Electroless nickel alloys are applied because of their useful physical and mechanical properties.1,2 Black electroless nickel coatings which are used in optical instruments, absorbing material, decorative coatings and aerospace industries are specified due to their stability against sunlight exposure, electrical conductivity and wear resistance.3
The black coating is formed by oxidization of a low nickel-phosphorus alloy. This alloy is proposed for its improved wear resistance and deeper black finish when compared to medium or high phosphorus alloys.
Steel Hull cell panels processed with a proprietary electroless nickel** was used to study the blackening coating and its properties. First the panels were plated in high nickel-phosphorus for one hour to increase the corrosion resistance. They were then plated in low phosphorus-nickel for a one additional hour. Following water rinsing, they were immersed in the oxidizing solution. The immersion time was studied, including 30 sec, 1 min, 2 min and 5 min. The temperature of the solution was varied at 25°C, 35°C and 50°C. The parts were subsequently rinsed and dried. Heat treatment was performed at 190°C for 2 hr.
Investigative techniques included dispersive x-ray microanalysis (EDX) and scanning electron microscopy (SEM), which was also used to measure cross-section thicknesses. The results were analyzed to determine the optimum immersion time, temperature, post-heat treatment and top coating. The deposit morphology and the components of the pre-etch and post-etch processes were compared to examine phosphorus, nickel, oxygen and sulfur contents on the black surface. The corrosion resistance, with and without heat treatment, with and without top coatings, was also investigated by salt spray test in 5% NaCl solution for 48 hr. The black coating abrasion resistance was check by a finger rub test, ten times on each sample. Finally, the electrochemical corrosion resistance, with or without heat treatment, was made using a potentiostat.***
Results and discussion
Effect of immersion time on as-processed black Ni-P
The black color of Ni-P coating arises from its unique surface morphology combined with the formation of nickel oxides and nickel phosphate.