The participation of bound chloride in passive film breakdown on steel in concrete

It is well known that solid calcium hydroxide may act to inhibit chloride induced corrosion of steel in concrete. This hypothesis has been recently extended to include the inhibitive and aggressive nature of other solids that exhibit pH dependent dissolution behaviour. Concrete constituents that resist a local fall in pH may inhibit corrosion, while bound chloride released by such a fall may participate in corrosion initiation. In this work the pH dependent solubility of chloride in concrete is demonstrated. It is shown that most of the bound chloride is released as the result of the rapid dissolution of at least two hydrated phases in chloride contaminated OPC concrete. This occurs at pH values that are high (above 11.5) compared to that considered necessary to sustain local passive film breakdown at the site of a nucleating pit. Thus, in theory, the corrosion risk presented by bound chloride at the steel–concrete interface may be very similar to that presented by free chloride.