typical of auto bodies except in extreme cases in wheel wells
May be accelerated by cavitation

28. Fretting Corrosion
Combination of a corrosive medium (e.g. salt water) and friction
Similar to erosion
Starts attack at surface asperities

29. Intergranular Corrosion
Corrosion along grain boundaries
May be a function of material segregation along grain boundaries
May attack precipitates along grain boundaries (Cr in stainless steel)
Typical problem in welds

30. Exfoliation
A type of intergranular corrosion typical of high-strength aluminum alloys
Starts (usually) at exposed grains, typically on a machined surfaces such as holes or edges
Attacks following grain boundaries
Volume of corrosion products separates grains (leafing)

31. Stress Corrosion Cracking
Combination of a corrosive medium (e.g. salt water) and tensile stress
Stress can be external or internal (residual)
Not always visible without microscopic evaluation
May cause transcrystalline or intercrystalline fissures

32. Vibration Corrosion Cracking
Stress corrosion with fatigue loads
Typically results in transcrystalline fissures
Not always visible

33. Controlling Factors Material Environment Stress Geometry Temperature
Time