The Formation of Ferrous Monosulfide Polymorphs during the Corrosion of Iron by Aqueous Hydrogen Sulfide at 21°C

The initial stages of corrosion of iron by unstirred saturated aqueous solutions at 21°C and atmospheric pressure have been examined as a function of time, p H (from 2 to 7, adjusted by addition of or ), and applied current. Detailed examination of the morphology and phase identity of the corrosion products has led to a qualitative mechanistic understanding of the corrosion reactions. Mackinawite (tetragonal ) is formed by both solid‐state and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between and , subsequent to metal dissolution upon cracking of a mackinawite base layer formed by a solid‐state mechanism. The corrosion rate, and the relative amounts of these phases produced, are controlled by p H, applied current, and the degree of convection. The corrosion rate increases with decreasing p H; the quantity of precipitated material peaks near , below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at , when the initial mackinawite base layer remained virtually intact. The solid‐state conversion of cubic ferrous sulfide to mackinawite at 21°C was monitored by x‐ray diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.