Abstract

In this investigation, various surface treatment techniques were applied to gray cast iron used in different environments to prevent cavitation erosion damage. First, to avoid cavitation erosion damage of diesel engine cylinder liners, superhydrophobic coating, plasma ion nitriding, electroless nickel plating(ENP), and hard chrome plating were applied to the gray cast iron as surface treatment technology widely used in industry. Cavitation erosion characteristics of various surface-treated gray cast iron were investigated by weight loss measurement and surface analysis using 3D microscope and SEM after cavitation erosion test in diesel engine coolant. As a result, the hard chrome plating was the excellent surface treatment technique to prevent cavitation damage of the diesel engine cylinder liners, and the ENP was selected as the second best plan.
Second, optimal ENP process conditions were investigated to minimize cavitation erosion damage of gray cast iron under marine environment. The ENP process conditions were composed of the nickel source, a reducing agent, complexing agent, stabilizer, which constitute the plating bath, and the pH of the plating bath. The effect of each process condition on the cavitation erosion-corrosion was investigated. The ENP optimal conditions for the nickel source, the reducing agent, and complexing agent were determined by robust optimization design(so-called Taguchi method) and cavitation erosion experiments. The process conditions of stabilizer and pH were selected by cavitation erosion experiment and electrochemical polarization experiment according to the concentration of stabilizer (0.5-5 ppm) and pH (4.0-6.8) in the plating bath. The characteristics of the ENP layer were analyzed by XRD, SEM, EDS, roughness and micro-Vickers hardness measurement. As a result, the plating rate, chemical composition, crystallinity, and surface morphology of the ENP layer were determined by the change of process conditions. Hence cavitation erosion resistance and corrosion resistance were influenced. Besides, the microstructure of the substrate significantly affected the plating surface morphology, cavitation erosion resistance, and corrosion resistance. The optimal ENP layer has an amorphous-nanocrystalline mixed structure with a P content of 4.3 wt.% and an average crystal size of about 10 nm. Its average hardness value was measured to be approximately 900 HV0.1. Therefore, the cavitation resistance was evaluated to be excellent. Besides, cavitation erosion damage mechanism is proposed for the optimal ENP layer.