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Your Complete Guide to Zinc Plating for Fasteners

If you're looking for a reliable and cost-effective protective finish for fasteners, zinc electroplating is one of the most popular choices in the world. But what exactly is it, how does it work, and what are the crucial differences between the types available?

This guide answers all your key questions.

What Exactly is Zinc Plating?

Zinc plating is an electrochemical process where a thin layer of zinc is bonded onto the surface of a metal component, such as a steel fastener. This process is also known as electro-galvanizing.

The primary purpose of the zinc layer is to provide corrosion protection. Zinc is more reactive than steel, meaning it acts as a 'sacrificial anode'. When exposed to a corrosive environment like moisture, the zinc layer will corrode first, sacrificing itself over a long period to protect the steel underneath. It's a fantastic and economical way to significantly extend the service life of fasteners.

How is the Zinc Plating Process Carried Out?

The process is highly controlled and involves several critical stages to ensure a perfect finish.
1. Thorough Cleaning & Preparation: This is the most vital step. The fasteners must be completely free of oils, grease, rust, and scale. This is typically done through a series of alkaline cleaning baths, acid pickling, and rinsing. If the surface isn't perfectly clean, the zinc will not adhere properly.
2. The Plating Bath (Electrolysis): The clean fasteners are immersed in an electrolytic solution (the 'bath') containing dissolved zinc salts. The fasteners are connected to a power supply and act as the cathode (negative electrode). An electric current is passed through the solution, causing the positively charged zinc ions to migrate and deposit evenly onto the surface of the fasteners.
3. Passivation & Sealing: After plating, the fasteners are rinsed and then immediately submerged in a passivation solution (also called a chromate conversion coating). This creates a protective top-coat over the zinc, which dramatically increases corrosion resistance and gives the fastener its final colour. This is the step where the choice between Trivalent and Hexavalent processes is made.

The process can be done in two main ways:

Barrel Plating:Used for large batches of smaller parts like nuts, bolts, and screws. The fasteners are placed inside a perforated barrel which slowly rotates in the various solutions, ensuring all parts are coated evenly.

Rack Plating:Used for larger, more complex, or delicate parts. Components are individually hung on racks to prevent damage and ensure a uniform, high-quality finish.

What's the Difference Between Trivalent (Cr3+) and Hexavalent (Cr6+) Passivates?

This is the most important distinction in modern zinc plating. The passivation layer contains chromium, which can exist in two main states: Trivalent (Cr3+) and Hexavalent (Cr6+).

Hexavalent Chrome (Cr6+)

This is the traditional method, known for its exceptional performance. The key benefit of a hexavalent passivate is that it is 'self-healing'. If the surface gets a minor scratch, the hexavalent chromium compounds in the coating can migrate to the damaged area and re-passivate the exposed zinc, maintaining corrosion protection.

However, Hexavalent Chrome is a known carcinogen and a hazardous substance. Its use is now heavily restricted by environmental and health regulations like RoHS (Restriction of Hazardous Substances) and REACH in Europe. It is not compliant with these regulations, for this reason Trojan Special Fasteners Ltd does not offer Hexavalent Chrome Passivated plating.

Trivalent Chrome (Cr3+)

This is the modern, environmentally friendly alternative. Trivalent processes are free from Hexavalent Chrome, making them fully compliant with RoHS and REACH regulations. While they don't have the same "self-healing" properties as Cr6+, modern heavyweight trivalent passivates offer excellent corrosion resistance that is often comparable to, or even better than, traditional hexavalent finishes for many applications.
For any new project, especially in the automotive, electronics, or consumer goods sectors, Trivalent plating is the standard choice.

What Do the Different Colours of Zinc Plating Mean?

The colour of a fastener is determined by the type of passivation applied after the zinc plating. Each offers a different level of corrosion resistance.

Trivalent (RoHS Compliant) Finishes

Clear / Blue Passivate (Zinc & Clear): Gives a bright, silver or slightly blueish sheen. It offers good corrosion resistance and a clean, aesthetic appearance.

Iridescent / Yellow Passivate: This is typically a heavyweight trivalent passivate that provides very high corrosion resistance, often superior to traditional hexavalent yellow. The colour can range from pale yellow to a multi-hued iridescent shine. It is a RoHS compliant alternative to the usual Hexavalent Zinc & Yellow plating that is often called for.

Black Passivate: Provides a deep black finish for aesthetic purposes, often used where a non-reflective surface is needed. It offers good corrosion protection, which is enhanced with a final sealant.

Hexavalent (NON-RoHS Compliant) Finishes

Yellow / Gold Passivate (Zinc & Yellow): The classic golden-yellow finish known for its self-healing properties and excellent corrosion resistance.

Olive Drab Passivate: A dark, military-green finish specified for its high corrosion resistance and non-reflective properties, often used in defence applications.

Black Passivate: A hexavalent black finish, also offering very good corrosion resistance.