◎: a very large amount, ○: a large amount
△: a very small amount, blank: none
g: granular, p: powdered
G: general corrosion, I: intergranular corrosion
P: pitting with intergranular corrosion
That is, the micropits resulted from the preferential dissolution of Al matrix around the intermetallic compounds, because the pitting potential of Fe and Si is quite noble compared to aluminum matrix and Mg.
Fig. 3 (b) shows the different type of intergranular corrosion from Fig. 3 (a). This was observed in 6I-2, 3 and 4. The development of the corrosion with a soaking period was confirmed by being larger in corrosion area. The width of the dissolved zones along the grain boundaries didn't become large, and the evidence of losing a grain was not found. Therefore, corrosion loss was about the same amount as the other samples, as shown in Fig.2. It was thought to result from the preferential dissolution of the particles that distributed on the grain boundaries. Although this type of corrosion had been restricted to the surface layer under these experimental conditions, the corrosion loss will increase abruptly by losing a grain unit by further long-term soak if the corrosion depth keeps increasing.
The results of corrosion behavior of all samples were summarized in Table 3. The correlation between the corrosion behavior in Table 3 and the chemical composition and mechanical properties in Table 1 and 2 is not clear. It is important that the samples among the industrially manufactured materials showed many pits accompanied by the intergranular. Although the corrosion loss itself was enough small to use practically, when corrosion morphology is considered, it also has a possibility that a problem will arise in more long-term use. In order to raise the reliability of 6000 series alloy as a material for vessels, it is thought that the variation in these materials needs to be suppressed to the minimum extent.
3.2 Metallurgical analyses of as-received materials
The results of the corrosion behavior showed that there is a difference in corrosion morphology or seawater resistance between materials, even if they have the similar chemical composition and mechanical properties. If the corrosion behavior results from the electrochemical interaction between the matrix and various kinds of particles formed in materials, this difference may depends on their microstructure.
3.2.1 Microstructural analyses by EPMA
Figure 7 shows the analyses on the surface of the as-received materials by EPMA. Each window exhibits the relative concentration distribution of the element, and the area where the element exists at the higher concentration appears the brighter. That is, it is thought that at the point where it seems white in Fig. 7, the element exists as a constituent of the particle. The analyses revealed that the distribution patterns of Fe and Si did not have big difference between the samples, whereas the distribution of Mg is very distinctive in the number and the size of the particles or the concentration distribution. While a few particles containing Mg appeared in 6I-I and 5, many particles distributed comparatively uniformly in 6I-4 and 7. 6I-8 has low-concentration zones of Mg, which were discontinuously located along with grain boundaries. Mg distribution patterns might be influenced by the heat-treatment conditions rather than chemical composition. This is supposed from the following facts: (a) the practical-use materials do not have a big difference in chemical composition, (b) the size and the distribution state of coarse particles are greatly influenced by the homogenization process before extrusion, and (c) such a wide variation of element distribution patterns in practical-use materials was not investigated in the materials produced in the laboratory. The microstructural variation of practical-use materials might be caused from the difference of the heat-treatment conditions between companies and/or between the profile of a product and others.
3.2.2 Microstructural analyses by TEM
The detailed microstructural analyses were performed on some samples by TEM. Figure 8 shows TEM images of as-received samples. In 6I-1, a large number of fine precipitates distributed uniformly, and the coarse particles, which seemed sometimes beltlike, were investigated at the grain boundaries.
In 6I-8 the very coarse particles were observed mainly at the grain boundaries and the triple points of the boundarieS. These results of the analyses by TEM agreed with those by EPMA. The chemical constituents of the coarse particles and the matrix were analyzed by Energy Dispersive Spectrometry (EDS). It revealed that the coarse particles at the grain boundaries were Mg2Si and that the others in the grains were intermetallic compounds which consisted of (Al, Fe, Si, Mn) or (Al, Fe, Si, Mn, Cr). Moreover, it showed aluminum, copper and a little silicon in the matri. Although the free zones of Mg and the solid solution Si near the grain boundaries could not confirmed by TEM. The results of the TEM analyses on the chemical constituents of the coarse particles agreed with those of EPMA analyses.
According to the results of detailed microstructural analyses and corrosion behavior of samples, the distribution of Mg will be more influenced by the difference between their received heat-treatment conditions compared with the other elements.
