Macro- and micro-galvanic corrosion of friction stir welding 6061-T6 Al and C1100 Cu dissimilar joints

Friction stir welding (FSW) is one of the solid-state methods for welding dissimilar metals to inhibit the formation of intermetallic phases. The effect of the heterogeneous microstructures of FSW 6061-T6 Al and C1100 Cu dissimilar joints on corrosion in 3.5 wt % NaCl was studied, including macro- and micro-galvanic corrosions. The stir zone contained Cu particles embedded in Al, and CuAl 2 and Cu 9 Al 4 intermetallic phases formed via interdiffusion between Al and Cu. Consequently, the microstructure of the stir zone was heterogeneous. The Volta potential measurement revealed that Cu had a Volta potential of +200–400 mV relative to Al. An immersion test of the entire dissimilar joint revealed that the macro-galvanic coupling between 6061-T6 Al and C1100 Cu accelerated the corrosion of the stir zone. The stir zone tested individually showed that Al/CuAl 2 /Cu 9 Al 4 /Cu micro-galvanic coupling resulted in a uniform Al oxide/hydroxide layer on Al. Conversely, the surface of CuAl 2 was porous and rich in Cu because of the dealloying of Al. The corrosion was not uniform and concentrated in Al-rich phases (Al, CuAl 2 ). Moreover, the macro-galvanic couple between 6061-T6 Al and C1100 Cu base metals accelerated the corrosion of the heterogeneous stir zone. This study fills the gap between micro- and macro-galvanic corrosion of FSW Al and Cu dissimilar joints, which are essential for the applications.