Process characteristics of chemical nickel coating on stainless steel

Process characteristics of chemical nickel coating on stainless steel

1. Thickness uniformity

Uniform thickness and good plating ability are a major characteristic of electroless nickel plating, which is also one of the reasons for its wide application. Electroless nickel plating avoids the uneven thickness caused by uneven current distribution in the electroplating layer. The thickness of the electroplating layer varies greatly throughout the entire part, especially for parts with complex shapes. The coating is thicker at the edges and near the anode of the part, while the coating is thin on the inner surface or far from the anode, and even cannot be plated. Using electroless plating can avoid this deficiency of electroplating. During chemical plating, as long as the surface of the part comes into contact with the plating solution, the consumed components in the plating solution can be replenished in a timely manner. The coating thickness of any part is basically the same, even for grooves, gaps, and blind holes.

2. There is no issue of hydrogen embrittlement

Electroplating is the use of a power source to convert nickel cations into metallic nickel and deposit it on the anode. The method of chemical reduction is to reduce nickel cations to metallic nickel and deposit it on the surface of the substrate metal. Experiments have shown that the inclusion of hydrogen in the coating is not related to the chemical reduction reaction, but is closely related to the electroplating conditions. Generally, the hydrogen content in the coating increases with the increase of current density.

In nickel plating solution, except for a small portion of hydrogen produced by the reaction between NiSO4 and H2PO3, most of the hydrogen is produced by hydrolysis caused by electrode reaction when the two poles are electrified. In the anode reaction, a large amount of hydrogen is produced, and hydrogen on the cathode precipitates simultaneously with the metal Ni-P alloy to form (Ni-P) H, which adheres to the deposition layer. Due to the formation of an excessive amount of atomic hydrogen on the cathode surface, some desorbs to form H2, while those that do not desorb in time remain in the coating. Part of the hydrogen left in the coating diffuses into the base metal, while another part of the hydrogen accumulates at the defects of the base metal and coating to form hydrogen gas clusters. Under high pressure, cracks are formed at the defect site, and under stress, a fracture source is formed, leading to hydrogen embrittlement fracture. Hydrogen not only penetrates into the base metal, but also into the coating. According to reports, electroplated nickel requires a heat treatment of 400 ℃× 8h or 230 ℃× 48h to basically remove hydrogen from the coating. Therefore, it is difficult to remove hydrogen from electroplated nickel, while chemical nickel plating does not require hydrogen removal.

3. The functions of many materials and components, such as corrosion resistance, high-temperature oxidation resistance, etc., are reflected by the surface layer of the materials and components. In general, certain chemically plated nickel layers with special functions can be used to replace solid materials prepared by other methods, or inexpensive substrate materials can be used to replace components made from valuable raw materials. Therefore, the economic benefits of chemically plated nickel are very significant.

4. It can be deposited on the surface of various materials, such as steel nickel based alloys, zinc based alloys, glass, ceramics, plastics, semiconductors, etc., creating conditions for improving the performance of these materials.

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