INTERGRANULAR CORROSION BEHAVIOR OF ALUMINUM-ALLOYS EXPOSED TO ARTIFICIAL SEAWATER IN THE PRESENCE OF NITRATE ANION

The corrosion behavior of several high-strength aluminum alloys was characterized. The alloys were exposed to solutions of artificial seawater, some of which contained nitrate ion. Alloys were tested in the seawater solutions at room temperature and elevated temperature conditions. A unique synergism was discovered when nitrate anion was present in seawater, the synergism between nitrate anions and sea water greatly accelerated the intergranular corrosion of AA7075 alloys. This environment was relatively benign toward the alloys AA6061, PM7XXX, IN9052, and IN905XL. A mechanism to account for the observed synergism, which was recreated by mixing nitrate salts with artificial seawater, is proposed herein. The importance of grain boundary microstructure and chemical composition is discussed with respect to intermetallic reactivity and connectivity.