Effect of welding conditions on corrosion behaviour of spot welded coated steel sheets

Steel in the galvannealed condition is preferred over simple galvanised steel for autobody components due to its superior corrosion resistance and relatively easy weldability. However, during resistance seam welding, the area at the weld nugget completely loses its zinc coating due to melting. On the other hand, the zinc coating in the heat affected zone (HAZ) is not totally removed although it is affected by the heat input consequent from the welding process. Although this is potentially a problem, the weld nugget has generally less probability of exposure to the open atmosphere as it is usually enveloped within the HAZ. In the present investigation, a reduction in zinc coating thickness in the HAZ from 9 to 1.5 mu m has been observed with increasing resistance weld heat input from 775 to 1310 kJ. This reduction is responsible for increasing the corrosion rate from similar to 1 to similar to 5 mpy. Therefore, in order to reduce corrosion at the HAZ, the weld heat input has to be restricted.