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Frequently Asked Questions (FAQs)

Your Complete Guide to Zinc Plating Finishes

Here we answer the most common questions our customers ask about the zinc plating process, its benefits, applications, and technical specifications. If you have a question that isn't answered here, please don't hesitate to contact our technical team.

Section 1: The Fundamentals of Zinc Plating

What is zinc plating?

Zinc plating, also known as electro-galvanising, is an electrochemical process where a thin layer of zinc is bonded to the surface of a steel or iron part (the substrate). This is achieved by immersing the fastener in a specially formulated zinc salt solution (an electrolyte) and applying a direct electrical current. The process creates a durable, corrosion-resistant finish.

Why is zinc plating used on fasteners?

The primary reason is for corrosion protection. Steel, while strong, is susceptible to rust (oxidation). The zinc coating acts as a sacrificial barrier, meaning it corrodes in preference to the steel fastener underneath, significantly extending the component's service life. It also provides a clean, uniform finish and some lubricity for easier installation.

Is there another name for zinc plating?

Yes, it is commonly referred to as electro-galvanising, zinc electroplating, or simply plating. These terms are generally interchangeable.

How does the sacrificial protection work?

Zinc is more electrochemically active than steel. When the plated fastener is exposed to moisture and oxygen, a galvanic cell is formed. The more active zinc becomes the anode and corrodes, while the less active steel becomes the cathode and is protected. This protection continues even if the coating is scratched, as long as zinc is present nearby.

What is 'passivation' or a 'chromate conversion coating'?

After the zinc is plated onto the fastener, it is dipped in a passivation solution. This creates a thin, gel-like chromate conversion coating over the zinc. This secondary layer seals the zinc, significantly slowing down its corrosion rate (preventing 'white rust') and increasing the overall protection. The passivation process also determines the final colour of the fastener (e.g., clear/blue, yellow, or black).

What is 'white rust' on zinc plating?

White rust is a white, powdery corrosion product (zinc hydroxide) that can form on the surface of the zinc plating when it's exposed to moisture. While it looks unsightly, it is a sign that the zinc is doing its job of protecting the underlying steel. A high-quality passivation layer is the primary defence against premature white rust.

Section 2: Plating Types, Standards, and Compliance

What are the different colours of zinc plating?

The colour is determined by the type of passivate applied after plating:

• Clear/Blue Passivate: Gives a shiny, silver-like or slightly blue-tinged appearance. It's a very common, cost-effective finish.
• Yellow Passivate: Traditionally offered the best corrosion resistance and has a distinctive yellow or iridescent gold colour.
• Black Passivate: Provides a deep black finish, often used for aesthetic reasons or to reduce light reflection. It offers good corrosion resistance, similar to yellow passivate.

What is the difference between Trivalent and Hexavalent passivation?

This refers to the chemical state of the chromium used in the passivation solution. Hexavalent chromium was the traditional choice, particularly for yellow passivates, due to its excellent 'self-healing' properties and corrosion resistance. However, it is a hazardous substance. Trivalent chromium is the modern, environmentally safer alternative that is compliant with international regulations like RoHS.

What is RoHS and why is it important for plating?

RoHS stands for the Restriction of Hazardous Substances. It's an EU directive that restricts the use of specific hazardous materials, including hexavalent chromium, in electrical and electronic products. Trivalent passivates are used to ensure our plated fasteners are RoHS compliant for customers who require it.

Which industry standards apply to zinc plating?

Several key standards govern zinc plating thickness, testing, and performance. The most common are:

• BS EN ISO 2081: The primary European and international standard for electroplated zinc coatings on iron or steel.
• BS EN ISO 4042: Specifically covers electroplated coating systems for fasteners.
• ASTM B633: A widely used American standard specifying coating types and classes for zinc on iron and steel.
• DEF STAN 03-20: A UK Ministry of Defence standard for the electro-deposition of zinc.

Section 3: Performance and Technical Specifications

How thick is a typical zinc plated coating?

Coating thickness is crucial for performance. It's typically measured in microns (µm). A standard commercial coating might be 5µm, while a more protective coating specified for outdoor use could be 8µm, 12µm, or even 25µm. The required thickness is often dictated by the standard and the service environment.

How is corrosion resistance measured?

It's measured using a Neutral Salt Spray (NSS) test, as defined in standards like ISO 9227. Plated fasteners are placed in a sealed cabinet and exposed to a continuous salt fog. Performance is rated by the number of hours the fastener can resist developing 'white rust' (zinc corrosion) and 'red rust' (steel corrosion).

Does zinc plating affect the fastener threads?

Yes. The addition of the coating increases the fastener's dimensions. For external threads (bolts), this increases the pitch diameter. For internal threads (nuts), it decreases it. This must be accounted for. Standard nuts are often tapped oversize before plating to ensure a proper fit with a plated bolt after coating. We manage these allowances as part of our manufacturing process.

What is hydrogen embrittlement?

