ABSTRACT
Iron sulfide, frequently found on carbon steel exposed to sour conditions, is under certain conditions expected to act as a (large area) cathode thereby increasing the corrosion rate of the underlying steel through galvanic coupling. In this work electrochemical reactions taking place at mackinawite (the most common low temperature polymorph of iron sulfide) electrodes in sour aqueous solutions have been studied using electrochemical techniques and the effect of mass transport was obtained using a jet impingement setup. The main objective was to obtain current-potential relations for iron sulfides which can be taken into a modelling framework, thereby allowing the estimation of the galvanic coupling between carbon steel and iron sulfides under different environmental conditions. In order to do so a method to produce mackinawite electrodes was developed. Part of this paper is therefore devoted to describing the FeS manufacturing process and to some physical characteristics to the produced mackinawite and the electrode itself.
INTRODUCTION
Iron sulfides in various polymorphs may form on steel surfaces under sour conditions. The layer formed can be either protective or increase the corrosion rate of the underlying steel, depending on conditions. Sour corrosion can therefore 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.4,5 When in contact with steel, iron sulfide is expected to act as a cathode thereby increasing the corrosion rate of the underlying steel through galvanic coupling. If we imagine a situation with an iron sulfide film which has partly lost its protective ability due to some film flaw (typically cracking or delamination) with a layer of porous deposits on top, the cathode to anode ratio can become very large and the outcome may be severe localized corrosion. According to reference 4, 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.
