Corrosion of Al alloys in repeated wet-dry cycle tests with NaCl solution and pure water at 323 K

Corrosion of Al alloys was examined by repeated wet-dry cycle tests with 5.5 M NaCl solution and pure water at 323 K and compared with immersion test corrosion. All Al alloy specimens after the immersion tests for 7 days in the NaCl solution showed preferential dissolution of the aluminum matrix around Si phases and Al3Fe intermetallic compounds, and in pure water showed formation of thick amorphous hydroxide films. The corrosion rate in pure water was higher than that in the NaCl solution. In the repeated wet-dry cycle test with NaCl solutions, a 0.02-ml droplet of 5.5 M NaCl solutions was initially dripped on specimens at 323 K and left for 690 s to allow the dry up of the droplet, resulting in solid NaCl precipitation. Then, the dissolution and precipitation of NaCl were repeated 150 times by dripping 0.02-ml pure water at 690-s intervals. At the edge of the droplet, pits with 20–100-μm diameter had formed after the 150 cycles, while, at the central areas, the corrosion behavior was similar to that observed in the NaCl immersion test. In similar cycle tests with pure water, dripping of water droplet was also repeated 150 times. Here, needle-like crystalline hydroxides formed at the edge of the droplet, while thick hydroxide films formed at the central areas, like in the pure water immersion test.