Short- to Medium-Term Corrosion of Mild Steel in Highly Calcareous Seawaters: Effects of Calcium Carbonate Concentration, Coupon Orientation, and Nutrient Addition

Mild steels exposed to marine immersion conditions in which carbonate solubilities are exceeded, leading to deposition of calcareous material, have been shown to exhibit much-reduced corrosion rates compared with corrosion under conditions in which there is no exceedance of solubility limits and therefore no calcareous deposition. However, the differences in corrosion including the potential effect of microbiologically influenced corrosion (MIC) in these conditions have not been examined systematically in the literature. The study reported herein involves short- to medium-term marine immersion exposures (up to 18 month) of mild steel coupons in natural seawater (control) and in environments using the same seawater source dosed with (1) CaCO₃, acting as an inorganic particulate suspension representative of that encountered in seawater with elevated calcareous content; and (2) a nitrate salt, acting as an industrial pollutant and bacterial nutrient source. The results show the extent to which the deposition of particulate CaCO₃ from suspension in seawater onto and into the rusts reduces the rate of corrosion of mild steel; a rate that was found to be dependent on the coupon orientation within the seawater. Coupons that were permanently buried beneath a cap of settled CaCO₃ particles exhibited a 70% reduction in corrosion rate compared with the representative control coupons. The results also show that for these exposure durations and conditions, while iron-related bacteria and sulfate-reducing bacteria populations were identified in high concentrations (respectively ∼1 × 10⁶ cfu/g and ∼1 × 10² to 4 × 10⁶ cfu/g, depending on coupon orientation) within the rust products themselves after 18 month of exposure, there was no clear evidence of MIC. These findings indicate that short and medium-term (nonaccelerated) studies of MIC for mild steel are insufficient or even misleading for the prediction of long-term corrosion rates in these environments.