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The effect of chlorides on corrosion of reinforcing steel is well documented in the literature. However, few studies have focused attention on the effect of sulphates on the corrosion of low carbon steel in alkaline media. This paper aims to address this issue.

This paper describes the results of the investigation into the corrosion behaviour of low carbon steel (typically used as reinforcing steel), exposed to alkaline media at a pH of 9 and 12 with varying concentrations of sulphate and chloride ions, as well as, in sulphate only solutions. An attempt was made to inhibit corrosion of steel exposed to the corrosive media using laser surface treatment of the steel specimens. Mass loss tests and electrochemical tests were conducted to evaluate the effect of sulphate only and the combined effect of sulphate and chloride ions in an alkaline media on steel.

It was found that sulphate ions could cause corrosion of steel in alkaline media. The severity of the attack increased with increasing sulphate ion concentrations, as well as with combinations of sulphate and chloride ions. The pH 12 conditions seem to show much more critical attack with evidence of pitting corrosion as compared to uniform corrosion for test conditions at a pH of 9. The attempt to inhibit the observed corrosion of steel exposed to the corrosive high alkaline media containing sulphate ions on their own and in combination of chloride ions by laser surface treatment proved to be successful.

The research undertaken here adds to the body of knowledge relating to the effect of sulphate ions on reinforcing steel corrosion in highly alkaline media. Most literature points out that sulphate ions on their own do not have a corrosive effect, but rather, a passivating effect!