  Experiments on the diffusion-limited corrosion of porous copper clusters in
thin gap cells containing cupric chloride are reported. By carefully comparing
corrosion front velocities and concentration profiles obtained by phase-shift
interferometry with theoretical predictions, it is demonstrated that this
process is well-described by a one-dimensional mean-field model for the generic
reaction A + B (static) -> C (inert) with only diffusing reactant (cupric
chloride) and one static reactant (copper) reacting to produce an inert product
(cuprous chloride). The interpretation of the experiments is aided by a
mathematical analysis of the model equations which allows the reaction-order
and the transference number of the diffusing species to be inferred. Physical
arguments are given to explain the surprising relevance of the one-dimensional
mean-field model in spite of the complex (fractal) structure of the copper
clusters.
