Surface Technology Part 4 Corrosion

1. Surface TechnologyPart 4Corrosion Professor Kenneth W Miller Office A108 Phone 0841 9348 0324 Surface Technology

2. Mechanisms of Corrosions Types Causes Outline

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 • 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 Fe2+ + 4 (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 + SO4-2 + 2H+→ PbSO4 + 2e- + 2H+ • Pb + PbO2 + 2SO42- + 4H+→ 2PbSO4 + 2H2O • PbO2 + SO4-2 + 4H+ + 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

14. Potential Fe – Cu / Fe - Zn

15. Reactions and Rates • Nernst Equation, addresses temperature and concentration • R – Universal gas constant • R = 8.3145 J / mole °K • F – Faraday constant • F = 1.6027733 x 10-19 C / electron • F = 96,485 C / (mole of electrons)

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

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 • 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