Finishes & Coatings for Fasteners
Not all fasteners are plated. Materials such as A2 stainless steel, A4 stainless steel and brass are naturally corrosion‑resistant and are supplied in their untreated state. This is often referred to as self colour – the natural appearance of the base material without any applied coating. Steel fasteners are also available in self colour, but unlike stainless or brass they will corrode if left unprotected, so a suitable finish or coating is normally required for most applications.
Fastener finishes and coatings provide corrosion protection, improve appearance, and in some cases enhance lubricity or electrical performance. The correct finish depends on the environment, required lifespan, and the material of the fastener itself.
Below is an introduction to the most common finishes used on steel fasteners, along with typical applications and key considerations.
Common Fastener Finishes
Zinc Plating
A thin electroplated zinc layer providing basic corrosion resistance for indoor or dry
environments. Available in clear, yellow, and black passivates. Older zinc plating systems
traditionally used hexavalent chromium (Chrome 6) in the passivation layer, which is now
restricted due to environmental and health concerns. Modern zinc finishes are
Chrome 6–free, using safer trivalent chromium systems while still providing effective
corrosion protection. Not recommended for high‑strength fasteners without controlled processing
due to hydrogen embrittlement risk.
Zinc Flake Coatings
A non‑electrolytic coating system that provides very high corrosion resistance without introducing
hydrogen into the steel. Unlike traditional zinc plating, zinc flake coatings are applied by
dip‑spin or spray processes rather than electroplating, making them ideal for high‑strength
fasteners (10.9 and 12.9) where hydrogen embrittlement is a concern. Widely used in the
automotive industry, construction fixings, and applications requiring long‑term outdoor
durability, zinc flake systems offer excellent performance with a thin, uniform coating that does
not significantly affect thread fit.
Hot Dip Galvanised (HDG)
A thick, highly durable zinc coating applied by immersing the fastener in molten zinc. HDG provides
excellent long‑term corrosion resistance, especially in outdoor, marine‑adjacent, or exposed
structural environments. The coating is much thicker than electroplated zinc, giving superior
protection but reducing thread precision — so HDG fasteners and nuts are typically supplied as
matched sets with oversized threads. Unlike zinc plating, HDG is not suitable for small precision
screws or fine‑pitch threads, but it is the preferred choice for structural bolts, fencing,
agricultural hardware, and general outdoor fixings where maximum durability is required.
Finish Suitability by Strength Class
| Finish / Coating | Mild Steel | Grade 8.8 | Grade 10.9 | Grade 12.9 |
|---|---|---|---|---|
| Zinc Plating | ✔ Suitable | ✔ Suitable | ⚠ Requires controlled processing (hydrogen risk) | ⚠ Generally not recommended |
| Zinc Flake | ✔ Suitable | ✔ Suitable | ✔ Ideal (no hydrogen risk) | ✔ Ideal (automotive standard) |
| Hot Dip Galvanised (HDG) | ✔ Suitable | ✔ Suitable | ⚠ Limited — coarse threads only | ✖ Not used (thread fit + mechanical limits) |
| Self Colour (Uncoated Steel) | ✔ Indoors only | ✔ Indoors only | ✔ Indoors only | ✔ Indoors only |
| Stainless A2 / A4 | Not plated — inherently corrosion‑resistant | |||
| Brass | Not plated — supplied in natural self‑colour | |||
Hydrogen Embrittlement Considerations
High‑strength steels — especially 10.9 and 12.9 — are susceptible to hydrogen embrittlement if exposed to processes that introduce hydrogen, such as electroplating (zinc, nickel, chrome) or acid pickling. Hydrogen atoms can diffuse into the hardened steel and cause delayed brittle failure under load.
To reduce the risk:
- Use non‑electrolytic coatings such as zinc flake where possible.
- If electroplating is required, ensure strict process control and post‑plating bake‑out.
- Avoid acid pickling on high‑strength fasteners unless absolutely necessary.