Wet Corrosion of Materials

Why the dramatic effect of water on the rate of loss of material As an example we shall look at iron, immersed in aerated water (Fig. 23.2). 

Iron atoms pass into solution in the water as Fe leaving behind two electrons each (the anodic reaction). These are conducted through the metal to a place where the oxygen reduction reaction can take place to consume the electrons (the cathodic reaction). This reaction generates OH ions which then combine with the Fe ions to form a hydrated iron oxide Fe(OH)2 (really FeO, H2O) but instead of forming on the surface where it might give some protection, it often forms as a precipitate in the water itself. The reaction can be summarised by 

Now the formation and solution of Fe is analogous to the formation and diffusion of M in an oxide film under dry oxidation and the formation of OH is closely similar to the reduction of oxygen on the surface of an oxide film. However, the much faster attack found in wet corrosion is due to the following  

The result is that the oxidation of iron in aerated water (rusting) goes on at a rate which is millions of times faster than that in dry air. Because of the importance of (c), wet oxidation is a particular problem with metals. 

In dry oxidation we quantified the tendency for a material to oxidise in terms of the energy needed, in kj mol of O2, to manufacture the oxide from the material and oxygen. Because wet oxidation involves electron flow in conductors, which is easier to measure, the tendency of a metal to oxidise in solution is described by using a voltage scale rather than an energy one. 

---