Coatings electroless nickel plating

Typically, before Ni coating on Al, a zincation pretreatment of the A1 is essential to enhance the Al-Ni interfacial contact, acting as a sacrificial layer during the autocatalytic electroless nickel plating process [3], This paper focuses on the zincation treatments for electroless nickel plating by analyzing the surface morphology and the deposited Ni properties. 

Electroless nickel plating is used to deposit a coating of nickel or nickel alloy on a substrate. It relies on the presence of a reducing agent that reacts with the metal ions to deposit metal. This process uses a unique chemical bath without any electrical current. The bath chemistry is constantly replenished during the plating process. 

Coatings are also produced by electroless plating—that is, by chemical reduction of metal-salt solutions, with the precipitated metal forming an adherent overlay on the base metal. Nickel coatings of this kind are called electroless nickel plate. 

The Niphos process (1955) represents an alternative method of electroless nickel plating [45], A paste of composition NiO = 70%, (NH4)2HP04 = 15%, H2O = 15% is coated on to a clean metal surface which is then processed in a hydrogen atmosphere at 900°C. It is claimed that reduction to the metal occurs and a coating of nickel is obtained, similar to that from the hypophosphite process above. 

Surfaces can also be coated without involving electricity. Electroless nickel plating, for example, involves pretreating the surface of any material, including nonconductive materials, with a catalyst such as sodium hypophosphite. This treated surface is then immersed in a heated nickel-phosphorous or nickel-boron solution. The metal ions from the solution are reduced to metal in contact with the catalyst and form a dense alloy layer on the treated surface. 

The most popular electroless nickel plating process is the one in which hypophosphite is used as the reducer. Autocatalytic nickel ion reduction by hypo-phosphite takes place in both acid and alkaline solutions. In a stable solution with a high coating quality, the deposition rate may be as high as 20-25 p,m/h. However, a relatively high temperature of 194°F/90°C is required. Since hydrogen ions are formed in the reduction reaction,... 

Copper/beryllium alloys (thermal conductivity 200 W/mK) and copper/cobalt/ beryllium alloys (225 W/mK) for nozzle casings and tips in applications up to aromid 280 C (see Figure 4.5a) (because of the mechanical strength of the alloy, which rapidly falls when the temperature rises). Treatment involving an overlayer of silicon carbide increases injection abrasion resistance with, for example, PA and GF. Nickel coating eliminates the influence of copper on the melt (electroless nickel plating) ... 

The Fe, Co, and Ni deposits are extremely fine grained at high current density and pH. Electroless nickel, cobalt, and nickel—cobalt alloy plating from fluoroborate-containing baths yields a deposit of superior corrosion resistance, low stress, and excellent hardenabiUty (114). Lead is plated alone or ia combination with tin, iadium, and antimony (115). Sound iasulators are made as lead—plastic laminates by electrolyticaHy coating Pb from a fluoroborate bath to 0.5 mm on a copper-coated nylon or polypropylene film (116) (see Insulation, acoustic). Steel plates can be simultaneously electrocoated with lead and poly(tetrafluoroethylene) (117). Solder is plated ia solutioas containing Pb(Bp4)2 and Sn(Bp4)2 thus the lustrous solder-plated object is coated with a Pb—Sn alloy (118). 

A principal commercial appHcation of the hypophosphites is ia the electroless plating (qv) process. Nickel salts are chemically reduced by hypophosphites to form a smooth adherent nickel plating to protect the iateriors of large vessels and tank cars. The coating, which can be hardened by heat treatment, usually contains 8—10 wt % phosphoms and is highly impervious. 

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is suppHed by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphoms is used as the catalyst. Materials that are commonly deposited by electroless plating (qv) are Ni, Cu, Au, Pd, Pt, Ag, Co, and Ni—Fe (permalloy). In order to initiate the electroless deposition process, a catalyst must be present on the surface. A common catalyst for electroless nickel is tin. Often an accelerator is needed to remove the protective coat on the catalysis and start the reaction. 

Modem electroless plating began in 1944 with the rediscovery that hypophosphite could bring about nickel deposition (7,8). Subsequent work led to the first patents on commercially usable electroless nickel solutions. Although these solutions were very useful for coating metals, they could not be used on most plastics because the operating temperature was 90—100°C. The first electroless nickel solution capable of wide use on plastics was introduced in 1966 (9). This solution was usable at room temperature and was extremely stable (see Nickel and nickel alloys). 

The advantages of electroless nickel over hard chromium include safety of use, ease of waste treatment, plating rates of as much as 40 p.m/h, low porosity films, and the ability to uniformly coat any geometric shape without burning or using special anodes. Increased chemical safety is another... 

Electroless nickel or nickel—lead alloys can improve the solderabiUty and braisabiUty of aluminum even when a continuous film is not present. Electroless nickel systems based on dimethylaminehorane reduciag agents are used to coat aluminum contacts and semiconductors (qv) ia the electronics iadustry. Newer uses iaclude corrosion-resistant electroless nickel topcoatings on electroless copper plating for radio frequency... 

A major advantage of the electroless nickel process is that deposition takes place at an almost uniform rate over surfaces of complex shape. Thus, electroless nickel can readily be applied to internal plating of tubes, valves, containers and other parts having deeply undercut surfaces where nickel coating by electrodeposition would be very difficult and costly. The resistance to corrosion of the coatings and their special mechanical properties also offer advantages in many instances where electrodeposited nickel could be applied without difficulty. 

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