Corrosion stationary dissolution conditions

Dissolution of alloys follows different principles. Dissolution of a freshly prepared surface can be described by the dissolution mechanism of a pure metal but now with different rate constants for the different alloy components (section General mechanisms). Because of the different corrosion rates a depletion of one component soon occurs. An intermediate region of different composition can develop (section Stationary dissolution conditions). In some cases the matrix of the nobler component is retained and a sponge-like structure develops. Again, this all concerns an oxide-free surface. The complications connected with oxide layer formation will be discussed in Section 10.2. 

Finally, we should briefly discuss the rate of steady state dissolution of a compact corrosion product in aqueous solution. In order to calculate this stationary corrosion rate, it is necessary to know the rate-determining step for the removal of metal ions from the corrosion product into the electrolytic solution. It has been found that the stationary corrosion rate of a passivated layer on iron between 0.7 and 3.9 in an aqueous H2SO4 solution does not depend upon the stirring rate, nor upon the Fe /Fe ratio in the solution, nor, most importantly, does it depend upon the electrode potential h [34]. From these observations it has been concluded that the rate-controlling step is a chemical reaction between the iron ions, which are leaving the corrosion product, and the solvent ions to form complex ions immediately next to the surface. Thus, in the case of iron under the given conditions, this step determines the overall corrosion rate and thus it also determines the stationary thickness of the passivating corrosion product. 

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