Pitting corrosion of mild steel in marine immersion environment - Part 1: Maximum pit depth

For modem mild and low-alloy structural steels under marine immersion conditions, pitting corrosion typically is not considered an important issue. However, the literature shows that in the laboratory micro-pitting can occur within a very short time of exposure. This is confirmed for real seawater by recent new field observations. Most previous field investigations have failed to report such pitting; Instead, they have reported on macro-pitting, which usually is very mild in the earlier stages of corrosion but increases substantially for longer exposures. For tropical waters, however, significant macro-pitting has been reported within one year of exposure. Me various data suggest that conventional pit growth models derived from short-term observations are not appropriate. The present paper presents new field data and interprets literature data for pitting corrosion depth as a function of time using a modification of the recently proposed multi-phase model for marine immersion corrosion. This shows that macro-pitting for mild and low-alloy structural steels tends to become significant only after overall anaerobic conditions have set in and, by implication, when sulfate-reducing bacteria (SRB) become active. As was shown previously, the time to commencement of overall anaerobic conditions is strongly influenced by seawater temperature. This allows a preliminary calibration of the model for maximum pit depth for a given surface area. With the aid of the model, the effects of surface finish and mill scale can be distinguished.