Characterisation and corrosion of spot welds of austenitic stainless steels

The corrosion resistance of spot-welded and induction-heated austenitic stainless steels EN 1.4301 and EN 1.4318 was investigated in 3.5% sodium chloride solution at ambient temperature. In potentiostatic measurements pitting corrosion of spot-welded and induction-heated samples occurs at lower potentials than in the base materials. The corrosion pits initiate on the heat-tinted areas. In immersion tests the surface of the spot welds was not attacked but crevice corrosion was found between the sheets around the spot welds. The compositions and structures of the heat-tinted oxides were investigated by surface spectroscopy. The results indicate the yellow, red and blue oxides to have a double-layered structure with an iron-rich and chromium-depleted outer layer and a less iron and more chromium containing inner layer. A chromium-depleted layer was found in the base material under the yellow heat-tinted oxide on an induction-heated sample. Significant amounts of copper contamination were found on the spot-weld surface near the weld edge.