A Comparison Between Microstructure, Tensile Properties and Pitting Corrosion Resistance of Friction Stir and Gas Tungsten Arc Ferritic Stainless Steel Welds

In the present work an attempt was made to study the pitting corrosion susceptibility of AISI 430 ferritic stainless steel welds. Gas tungsten arc (GTA) welding and friction stir welding (FSW) processes were used to make welds. Potentio-dynamic polarization test was used to evaluate pitting corrosion resistance of the welds. Optical and electron microprobe analysis were used to study the microstructure and corrosion mechanism respectively. It was observed that the weld microstructure of GTA weld was predominately columnar. Friction stir weld exhibited fine equiaxed grain structure. Investigations clearly revealed that pitting corrosion resistance of friction stir weld zone was inferior compared to GTA weld in which breakdown potential was reduced by chrornium depletion. This was due to enhanced formation of intermetallic phase in friction stir welds compared to GTA welds. This was attributed to the slower cooling rate of friction stir welds, which results in longer exposure to the temperature range for precipitation during cooling cycle. Fine grain structure of friction stir weld has resulted in greater chromium depleted area than that of weld zone of GTA weld. In addition to the above, lower pitting corrosion resistance of friction stir welds was correlated to the formation of strain induced martensite in the nugget zone. Presence of residual stresses was also found to be affecting localized corrosion resistance by increasing number of active anode sites on the surface.