Effect of Pyrrhotite on Mild Steel Corrosion in Aqueous CO₂ and H₂S Solutions

ABSTRACT

Due to the electrical conductivity of pyrrhotite it was hypothesized that its presence in the corrosion product layer on a steel surface could lead to localized corrosion. Mild steel specimens (API 5L X65) were pretreated to form a pyrrhotite layer on the surface using high temperature sulfidation in oil. The pretreated specimens were then exposed to a range of aqueous CO₂ and H₂S corrosion environments at 30 and 60°C. X-ray diffraction data showed that the pyrrhotite layer changed during exposure; in an aqueous CO₂ solution it underwent dissolution while in a mixed CO₂/H₂S solution it partially transformed to troilite, with some mackinawite formation. This led to initiation of localized corrosion in both cases. Propagation of the localized attack was enhanced due to a galvanic coupling between the pyrrhotite layer and the steel surface. The intensity of the observed localized corrosion varied with solution conductivity (NaCl concentration); a more conductive solution resulted in higher localized corrosion rates consistent with the galvanic nature of the attack propagation.

INTRODUCTION

In H₂S containing environments encountered in the oil and gas industry, localized corrosion is a potential cause leading to facility failure. There can be a high rate of metal loss in a very limited area, which may be covered by a corrosion product layer. This makes H₂S localized corrosion more difficult to predict and detect prior to failure by using the conventional corrosion inspection and monitoring methods.1,2 Considering the often random spatial distribution of localized attack and the limited number of monitoring probes that can be installed in any given facility, the chances of detecting localized corrosion this way are slim at best. Internal line inspection techniques which could theoretically detect localized attack are complicated, expensive and therefore are used infrequently. Thus, a better understanding of localized corrosion mechanisms would be essential for the development of predictive models and implementation of corrosion mitigation strategies.

There are complicating factors associated with the investigation of H₂S corrosion mechanisms. This includes the recently found electrochemical mechanisms involving direct reduction of H₂S at the metal surface ^3-5 and the role of different iron sulfides ^6-12 that can form on the metal surface in the corrosion process. In H₂S solutions, the corrosion product layer can be composed of various iron sulfides with distinct physicochemical and electrical properties ^13-18. The electrical conductivity of various iron sulfides is one of the key parameters. For example, pyrrhotite (Fe_1-xS), troilite (FeS) and pyrite (FeS₂) all occur as stable corrosion products and have similar electrical conductivities^13-15 while for the more unstable mackinawite (FeS) there are far fewer values reported for its conductivity¹⁹. Mackinawite has anisotropic electrical properties, being conductive in the direction of oriented layers in its crystal structure and much less conductive in the perpendicular direction ¹⁹. The existence of conductive phases on a steel surface significantly impacts the electrochemically driven corrosion process ^3,20,21. The conductive corrosion product layer may intensify the electrochemical reaction rate through providing a larger cathodic surface area, locally or uniformly across the corroding steel surface.