Addition of surface polysulfide film to the inhibitor protective action against hydrosulfide corrosion of carbon steel

Abstract

Protection against carbon steel corrosion in the inhibited hydrosulfide media is conditioned by two effects: (i) a formation of low-soluble film of iron sulfides on the steel surface in the presence of hydrosulfide and (ii) an inhibitor action. The theoretical basis of the method allowing differentiation of the deposits is suggested. With this view, the steel corrosion rate (K) is measured in time in uninhibited solutions by the polarization resistance method and the protective effect of the polysulfide film formed is estimated in time and at the stationary state (Zf). Then, similar data are obtained from the solutions of the same composition containing known inhibitor concentration, and the sum (total) protective effect (Zsum) is calculated. The inhibitor protective effect (ZInh) is calculated according to the equation ZInh = Zsum - Zf. In the high mineralized media (5% NaCl) containing 50-200 mg/l H2S, the protective effect of the polysulfide film with respect to the initial surface of carbon steel is equal to 40-75% increasing in time. The protective effect of the inhibitor (200 mg/l) on the tertiary amine base does not exceed 20-24% at the stationary state of the metal surface. The sum protective efficiency of the film and the inhibitor is equal to the integral protective effect that reaches 92-99%. Copyright © 2010 John Wiley & Sons, Ltd.