The presence of acetic acid (HAc) has been identified as one factor that may contribute to enhance localized top-of-line corrosion attacks in gas condensate pipelines. The role of HAc on the growth of localized attacks in CO2 corrosion of carbon steel pipelines was studied by means of a pre-initiated localized attack electrode assembly (“artificial pit electrode”). The current flowing between the localized attack and the outer surface was measured with a zero resistance ammeter. It is shown that the corrosion potential increases with increasing HAc concentration. Depletion of HAc inside the attack imposed a potential difference that triggered the propagation of the attack at room temperature. The attack did not propagate in absence of HAc. The growth of the attack was self-sustained only to a certain depth, beyond which the dissolution current at the bottom of the attack vanished. This is in good agreement with field observations in the case of top-of-line corrosion phenomena.
Internal CO2 corrosion of pipelines for transportation of unprocessed oil and gas constitutes a major problem to the petroleum industry, where carbon steel is still the principal material of choice [1]. In the absence of preventive mitigation efforts like injection of inhibitors, the corrosion rates may be several mm per year, thus leading to premature failure of installations [2]. Top of Line Corrosion (TLC) is recognized as one of the most serious concerns encountered in the transportation stage of hydrocarbons [3]. TLC occurs in wet gas transportation when the co-produced water vapour condenses on the internal upper part of the pipe due to external cooling. The TLC rates significantly increase with water condensation rate, which can be too high if the pipe is not properly thermally insulated. TLC is difficult to mitigate by corrosion inhibition in wet gas lines operated in stratified flow regime, since the inhibitors are not easily able to reach and protect the upper part of the pipe. The TLC rate can be dramatically increased in presence of short chain carboxylic acids. Acetic acid (CH3COOH), denoted as HAc, is frequently the most abundant organic acid found with high concentrations up to thousand ppm in the co-produced aqueous phase. Increasing attention has been paid to the effect of HAc on the corrosion of carbon steel over the last years. On the basis of results issued from field assessments and laboratory investigations, HAc is recognized to considerably contribute to the overall corrosion rate [3-5]. Recent results showed that the type of corrosion in the presence of HAc is influenced by the operating temperature [4]. HAc was found to inhibit general corrosion while promoting the propagation of few, but deep localized attacks at room temperature. At elevated temperatures, however, the corrosion was high but uniform with rates exceeding several tens of mm per year. In previous laboratory study on CO2 corrosion of carbon steel, results obtained with artificial pit electrodes at room temperature also identified HAc as a driving factor that triggers and sustains the growth of localized attacks through coupling effect.
