Which Is the Best Electroplating Process for Zn-Ni Coatings?

Turkchem 28 Nov 2019 95 5 dk okuma

TURKCHEM

Zinc-Nickel Electroplating

Many industries, particularly the automotive sector, are focusing on zinc-nickel coatings. Advances in coating formulations and improved performance make these coating applications increasingly attractive. Zinc-nickel coatings have been developed primarily for the automotive industry to meet rising demand for corrosion protection based on exposure to high temperatures, road salts, and climatic conditions. Zinc-nickel coatings are generally preferred in high-temperature and aggressive operating conditions where zinc-based coatings alone are insufficient. The nickel content in the coating composition typically ranges from 10-15%. Research highlights the effect of the Ni ratio in the bath composition on performance. In addition, current density and temperature are the most critical parameters throughout the entire process. Zn-Ni coated surfaces provide cathodic protection and can withstand temperatures up to 120°C. These coatings are most commonly applied to ferrous materials such as steel and cast iron. Selecting the most suitable coating method is essential for achieving the desired coating quality and minimizing coating defects. Each method offers its own distinct advantages in meeting customer requirements. The coating composition and type are determined based on working environment conditions. Coating corrosion (salt spray) performance is controlled by thickness and chemical composition. Coating thicknesses vary between 2-25 microns depending on working environment conditions. Zn-Ni based electroplating coatings on carbon steels are applied similarly to alkaline zinc coatings for extended corrosion life. Typical coating thicknesses range from 8-14 μm. On top of this, a passivation layer of 0.06-0.15 μm is applied. These coatings are preferred for components with high corrosion resistance requirements, wear resistance, and exposure to elevated temperatures. A hard and thin Zn-Ni layer possesses excellent surface quality and increases wear resistance and reduces friction in components exposed to wear through its hardness. Zn-Ni coatings can achieve hardness values up to 450HV (zinc coatings have a hardness of 150 HV). In SAE J2334 cyclic corrosion tests, Zn-Ni coatings demonstrate 4 times superior corrosion performance.

Barrel Plating

As the name suggests, parts are placed inside a stepped barrel made of polypropylene. Conductive electrodes are mounted at the center of the barrel. The coating process is carried out as the barrel enters the electroplating solution bath and rotates slowly (5-7 rpm). The movement of parts inside the barrel enables uniform coating coverage. This allows for a bright appearance from an aesthetic standpoint and a coating resistant to wear and corrosion from a surface engineering perspective. Barrel plating is an economical coating application for high-volume, small-sized parts (such as fasteners). It is possible to coat many different numbers of parts with different geometric shapes in the same barrel. These coating applications require lower investment while requiring less labor. This also provides a more economical solution for customers. Advantages of barrel-type coating applications for higher efficiency coating: • Small parts can be coated much faster with barrel-type compared to large and bulky parts. • Occupies less space and consequently reduces equipment investment, providing cost savings for operations. • Requires less labor compared to rack plating. • The entire coating process occurs within the barrel, so only one person per barrel is needed. • Versatile—parts of different sizes and geometries can be coated simultaneously. • The stepped motion of the barrel enables formation of a uniform coating layer. • Barrel motion delays sludge and sediment formation in the bath, extending bath life.

Rack Plating

Rack plating is a different application compared to barrel plating, performed by suspending metal parts with screws, wires, and springs. These suspension fixtures help keep the part fixed in the solution during plating. Rack application is a more convenient method when the size and dimensions of the parts to be coated are large. It is used for parts that cannot withstand barrel plating. This application reduces part damage, improves coating of sensitive sections on the part surface, and eliminates shadowing. In this way, a uniform and precise surface finish can be obtained. Rack plating is the preferred method in industries with high surface quality and coating requirements such as military and defense, automotive, medical, and electronics. It is more costly than barrel plating and requires more labor. Suspension design for each part shape is necessary according to customer requirements. When evaluating a potential application, the following considerations should be noted: • Part geometry, • End use, • Material type, • Part volume and quantity, • Efficiency, • Process type (flow, electroless, immersion, etc.).

ZnNi Plating Applications

• Automotive parts, fasteners, • Fuel and hydraulic system components, • Parts to be bent, formed, rolled, or stamped after coating, • Precision sealing parts, • Automotive Industry: Used when corrosion protection superior to standard zinc plating is required. • Electrical Transmission Industry: Used for anchors, locks, bolts, and coaxial cables. • Alternative to cadmium coatings in fasteners, • Aerospace and Space Industry: To reduce galvanic corrosion risk in stainless steel and aluminum couples. • Defense Industry: Alternative to cadmium coatings.

ZnNi Plating Advantages

• Excellent corrosion resistance—up to 5 times better than zinc plating, • 250 hours white corrosion in salt spray tests, • Over 1200 hours red corrosion with passivation layer, • Corrosion resistance at elevated temperatures, • High wear resistance, • Alternative to cadmium coatings, • Prevents galvanic corrosion in parts in contact with aluminum, • Bright or black passivation can be applied. ***Electroplating process is a procedure requiring skill, experience, and proper equipment use. Dr. Ekrem Altuncu - ETSA / TESLAB - Sakarya Üniversitesi, Teknoloji Fakültesi, Metalurji ve Malzeme Müh. Bölümü - Sakarya University, Technology Fac., Metallurgical and Materials Eng. Dept. Which is the Best Electroplating Application for Zn-Ni Coatings? Barrel Type or Rack Type