Influence of the molybdenum content of high‐alloy stainless steels and of the precipitation of intermetallic phases on pitting resistance in chloride‐containing media

Abstract

AbstractThe influence of molybdenum on the resistance of stainless steels and nickel alloys (NiCrMoFe) to pitting corrosion is determined by means of electrochemical measurements (0.5 M NaCl/N2, 0…90°C) in both homogeneous and heterogeneous materials. As far as homogeneous materials are concerned, this influence is due to the inhibiting effect of molybdate formed at the interface passive layer/electrolyte and to a change in structure of the passive layer. The molybdate becomes effective as inhibitor of pitting corrosion only after the transpassive dissolution potential of molybdenum has been exceeded.Due to the precipitation of intermetallic phases, the pitting potential is shifted in cathodic direction. From a certain amount of precipitated intermetallic phases onward, the pitting potential remains constant. Consequently, the pitting potential is not dependent on the concentration of the alloying elements in the depleted matrix. Pitting corrosion preferably occurs at the phase boundaries intermetallic phase/depleted matrix and is due to lattice irregularities. Pit nucleation is observed at potentials at which the molybdate required for the inhibition of pitting corrosion has not yet been formed. Additions of molybdate to the electrolyte inhibit nucleation at the phase boundaries.Molybdenum segregates heavily in the weld microstructure of the afore‐mentioned materials when compared with chromium. Pitting corrosion occurs at the dendrites that are poorer in molybdenum. A comparison of solution‐annealed materials with different molybdenum contents shows that the molybdenum concentration of the dendrites imparts the same pitting resistance as is observed with solution‐annealed materials with the same constant molybdenum content, respectively.