Electroless nickel plating is to restore the deposition of nickel ions in the solution with the reducing agent in the catalytic activity of the surface. Electroless nickel plating can choose a variety of reducing agents, the most common is sodium hypophosphite as the reducing agent of chemical nickel plating process in industrial application. The widely accepted reaction mechanism is the theory of atomic hydrogen and the hydride theory.
Atomic hydrogen theory is that the solution of Ni2 atoms depends on the reducing agent of sodium hypophosphite (NaH2P02) release active hydrogen reduction of nickel, instead of H2PO2-and Ni2 direct role. The first magnesia is under heated conditions, sodium hypophosphite explain the release of atomic hydrogen in the catalytic surface, or by H 2PO2-catalytic dehydrogenation to produce atomic hydrogen, that is, then the H atom adsorbed on the surface of active metal restore the case of Ni sedimentary deposit T pieces bright. The same time a phosphate atomic hydrogen reduction of phosphorus, or the occurrence of their own redox reactive deposition of phosphorus, namely, the precipitation of H2 either by H2POf hydrolysis can also be a combination of atomic states of hydrogen.
Hydride theory is that the decomposition of sodium hypophosphite not release state atomic hydrogen, but the release of stronger reducing ability of hydride ion (hydrogen anion H), and nickel ions to restore the negative ions of hydrogen. In the acid bath, H2PO2-catalytic surface reaction with water, that is, in the alkaline bath, compared with nickel ions by hydride reduction, the hydrogen negative ion H with H20 or H reaction to release hydrogen.
So far, the development of electroless nickel plating for over 50 years of history. After half a century of research and development, electroless nickel plating has entered a mature stage of development, its present situation can be summarized as follows: the technology is mature, stable performance, versatile, wide range of uses.Chemical nickel oxide plating deposition of the coating, there are some characteristics different from the electrodeposited layer. Hypophosphite as reducing agent, phosphorus precipitation, the occurrence of the co-deposition of phosphorus and nickel, electroless nickel plating phosphorus was the dispersion state of the nickel-phosphorus alloy coating, the phosphorus content in the coating of 1% to l5% control phosphorus content of the nickel-phosphorus plating dense, nonporous, corrosion resistance far superior to electroless nickel. Borohydride or amino-borane as a reducing agent, the electroless nickel plating layer is a nickel-boron alloy coating, the boron content of 1% to 7%. Only to hydrazine as the reducing agent to get the coating is a pure nickel layer, the nickel content of 99.5%.
High hardness, good wear resistance. Only l60 ~ 180HV hardness of the electroless nickel layer, the hardness of the electroless nickel plating layer of 400 ~ 700HV, after appropriate heat treatment can be further increased to close to or exceed the hardness of chromium plating, it is a good wear resistance, the more rare both good corrosion and wear resistance of electroless nickel plating layer. High chemical stability, and coating with good adhesion. Higher than the chemical stability of the electroless nickel layer in the atmosphere, as well as in other media, the chemical stability of the electroless nickel plating layer. As usual the bonding of steel, copper and other good binding capacity of not less than the electroless nickel layer and the substrate binding.
According the different electroless nickel plating layer phosphorus (boron) containing and different heat treatment process after plating, the physical and chemical properties of the nickel plating layer, such as hardness, corrosion resistance, wear resistance, electromagnetic properties have a variety of changes. Therefore, the industrial applications and process design of electroless nickel plating have the characteristics of diversity and specific. Because electroless nickel plating with excellent physical and chemical properties, the technology has extensive applications in various industrial sectors of electronics, computers, machinery, transportation, energy, chemical, aerospace, automobile, metallurgy, textile and mold.Source:http://www.mhcocm.com
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