Corrosion resistance of plasma transferred arc (PTA) Inconel 625 deposits

Abstract

Abstract Although the industrial sector is very restrictive in terms of the amount of dilution of the substrate allowed in welded Inconel coatings (often below 10%), the real impact of small changes in dilution on corrosion resistance of Inconel deposits is not clear yet. To evaluate this, welding current, welding speed and powder feed rate were varied within narrow ranges during PTA deposition of Inconel 625 onto carbon steel substrates. The effects of welding parameters on dilution and convexity index of the beads were analyzed within the established operational envelope. Potentiodynamic corrosion tests were carried out using solutions with different corrosion aggressiveness for specimens with different dilutions. The current of 180 A appeared as a transient point for the effects of the welding current on dilution and convexity index, suggesting a complex interaction between welding current, welding speed and powder feed rate. XRD results showed that for the specimens with higher dilution, only the region very close to the interface with the substrate was affected, suppressing the formation of the M2C. Interdendritic corrosion with preferential dissolution of some phases was observed as the main corrosion mechanism for all the deposits. The specimens with higher dilution showed tendency for secondary passivation in NaCl and NaCl + FeCl3 solutions. Moreover, the corrosion potential and corrosion current density values obtained for the specimens with lower and higher dilution were very close, with negligible differences, regardless of the position in the bead. The presence of FeCl3 in the electrolyte shifted the corrosion potential to more positive values. In the NaCl + H2SO4 solution, the deposits showed significant differences only in terms of passivation current between higher and lower dilution specimens. However, in general the differences between the deposits were negligible, showing good overall aqueous corrosion resistance in a wide range of highly aggressive environments. Therefore, this work found a relatively wide operational envelope for Inconel 625 PTA deposits, showing that small changes in deposition conditions, and thus in dilution, do not affect the coating applicability.