Corrosion is an electrochemical cell
A corroding metal is a short-circuited electrochemical cell on one surface. At some places the metal is oxidised and goes into solution, the anodic reaction. The electrons it releases travel through the metal to other places, where something in the environment is reduced, the cathodic reaction. In water the cathodic reaction is usually oxygen reduction in neutral or alkaline solutions, and hydrogen evolution in acids.
Both reactions must run at the same rate, because no charge can build up. That simple balance sets how fast the metal corrodes.
Mixed potential and the Evans diagram
Each half reaction has its own equilibrium potential and its own kinetics. On a freely corroding metal, the potential settles where the anodic current of metal dissolution equals the cathodic current of the oxidant. This is the mixed potential or corrosion potential Ecorr, and the current at that point is the corrosion current icorr. Wagner and Traud described this idea in 1938, and plotting both reactions as potential against log current gives an Evans diagram.
- Choose Steel in aerated neutral water. The cathodic line bends vertical where oxygen cannot diffuse in fast enough. Corrosion is then controlled by oxygen supply, which is why stirring or flow can speed it up.
- Choose the two inhibitor examples. An anodic inhibitor slows metal dissolution and moves Ecorr up. A cathodic inhibitor slows the cathodic reaction and moves Ecorr down. Both lower icorr.
The solid line is what a potentiostat measures when it polarizes the sample: the difference between the two half reactions. Near Ecorr it passes through zero current, which is why a measured polarization curve shows a sharp dip on a log scale.
Passivity
Some metals, such as stainless steel, aluminium, titanium and nickel, form a thin oxide film at higher potentials that slows dissolution by orders of magnitude. On an anodic polarization curve the current first rises, then drops to a low passive current, and rises again at the breakdown or pitting potential. Chloride ions can break the film locally and start pits, which is why stainless steels can fail in seawater even though they look fine elsewhere.
Forms of corrosion
| Form | What happens | Typical example |
|---|---|---|
| Uniform | The whole surface corrodes at a similar rate | Carbon steel in acid |
| Galvanic | The less noble of two metals in contact corrodes faster | Steel screws in copper, aluminium next to steel |
| Pitting and crevice | Local breakdown of a passive film, small but deep attack | Stainless steel in chloride |
| Intergranular and stress corrosion cracking | Attack along grain boundaries, or cracks under stress | Sensitised stainless steel, pipelines |
| Microbially influenced | Bacteria change the local chemistry | Pipelines and tanks in oil and gas |
Electrochemical rate measurements describe uniform corrosion best. For local attack they give an average that can hide a deep pit, so they should be combined with inspection of the surface.
How corrosion is controlled
- Materials that passivate, or alloys suited to the environment.
- Coatings and paints that keep water and oxygen away.
- Inhibitors added to the fluid, which adsorb on the metal and slow the anodic reaction, the cathodic reaction, or both. Many organic inhibitors used in the oil and gas industry contain nitrogen, sulfur or aromatic groups that bind to steel.
- Cathodic protection, which shifts the metal to a potential where it no longer dissolves, using a sacrificial zinc or magnesium anode or an impressed current.
Check your understanding
Quiz for lesson 5.1
Pick one answer per question. You see the explanation straight away.
References and further reading
- R. W. Revie, H. H. Uhlig, Corrosion and Corrosion Control, 4th ed., Wiley, 2008.
- E. McCafferty, Introduction to Corrosion Science, Springer, 2010.
- M. Stern, A. L. Geary, Electrochemical polarization I. A theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–63.
- C. Wagner, W. Traud, On the interpretation of corrosion processes through the superposition of electrochemical partial processes and on the potential of mixed electrodes, Z. Elektrochem. 44 (1938) 391–402, in German.