Electroless nickel-phosphorus coatings

Electroless nickel-phosphorus should not be used with either fused or hot, strong, aqueous caustic solutions because the coating offers lower resistance to attack than does electrodeposited nickel. As-deposited electroless nickel-boron, however, offers good resistance to hot aqueous caustic solutions It is also resistant to solutions of oxidising salts such as potassium dichromate, permanganate, chlorate and nitrate. 

Resistance to abrasion The resistance to abrasion of electroless nickel-phosphorus hardened to 600 Hy, assessed by Taber abrasion tests, has been found to be double that of electroplated nickel However, electroless nickel coatings are not suitable for applications where two electroless nickel surfaces rub together without lubrication unless the values of hardness are made to differ by over 200 Hy units. Galling of aluminium, titanium or stainless steel may be overcome by applying electroless nickel to one of the two mating surfaces. 

As deposited, the microhardness of electroless nickel-phosphorus coating is about 5(X) to 600 HVN (48-50 HRC), equivalent to many hardened steels. After precipitation hardening, hardness values as high as 1100 HVN are reported, which is equivalent to commercial hard-chromium... 

A wide range of applications for hard, wear-resistant coatings of electroless nickel containing silicon carbide particles have been discussed by Weissenberger . The solution is basically for nickel-phosphorus coatings, but contains an addition of 5-15 g/1 silicon carbide. Hiibner and Ostermann have published a comparison between electroless nickel-silicon carbide, electrodeposited nickel-silicon carbide, and hard chromium engineering coatings. 

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is supplied by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphorus 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. 

Some firearms are plated with anodized aluminum, nickel, or chromium which gives durability and good looks, and some are made from stainless steel which is much less prone to rust than conventional steel. Electroless nickel coating is an alloy coating of 88% to 96% nickel and 4% to 12% phosphorus, which is produced by chemical (not electrical) reduction of nickel on to the metal surface. 

Note All data are in ttm/yr, as detomined from 100-day exposure. EN, electroless nickel % to phosphorus content of coating. 

Electroless Ni coatings are produced by autocatalyt-ical reduction of Ni ions from aqueous solution. Three electroless coatings are applied most frequently nickel-phosphorus (6-12wt%P), nickel-boron ( 5wt%B), and composite coatings (Ni—P with SiC, fluorocarbons, and diamond). A more extensive account is given in [1.96]. 

The exact mechanisms of the chemical and electrochemical reactions occurring during electroless plating are still obscure. In the case of electroless nickel, they are far more complex than those suggested by reactions (8.21) aifd (8.22). Other reactions are possible besides the formation of a metallic coating. For example, the hypophosphite bath can lead to the formation of phosphorus, resulting in a Ni-P alloy deposit. 

The corrosion resistance of electroless nickel is superior to that of electrodeposited nickel. The coating may contain 6-12% phosphorus which increases the resistance to corrosion. A uniform coating thickness is obtained by electroless nickel. 

Electroless coatings form by the reduction of nickel ions by hypophosphite (usually monobasic sodium hypophosphite, NaH2P02>. The treating solution contains an excess of nickel. Rates of precipitation depend on temperature and pH, and control of these variables allows control of the properties of the coating, in particular its phosphorus content [144]. The oxidized phosphorus compound releases hydrogen ions, which tend to reduce the pH of the solution. The formulation therefore usually contains acetate or citrate buffers. 

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