Table 2 Mechanical properties of samples.
This suggests the A6N01 products has possibility to show the different seawater resistance, if they designated by a temper treatment. However the seawater resistance of all the materials was satisfactory, on the whole. Their corrosion loss after the soak for a year was very little about 7g/m2. Under this experimental condition, the dependence of the corrosion loss on alloy grades, chemical composition, or temper conditions was not significant.
Fig. 2 Corrosion loss of each sample after a soak in synthetic seawater at 25℃ for 6 months and a year.
3.1.3 Corrosion morphology
Figure 3 shows the surface observation by SEM of some samples after removing the corrosion products. It shows the macroscopic pit was accompanied by the intergranular corrosion. The intergranular corrosion is one of the dangerous types of corrosion, because at the initial stage of the corrosion it goes on deeper in the thickness direction with less corrosion loss than the simple pitting corrosion. Fig. 3 (a) shows that the width of dissolved grain boundaries was wide, and some grains inside a pit might be lost, this is why it enlarged corrosion loss in 6I-1 and 6. The grains in the pits were etched, showing crystallographic planes, and the orientation of the etching changed from one grain to another. This type of the morphology has been reported in some kinds of pure aluminum and aluminum alloys, and in Al-Mg-Si alloy, they are explained the dissolution of solid solution Si free zones near the grain boundaries following the preferential dissolution of Mg2Si particles at grain boundaries [5] [6].
The other type of pitting corrosion, which were much smaller than the macroscopic one, was observed on all distributed over the surface of all samples, presenting the aspect of the general corrosion. This is that the circumference of the coarse particles, the size of about 2-3μm, was dissolved and became a crater-like pit to the diameter of about 20μm. These microscopic pits sometimes might become deep and clear as seen in Fig. 3 (c) and (d), and sometimes relatively shallow as seen in Fig. 3 (b). It depends on the composition, the size and the distribution of the particles. According to EPMA analyses on these particles inside the micropits, it revealed that it was the intermetallic compounds, which contains Fe, Si, Cr, Mn, etc. as shown in Fig. 4.
