Table 1 Chemical composition of samples in mass%. (Al bal.)
6E-1-7 samples were provided in order to investigate the influence of the Cu content on the corrosion behavior. 6E-1-4 and 5-7 were, however, produced at the different time, and Fe content of these two groups was slightly different. 6E-8 was rolled after extrusion for thickness adjustment, followed by T6 temper treatment. Samples of a 5083 alloy were prepared as a reference material, since a 5083 alloy is most commonly used in shipbuilding today.
Table 2 shows the thickness and the mechanical properties in the mill sheet of materials. It shows 6I-3 and 7 are lower than others in the strength and that 6I-6 and 8 are lower in the elongation. But it is thought that there is no big difference when the effect of the difference in sample thickness is taken into consideration.
2.2 Corrosion test
A set of two specimens for each experimental condition was prepared in the form of 5cm square. These specimens kept as-extruded surface, being only ultrasonically cleaned with acetone; thus they had different thickness as shown in Table 2. They were soaked in synthetic seawater of pH 8.2 at 25℃ for 6 months and one year. The synthetic seawater kept saturated with oxygen by supplying air during the soak. After the soak the samples were treated with solution containing Cr2O3 and H3PO4 for removing the corrosion products, and then rinsed with demineralized water, and dried for over one day in the vacuumed oven. Their corrosion behavior was determined by measurement of corrosion loss and SEM observation of corrosion mopohologies.
2.3 Metallurgical analyses
Details of microstructure and corrosion morphologies were investigated using by scanning electron microscopy (SEM), electron-probe microanalysis (EPMA), transmission electron microscopy (TEM) and X-ray diffractometry (XRD).
3. Results and discussion
3.1 Corrosion behavior
3.1.1 Sample appearance
The surface of all specimens was covered with dark gray films after the soak, and some of them, 6I-1, 5, 6, 8, 6E- 4-, 7, 9 and 5E-2, had white powdered and/or granular corrosion products. Especially large amounts of corrosion products were observed on 6I-1 and 6. The sample 6I-1 showed the different type of corrosion products on each side: only powdered ones were observed on one side and mainly granular ones on the other side. The removing of the corrosion products revealed that the granular ones had occluded macroscopic pitting corrosion, that is, the pitting corrosion was observed under every granular corrosion product. Those products appeared in the very early stage of the soak, when the rapid decrease of the pH value of the synthetic seawater occurred. According to XRD analyses the white corrosion products mainly consisted of the hydroxide of aluminum and magnesium, as shown in Fig. 1. This suggests aluminum and magnesium were dissolved out in preference to the other elements in the initial stage of corrosion.
3.1.2 Corrosion loss
Figure 2 shows the mean weight loss of each material after the soak in the synthetic seawater for 6 months and for one year respectively. 6I-1, which is one of the practical-use materials, showed the poorest seawater resistance, whereas most of the 6I-series samples had the similar chemical composition and were in the same temper.
