The Pitting Behavior of Newly Modified 17-4 Precipitation Hardened Stainless Steel with Different Nb, N, and Mo Contents

• DOI: 10.1007/s11665-019-04546-6
• Publication Date: 2020-01-06T15:03:25Z
• Authors: Yunlong Wu; Qi Guo; Wangyan Lv; Feng Huang
• Keywords: Pitting Corrosion; Martensitic stainless steel; Austenitic stainless steel; Hardening (computing)

The effect of the addition of Nb, N, and Mo to 17-4 precipitate hardening stainless steel (17-4 PH steel) on its pitting behavior in a simulated geothermal environment was investigated using a cyclic polarization test, and the pitting potentials and the repassivation potentials of the materials were acquired to evaluate their pitting resistance. Nb and N were found to lower the pitting potential of 17-4 PH steel, while Mo increased it. In the case of repassivation potential, the addition of all these elements seemed to have a slightly negative impact. Cr-rich oxide, Nb carbonitride (Nb (C, N)), and reversed austenite were found in the materials. The element distribution of/around the Cr-rich oxide inclusion and the Nb (C, N) precipitate of the material were studied. A Cr-depleted zone was found around the inclusion/precipitate, which rendered the surrounding area to be more susceptible to pitting corrosion. Pit embryos and well-developed pits were observed after the cyclic polarization tests. Pits tended to initiate in the vicinity of the inclusion/precipitate, and the dissolution of the oxide around the pit embryo was seen, which was believed to cause the local deterioration of the electrochemical environment. Lacy cover with narrow strip-shaped holes was found above the well-developed pit, and the forming mechanism of these holes was believed to be the dissolution of martensitic.