Abstract
Iron sulfide, frequently found on carbon steel under sour conditions, is under certain circumstances expected to act as a (large area) cathode thereby increasing the corrosion rate of the underlying steel through galvanic coupling. In the present work electrochemical reactions taking place at electrodes of different iron sulfide polymorphs (namely troilite, pyrrhotite and pyrite) in sour aqueous solutions have been studied using electrochemical techniques. Also, the effect of galvanic coupling between different iron sulfides and steel electrodes has been investigated. The results show that for the conditions studied in this work, all polymorphs act as cathodes when coupled to steel electrodes, with pyrrhotite having the largest effect of the polymorphs studied.
Introduction
Iron sulfides in various polymorphs may be formed on steel surfaces under sour conditions. The layer formed can be either protective or increase the corrosion rate of the steel, depending on conditions. Thus, sour corrosion can be a big problem in pipelines connected to reservoirs containing H2S. Even though alloys resistant to both CO2 and H2S exist, the use of carbon steel is still attractive and widely used due to its lower cost.1
A significant body of work has been done on the corrosion of steel under various sour conditions. Models, also including the effect of multiphase flow 2, have been demonstrated to give good prediction on general corrosion rates. 3 When it comes to localized corrosion the understanding is more limited, but some factors promoting localized corrosion have been identified. Studies of corrosion under deposits (under-deposit corrosion, UDC) of iron sulfide indicate increased corrosion rates and susceptibility to localized attack. 4 According to the reference, mackinawite deposits result in significantly increased corrosion rates, while troilite/pyrrhotite deposits do not. However, in-house (unpublished) experiments indicate that under deposit corrosion is significant also under troilite/pyrrhotite.
