A coating should be selected as part of the bolted-joint design—not as a colour choice. Exposure, fastener hardness, thread tolerance, friction, operating temperature, mating components and regulatory requirements all affect the right answer.
Fast comparison
| System | Typical reason to consider it | Critical engineering check |
|---|---|---|
| Black phosphate + oil | Indoor handling, appearance, run-in | Limited exposed corrosion protection |
| Electroplated zinc | Thin general-purpose sacrificial finish | Hydrogen controls and friction |
| Hot-dip galvanized | Thicker zinc for outdoor exposure | Thread accommodation and nut system |
| Zinc flake | Corrosion performance with non-electrolytic application | Approved system, friction and thickness |
| Zinc-nickel | Higher-demand thin alloy coating | Electroplating controls and topcoat |
| PTFE-based system | Barrier protection and low friction | Coating-specific torque procedure |
| Ceramic system | Qualified chemical or temperature exposure | Exact product data and cure |
| Cadmium | Legacy/special controlled applications | Legal and customer restrictions |
A six-step selection method
- Define exposure. Record atmospheric category, immersion, chemicals, temperature, UV, maintenance interval and intended life.
- Confirm the base fastener. Identify grade, heat treatment, hardness, dimensions and whether the processing route introduces hydrogen risk.
- Protect fit. Calculate coating allowance on external and internal threads, then define the post-coating gauging condition.
- Control friction. Specify lubricant/topcoat condition and validate torque–tension behaviour on the actual assembly.
- Name the tests. Set coating thickness, adhesion, corrosion, appearance and any system-specific acceptance criteria.
- Close the documentation loop. Require traceability, certificate content, packing and approved repair procedures.
Standards and project requirements
Relevant routes can include ASTM F1941/F1941M for electrodeposited fastener coatings, ASTM F2329/F2329M for hot-dip zinc coatings, ISO 10683 for zinc flake systems and ISO 10684 for hot-dip galvanized fasteners. The project specification takes precedence, and the current edition should always be confirmed.
Primary sources: ASTM F1941/F1941M, ASTM F2329/F2329M, and ISO 10683:2018.
Send a complete coating enquiry
Provide the fastener drawing or standard, material grade, dimensions, quantities, mating nuts and washers, service environment, named coating system, performance tests, friction requirement, inspection documents and destination. FERROBEND can then review manufacturability and identify items that require project approval.