Mathematical modelling of the diffusion controlled phase in marine immersion corrosion of mild steel

In the mathematical modelling of the ‘general’ corrosion of mild and low alloy steels and of some other metals oxygen diffusion through the corrosion products to the corroding surface controls the rate of corrosion at least during some period in the corrosion–time behaviour relationship. Under slightly different theoretical assumptions various early authors derived expressions for this phase. It was usually assumed the derived relationship applied from initial immersion. Only very limited laboratory support for the derived relationships exists. They have been applied to field data for atmospheric corrosion but with constants and exponents modified empirically. In the present paper the theory is reviewed and extended. It is argued that diffusion control cannot apply from first immersion and that this provides considerably greater freedom to mathematical modelling. Allowance is made for non-uniform corrosion product density and permeability and for loss of corrosion product. Recent detailed ‘at-sea’ experimental observations obtained on the Eastern Australian seaboard are applied to test the theory for mild steel. It is shown that the most likely competing mathematical relationships produce relatively similar results. The derived relationship is fitted to literature data to deduce its temperature dependence.