Fastener Surface Finishes: The Complete Guide

1. A salt spray surprise

In 2021 an outdoor EV charger supplier specified blue-white electroplated zinc with a 24-hour salt spray report. Six months after installation, customers on the southern coast complained that every enclosure screw was rusty and bleeding stains down the housing.

The causes:

1. Blue-white zinc at 24 h is only the minimum threshold for no red rust. Real outdoor exposure combines UV, rain and salt, and the coating decays much faster.
2. The product was specified to IP54 outdoors, but the finish was an indoor standard.
3. Zinc flake or stainless was never considered.

Replacing everything with zinc flake at 720 h solved it. The finish must match the real environment, not the supplier's standard salt spray figure.

2. What a finish actually does

2.1 Corrosion protection

Carbon steel rusts in air. A finish protects in three ways: a barrier that keeps air away, a sacrificial anode where zinc corrodes before the steel, or a passive film as on stainless.

2.2 Friction control

Tightening torque follows T = K·F·d, where K is the friction coefficient. Each finish gives a different K:

3. Eight finishes compared

4. Electroplated zinc

4.1 Principle

The part is immersed in a zinc salt solution and current deposits zinc on the surface. Zinc is more active than iron, so even a scratched coating keeps protecting the steel as a sacrificial anode.

4.2 Passivation colours

• Blue-white: chromate passivation, transparent blue film, 24–48 h.
• Yellow: rainbow passivation, 72–96 h.
• Black: black passivation, 48–72 h, common on visible parts.
• Chrome-free: compliant with RoHS, slightly lower salt spray.

4.3 Hydrogen

Plating drives hydrogen into the steel, and above 1,000 MPa — class 10.9 and higher — that causes delayed fracture. Bake at 200 °C for four hours within four hours of plating. See the hydrogen section below.

5. Zinc flake coatings

5.1 Principle

Zinc and aluminium flakes with a binder are applied by dipping or spraying and then cured. There is no electrolysis, so no hydrogen is generated.

5.2 Advantages

• 720–1,000 h of salt spray, ten times electroplated zinc.
• No hydrogen, so safe on high-strength steel.
• Resists 300 °C.
• Stable friction coefficient.

5.3 Cautions

Dacromet is a trademark. The generic standards are zinc-chromium coating (GB/T 18684) and non-electrolytic zinc flake coating (Geomet, Delta-Protekt). Hexavalent chromium versions do not meet RoHS, so the mainstream today is chrome-free.

6. Anodising and black oxide

6.1 Anodising (aluminium)

Aluminium is anodised in sulphuric acid to grow a dense Al₂O₃ film, which can be dyed black, red or gold. It is hard and attractive, and applies only to aluminium and titanium, never steel.

6.2 Black oxide (steel)

Steel is treated in an alkaline bath to form a black Fe₃O₄ film a few micrometres thick. It does not protect against rust; it is cosmetic plus short-term oxidation resistance, and must be oiled. Common on internal machinery.

7. PVD and DLC

7.1 PVD

Physical vapour deposition deposits titanium or chromium ions in vacuum to form hard films such as TiN (gold) or TiCN (grey-black). Wear resistant and attractive, but expensive.

7.2 DLC

Diamond-like carbon is an ultra-hard carbon film with friction around 0.1. Used on premium tooling, precision parts and engine components, at five to ten times the cost of zinc.

8. Hydrogen embrittlement

8.1 What it is

High-strength steel absorbs hydrogen during pickling or plating. The hydrogen collects at grain boundaries and causes delayed fracture, typically hours to weeks after assembly, with no warning.

8.2 Where to worry

• Tensile above 1,000 MPa, class 10.9 and above.
• Spring steel and martensitic stainless.
• Sharp notches that concentrate stress.

8.3 Countermeasures

1. Bake at 200 °C for four to eight hours within four hours of plating.
2. Use a process with no hydrogen, such as zinc flake or mechanical zinc.
3. Avoid acid pickling; use blasting instead.

9. Reading a salt spray report

Salt spray testing (GB/T 10125, ASTM B117) exposes samples to 5 % NaCl mist and records time to red rust. Note that:

• It is an accelerated test; 1,000 h in a cabinet is not 1,000 hours outdoors.
• Acceptance criteria differ between standards (5 % versus 1 % red area).
• Real exposure adds UV and thermal cycling, which are harsher.

10. Decision tree and FAQ

Step 1 — What material?

Steel → zinc or zinc flake. Aluminium → anodising. Stainless → passivation only.

Step 2 — Indoor or outdoor?

Indoor → blue-white zinc. Outdoor → yellow zinc or zinc flake. Coastal → zinc flake or 316.

Step 3 — What property class?

Class 10.9 and above → baking is mandatory, or use zinc flake.

Step 4 — Appearance?

Black → black zinc, black oxide or PVD. Coloured → anodising on aluminium.

Step 5 — Budget?

General parts 1× zinc; demanding 2.5× zinc flake; premium 5–10× DLC.

FAQ

Q. Blue-white or yellow zinc? Yellow resists corrosion better; blue-white looks cleaner. Indoors blue-white, outdoors yellow.

Q. Why is zinc flake expensive? The process is more complex, but it gives ten times the salt spray and no hydrogen risk on high-strength steel, so total cost is often lower.

Q. Why does black oxide not stop rust? The film is only micrometres thick and provides no sacrificial action. Oiling only protects against fingerprints short term.

Q. Does stainless need a finish? It needs passivation to remove free iron. See the passivation guide.

Q. Can PVD go on any screw? No. Equipment is expensive and small batches are uneconomic; it is for high-value parts only.