INVESTIGATION OF PERMANGANATE AND CERIUM AS ENVIRONMENTALLY FRIENDLY CORROSION INHIBITORS ON AA2024-T351

Precipitation hardened aluminum alloys rich in copper and iron are susceptible to corrosion due to local galvanic coupling between intermetallic phases and the surrounding aluminum matrix. Hexavalent chromates are recognized as the most effective chemical species used to mitigate corrosion on AA2024. Unfortunately, as a human carcinogen, the OHSA has, as recently as 2008, imposed strict new exposure limits. 

One aspect of current research in the field of corrosion inhibition has focused on oxoanions of the form MeO₄^(n-8), structural analogues of hexavalent chromium. In this study permanganate (MnO₄^-) has been synergistically paired with cerium (Ce⁺) and evaluated as a replacement for CrO₄^-. Permanganate and cerium were considered as both solution phase additions and as pretreatments with subsequent exposure in NaCl solution.

Anodic, cathodic, and mixed inhibition were investigated. Pitting potentials, oxygen reduction reaction (ORR) kinetics using the Koutecky-Levich correction, and copper replating characteristics were all considered.  Electron transport through the solid/electrolyte-interface was also characterized. Open circuit exposure experiments were conducted to examine the resulting surface chemical compositions. Permanganate solution phase additions resulted in higher E_pit, however significant increases in OCP accompanied these changes. Permanganate increases cathodic reaction rate since it is a strong oxidizer and is reduced in addition to O₂. In contrast, permanganate pretreatments decreased OCP as copper rich sites were covered with manganese oxide. On-going efforts seek to characterize the valence state of Mn in these oxides. Pitting potential was not statistically higher. Copper replating was decreased however not to the same extent as when permanganate was added as a solution phase inhibitor. Addition of a cerium pretreatment prior to a permanganate pretreatment lead to a decrease in ORR rates. Pitting potentials were not affected. OCP exposure to permanganate and cerium decreased copper replating which suggests reduced corrosion and electrochemical activity of the Al-Fe-Mn-Cu and Al-Cu-Mg phases.