The mechanism of trenching and pitting at intermetallic particles in AA1050 aluminum was investigated by open circuit potential measurements and microscopic polarization measurements, and the role played by the two types of intermetallic particles, Al-Fe and Al-Fe-Si, in this process was determined. Trenching was observed only around the Al-Fe-Si particles in the naturally aerated 0.1 M NaCl solution. Under the anodic polarization of AA1050 in naturally aerated 0.1 M NaCl, a crystallographic pit was initiated in the trench. However, neither trenching nor pitting occurred in the citric-citrate buffer with 0.1 M NaCl under naturally aerated conditions. Trenching was confirmed to be the result of the local alkalization induced by the oxygen reduction reaction on the particles. Microscale polarization showed that trenching occurred around the Al-Fe-Si particle from −0.9 to −0.6 V and did not occur above −0.5 V. While the surface of pure Al dissolved at pH 14 in 1 M NaCl, the corrosion morphology was different from pitting. Crystallographic pits occurred when the pure Al was immersed in 1 M NaCl at pH 0.0 after preimmersion in 1 M NaCl (pH 14). It was concluded that the change in local pH from alkaline to acidic triggers the morphological change from trenching to pitting.

The relationship between the change in the open-circuit potential (OCP) and the morphology of metastable pitting at Al–Fe–Si particles of AA1050 aluminum in 0.1 M NaCl was clarified. First, a metastable pit grew in the depth direction as the OCP decreased suddenly. Second, the pit started to grow on the Al surface, and the OCP remained low. Finally, the pit was repassivated with the reincrease of the OCP. In the early stage of OCP measurements of the electrode area of 1 cm 2 , the potential oscillations and values were found to be associated with the growth of many metastable pits rather than the initiation and repassivation of each metastable pit. The amplitude of the OCP oscillation decreased with time and disappeared. Then, the OCP decreased slowly and became constant. The transition from metastable to stable pitting was determined to occur gradually.

In an effort to improve the tensile strength of aluminumsilicon (AlSi) alloys used in heat exchangers, we investigated the influence of Si concentration and heat-treatment at 453 K on the susceptibility of AlSi alloys to intergranular corrosion. It was found that the susceptibility to intergranular corrosion increased with an increase in Si concentration. It also initially increased with heat-treatment at 453 K, but then decreased with long-term heat-treatment at 453 K. The addition of Mg and Mn, which affect the precipitation of Si, promoted precipitation and reduced the susceptibility of the AlSi alloys to intergranular corrosion. With longer heat-treatment at 453 K, large Si precipitates were observed in the grains and at the grain boundaries, which reduced the susceptibility to intergranular corrosion. Short-term heat-treatment at 453 K formed a continuous Si-depleted layer along the grain boundaries, which increased the susceptibility to intergranular corrosion. It is suggested that the susceptibility to intergranular corrosion was dependent on the addition of Mg and Mn.

Measurements of the open circuit potential of AA1050 were performed in 10 mM CrO 3 , and the Al-matrix around the intermetallic particles was found to dissolve locally. Pre-immersion in 1 M NaOH promoted the dissolution of the Al-matrix around intermetallic particles, resulting in the improvement of pitting corrosion resistance in NaCl solutions. The pitting corrosion resistance provided by molybdate treatment is effectively improved by pre-immersion in NaOH. The maximum pit depth of the specimen treated in MoO 3 HNO 3 solution after preimmersion in 1 M NaOH was small compared to the chromate-treated specimen. It was determined that immersion in an acidic solution containing an inhibitor after pre-immersion in NaOH is an effective method to improve the pitting corrosion resistance of aluminum alloys.

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