ABSTRACT
The influence of Bi, Pb/Cd and Te additions as bath stabilisers on the structure composition and corrosion behaviour of two types of electroless Ni–P coatings was compared. Bi and Te additions seem to be less effective than Pb/Cd towards bath stabilisation, but they have a similar smoothening action on surface topography and a refining effect on coating texture. The presence of these stabilisers up to concentrations of a few ppm leads to a somewhat higher deposition rate and to a slight lowering of P content in the Ni–P coatings. At the open circuit potential, the electroless Ni–P coatings deposited from a series of baths are less resistant to corrosion in a 3.5 wt % NaCl aerated solution than a Ni coating electroplated from a conventional sulphamate bath, but the corrosion rate is lower as the P concentration increases; as a consequence, the coatings deposited from stabilised baths tend to be less corrosion resistant. On the other hand, the refining effect of the stabilisers on the coating texture can be beneficial to the protectiveness of the coating by making it less sensitive to localised corrosion, such as /tunnelling, or internodular cracking. The Ni–P coatings however, show a greater passivation capability than electroplated Ni owing to the formation at more noble potentials of a thin protective layer rich in phosphate anions.
