Electrodeposition of Ni-P alloy coatings: a review

Ni[sbnd]P coatings are extensively employed in engineering, owing to their mechanical and tribological properties which confer protection against corrosion and wear. Classically, such deposits can have a thickness of <500 μm, although there is a trend to thinner coatings to achieve faster deposition and lower costs. Depending on their composition and structure, as-plated deposits demonstrate good mechanical, tribological and electrochemical features, catalytic activity but also beneficial magnetic characteristics. Following thermal treatment, the hardness of Ni[sbnd]P metal deposits can approach or exceed that of hard Cr coatings. This paper provides a general survey of research on the electrodeposition of Ni[sbnd]P binary alloy coatings. Proposed phosphorus incorporation mechanisms, Ni[sbnd]P alloy microstructure before and after thermal treatment. Mechanical, tribological, corrosion, catalytic and magnetic properties are considered, as are the key process variables influencing the phosphorus content of deposits and the roles of the major electrolytic bath constituents. The merits of employing pulse plating and fabrication of newer (layered and functionally graded)structures are concisely explored. Interaction of four aspects: substrate state, electrolyte composition, process conditions and deposit properties is seen to be important during electrodeposition of Ni[sbnd]P alloys; areas deserving further study are identified.