In this study, the effect of pure Ni and Ni–SiO2 nanocomposites coatings on corrosion, wear resistance and thermal conductivity of 316 stainless steel substrates was investigated with the purpose of extending the service life of 316 stainless steel plate heat exchangers. The nanocomposite coatings were developed by electroplating process in a Watts bath in different concentration values of SiO2 nanoparticles (10, 20 and 30 g l−1). Electrochemical corrosion was run to examine the corrosive performance of the coatings. The results showed that the Ni–SiO2 nanocomposite with concentration of 30 g l−1 had a higher corrosion resistance. A pin on disk wear test demonstrated that, in comparison to 316 stainless steel, the wear resistance of the Ni–SiO2 nanocomposite (30 g l−1) was up to 25% lower while its friction coefficient was almost the same. In addition, as measured via the laser flash method and differential scanning calorimetry, the thermal diffusivity and specific heat capacity of the sample respectively were found to be 32 and 43% lower in comparison to 316 stainless steel. Microhardness measurement via a Vickers microindenter showed that the microhardness of the Ni–SiO2 nanocomposite coating was more than three times higher than that of 316 stainless steel for all the reinforcement concentrations.
Failure analysis of a ruptured superheater tube from a boiler has been carried out. Experiments revealed that the chemical composition of the tube was in correspondence to SA213-T22 standard and had ferrite-pearlite microstructure of 2.25Cr-1Mo steels. Investigations showed that caustic stress corrosion cracking (CSCC) was the predominant mechanism of cracking and failure of the tube. Misuse of boiler feed water, as the supplying water through the attemperator to the superheated steam, in the secondary superheater, and carryover from the steam drum to the superheater resulted in scale buildup and cracking of the CSCC type.
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