Surface Technology Part 4 Corrosion
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Transcript of Surface Technology Part 4 Corrosion
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Surface Technology
Surface TechnologyPart 4
Corrosion
Professor Kenneth W MillerOffice A108
Phone 0841 9348 0324
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Outline
• Mechanisms of Corrosions
• Types
• Causes
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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
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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)
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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
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Galvanic Couple
• Oxidation is a half reaction
• Reduction is a half reaction
• They must happen together
• galvanic couple
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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
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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
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Reactions and Rate
• Reactions depend on “Standard Electrode Potential”
• Reaction rate depends on temperature
• Reaction rate depends on concentration
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Electrical Potential
• Standard emf series shows half reactions
• Two reactions are required– oxidation– reduction
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Standard emf Series
• Idealized reactions with solutions of the metal ions
• Does not address effects of dilution, formation of protective layers, or secondary reactions
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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
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Potential Fe – Cu
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Potential Fe – Cu / Fe - Zn
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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
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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)
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Very Base Metals
• emf < -0.4V
• Corrode in neutral aqueous solutions, even without oxygen
• includes Na, Mg, Be, Al, Ti, and Fe
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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
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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
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Noble Metals
• emf between > +0.7V
• includes Pd, Pt, Au
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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
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Uniform or General Surface Corrosion
• Evenly distributed loss of material over a surface
• Allows corrosion evaluation through material thickness
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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
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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
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Galvanic Corrosion
• Occurs around the junction of dissimilar metals
• Typical of riveted and bolted joints
• Corrosion products (reduction) can cause problems through volume increase
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Rust Formation
• Formation of ferriferous oxide and hydroxide corrosion
• Iron and Steel
• Most common problem in steel bodies and frames
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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
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Fretting Corrosion
• Combination of a corrosive medium (e.g. salt water) and friction
• Similar to erosion
• Starts attack at surface asperities
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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
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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)
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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
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Vibration Corrosion Cracking
• Stress corrosion with fatigue loads
• Typically results in transcrystalline fissures
• Not always visible
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Controlling Factors
1. Material
2. Environment
3. Stress
4. Geometry
5. Temperature
6. Time