Surface Technology

Surface TechnologyPart 4

Corrosion

Professor Kenneth W MillerOffice A108

Phone 0841 9348 0324

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– 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)Fe(OH)33

• 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

0 02 1E V V V

Potential Fe – Cu

Potential Fe – Cu / Fe - Zn

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)

0 lnR T

E E Qn F

Reactions and Rates

• Nernst Equation at 25°C

100 0.059

logE E Qn

•Numerator components are anode materials•Denominator components are cathode materials •Result still must be positive

m nM Na bA B

a aQ

a a

molar concentrations (a)

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– 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

1. Material

2. Environment

3. Stress

4. Geometry

5. Temperature

6. Time