Abstract
An electrochemical model was developed to predict the transient distribution of species generated during corrosion of carbon steel (CS) /aluminum alloy (AA) galvanic couples under thin NaCl electrolyte films at room temperature (24 °C). Using specific geometrical configurations, strong acidification of the electrolyte, localized above the AA surface and close to the carbon steel electrode, was predicted. However, a small zone evolved (also above the AA surface) between the strong acidified area and the AA/CS joint, where the pH was neutral due to the OH- production on the CS surface. Moderate acidification of the electrolyte was predicted above the rest of the AA surface. Both diluted and concentrated electrolytes were taken into account. Surprisingly, the less concentrated the electrolyte, the more significant the drop of pH on top of the AA electrode.
Transient experimental measurements were achieved with the help of an innovative setup, and findings corroborated the theoretical predictions. These results provide enough evidence to properly discuss whether such an acidification process is due to the production of intermediate aluminum compounds or by the fast consumption of hydroxyl ions (OH-) produced by the cathodic reaction on the steel surface.
