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Zinc Plating Process and Characteristics
- Dec 19, 2017 -

 Here’s an overview of how a typical zinc plating process works

  • Preparing and cleaning the surface — It’s essential to thoroughly clean the surface of the substrate prior to plating. Any debris or contaminants remaining on the surface will prevent proper adhesion of the zinc coating. An alkaline detergent solution is typically used for surface cleaning, which is followed by the application of an acid treatment to remove surface rust. This latter procedure is referred to as pickling.

  • Preparing the plating solution — Zinc plating requires the immersion of the substrate into a specially formulated electrolyte solution, which is referred to as the plating bath. The bath consists of the zinc metal ionic solution and various chemicals that facilitate plating processes. They also help produce the desired chemical and physical properties of the finished product. Specific types of zinc electrolyte solutions include:

  • Acid zinc — This is a widely used plating technology known for its high efficiency, fast deposition and superior covering power. However, acid zinc also provides poor throwing power and thickness distribution.

  • Alkaline zinc — This offers less plating efficiency than acid zinc and a slower electrodeposition rate, but it provides better thickness distribution and ductility

  • Choosing the appropriate zinc plating procedure — After solution preparation, the parts are ready for plating. The chosen method could involve rack plating, where larger parts are affixed to metal racks which are placed inside the tank containing the plating bath. The parts remain stationary during plating. Barrel plating is normally used for smaller parts — instead of a plating tank, the parts are placed inside a barrel and rotated, which provides a more uniform finish.

  • Introducing the electrical current — Electroplating is also known as electrodeposition because an electrical current is used to deposit metal ions onto the surface of the substrate. In the case of zinc plating, the substrate serves as the cathode. A DC current originating at the anode is introduced into the bath and flows to the substrate. The zinc ions are then deposited onto the surface. The current flows from the cathode back to the anode to complete the circuit.

  • Post-treatment procedure — Upon completion of the electrodeposition process, the parts are ready for post-treatment. This normally involves rinsing the parts in water to remove any remaining contaminants and plating bath remnants. In cases of heavy contamination, the parts may need to be rinsed several times. The final step is to thoroughly dry the zinc-plated parts. In situations where additional corrosion protection is required, the application of passivates and sealers can be included in the post-treatment process.

Advantages of Zinc Electroplating

Zinc oxide is a fine white dust that, unlike iron oxide, doesn’t break down the substrate’s surface integrity as it is formed. Much like aluminum or stainless steel oxidation, zinc plating forms a sacrificial barrier around the substrate – it corrodes first, thereby protecting the metal underneath. Protection is typically measured in salt spray performance, such as specified in ASTM B117.

A chromate or passivate is applied over the zinc for additional corrosion protection. This chromate or passivate can be clear, yellow, and under certain circumstances, olive drab.

Coating Characteristics

The normal zinc-plated coating is dull gray in color with a matte finish, although whiter, more lustrous coatings can be produced, depending on the process or agents added to the plating bath or through post-treatments. The coating is thin, ranging up to 1 mil (25 µm), restricting zinc-plated parts to very mild (indoor) exposures. ASTM Specification B 633 lists four classes of zinc plating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25. The number indicates the coating thickness in microns (µm). The coating finds application in screws and other light fasteners, light switch plates and other small parts. Materials for use in moderate or severe applications must be chromate conversion coated. The coating is entirely pure zinc, which has a hardness about one-third to one-half that of most steels.

Transformation of the Zinc Layer

The zinc forms a bond with the steel part, resulting in a transition layer of zinc-steel alloy between the metals. The zinc layer cannot be peeled off like a coat of paint because it is integrated atomically with the steel. After one to two days of exposure to the atmosphere, the outer surface of the zinc layer becomes zinc oxide. This transformation increases the protection that the zinc layer provides. After an extended period of time of environmental exposure, the zinc oxide converts to zinc carbonate, which also acts as a protective layer.

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