Effect of formic acid on aqueous corrosion mechanisms of mild steel

The effect of formic acid (HCOOH or shortly HFr) on corrosion mechanisms of X65 steel was systematically investigated in pH controlled, N2-sparged and CO2-sparged solutions using potentiodynamic polarization and linear polarization resistance (LPR) techniques. The results from the electrochemical experiments conducted in 1 wt.% NaCl solutions at 1 bar (total pressure) confirm that HFr is not significantly electroactive and therefore is not directly reduced on the steel surface under the conditions studied here. Experiments at various HFr concentration, temperature, pH, and flow rate also confirm that the main role of HFr on the corrosion of X65 steel is through the "buffering effect" mechanism. According to this mechanism, weak acids such as HFr increase the cathodic limiting current by providing hydrogen ions (H+) through their chemical dissociation similarly as is seen with acetic acid (CH3COOH or HAc). A comparison between HFr and HAc shows that these two organic acids have some differences in their behavior. First, HAc seems to retard the anodic reaction and decrease the corrosion rate, especially at lower temperatures and higher concentrations, while HFr does not. Second, the influence of these two acids on increasing the limiting current density seems to be similar at 30 degrees C, but at higher temperatures (50 degrees C, 80 degrees C), the cathodic limiting current density in the presence of HAc is greater than that with HFr at the same molar concentration. Under similar experimental conditions, HAc was observed to be less corrosive at lower temperature (30 degrees C), and more corrosive at higher temperatures (50 degrees C and 80 degrees C) compared to HFr at the same molar concentration. Experimental data collected in this study were used to validate a newly developed mechanistic model. According to our investigations this model shows an accurate fit to the experimental data at different HFr concentrations and pH at 30 degrees C. However, the model deviates from the experimental limiting current density value at higher temperatures (50 degrees C and 80 degrees C). It is speculated that this deviation results from a potential inaccuracy in the available temperature function for equilibrium constant of HFr dissociation reaction, which requires further investigations.