KJM 3110 Electrochemistry

Chapter 11. Corrosion

Summary Ch. 10 Electrode polarisation

• 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

• Complexing agent

• And thereby dissolving protective layers

Effects of salt, chloride ions

• 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

• 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

Corrosion of iron Fe and its alloys is autocatalytic

• Corrosion potential; mixed potential caused by two electrode processes

• Limited by kinetics (not transport or ohmic polarisation)

Corrosion potential and corrosion current

• Corrosion potential; mixed potential caused by two electrode processes

• Tafel regions of each of the two reactions

Corrosion potential and corrosion current

Logarithmic (Tafel) plot

• Tafel equations

• Tafel slopes

Logarithmic (Tafel) plot

Polarisation curve

• Linear polarisation

• Polarisation resistance

• Can also be found from AC impedance or from weightloss rate of metal

• Textbook speculations

Pitting corrosion

• Same mechanisms as in pitting corrosion

Crevice corrosion

• 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

• Zn as sacrificial anode («offeranode»)

Cathodic protection

• 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

pH-dependent protection

• 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

Ch. 11 Corrosion - Summary