Weight loss (WL) and linear polarization resistance (LPR) measurements have been used to verify the effect of acetic acid (HAc) on the anodic and cathodic reactions in CO2 corrosion of carbon steel. The experiments were performed using a standard rotating cylinder three-electrode system, in a 3% NaCl solution, in a temperature range of 22-60°C and at pH 4. The HAc concentration range used in the study was 0-1000 ppm. An electrochemical model has also been developed to predict the experimental potentiodynamic sweeps and corrosion rates which were compared with the weight loss and LPR measurements. In another modeling development, an additional term has been added to the latest de Waard corrosion model to account for HAc. The modified de Waard model was also compared to the experimental measurements.
When a gaseous phase of HAc is present in multiphase pipelines it, in addition to carbon dioxide, dissolves into the aqueous solution. The HAc then dissociates into hydrogen and acetate ions Since HAc is a stronger acid than carbonic acid (pKa 4.76 vs 6.35 at 25°C ), it is the main source of hydrogen ions when the two acid concentrations are similar. The acetate ions form iron acetate upon reaction with iron
But, iron acetate?s solubility is much higher than iron carbonate?s, so protective film formation by iron acetate does not readily occur. Without formation of a stable protective film, the corrosion rates of the steel can remain at a high value.
Some understanding of the role of HAc in CO2 corrosion comes from field experience as related to the so-called Top-of-Line-Corrosion (Gunaltun 2000). But, very few systematic studies have been performed in the laboratory. Little or no information exists about the basic effect of HAc on the anodic and cathodic reactions. Hedges and McVeigh (1999) reported a mild increase in the cathodic reaction in the presence of HAc although their results were not fully conclusive. The work of Crolet et al. (1999) suggests that the presence of HAc inhibits the anodic (iron dissolution) reaction.
Crolet et al. (1999) were of some of the first to report on low concentrations of HAc (6-60 ppm) affecting the corrosion rates of carbon steel. They argue that the increase in the rate of corrosion in the presence of HAc occurs due to an inversion in the bicarbonate/acetate ratio. At this inversion point, HAc is the predominant acid compared to carbonic acid and is therefore the main source of acidity.
Hedges and McVeigh (1999) published results on acetate?s role in CO2 corrosion. Experiments using both HAc and sodium acetate as a source of acetate ions in various media (3% NaCl and two synthetic oilfield brines) were performed using rotating cylinder electrodes. Both sources of acetate ions were shown to increase the corrosion rate, while acetic acid decreased the pH while sodium acetate increased it. The increased corrosion rates were attributed to the forming of thinner iron carbonate films since acetate ions have the ability to form iron acetate and transport iron away from the steel surface. However, no attempt was made to quantify the thickness or morphology of the films formed in their experiments.
Garsany et al. (2002) published work using voltammetry to study the effect of acetate ions on the rates and mechanisms of corrosion using a rotating disc electrode (RDE) on film-free surfaces. Their voltammograms show two waves, which are attributed to hydrogen ion and HAc reduction on the steel surface. They argue that since HAc dissociation can occur very quickly it is not possible to distinguish the reduction of hydrogen ions from direct HAc reduction at the electrode surface.
Sun et al. (2003) re