Effect of heat treatment, top coatings and conversion coatings on the corrosion properties of black electroless Ni–P films

Abstract

Electroless black nickel–phosphorus plating is an advanced electroless nickel plating process formulated to deposit a black finish when processed through an oxidizing acid solution. Heat treatment, five types of top organic coating techniques and one conversion coating technique with three different experimental conditions were investigated to stabilize the black film and increase the hardness and corrosion resistance. Morphology and compositions of electroless nickel–phosphorous films with or without heat treatment, with five types of top organic coatings, and with three conversion coatings were compared to examine nickel, phosphorus, oxygen, carbon, silicon and chrome contents on the corrosion resistance of black surfaces by energy dispersive X-ray microanalysis and scanning electron microscope. Corrosion resistance of black electroless nickel–phosphorus coatings with or without heat treatment, with five types of top organic coatings, and with three conversion coatings was investigated by the polarization measurements and the salt spray test in 5% NaCl solution, respectively. HydroLac as the top organic coating from MacDermid showed the excellent corrosion resistance and the black EN film did not lose the black color after 48h salt spray test. Electrotarnil B process with 0.5 ASD for 1min stabilized the black Ni–P film immediately and increased the hardness and corrosion performance of the black Ni–P film. The black Ni–P coating with Electroarnil B process passed the 5% NaCl salt spray test for 3000h in the black color and had a minimal corrosion current 0.8547μA/cm2 by the polarization measurement.