Electrode potential, electrode polarisation and electrical resistance probe measurements have been used to investigate the corrosion of mild steel in calcium hydroxide solutions approximating to the solution found in the pores of concrete. The usefulness of the corrosion data obtained from these measurements is discussed and corrosion threshold sodium chloride concentrations reported for saturated calcium hydroxide and 0.002 m calcium hydroxide solutions. This threshold sodium chloride concentration data fell into two categories depending on the method of investigation. Electrode potential and polarisation techniques indicated threshold concentrations an order of magnitude lower than the polarisation resistance and electrical resistance probe methods. This demonstrates that although the corrosion reaction may be thermodynamically feasible, the rate of corrosion can be limitingly low, thus emphasising the advantages of the polarisation resistance and electrical resistance probe techniques which deliver the results in the form of corrosion rates. The data recorded in this report cannot be quantitatively applied to a reinforced concrete structure.

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