Coatings electroless

Coatings, 7 1, 77-150. See also Antifouling coatings Electroless coatings ... 

See electrophoresis protective coating electroless coating throwing power current density. 

In contrast to electrolytic nickel coating, electroless nickel coating, is deposited without any current as the name indicates. The metal is formed by the reduction of nickel ions in solution by a reducing agent. Sodium hypophosphite is used as a reducing agent. The following is the mechanism of reduction. 

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). 

Nickel-based aUoys have superior corrosion resistance to Hon-based aUoys. The only aUoys recommended for hot hydrochloric acid use are Ni—Mo aUoys containing 60—70% Ni and 25—33% Mo. Chlorimet (63 Ni, 32 Mo, 3 Fe) and HasteUoy (60 Ni, 28 Mo, 6 Fe) are found to be stable at aU acid concentrations in the absence of aH and Hon chlorides. Electroless nickel, a Ni—P aUoy containing 2—10% P, shows exceUent resistance to hot hydrogen chloride (71). The corrosion resistance increases with phosphoms content. This coating can be deposited on cast Hon, wrought Hon, mild steel, stainless steels, brass, bron2e, and aluminum (qv). 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

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. 

The growth of electroless plating is directiy traceable to (/) the discovery that some alloys produced by electroless deposition, notably nickel phosphoms, have unique properties (2) the growth of the electronics industry, especially the development of printed circuits (see Electronic coatings Integrated circuits) and (i) the large-scale introduction of plastics into everyday life. 

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). 

A primary advantage of electroless solutions is the abUity to produce conductive metallic films on properly prepared nonconductors, along with the abUity to uniformly coat any platable object. The most complex geometric shapes receive a uniform plated film. Film thicknesses range from <0.1 /tm, where only conductivity or reflectivity is wanted, to >1 mm for functional appHcations. 

Fig. 1. Lacquer-coated optical readout laser disk master. Plating by (a) electroless silver spray coating and by (b) vacuum evaporation. Scale bar, cm.

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... 

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