Propagation and arrest of localized attacks in carbon dioxide corrosion of carbon steel in the presence of acetic acid

The presence of acetic acid (HAc) has been identified as one factor that may contribute to enhance localized top-of-the-line corrosion attacks in gas condensate pipelines. The role of free HAc on the growth of localized attacks in carbon dioxide (CO₂) corrosion of carbon steel pipelines therefore was studied by means of a preinitiated 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 free HAc concentration. Depletion of free HAc inside the attack imposed a potential difference that triggered the propagation of the attack at room temperature. The pit growth was self-sustained in CO₂ and HAc environments 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-the-line corrosion phenomena.