Mechanism for the Morphological Change from Trenching to Pitting around Intermetallic Particles in AA1050 Aluminum

Abstract

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.

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“…As can be observed, the surface of the blank sample has been degraded by trenching of the intermetallic compounds. This finding is in good agreement with the previous work conducted to study the corrosion behavior of the aluminum alloy in the presence of intermetallic compounds [66][67][68] . The trenching attack is strengthened at prolonged exposure times.…”

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“…Figure 3(c) confirmed that the Al-matrix was locally dissolved during the chromate treatment for 10 h. From micro-scale electrochemical measurements of AA1050, it was concluded that not all of the IMPs acted as initiation sites for pitting. 6,18) As shown in Fig. 3(c), also in this case, not all of the IMPs acted as A d v a n c e V i e w initiation sites for pitting.…”

“…The AA1050 used in this study was the same as that used in our previous study. 6,18) The specimens were annealed at 623 K for 1 h and slowly cooled in a furnace (the cooling rate was approximately 65 K h ¹1 ). The specimen surface was mechanically ground with SiC paper to 4000-grit and polished with diamond pastes (15, 6, 1, and 0.25 µm).…”

“…In our previous study, the existence of Fe-rich intermetallic particles (IMPs) in the same AA1050 used in this paper was confirmed. 6,18) The small gray particles in Fig. 1(a) are the Fe-rich IMPs.…”

“…The pH around these particles locally increases due to the oxygen reduction reaction, and the Al-matrix around the particles is slowly dissolved in an alkaline environment, followed by the formation of trenches around the particles. 4,68) After trench generation, it has been reported that crystallographic pitting is initiated inside the trenches due to the decrease in pH, 6,7) and the pits grow rapidly, resulting in corrosion damage.…”