Anomalous codeposition of iron group metals - II. Mathematical model

A mathematical model for anomalous codeposition of iron group metals is presented which describes effects of inhibition and enhancement observed experimentally during codeposition of FeNi, NiCo, and FeCo alloys. The model assumes three parallel reaction paths each one proceeding in two consecutive steps and it takes into account the effect of mass transport. The model assumes that deposition involves an adsorbed reaction intermediate containing both metal ions in partly reduced form. This reaction intermediate is responsible for both the inhibition of the more noble species and the enhancement of the less noble species. Differences in the deposition behavior of the three alloy systems considered are due to differences in the surface coverage of the mixed reaction intermediate. The theoretical predictions of the model are compared with experimental data obtained under well-defined mass transport and current distribution conditions. The model can simulate in a satisfactory way the observed effects of inhibition and enhancement due to codeposition and it predicts correctly the influence of mass transport. It is less successful in the simulation of the influence of the concentration of reacting species. Possible reasons for this behavior are discussed and research directions for improving the model are outlined. (C) 1999 The Electrochemical Society. S0013-4651(98)10-089-7. All rights reserved.