Pitting Corrosion of Mild Steel in Marine Immersion Environment—Part 1: Maximum Pit Depth

For modern 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. The 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.