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KJM 3110 Electrochemistry
Chapter 11. Corrosion
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Summary Ch. 10 Electrode polarisation
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• All metals except gold, Au, are thermodynamically unstable vsoxidation under ambient conditions.
• Pollutants like sulphur can enhance thermodynamics for oxidation
• Sulphurisation
• Sulphatisation
• Water
• Water+oxygen
• Submersion in water not necessary – a film of physisorbed water provides H3O+ and H2O and transport; «surface protonics»
• Largely enhanced by >60& RH and presence of acid-base adsorbants
Corrosion ≈ unwanted oxidation of metals
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• Complexing agent
• And thereby dissolving protective layers
Effects of salt, chloride ions
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• Noble metals
• Au is thermodynamically stable
• Pt, other Pt group metals, Ag, Hg, and to some extent metals like Cu areoften practically stable – small driving forces for oxidation
• Corrosion resistive metals
• Aluminium is very unstable, but therefore forms a very stable protectiveoxide layer Al2O3.
• The same is true for many other non-noble metals; Mg, Ti, Cr, Ni, Zn, Sn….
Noble metals and other corrosion-resistive metals
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• Corrosion reactions we have written have been redox reactions, not electrochemical red and ox reactions
• In reality, they are often split in red and ox taking place at different locations• Electrochemistry
• Transport of electrons and ions between the two locations
• Often an ever-changing mosaic of sites due to fluctuating driving forces and kinetics (seetextbook pages 215-216).
Corrosion cells: Two electrodes on the same surface
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Corrosion of iron Fe and its alloys is autocatalytic
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• Corrosion potential; mixed potential caused by two electrode processes
• Limited by kinetics (not transport or ohmic polarisation)
Corrosion potential and corrosion current
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• Corrosion potential; mixed potential caused by two electrode processes
• Tafel regions of each of the two reactions
Corrosion potential and corrosion current
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Logarithmic (Tafel) plot
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• Tafel equations
• Tafel slopes
Logarithmic (Tafel) plot
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Polarisation curve
• Linear polarisation
• Polarisation resistance
• Can also be found from AC impedance or from weightloss rate of metal
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• Textbook speculations
Pitting corrosion
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• Same mechanisms as in pitting corrosion
Crevice corrosion
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• Coatings
• Electropainting with carboxylate COO- groups• Negatively charged paint polymer particles migrate, are neutralised, and
adhere• Fe dissolves electrolytically and forms insoluble Fe carboxylates• Uniform
• Electroplating – electrochemical reduction of plating metal
• Electroless plating – chemical reduction of plating metal
• Corrosion inhibitors• Oxidising agents – forms protective film
• Nitric acid • Electrochemical anodisation (e.g. for aluminium)
• «Phosphating» - hot phosphoric acid or acidic phosphate solution
Corrosion protection
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• Zn as sacrificial anode («offeranode»)
Cathodic protection
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• Some metals are naturally protected by oxidic scales (e.g. Al, Cr, Ni…)
• For others, we may apply chemically or electrochemically forced formationof an oxidic scale; passivation.
Anodic protection; passivation
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pH-dependent protection
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• Crack corrosion• High energy in crack tips
• Stress corrosion cracking• Alloys, not pure metals
• Hydrogen embrittlement
• Corrosion fatigue• Dislocations entangle
• Stress – enhanced corrosion
Stress corrosion, H2, fatigue
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Ch. 11 Corrosion - Summary