A contribution to the theory of cementation (metal displacement) reactions

Abstract

In this paper, cementation (metal displacement) reactions are discussed in terms of Evans diagrams, i.e. diagrams constructed by the superposition of cathodic and anodic polarization curves. The conditions under which the rate of a cementation reaction will be controlled by cation diffusion to the reacting metal surface or some chemical step at the surface are explored. It is found that for the case in which the effective cathodic and anodic areas are approximately equal, the cementation is likely to be diffusion-controlled when its two component oxidation-reduction reactions have standard electrodepotentials which differ by more than 0.36 V. The ratio of cathodic to anodic area is also found to be important in determining the reaction mechanism. The presence of an oxide film on the metal is shown to be another factor which influences the reaction mechanism. Evans diagrams are used to discuss the composition of the product layer formed during a cementation reaction.