Nickel-phosphorus coatings

A.STM B733, Std. Spec, for Autocatalytic Nickel—Phosphorus Coatings on Metals, American Society for Testing and Materials, Philadelphia, Pa., 1990. 

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. 

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. 

Sudin, M. B., Leyland, A., James, A. S., Matthews, A., Housden, J., and Garside, B., Substrate Surface Finish Effects in Dupiex Coatings of PAPVD TiN and CrN with Eiectroiess Nickel-Phosphorus Interlayers," Surf. Coat. Technol., Vol. 

G. Nickel. Nickel (nip 1,453°C), finds its primary use in the construction of apparatus to handle fluorine and volatile fluorides. In this situation the metal is rendered passive by a fluorine coating. Nickel plating is easily performed and provides a means of imparting corrosion resistance. The metal may be machined, silver soldered, copper brazed, or welded. However, the weld should be performed on clean surfaces because the presence of impurities containing lead, sulfur, phosphorus, and various low-melting metals leads to embrittlement and failure at the weld. 

The tramp elements listed in this table are also typical coating materials. The reason that they are included in scraps is not always dependent on their plating materials. The nickel-plated steel scrap seems to become the raw material for stainless steel production. However, the nickel- phosphorus plated steel scrap cannot be used as the raw material because phosphorus in the molten stainless steel cannot be removed with the present refining method. 

This particular formulation deposits nickel at the rate of about 0.015 mm (0.6mil)/h in the form of a nickel-phosphorus alloy [13]. The usual range of phosphorus content in coatings of this kind is 7-9%. Various metal surfaces, including nickel, act as catalysts for the reaction so that deposits can be built up... 

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]. 

Low alloy steels, also called weathering steels, contain small amounts of copper, chrome, nickel, phosphorus, silicon, and magnesium (< 1 %, typically). Their resistance to atmospheric corrosion generally exceeds that of carbon steel. Indeed, when exposed to environments that are not too strongly polluted a dark brown patina forms over some years that slows down the corrosion rate. On buildings this natural layer can thus replace a paint coating. For this reason, alloyed steels find numerous applications in architecture. 

Nickel-boron coatings have excellent resistance to wear and abrasion, but because they are not completely amorphous they have reduced resistance to corrosive envirorunents. Furthermore, they are much more costly than nickel-phosphorus coatings. 

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... 

The dissolved inorganic pollutants for the coil coating category are hexavalent chromium, chromium (total), copper, lead, nickel, zinc, cadmium, iron, and phosphorus. Removal of these inorganics is often... 

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. 

Although this was a simple, relatively safe mixture, and was a satisfactory primer, it was discontinued after a very short period because of two major disadvantages. It was shown that copper, bismuth, silver, iron, and nickel increased the oxidation rate of red phosphorus to acidic compounds. Primer cups had to be zinc plated to prevent contact with copper. The red phosphorus had to be of high purity, and it was necessary to remove the major impurities (iron and copper) from commercial red phosphorus before use, and to coat the purified material with up to 7.5% aluminum hydroxide which inhibited oxidation. 

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. 

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