Influence of dissolved inorganic nitrogen concentration on short- and long-term marine corrosion of steel

Elevated concentrations of dissolved inorganic nitrogen (DIN) that stimulate microbiologically influenced corrosion (MIC) likely much increase longer-term seawater immersion corrosion of mild and low alloy steels. In contrast, in shorter-term exposures, DIN less than about 0.25 mgN/L concentration usually has little or no effect. This is illustrated with data for 12 independent cases with corrosion loss trending interpreted using the bimodal model. Its early part is predominantly under oxygenated conditions suitable primarily for iron and sulphur oxidising bacteria that are less aggressive for corrosion. These conditions also are suitable for the nitrogenous components of DIN to act as corrosion inhibitors. Longer-term exposure conditions predominantly are anaerobic suitable for sulphate-reducing microorganisms, typically the main actors in MIC. The results demonstrate it is problematic to extrapolate short-term (days – months) observations to long-term corrosion, particularly when MIC or DIN are involved. The results also highlight that the change from oxygen reduction as the critical cathodic reaction in shorter exposures to water reduction (or hydrogen evolution) as the critical cathodic reaction in extended exposures has a profound effect on the effect of DIN concentration and its effect on corrosion.