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The transition from short- to long-term marine corrosion of carbon steels - 1. experimental observations

This paper is concerned with development of understanding of the transition between modes 1 and 2 of the bi-modal characteristic often observed for corrosion (mass) loss of many different alloys and in a variety of longer-term (years) exposure conditions. Corrosion losses and pit depth and size for carbon steels (0.01, 0.04 and 0.06 %wt. C) immersed continuously in sheltered 20˚C (av.) seawater, 30˚C (constant) seawater and 70˚C (constant) distilled water environments are reported with observations at 14 and 28 day intervals over 1.4 years. The data allow clear demonstration of bi-modal behaviour and variability in mass loss much less than typical for unprotected environments. All cases showed significant development of pit depths and pit areas particularly later in mode 1, through the transition phase and into mode 2. Using established observations of pitting mechanisms, the development of pitting is interpreted as facilitating the reduction in pH at the corrosion interface that permits the cathodic reaction to change from oxygen reduction in mode 1 to hydrogen evolution in mode 2, consistent with longer-term predominantly anaerobic corrosion in the later part of the bi-modal model. The observations also shed light on the relationship between pitting corrosion and what is conventionally referred to as ‘uniform’ or ‘general’ corrosion.