Corrosion of AlZn5Mg1 alloy weldments in natural seawater

AlZn5Mg1 alloy extrusions with normal and very low iron concentrations have been MIG welded and corrosion tested in natural seawater for 50 days. The corrosion behaviour of the alloys depended strongly on the temper before welding. Artificially aged material exhibited preferential pitting in a low hardness transition zone between the HAZ and unaffected base metal (BM). No preferential corrosion of the HAZ/BM transition zone occurred when the materials were naturally aged before welding. The susceptibility to corrosion of the low hardness transition zone was promoted by iron impurities in the alloy, present in the form of Al 3 Fe particles that are cathodic to the aluminium matrix. Reduction of the Fe concentration to a very low level caused the corrosion potential to shift in the negative direction, thereby making the alloy more susceptible to galvanic corrosion. Hence, when a normal Fe and a low Fe profile were welded together, the deepest attacks developed in the HAZ/BM transition zone of the low Fe profile.