INVESTIGATION OF THE CORRELATION BETWEEN SURFACE-PROPERTIES AND SURFACE-CHEMISTRY OF ELECTROLESS NICKEL COATINGS

Corrosion and erosion resistance of high phosphorus (more than 10 wt.%) and low phosphorus (1-2 wt.%) electroless nickel (EN) coatings are investigated using scanning Auger microscopy and X-ray photoelectron spectroscopy. The results support the observations of others showing the correlation between nickel-to-phosphorus ratios and the chemical-mechanical properties of the coatings. The present study gives further insight into the mechanisms responsible for surface chemistry modification and subsequent change in properties as a result of process variables and heat treating. Angle-dependent X-ray photoelectron spectroscopy results showed that the thickness of the passivated surface oxide formed on as-plated EN materials was proportional to the phosphorus content of the deposit. The model derived from this and Auger data implied a layered structure, with a nickel oxide over a phosphate film on the EN alloy surface. Heat treating thickened the oxide and depleted the underlying alloy of nickel.