Surface Technology Part 4 Corrosion Professor Kenneth W Miller Office A108 Phone 0841 9348 0324 Surface Technology

Outline Mechanisms of Corrosions Types Causes

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

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)

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

Controlling Factors Material Environment Stress Geometry Temperature Time