Microstructure and corrosion resistance of Ni–P gradient coatings

Abstract

Ni–P gradient coatings, gradient Ni–P coatings with rare earth (RE) Yttrium (Y) (coating A) and non-gradient Ni–P coatings with RE Yttrium (coating B) were prepared separately on LY12 aluminium alloy by an electroless plating technique. The corrosion resistance of the three kinds of coatings in different corrosive environments were evaluated by corrosion weight loss rate and polarisation curve analysis. The Ni–P gradient coating had a thickness of about 40 μm, with phosphorus content gradually increasing from 6.27 wt-% in the innermost layer (crystalline structure) to 14.74 wt-% in the outermost layer (amorphous structure), thus protecting the substrate from corroding. Furthermore, the introduction of RE Yttrium (Y) into the Ni–P matrix can correspondingly improve the corrosion resistance of the coatings. Consequently, the corrosion resistance of the coatings in acidic and alkaline environments is characterised by coating A>coating B>Ni–P gradient coating, while in neutral environment is characterised by coating A>Ni–P gradient coating>coating B.