Investigation of the corrosion behaviour of electroless nickel-boron and nickel-phosphorus coatings in basic solutions

One of the major drawbacks to using aluminum parts in automotive applications is poor wear resistance. Various techniques have been used to address this concern and the purpose of this work was to produce a hard and wear resistant Ni-B coating on AA6061. This was accomplished using an electroless nickel-boron (EN-B) bath preceded by a protective zincating/electroless nickel-phosphorus pretreatment. The experimental parameters for Ni-P and Ni-B baths were optimized, and the effect of various experimental parameters on the plating rate were examined. The phosphorus and boron contents of each deposit were measured using electron probe microanalysis (EPMA) and atomic absorption spectroscopy (AAS), respectively; the surface morphology of each coating was examined using scanning electron microscopy (SEM). Results showed that the surface morphology of the Ni-B coating varies with that of the intermediate EN-P coating. In turn the surface morphology of the intermediate EN-P coating depends on the thickness of the coating and the EN-P plating bath condition. As well the corrosion behaviour of electroless nickel-phosphorus (EN-P) coatings with phosphorus content ranging from 2 to 12.5 w% was investigated using potentiodynamic (PT) and cyclic voltammetry (CV) techniques in 0.5 M sodium hydroxide. It was found that although the corrosion resistance of EN-P coatings generally decreased with an increase in phosphorus content the trend was observed only for samples where the difference in the P content was large. For example, a very low P EN coating (2 w%) was more resistant in alkaline solution than two medium P (6.5, 8 w%) samples, and these in turn were more resistant than the high P (12.5 w%) coated sample. However, when comparing the 6.5 and 8 w% samples the higher P content showed more resistance. This behaviour may be explained in terms of the two counteracting effects, activation and inhibition, of phosphorus on corrosion resistance. The activating effect is due to the lower protectiveness of the passivating film because of its lower thickness. As the P content increases, the inhibiting effect increases as a result of phosphate formation. As well, the corrosion products and cyclic voltammetric behaviour of the EN samples varied with bath pH. Finally the corrosion behaviour of electroless nickel-boron (EN-B) coatings with boron content of 6 w% was also investigated using potentiodynamic technique in 0.5M sodium hydroxide, and the results were compared with those of nickel-phosphorus coating with phosphorus content of 6 w%. The overall results are discussed with a view to providing an optimum process to inhibit the corrosion of AA6061.