Scale as a Diffusion Barrier

Once a corrosion product layer is formed, the corrosion process may continue through the diffusion of at least one of the reactants through the corrosion product layer. Let us consider, for example, the case of nickel exposed to air at high temperature. Corrosion can theoretically continue through the nickel oxide layer by means of diffusion in either direction, alone or by counter-current diffusion, as illustrated in Fig. 15.9. 

For this type of oxide, the controlling step in increasing scale thickness is the ability of the nickel ions to migrate outward toward the surface. If a small quantity of a higher valence ion (Cr +, for example) is added to the nickel, as in Fig. 15.11, charge balance will require a greater number of nickel vacancies and hence an easier. 

Fioure 15.9 Schematic diagram showing countercurrent diffusion, that is, diffusion of oxygen inward and metal ions plus electrons outward. Thus, the anode is at the metal/ metal oxide interface where oxidation (M - + 2e ) occurs. The cathode is at the oxide/air interface where 

Nickel oxide as an example of metal-deficit scale. Schematic of NiO lattice shows a few Ni sites among the Ni. For every two Ni sites, there is a vacant Ni site. 

The opposite situation, a metal-excess or -type scale, is also possible (zinc is an example). In this case, extra metal ions are believed to occupy intermediate positions in the scale. The introduction of a higher valence metal ion will reduce the number of zinc ions in the scale and thereby lower the oxidation rate. A lower valence metal ion will, of course, have the opposite effect, causing the metal-excess scale to respond to the addition of other metal ions in a manner just the reverse of that in metal-deficit scales. 

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