Influence of the Dissolved Oxygen Concentration on the Passive Oxide Film of Al Alloy in Different Media

This paper employs the electrochemical current noise (ECN) and electrochemical impedance spectroscopy (EIS) techniques to better evaluate the dissolved O2 concentration on the passive oxide film of AA6162 Al alloy. The ECN measurements were done on the asymmetrical electrodes with different sizes (2-200 mm2) after 5 min from immersion in each of 0.4% NaCl, 0.4% NaCl + 0.1% NaNO2 and 3.5% NaCl solutions containing different O2 concentrations (5-25 ppm). EIS measurements were used to calculate the thickness of the passive oxide film. In dilute NaCl solution, the dissolved oxygen played an active role with an increase in oxygen concentration up to 15 ppm, while with further increase of oxygen concentration it acted as a passive factor to decrease the corrosion activity. With increasing the concentration of oxygen from 5 to 25 ppm, in the concentrated NaCl solution, the oxide film thickness decreased as an evidence of the active role of the oxygen reduction reaction. The behavior of nitrite-containing NaCl solution was in accordance with the property of nitrite ion which assists the formation of the passive film according to the adsorption theory. Keywords