Corrosion of AA6061 Brazed with an Al-Si Alloy: Effects of Si on Metallurgical and Corrosion Behavior

Aluminum alloy (AA) 6061 (UNS A96061) and AA4047 (UNS A94047) were brazed to construct a tapered model braze joint with variable braze clearances. Isolated AA6061-T6 was also exposed to the thermal cycle of a typical dip brazing process without applying filler metal. The braze, mixed, and heat-affected zones were modeled by thermochemical computations and characterized experimentally. Corrosion exposures were conducted in accordance with BS 11846 Method B for detection of intergranular corrosion (IGC). Each zone developed its own corrosion morphology traced to the unique chemical composition and metallurgical characteristics. The brazing thermal cycle promoted IGC of isolated AA6061. Moreover, the Al-Si braze and AA6061 joint promoted additional intergranular corrosion in the Si-enriched mixed zone exacerbated by grain boundary Q (Al4Cu2Mg8Si7) and beta phases as well as eutectically formed Si needles at solidification boundaries. Pitting occurred in the Al-Si braze zone. Anodic and cathodic polarization scans were conducted on Al, Cu, heat-treated AA6061 sheet, and selected synthesized intermetallic phases (Al2Cu, Si, Mg2Si, Al2CuMg + AlCuMg) in various sodium chloride (NaCl) solutions. These tests confirmed microgalvanic couples between grain boundary Mg2Si and likely Q relative to both the Al-Cu-Mg alloy matrix and eutectically formed Si.