The physical and electrochemical properties of electroless deposited nickel-phosphorus black coatings

The manufacture of electroless black nickel surfaces had been obtained, through etching electroless nickel deposits by oxidizing acid solution. The morphology, chemical composition and reflectance of pre-etch and post-etch coatings were compared to examine influence of phosphorus and sulfur content on preparation of black surfaces. The non-metallic elements content of electroless nickel deposits was greatly improved after black treatment. It indicated the etching treatment was a selective dissolving process. The nickel atoms can be preferentially removed during the period. Optimum phosphorus content range to produce low reflectance black surface was proposed, after reflectance measurement of black surfaces. Additionally, the effect of co-deposited phosphorus and sulfur atoms on corrosion resistance of electroless nickel deposits was evaluated by electrochemical impedance spectroscopy (EIS). The fitting results indicated that corrosion resistance of high-phosphorus electroless nickel deposits will be significantly reduced after black treatment. However, that of low-phosphorus deposits will be improved.