Long-term corrosion of mild steel in natural and uv-treated coastal seawater

Highly polished coupons (25 by 25 by 1.5 mm) sourced from the same steel sheet were continuously immersion-exposed either to natural coastal seawater or to seawater from the same source subjected to filtration and UV irradiation to eliminate microbiologically influenced corrosion as much as possible. This was continued for 943 days (2.6 years). Dissolved oxygen levels were very similar in both environments. On average the UV-treated seawater was 2°C warmer, but all coupons exposed to it showed less localized corrosion than those exposed to natural seawater. The typical topographical difference was about 60% as measured by surface roughness parameter Sa. Mass losses in UV-treated seawater were about 10% higher than in natural seawater, but after temperature correction were similar to natural seawater for the first year and tended to be lower subsequently. At all exposure periods the rusts in UV-treated seawater were less voluminous than the rusts in natural seawater. Eventually they also contained a higher proportion of magnetite.