Chromium-free consumable for welding stainless steel: Corrosion perspective

Arc welding of stainless steels generates welding fumes containing carcinogenic hexavalent chromium (Cr+6). To mitigate this problem, a Cr-free filler metal needs to be developed to reduce the health risks for welding stainless steel structural alloys. An important aspect of the corrosion of welded structures is the galvanic interaction between the weld and base metals.; Ni-Cu alloy, Monel K400, was selected initially based on its galvanic compatibility with types 304 and 316 SS in chloride environments Type 304L (UNS S30403) stainless steel plate was successfully welded with Monel filler wire, resulting in high quality welds with no cracks. The welds survived long term exposure in 0.1 M NaCl with no evidence of corrosion. However, segregated regions in weld that are rich in Cu are the weak spots for corrosion susceptibility.; The effects of alloying elements on the corrosion properties of Ni-Cu alloy were investigated to optimize the composition of a Cr-free consumable for welding of stainless steels. Cyclic polarization experiments were performed in 0.1 M NaCl on Ni-base alloys containing different amounts of Cu, Pd, Mo, Fe, and Cr. The localized corrosion behavior, as exhibited by the breakdown and repassivation potentials, improved as the Cu content decreased from 30 to 5 wt% for both of as-cast and homogenized Ni-Cu alloys. The addition of Mo up to 10 wt% resulted in a detrimental influence on the localized corrosion with a decrease of the breakdown potential. Additions of Fe and Cr slightly increased the breakdown potential, but decreased the repassivation potential. The addition of 1 wt% Pd ennobled the alloy and greatly improved the localized corrosion properties. The optimized composition for a new welding consumable is a Ni-based alloy containing 5-10 wt% Cu and 1 wt% Pd. This alloy exhibited slightly higher corrosion potential, slightly lower breakdown potential, and much higher repassivation potential compared to Type 304L (UNS S30403) stainless steel, satisfying the design criteria.; Ni-10Cu-1Pd alloy across the entire dilution range exhibited higher repassivation potential than Type 304L stainless steel in 0.1 M NaCl solution. The repassivation potential was also higher in aerated solutions with Cl- concentrations of 105-35000 ppm. The breakdown behavior of the actual weld, made using a Bead-On-Plate welding technique, was similar to that of conventional welds made with Type 308L filler metal. After 31 days exposure of samples with crevice-formers in 500 and 1000 ppm Cl- solutions, the bead-on-plate weld showed much shallower attack than the 308L weld.; The passive film on Ni-10Cu-1Pd alloy mainly consisted of outer Ni-hydroxide and inner oxide, but the noble elements of Pd and Cu did not contribute to the formation of the passive film. However, Pd catalyzed the reduction of Cu at the bottom of the artificial pit, which enhanced the cathodic reaction and thus ennobled the deactivation potential making stable pit growth more difficult. This catalytic effect of Pd also prevented the propagation of deep pits in the test using thin foil crevice samples.