A new model for the anomalous codeposition of alloys is based on the kineti
c characteristics of metal electrodeposition and th effect of hydrogen evol
ution accompanying the deposition. The less noble metal has intrinsically f
aster kinetics and so tis deposition rate can be controlled manly by diffus
ion mass transfer in a certain range of over potentials; the convection ind
uced by hydrogen evolution can speed up the relative deposition rate of thi
s less noble metal when it is mainly diffusion-influenced. In contrast with
previous models, such as the hydroxide suppression hypothesis, the monohyd
roxide-discharge-controlled mechanism, and the competitive adsorption effec
t, our model can explain naturally and completely anomalous codeposition, i
.e. the preferential deposition of the less-noble metal, oa problem in allo
y electrodeposition that has persisted for a century.