Corrosion Behaviour of some Electroless Nickel—Phosphorus Coatings

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Abstract
Electroless nickel-phosphorus coatings are claimed to possess good corrosion resistance against a number of agressive environments, containing chlorides, alkalis, hydrogen sulphide etc. In practice, in certain difficult situations such coatings have failed. Long term exposure tests in salt spray chambers have demonstrated differences between coatings produced from a number of proprietary baths. This paper set out to observe the electrochemical behaviour in aerated 3% NaCl solution at 60°C of Ni-P deposits produced from two baths which had different stabilisers. The corrosion study was carried out with a plated rotating disc electrode to give reproducible flowing conditions. The two coatings differed in their ability to passivate. Under free corrosion potential conditions the deposit which was capable of passivation resisted the onset of pitting attack for at least twenty times the period that was found for the coating which did not passivate. The nucleation and propagation of pitting attack on Ni-P deposits is considered in respect of the breakdown of passive films, together with the influence of surface topography and the internal growth patterns within the coating.