Influence of Seawater Nutrient Content on the Early Immersion Corrosion of Mild Steel—Part 2: The Role of Biofilms and Sulfate-Reducing Bacteria

The higher early corrosion rate of mild steel coupons under elevated nutrient levels (in particular total nitrogen) in seawater is examined using recently reported and new one-month empirical observations of corrosion losses. These are compared with field observations of the variation with exposure time of the bacterial and the iron sulfide contents in rust layers. Together these observations are interpreted using previously reported laboratory observations, including the marine bacterial survival strategy of colonizing (metal) surfaces in nutrient-poor ocean waters and the known heterogeneity of surface biofilms that permits the development of niches of anaerobic sulfate-reducing bacteria (SRB) activity that eventually become incorporated in the rust layers. It is proposed that this mechanism provides an explanation for the observed increased levels of short-term corrosion under elevated levels of nutrient exposure. A more detailed first estimate of the effect of nutrients on the calibration parameters for Phase 0 of the previously proposed marine corrosion loss model is given.