Corrosion of carbon steel in presence of mixed deposits under stagnant seawater conditions

•Steel specimens covered with deposits showed more severe pitting corrosion.•Filtration and UV irradiation reduced the severity of broad pits.•Nitrate addition enhanced the severity of localized corrosion and caused “stepped bench” marks.•Initial surface roughness influenced the general profile of pitting of steel surfaces.•The implications for the corrosion management of offshore water injection pipelines were proposed.

Channelling corrosion of steel pipelines used for offshore water injection pipelines can be severe. Both microbiologically influenced corrosion and under-deposit corrosion are suspected. Half-pipe steel specimens with mixed deposits of magnetite, calcium carbonate and sea sand exposed to near-deoxygenated seawater for 180 days showed more severe pitting corrosion under the deposits than elsewhere. Less severe under-deposit pitting occurred for seawater treated with filtration and ultraviolet (UV) irradiation. Nitrate addition, used in practice to control bacterial H2S production in oil reservoirs, resulted in the most aggressive localized corrosion under the deposits. This has implications for water injection pipeline management.