INHIBITION OF CORROSION OF AL-6061, ALUMINUM, AND AN ALUMINUM-COPPER ALLOY IN CHLORIDE-FREE AQUEOUS-MEDIA - PART 2 - BEHAVIOR IN BASIC SOLUTIONS

Electrochemical characteristics of Al 6061 (UNS A96061), Al and an Al- Cu. alloy were investigated in aqueous solutions. Inhibition of the co rrosion processes in basic solutions was studied using electrochemical impedance spectroscopy (EIS) and polarization techniques. Among a ser ies of inhibitors, molybdate (MoO42-) and dichromates (Cr2O72-) were f ound effective in passivating the metal or alloy surface. The high inh ibition action of Cr2O72- was traced and discussed. X-ray photoelectro n spectroscopy (XPS) of the different electrode materials revealed the presence of Cu peaks on the Al-Cu alloy surface. Immersion of the dif ferent electrodes in basic solutions containing the same concentration of Cr2O72-, MoO42-, and sulfate (SO42-) anions did not show character istic peaks of Cr, Mo, or S, which meant the surface layer consisted m ainly of aluminum oxide (Al2O3). Depth profiling experiments up to 6.0 -nm thickness showed the Cu peaks of the Al-Cu alloy appeared after et ching and that Mo was incorporated in the surface film. The presence o f Cu on the Al-Cu surface initiated flawed regions, identified by scan ning electron microscopy, which were responsible for increased corrosi on rates oft his alloy. Effectiveness of the dichromate as a corrosion inhibitor for these materials resulted from its powerful oxidizing pr operties, which led to formation of a stable passive film.