The changing topography of corroding mild steel surfaces in seawater

The corrosion of mild steel exposed to marine immersion conditions typically is not uniform although it is often idealized as such. Anodic regions and micro-pits develop very quickly after first exposure and eventually there is the development of shallow broad pits. This transition of the surface topography and the processes involved are still not completely understood. The present paper presents a number of images typical of the sequential evolution of the surface topology of mild steel corroding in sub-tropical coastal seawater. The sequence consists of the development of anodic areas, development of small pits and shallow broad pits, the apparent coalescence of small pits into larger localized corrosion and eventually the appearance of stepped or benched, perhaps irregular-shaped broad or macro-pits. This sequence is typical of that which has been observed many times and this suggests it is reasonable to infer a generic sequence that describes the changing surface of corroding mild steel in seawater. It will assist in developing a better understanding of the evolution of corrosion processes for mild steel in seawater and assist in identifying the various controlling processes.