Hydrogen embrittlement is a serious risk for high-strength steel fasteners (typically those with a hardness above 320 HV or a tensile strength over 1000 MPa). During the acid cleaning and plating processes, hydrogen atoms can be absorbed into the steel's grain structure. This can make the steel brittle and lead to sudden, catastrophic failure under load, sometimes hours or days after being installed.

How can hydrogen embrittlement be prevented?

The risk is managed by a post-plating baking process, also called de-embrittlement. The fasteners are heated in an oven at a specific temperature (e.g., 190°C - 220°C) for a set duration (e.g., 2 to 24 hours, depending on the material's strength and the standard). This process allows the trapped hydrogen atoms to diffuse safely out of the steel.

Can you plate oversized or undersized fasteners?

Absolutely. As a bespoke manufacturer, we control the entire process. We can machine threads to specific pre-plating dimensions to ensure the final coated product meets the required tolerance class perfectly.

Can you plate left-hand thread fasteners?

Yes. The plating process is independent of the thread direction. We manufacture and plate fasteners in a full range of sizes and pitches, in both right-hand and left-hand configurations.

Does zinc plating affect the torque-tension relationship?

Yes. The plated surface has a different coefficient of friction compared to plain steel. This means that for the same amount of applied torque, the resulting tension (or clamping force) in the bolt can be different. Lubricants or topcoats are sometimes applied to achieve more consistent torque-tension results.

Section 4: Comparison with Other Finishes

What is the difference between Zinc Plating and Hot-Dip Galvanising?

Process: Plating is an electro-chemical deposition ('cold' process), while galvanising involves dipping the part in molten zinc ('hot' process).
Thickness: Plating is very thin and uniform (typically 5-25µm). Galvanising is much thicker and less uniform (often 50-100µm+).
Application: Plating is ideal for fasteners where tight tolerances are critical. Galvanising is used for structural steel and large fabrications where maximum corrosion protection is needed and thread fit is less precise.

Is zinc plating better than stainless steel?

They serve different purposes. Stainless steel is an alloy that is inherently corrosion-resistant throughout its entire structure. Zinc plating is a surface coating on carbon steel. For harsh, corrosive environments (e.g., marine or chemical), stainless steel is superior. For general or moderately corrosive environments, zinc-plated steel is a highly effective and much more economical solution.

How does zinc compare to cadmium plating?

Cadmium plating offers exceptional corrosion resistance, excellent lubricity, and is very malleable, making it ideal for aerospace applications. However, cadmium is highly toxic and its use is now heavily restricted. Zinc plating, particularly with modern high-performance passivates, is the preferred and environmentally safer alternative for most applications. However, because it is not RoHS compliant and requires specialised handling, we do not offer this kind of plating.

What is Mechanical Plating?

Mechanical plating is a 'cold' process where zinc dust, glass beads (impact media), and proprietary chemicals are tumbled with the fasteners in a barrel. The mechanical energy causes the zinc to be cold-welded onto the parts. A key advantage is that it does not introduce hydrogen embrittlement, making it a good choice for very high-strength fasteners. The finish is typically less bright than electroplating.

What is Sherardizing?

Sherardizing is a thermal diffusion process where parts are heated in a sealed container with zinc dust. The zinc diffuses into the surface of the steel, forming a zinc-iron alloy. It creates a very uniform, hard, and wear-resistant coating that is excellent for threaded parts, but the finish is a dull grey. This is no longer offered by Trojan Special Fasteners Ltd.

Section 5: Practical Considerations

Is zinc plating expensive?

No, it is generally considered one of the most cost-effective corrosion protection finishes available for fasteners, offering an excellent balance of performance and price.

Can old, rusty fasteners be re-plated?

It is possible but often not practical. The process would involve chemically stripping the old plating and rust, which can be aggressive and may damage the threads. For standard fasteners, it is almost always more economical to replace them. For rare or bespoke components, re-plating can be a viable option.

Can you plate very small or very large fasteners?

Yes. We have the capability to plate our full manufacturing range, from small metric sizes like M3 up to large diameter nuts and bolts like M52, and their Imperial/Unified equivalents up to 2.1/4". The process (barrel plating for small parts, vat/rack plating for large parts) is adapted to the component size.

What is a 'sealer' or 'topcoat'?

A sealer is an optional final coating applied over the passivate. It fills the microscopic pores in the passivation layer, dramatically increasing corrosion resistance. Sealers can also contain lubricants to control the torque-tension relationship.

Does the bright, shiny appearance last?

The initial brightness of a clear zinc finish will dull over time with exposure to the elements. The rate of dulling depends on the severity of the environment. However, the protective properties of the zinc and passivate layers will continue to work long after the aesthetic shine has faded.

How do I specify a zinc plated finish on a drawing?

The best way is to reference the relevant standard. A typical specification might look like this: "Zinc Plate to BS EN ISO 4042, Fe/Zn 8c". This tells the plater to use the ISO standard for fasteners, apply a zinc coating (Zn) over steel (Fe), with a minimum thickness of 8µm, and a clear (c) passivate finish.