ABSTRACT

This paper presents the results of an investigation into the effect of ppm concentrations of acetic acid on the electrochemical corrosion behavior of API 5L X65 carbon steel in a sour environment. Electrochemical techniques, Linear Polarization Resistance (LPR), Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy (EIS), were used to characterize the electrochemical behavior of API 5L X65 carbon steel in a 3.5 wt% NaCl solution with 10 ppm of hydrogen sulfide (H2S) and acetic acid at 0, 100, 500 and 1000 ppm concentrations. All tests were conducted at 40°C temperature and 1.0 bar CO2 pressure.

The corrosion product film formed on the electrode surface is less protective in the presence of acetic acid than when acetic acid was not present. This is confirmed by a decrease in the polarization resistance and increase in the corrosion rates with increase in acetic acid concentration. EIS shows high impedance of the film with no acetic acid present, indicating a highly protective corrosion product on the electrode surface. However, the impedance significantly decreases as the acetic acid concentration increases. Examination of the tested electrodes in solutions containing acetic acid shows pitting corrosion on the surface. This appears to be due to the dissolution of iron acetate from the corrosion product scale.

INTRODUCTION

The effect of acetic acid on CO2 corrosion has been studied in the literature.1-4 Experimental work has also been done on the effect of small amounts of H2S on CO2 corrosion of carbon steel, and the different chemical and electrochemical reactions involved in CO2/H2S corrosion process in the presence of acetic acid. 5-7

These studies found that the presence of small amounts of H2S results in a rapid and significant reduction of the CO2 corrosion rate. The reduction of the corrosion rate is usually associated with the formation of a corrosion product film on the metal surface, even if the bulk conditions for super saturation of FeCO3 or FeS are not met. Analysis of the corrosion product film usually shows the presence of a very thin mackinawite film. It has been reported that the process of FeS film formation is linked to a solid state reaction where hydrogen sulfide (or sulfide ions) reacts directly with the iron at the metal surface.6 The chemistry of iron sulfide film formation is very complex. It was reported in a review paper that there are three main forms of FeS commonly found in the field: Mackinawite, Pyrrhotite and Pyrite.8

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