Seawater Corrosion Resistance Property Of 0Cr17Ni4Cu4Nb Martensite Precipitation Hardening Stainless Steel

The corrosion resistance of 0Cr17Ni4Cu4Nb martensite precipitation hardening (PH) stainless steel used as high strength output shaft in artificial seawater was studied in this paper. According to service character of high strength output shaft four heat treatments were carried out on the stainless steel based on preliminary demonstration and experiment. The corrosion behavior of 0Cr17Ni4Cu4Nb steel after heat treatments was investigated by immersion test, potentialdynamic polarization, cyclic polarization curves and electrochemical spectroscopy. The metallurgical structure and the surface micrographs after tests were observed by OM, XRD, SEM and EDS. The influence of microstructure after heat treatments on corrosion resistance, including solution treatment, intermediate treatment and aging treatment, was also analyzed.The immersion and electrochemical tests indicate that 0Cr17Ni4Cu4Nb showed the best seawater corrosion resistance when it is in solution treatment condition. Aging treatment largely decreases corrosion resistance, however, intermediate treatment before aging to some extent improves resistance.The microstructure after different heat treatments consists chiefly of martensite. In solution treated condition, only a small quantity of dispersed phase particles were found on the martensite matrix and the electrochemical homogeneity of microstructure led to excellent corrosion resistance. When the stainless steel was aging at 500-550℃, bulk precipitates, such as M₂₃C₆ and NbC, formed in the grains and grain boundaries. These precipitates increased inhomogeneity and worsen corrosion resistance of the steel. The incoherentε-Cu precipitates, created in the over-aging stage, also had disadvantage influence on the corrosion resistance. In the course of intermediate treatment at 816°C, the precipitation of NbC prohibited Cr depletion district on the grain boundaries. At the same time, refined martensite structure, which would retain in the following aging treatment, was observed by OM. Both of the two factors promoted seawater corrosion resistance of following aging-treated microstructure when it compared with directly aging-treated ones, so intermediate-aging treatment is a ideal heat treatment for 0Cr17Ni4Cu4Nb steel used as high strength seawater corrosion-resistant output shaft.