Abstract
Recent experience with electroless (autocatalytic) nickel coatings for the corrosion protection of mining components has been disappointing, particularly in view of their potential for certain applications. This paper presents some laboratory results concerning the corrosion resistance of various commercially available electroless nickel coatings when subjected to chloride containing waters. The laboratory experiments consisted of porosity and thickness measurements, of corrosion tests using a humidity and temperature cycle similar to that found on some coal faces, and of examination by a scanning electron microscope to observe the mechanism of breakdown. It was found that the porosity of the coatings increased with exposure to the corrosive environment, thereby allowing galvanically enhanced attack of the steel substrates. The porosity increase was a result of progressive pitting of the nickel coatings with time and the results supported previous field experience. The various coatings tested exhibited different degrees of corrosion resistance, but all of them eventually showed some pitting of the nickel. It appeared, with one exception, that the coatings with the best corrosion resistance had been plated using a strong complexing agent and an organic thiocompound stabilizer.
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