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To understand the structure and performance of Ni base coatings on 18Cr2Ni4WA steel, the key techniques of plasma arc cladding method are introduced. Microstructure and microhardness of the coating clad were analyzed by using Vickers hardness tester,scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).Results show that the phase of the coating is composed of γ-Ni solid solution， nickel boride Ni3B，carbide Cr27C6 and Cr2B3 type chromium boride. The coating has no cracks and does not exhibit porosity, the bonding between layers is compact and the boundaries are not observed. The coating is featured by a homogenized microstructure; small grain and no evident heat-affected zone.

Shift reactors are widely used in the petroleum and chemical industries, and especially in the coal chemical plants. The degradation of the reactor is critical to the safe operation of coal chemical plant. In this paper, a shift reactor after 8 year service was chosen. The test block taken from the reactor cylinder with the stainless steel clad plate was used and the test samples from the test block were prepared. And then, chemical composition analysis, metallographic examination, tensile and hardness tests were performed. The scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) method were used to study the degradation and degradation mechanism of the reactor. The metallographic analysis results show that there were a large number of carbides segregated in the grain boundary and the intergranular cracks were observed for the samples in the weld zone of the stainless steel clad plate. The SEM and EDS results indicate that the corrosion medium (i.e. polythionic acid) was found in the fracture surface of the samples from the weld zone of the stainless steel clad plate. The XRD test results show that the tensile residual stress occurred for the welding of the stainless steel clad plates. It is found that the degradation mechanism of the shift reactor was due to the polythionic acid stress corrosion cracking.