Atmospheric corrosion of aluminum in the presence of ammonium sulfate particles

The effect of (NH4)(2)SO4 particles on the atmospheric corrosion of aluminum has been investigated at 300 and 373 K at various relative humidity (RH) levels. Aluminum reacts with (NH4)(2)SO4 particles only at or above the critical relative humidity (CRH) of (NH4)(2)SO4 at either temperature. The corrosion rate increases with increasing RH and temperature. Above the CRH of (NH4)(2)SO4, droplets are formed on the par-tides at both temperatures, making electrochemical reactions possible. The (NH4)(2)SO4 decomposes and ammonia evaporates from the droplets. At 373 K mixed ammonium-metal-sulfates are formed, followed by basic metal sulfates; oxide formation is enhanced at 373 K compared to 300 K. At 300 K no solid corrosion products containing Al are found, but it was shown that Al dissolves in the droplets. A corrosion mechanism has been proposed that explains the experimental observations, including pH and corrosion potential changes with time.