Abstract
Plasma-sprayed ceramic coatings may spall catastrophically in service because corrosion products are developed at the coating-substrate interface. This phenomenon is likely to occur with ceramic coatings on steel substrates even when the substrates have first been coated with metallic bond coats. This work was aimed at developing zinc-nickel thermal-spray powders for cathodic protection of steel substrates. Powders were produced from zinc-30 wt.% nickel alloy with a particle size distribution of 90 - 150 μm and a study of their spraying parameters on steel substrates was carried out. The performance of these zinc-nickel coatings was determined by subjecting plasma-sprayed alumina and chromia coatings with and without a zinc-nickel underlayer to aqueous corrosion for various periods of time according to the ASTM B117-85 test. The effect of degradation by corrosion was measured by the ASTM C633-79 procedure. A drastic loss of bond strength, as a function of corrosion time, was noticed in ceramic coatings deposited directly on steel substrates. Similar behavior was also observed for ceramic coatings plasma-sprayed on to nickel-chromium bond coat. Results of electrochemical, salt-spray and bond strength tests showed that zinc-30 wt.% nickel coatings provide a cathodic protection. This cathodic protection confers an active corrosion protection on the substrate and is not affected by the presence of porosity in the ceramic coating. The adhesion strength of these zinc-30 wt.% nickel coatings is in the neighborhood of 40 MPa.
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