Pitting Corrosion of Mild Steel under Marine Anaerobic Conditions—Part 1: Experimental Observations

This paper is concerned specifically with the growth and development of pitting on mild steel surfaces under anaerobic conditions, that is, when sufficient corrosion product has built-up under marine immersion conditions for the corrosion environment at the rust-metal interface to have become anaerobic. Recent detailed microscopy observations of the changing nature of the pitting topography of the corroding surface will be presented for coupons exposed for up to 4 years in Pacific Ocean seawater. These show pit growth being mainly lateral and causing pit coalescence and the development of “macropits.” This is followed by renewed pit initiation. The patterns of pitting are similar to those on coupons recovered at various locations worldwide in long-term (8 years) field trials. It may be inferred from the observations that pit initiation and growth can occur under entirely anaerobic conditions. A model for anaerobic pit growth and development is proposed. The present observations have implications also for the statistical modelling of extreme pit depths for longer exposures.