Corrosion of steel by aqueous acidic solution when treating a wellbore with acid is inhibited by a procedure in which the steel is initially exposed to a first aqueous solution containing one or more corrosion inhibiting constituents which provide both corrosion inhibitor and hydrophobic liquid and deposit a corrosion inhibiting film on the steel surface and then exposed to an acidic second aqueous solution containing one or more such corrosion inhibiting constituents at a lower concentration. During this second period, the lower concentration of corrosion inhibiting constituents in the second aqueous solution maintains the film already established on the steel surface. Weight loss through corrosion in the second stage is lower than the weight loss which otherwise would be observed. Overall there can be reduction of both weight loss and consumption of corrosion inhibitor.