The Pertechnetate Ion as an Inhibitor Of the Corrosion of Iron and Steel^★

Theoretical consideration of possible mechanisms to account for the action of inorganic corrosion inhibitors of the XO₄^n- type led to the discovery that the pertechnetate ion, TcO₄^-, inhibits the corrosion of iron and carbon steels in aerated water at temperatures from 23 to 250 C, which was the highest temperature investigated. The chemistry and nuclear properties of technetium are such as to make it very useful in studying the inhibitory process. Thus, it has been shown that inhibition may be achieved without precipitating more than radiochemical traces of reaction products on the metal and even this amount is shown to be most probably of secondary origin. Exposures of two years have not been attended by a continuous increase in the amount of precipitated technetium. It was found that even at 250 C 5 ppm of technetium (5 × 10^-5 ƒ KTcO₄) in distilled aerated water effectively inhibited the attack on an SAE 1010 steel. The inhibition depends upon the maintenance of some minimum concentration of dissolved pertechnetate, the amount being dependent upon the nature of the metal and its surface activity. The evidence points to a weak, reversible adsorption of the inhibitor ion as the source of inhibition. The perrhenate ion, ReO₄^-, is very similar to the pertechnetate ion in geometry and charge, yet it was found to have no inhibitory properties under any of several conditions studied. The observations make it clear that inhibition arises from some intra-ionic property and it is tentatively suggested that this is a degree of internal polarity sufficient to induce a short-range electrostatic polarization at the interface, whereby the activation energy for the cathodic part of the corrosion process is increased.