Effects of Thickness of Electroless Ni-P Deposit on Corrosion Fatigue Damage of 7075-T6 under Salt Spray Atmosphere

摘要

Fatigue tests under reversed flexure have been conducted on samples of 7075-T6 Al-alloy with the application of electroless Ni-P deposits of thickness ranging from 5 to 50 mu m after salt spray corrosion (100, 500 and 1000 hours). The corrosion damage of the coated and uncoated samples was determined through the measurement of the mass loss and surface corroded morphology. The mean microhardness of the high phosphorous (Hi-P) deposits in 5, 25 and 50 mu m thickness were found to be 334, 457 and 606 Hv_(50), and the mean surface roughness (Ra) were measured at 0.197, 0.217 and 0.223 mu m for 5, 25 and 50 mu m thickness, respectively. The salt spray tests revealed that the corrosion damage increases as the exposure time increases for the 5 mu m coating. The corrosion damage literally stopped when the coating thickness was increased to 25 mu m. The fatigue strength of the coated samples was found decrease slightly as the deposit thickness increases to 25 mu m, whereas the reduction on the fatigue strength for the 50 mu m deposit is significant. It has been observed that the reduction in the fatigue strength is less or to none for the thicker coating system. For the 5 mu m Hi-P deposit sample, the fatigue crack initiation started at the surface corrosion pitting then extending to the substrate and onto failure, this phenomenon reduced the fatigue strength as compared to the uncoated samples. The fatigue strength is decreased slightly as the thickness increases to 25 mu m and 50 mu m. With or without corrosion damage, the fatigue crack initiation all started from the coating and extended to the substrate through the bonding between the coating and the substrate. Balancing between the need for the corrosion resistance and the fact on the fatigue strength reduction, the optimum coating thickness seems to be greater than 5 mu m but less than 25 mu m for the aerospace alloy 7075-T6 Al-alloy.