Mechanism of Intergranular Corrosion of Brazed Al–Mn–Cu Alloys with Various Si Content

Abstract

This study investigated the effect of post-brazing cooling rate and Si addition on the intergranular corrosion (IGC) susceptibility of brazed Al-Mn-Cu alloys by electrochemical analysis and microstructure observation. Water-quenched samples after brazing exhibited no IGC susceptibility, whereas slowly-cooled samples were prone to IGC. The results suggest that IGC is caused by precipitation during cooling. In addition, it was observed that IGC susceptibility depended on the Si content. An alloy sample with a low Si-additive content exhibited high IGC susceptibility because Mn/Cu-depleted zone was formed near the grain boundaries as a result of the preferential precipitation of Al 6 (Mn,Fe) and CuAl 2 on the grain boundaries. In contrast, moderate Si addition inhibited IGC because the decrease of the Mn content in the grain interiors due to enhanced precipitation of Al 15 (Mn,Fe) 3 Si 2 in the grain. Additionally, Cu-depleted zone also disappeared because preferential precipitation of CuAl 2 on the grain boundaries was prevented. The excess-Si alloy exhibited high IGC susceptibility because Si-depleted zone formed around the grain boundaries as a result of the preferential precipitation of coarse Si particles on the grain boundaries although the Mn/Cu-depleted zones were not formed.

Study of Intergranular Corrosion on Non-Heat Treatable Aluminum Alloys in a Compressor After-Cooler

“…7, the microstructures obtained are located around the WM (weld metal), HAZ (heat affected zone), and around the BM (base metal HAZ). The microstructure of weld metal generally shows a dendritic type, while the microstructure of the header material at its base metal shows typical wrought aluminum alloy microstructure containing fine particles of the intermetallic second phases [1], [2], [8]- [9]. Similarly, the microstructure obtained from the weld metal shown in Fig.…”

“…The corrosion damage on the weld metal surface in general shows typical interdendritic corrosion. This interdendritic corrosion may have been caused by sensitization that occurred on the weld metal due to formation of a number of intermetallic phases during welding process or in service at extended high-temperature exposure [1]- [2], [8]- [9].…”

“…As compressor heat exchanger (after-cooler) components are generally made of mediumstrength aluminum alloys such as the AA5xxx and AA3xxx. Besides that, these aluminum alloys are also widely used for components in car air conditioners and other structural applications because they have a good combination of strength and formability [1], [2]. Such properties can be achieved by the mechanism of solid solution hardening and enhanced by deformation due to the high strain hardening behavior [3], [4].…”

“…Although by addition of other solute elements may have produced different types of intermetallic phases and could increase the strength of the alloys, however, they may lead to a higher susceptibility to localized corrosion [8], [9]. Due to the limited solubility of Mg or Mn in the aluminum matrix in both alloys at lower temperatures, the alloys become supersaturated and the excess alloying atoms together with other solute atoms would precipitate and form various intermetallic phases, preferentially at grain boundaries [1]- [2], [9]. Under certain conditions, either during fabrication/welding or in extended service at high temperatures, the solubility of principal alloying elements in the aluminum matrix will further decrease because they may interact with the existing intermetallic phases and form other new precipitates.…”

“…This condition may result in a different concentration between the grains and the grain boundaries and makes the alloys sensitized and susceptible to intergranular corrosion, stress corrosion, or pitting corrosion [2], [8]- [9]. Many recently published works stated that the type of intermetallic phases that may play an important role in the intergranular corrosion and other localized corrosion on the non-heattreatable aluminum alloys include Mg2Si, Al3Mg5, Al6 (Mn, Fe), Al6 Mn, etc [1]- [2], [8].…”