Сorrosion resistance of the welded junction of aluminum alloy of the Al-Mg-Si-Cu system

Abstract

The results of complex investigations of corrosion resistance of aluminum alloy of the Al-Mg-Si-Cu doping system have been presented. By the method of metallography, it has been found that under the influence of the thermal cycle of welding, the secondary phases are separated and coagulation of insoluble phases occurs. The base alloying elements form metastable phases in the process of crystallization of seams, which are evenly distributed, but differ in size and shape. In the zone of thermal influence, the decomposition of the supersaturated solid solution and the dissolution of the strengthening phases, the characteristic feature of which is the instability of decomposition within the limits of one grain, the enrichment of the boundary areas with alloying elements, and the depletion of volume, are observed. This is accompanied by the formation of thin eutectic layers along the boundaries of crystallites. The results of electrochemical investigations have shown that the potential of the weld and the high effected zone (HAZ) is more noble compared to the base metal. Taking into account a larger area of the base metal in the construction compared to the weld (the weld area does not exceed 10% of the area of the base metal), a satisfactory resistance of the weld to the continuous corrosion can be expected in general. The depth of intergranular corrosion of the weld joint of the Al-Mg-Si-Cu system alloy is (0.245-0.350) mm, which is greater than for the base metal – from 0.082 to 0.086 mm. An increasing of the depth of destruction of the grain boundaries in the weld joint confirms the fact that the near-weld zone is weakening during welding. Welding does not worsen the resistance of welds to the exfoliation corrosion compared to the base metal, which is estimated by the ball 2-3 for the base metal and the ball 1 for the weld and HAZ. The stability of the weld under conditions of a simultaneous effect of constant loading and full immersion into the corrosive medium is reduced: the time until the destruction of the base metal is from 67 to 88 hours, of the weld – from 1 to 49 hours. Since the resistance to the exfoliation corrosion of the weld is satisfactory, the performance of the welded product as a whole, under conditions of a joint effect of corrosion-aggressive medium and mechanical stress, will be determined by the stability of HAZ against pitting and intergranular corrosion.