Effect of acetic acid on propagation and stifling of localized attacks in CO₂ corrosion of carbon steel

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.