PASSIVATION AND CORROSION OF STEEL IN SODIUM CHLORIDE/HYDROXIDE SOLUTIONS

In a critical range of concentrations of chloride ions in sodium hydroxide solutions, mild steel can be anodically polarised, but the polarised state cannot be maintained. It is, however, found that the polarised state can be restored by cutting off the current for a minimum amount of time or by substituting a purely inhibitive solution for the original corrosive solution. The anodic potential accelerates the depassivation caused by chloride ions. It has been observed that cathodic polarisation is only as effective as interruption of the anodic current in restoring the polarised state. The results are discussed in relation to the occurrence of depassivation at a few random spots, and the significance of the results is pointed out from the point of view of the technique for finding out the corrosive or inhibitive character of an environment.