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

TLDR: The initial stages of corrosion of iron by unstirred saturated aqueous H/sub 2/S solutions at 21/sup 0/C and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of H/Sub 2/SO/sub 4/ or NaOH), and applied current as discussed by the authors.

Abstract: The initial stages of corrosion of iron by unstirred saturated aqueous H/sub 2/S solutions at 21/sup 0/C and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of H/sub 2/SO/sub 4/ or NaOH), 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 FeS/sub 1-x/) is formed by both solid-state and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between pH = 3 and pH = 5, 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 pH = 4, below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at pH = 7, when the initial mackinawite base layer remained virtually intact. The solid-state conversion of cubic ferrous sulfide to mackinawite at 21/sup 0/C was monitored by x-raymore » diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.« less