AbstractMild steel was exposed at room temperature and in stagnant conditions to aqueous 1·0N‐ and 0·1N‐ammonium thiocyanate at various pH values, to ascertain the respective pH‐ranges of (1) uniform attack, (2) localized attack and (3) complete inhibition. For comparison, similar experiments were made with ammonium chloride and with ammonium sulphate. It was found that, with the more dilute solutions, a higher pH value was necessary for complete inhibition than with the stronger solutions, and that the pH range for localized attack was wider with the more dilute solutions. With ammonium thiocyanate, the solid corrosion products were mixtures of black iron ferrocyanide and a ferrous sulphide.A few experiments were carried out with ammonium thiocyanate in oxygen‐free conditions, to ascertain whether in these conditions, in which no measurable attack occurred at room temperature, corrosion was prornoted by raising the temperature to 90°c. The rate of attack by aqueous ammonium thiocyanate was found to be more severe than that by ammonium chloride of equivalent concentration in similar