Mechanism of Inhibitory Effect on Cavitation Erosion–Corrosion for Iron in a 3% Sodium Chloride Solution Available to Purchase

Cavitation erosion–corrosion tests by means of a magnetostriction vibratory unit were conducted on commercially pure irons in various corrosive solutions consisting of inhibitors added to a 3% sodium chloride solution. Three kinds of inhibitors were used, and the combined erosion–corrosion rate of the test specimens was reduced by adding a inhibitor in all cases. A chromate inhibitor, which was the most effective one, not only extended the incubation period but also changed the shape of the erosion–corrosion rate curve at its maximum rate. Moreover, the separate effects of the inhibitor on the erosion as well as on the corrosion rates could be obtained by using a magnet to separate the iron residue produced by the erosion–corrosion damage. The addition of the chromate inhibitor was found to decrease the corrosion rate under the combined erosion–corrosion conditions, but conversely, the chromate inhibitor was found to accelerate erosion under erosion conditions alone. The reason why the erosion–corrosion rate is inhibited by the chromate addition is due not to the presence of the chromate inhibitor itself, but to the adsorption of chloride ions that are present in the solution, on the metal surface. It can be considered, therefore, that the inhibitory effect of chloride ions occurs indirectly as a result of the presence of the chromate inhibitor.