Evaluation of the influence of dissolved nitrates on corrosion behaviour of ship structural steel exposed to seawater environment

Corrosion rates in marine structural steels differ significantly with the varying compositions of seawater particularly near harbours or coastal regions primarily due to the presence of untreated chemically active species from various sources. The reviewed literature reports accelerated steel corrosion losses in coastal seawater exposure conditions, which has widely been attributed to the presence of aggressive chemical compounds e.g., dissolved inorganic nitrogenous (DINs) compounds, sulphur containing compounds, in combination with various other environmental factors and their interdependent complex relationships. This paper aims to investigate the influence of nitrates, a DIN compound, on the corrosion behaviour of a low carbon ship structural steel, by exposing surface the cleaned coupons to an artificial seawater solution in a controlled laboratory environment. The uniform and localised corrosion damages were measured on steel coupons by using the standard weight loss and the dimensional metrology methods. A significant increase in corrosion losses was observed on coupons exposed to the nitrate-added artificial seawater than those exposed to similar seawater compositions with no additional nitrate content. Elemental compositions of corrosion deposits and corrosion morphologies investigated using various analytical tools such as SEM, EDS and Raman scattering techniques have shown different types of corrosion products in both exposure conditions.