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

Abstract

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, pH (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 pH, applied current, and the degree of convection. The corrosion rate increases with decreasing pH; 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.