Mild steel (1020 SAE steel) specimens in alkaline solutions containing 10−2M sodium sulfide subjected to a slow potential sweep show no pitting at pH values greater than 12.0.1 Conversely, if the pH is lowered by the addition of acetic acid, pitting corrosion and the simultaneous formation of ferrous sulfide are observed when the potential is raised above the breakdown potential. In this case, the localized acidification model for pitting corrosion predicts that ferrous sulfide will form before the critical hydrogen ion concentration is reached.2 Consequently, pitting may be assisted by the presence of acetate ions resulting from the acetic acid addition. This paper presents a study of the role played by sulfide, hydroxyl, and acetate ions in solution to explain pit initiation and growth on mild steel. Potentiostatic and potentiodynamic techniques described in previous publications were used.1,3
During the scan toward the positive, the potentiodynamic...
