Pitting and galvanic corrosion behavior of laser-welded stainless steels

Abstract

Autogenous welded specimens of austenitic (S30400 and S31603), duplex (S31803) and super duplex (S32760) stainless steels were fabricated by laser penetration welding (LPW) with a CW Nd:YAG laser in an argon atmosphere. The microstructure and the phases present in the resolidified zone of the laser-welded specimens were analyzed by optical microscopy and X-ray diffractometry, respectively. The pitting and galvanic corrosion behavior of the stainless steels in the laser-welded and unwelded conditions in 3.5% NaCl solution at 23 °C was studied by means of electrochemical measurements. X-ray diffraction analysis showed that the phases present in the weld metal depended on the composition of the base metal. While the laser weld for S31603 retained the original austenitic structure, the laser weld of S30400 contained austenite as the major phase and δ-ferrite as the minor phase. On the other hand, a slight change of δ-ferrite to austenite ratio was found in both the laser welds of S31803 and S32760, with austenite present at the δ-ferrite grain boundaries. The welds exhibited passivity but their pitting corrosion resistance was in general deteriorated as evidenced by a lower pitting potential and a higher corrosion current density compared with those of the unwelded specimens. The decrease in pitting corrosion resistance of the welds was attributed to microsegregation in the weld zone of S31603, and to the presence of δ-ferrite in S30400. For the duplex grades S31803 and S32760, the disturbance of the ferrite/austenite phase balance in the weld zone could be the cause of the decrease in corrosion resistance. The initial free corrosion potentials of the unwelded specimens were considerably higher than those of the corresponding laser welds, indicating that the welds were more active and were expected to act as anodes in the weldment. The ranking of galvanic current densities ( I G) of the couples formed between the laser-welds (LW) and the as-received (AR) specimens with area ratio 1:1, in ascending order, is: AR S31603/LW S31603 < AR S31803/LW S31803 < AR S32760/LW S32760 < AR S30400/LW S30400. The recorded I G in all couples was low (in the range of nA/cm 2).