The formation of pits of aluminum and its alloys in pure warer

Abstract

Although aluminum is a metal of high corrosion resistance, it sometimes suffers local corrosion in exposure test or water immersion test.With the object of proving the corrosion, tests were conducted in aerated and deaerated pure water at 35°C. In aerated pure water, a milk-white ring was produced around the metallic compound and a pit was formed in the ring.The results obtained were summarized as follows.(1) In annealed aluminum of high purity, a kind of zone (like denuded zone) was formed along the grain boundary (perhaps it may be due to a small amount of impurities contained) and intergranular corrosion occurred in these zones in aerated or deaerated pure water at 35°C.(2) When 1100 aluminum was immersed in aerated pure water at 35°C, a milk-white ring was produced around a compound α (Fe-Si) owing to the potential difference between the compound and aluminum matrix; and a pit was formed in the ring. Even after the formation of the pit, the depth of the pit more increased owing to the action of concentrated cell in aerated water. Aluminum was far more rapidly corroded in deaerated pure water at 35°C as compared with in aerated pure water.(3) When 3003 aluminum was immersed in aerated pure water at 35°C, a milk-white ring was produced around a compound (Fe-Si) and a pit was formed in the ring. However, no pit was formed around a compound (MnAl6). On the other hand, in deaerated pure water, a pit was generally formed around the both compounds.(4) When X8001 aluminum was immersed in aerated or deaerated pure water at 35°C, only a very small pit was formed.(5) The oxide film formed on aluminum of 99.99% purity, which had been corroded in aerated or deaerated water at 35°C, was bayerite.