On the corrosion behaviour of electroless nickel-boron coatings in basic solutions

The corrosion behaviour of electroless nickel-boron (EN-B) coatings on 70:30 brass and AA6061 with a boron content of 6 wt% was investigated using potentiodynamic techniques in 0.5 M sodium hydroxide and the results were compared with those of a nickel-phosphorus (EN-P) coating with a phosphorus content of 6 wt%. As well, the effect of heat treatment (5-120 hours at 493 K) on the corrosion resistance of the electroless nickel-boron coating was investigated. It was found that EN-B suffered a decrease in corrosion resistance with heat treatment. In addition, the EN-B coatings were found to be inferior to the EN-P with respect to corrosion resistance. Based on polarization curves and X-ray diffraction (XRD) spectra, it is postulated that the poor performance results from a shift from a largely amorphous to an increasingly crystalline material during the heat treatment process. The presence of a phosphate-based protective layer for the EN-P samples is viewed as a contributor to the better corrosion resistance of EN-P. (author)