Plating in which nickel is deposited autocatalytically by a reducing agent without a power supply. The deposit contains phosphorus and forms a uniform thickness even on parts of complex shape.
Japanese name: 無電解ニッケルめっき
Electroless nickel plating appears in 1 practice question across 1 topic, including Surface treatment and tribology.
In electroplating, current from an external power source deposits metal, so the coating is thick at corners and edges where current concentrates and thin deep in holes. In electroless nickel plating, a reducing agent in the bath supplies electrons and turns nickel ions into metal. The deposited nickel itself catalyzes the reaction, so once deposition starts, growth continues at almost the same rate over the whole surface and a uniform thickness is obtained regardless of the shape of the part. This reaction was found in 1946 by Brenner and Riddell in the course of research on electroplating. Coating the insides of holes and the bottoms of grooves is its greatest feature.
The most commonly used baths are acidic and use sodium hypophosphite as the reducing agent; they are operated at about 90 °C and a pH of about 4–5, with deposition rates of about 10–20 µm per hour as a guide. Hypophosphite ions are oxidized to phosphite ions, and some of the phosphorus is incorporated into the coating, so the coating is a Ni-P alloy. The phosphorus content changes with bath composition and pH and is classified roughly as low phosphorus at 2–5%, medium at 6–9% and high at 10–13%. The more phosphorus, the closer the coating is to amorphous. With reducing agents such as borohydride compounds, the coating becomes a Ni-B alloy.
The as-plated hardness is about 500–600 HV, and heat treatment at about 400 °C precipitates Ni₃P and hardens the coating to about 900–1000 HV, raising wear resistance. High-phosphorus coatings are amorphous with no grain boundaries, so they have excellent corrosion resistance and are used for chemical equipment parts and the like. Coatings with more than about 11% phosphorus are also almost nonmagnetic as plated, so they have been used to form a hard underlayer that does not affect magnetic recording on the aluminum substrates of hard disks. However, even high-phosphorus coatings become magnetic when heat treated above about 260–280 °C, as precipitation of Ni₃P proceeds.
Because it deposits uniformly, it is widely used for parts with many threads or holes, hydraulic components, molds and dies. If palladium, which acts as a catalyst, is applied in pretreatment, plastics and ceramics can also be plated, and on electronic circuit boards, layering electroless nickel and immersion gold over copper is used as a base for soldering. As the bath is used, nickel is depleted and phosphite accumulates, so it is used while replenishing its components, and its life is managed by how many replenishments it has had. There are also composite coatings that disperse PTFE or SiC particles in the coating to improve sliding or wear resistance.
Answer: C. It deposits autocatalytically via a reducing agent without external power, so it coats complex shapes almost uniformly and hardens on heat treatment
In electroless nickel plating, hypophosphite in the bath reduces nickel ions, and the deposited nickel itself catalyzes the reaction so that growth continues. Because it is not governed by current distribution, it deposits with uniform thickness inside holes and on parts of complex shape, and with suitable pretreatment it can even plate nonconductors such as plastics. The deposit contains a few to over ten percent phosphorus, and high-phosphorus deposits are amorphous. Heat treatment at about 400 °C precipitates Ni₃P and raises the hardness from about 500 HV to about 900–1000 HV. Thickening at corners and edges and poor coverage deep in holes are properties of electroplating, which passes a current. Heat treatment does not drive out the phosphorus and soften the deposit.
Electroless nickel plating is used for parts that need uniform thickness over complex shapes or inside holes, and for parts that need hardness or corrosion resistance. Electrolytic nickel plating deposits faster, its bath is easier to control and its cost is lower, so it suits parts of simple shape without strict thickness uniformity requirements and decorative plating.
An as-plated Ni-P coating consists of fine crystals supersaturated with phosphorus or is amorphous. When heated to about 400 °C, the hard compound Ni₃P precipitates finely and is dispersed in the nickel, so the hardness rises greatly. However, heating can also introduce fine cracks into the coating and lower its corrosion resistance.
It depends on the phosphorus content. Low- and medium-phosphorus coatings are magnetic, but high-phosphorus coatings with more than about 11% phosphorus are almost nonmagnetic as plated. However, magnetism appears when Ni₃P precipitates during heat treatment above about 260–280 °C.
In electroless nickel plating, hypophosphite in the bath reduces nickel ions, and the deposited nickel itself catalyzes the reaction so that growth continues.
Which statement correctly describes electroless nickel plat…Abrasive wear Adhesive wear Archard wear equation Carburizing Nitriding Real area of contact Stribeck curve