Models for the anaerobic phases of marine immersion corrosion

In the phenomenological model for the general marine corrosion of mild (and high strength low alloy) steel introduced previously, the corrosion process eventually enters a state governed predominantly by the activity of sulphate-reducing bacteria (SRB). This state has been modelled by two consecutive phases, the first describing a transient behaviour and the second the longer-term quasi-steady-state corrosion response. In earlier papers these have been described only by bounding parameters, calibrated to comprehensive field data. Herein a plausible idealized mathematical model is proposed for the corrosion losses within these phases. The model is based on an idealization of the distribution of nutrients within the corrosion product layer and its depletion by SRB as the anaerobic conditions necessary for their metabolism develop. With the gradual depletion of nutrients, the activity of the SRB reduces to a quasi-steady-state process governed by the rate of external nutrient supply and the loss of (protective) corrosion product. It follows that high levels of nutrients, such as in offshore oilfields and in nutrient-polluted waters can be associated with high rates of corrosion under anaerobic conditions. Also, in warmer (tropical) waters, the metabolism of SRB is enhanced. Both phenomena are consistent with field observations. A preliminary calibration of the models for phases 3 and 4 is attempted using the limited available field data.