Abstract

Aluminum sheet (Fe = 0.072%, Si = 0.037%, and Al = balance) was immersed in artificial water, LSW 1 (water for forming macro-pits) and macro-pits were formed on the aluminum surface.The composition of LSW 1 was composed of Na2SiO3 (5ppm of SiO32-) + NaCl (5ppm of Cl-) + pure water, and its pH value was 9.39.5.The formation of macro-pits consisted of 5 stages as follows.First stage: Microstructure of α (FeSi) contained in aluminum could not be observed by microscopy.Second stage:Microstructure of α (FeSi) was observed by microscopy, because of the dissolution of passive film on α (FeSi). It was proved by the weight decreases of the specimen after immersion and after the removal of the film.Third stage: Formation of macro-pits was observed around α (FeSi).Fourth stage: Etch pits, crystallographic or cubic corrosions was observed near α (FeSi).Fifth stage: Macro-pits were observed in the centre of the aggregate of etch pits. The formation of etch pits on the surface was proved by etching of aluminum by the etching solution (the composition was LSW 1 + any concentration of AlCl3) at pH 1.82.7(corresponding to 15.34.6% of AlCl3 concentration).As the results of these experiments, the formation of macro-pits on the aluminum sheet was presumed as follows:The thiner passive film covering α (FeSi) was dissolved by LSW 1, while, remainder parts of specimen surface being coverd with the passive film.Aluminium around naked α (FeSi) particles becomes anode and micro-pits, are generated. At selected points (somewhere among micro-pits) etch pits are generated as a result of etching the aluminum surface by the solution with spontaneous high concentration of AlCl3 resulted from dissolving aluminum. Therefore, necessary conditions generating macro-pit are presumed: existence of Cl-, and trace of inhibitor (SiO32-), thiner passive film covering on α (FeSi), and high pH of the artificial water(LSW 1).

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