Overview of Fastener Surface Treatments: Principles and Selection of Nine Main Processes
Executive Summary
Surface treatment is far more than "plating a layer." With the same batch of grade 10.9 bolts, the wrong process can rust through the thread root within three months, while the right one survives more than 1000 hours in the salt-spray chamber. This article places nine mainstream fastener surface treatments in one comparison table, breaking down principle, coating thickness, salt-spray hours, hydrogen embrittlement risk and friction coefficient, and gives an actionable four-step selection method.
Table of Contents
Wrong Coating: Rust Through the Thread Root in Three Months
At a project review meeting of a new-energy vehicle maker, an engineer showed comparison photos of two bolt groups: M10 flange bolts of the same specification and strength class, installed on the same assembly line, inspected after six months. Group A already showed clear red rust at the thread root, while Group B looked as good as new.
The only difference: Group A used ordinary blue-white zinc plating, Group B used a zinc-aluminium coating. The former costs about 30% less, but under the salt-laden, humid underbody environment its protection lasts only a few hundred hours, while the zinc-aluminium coating lasts beyond 1000 hours.
This shows that mistakes in surface-treatment selection are often more hidden and more costly than material-selection mistakes. They do not show up in a tensile test; they only surface at the corrosion site after six months in service.
Comparison of Nine Mainstream Processes
The surface treatments commonly used on fasteners fall into nine categories. The table below gives an engineering comparison of the key parameters (salt spray is the typical level under standard conditions).
| Process | Typical thickness | Neutral salt spray | Hydrogen embrittlement risk | Friction coefficient | Cost index |
|---|---|---|---|---|---|
| Electroplated zinc (blue-white / yellow) | 5–12 µm | 48–200 h | High | 0.15–0.25 | 1.0 |
| Zinc-nickel alloy plating | 8–15 µm | 720–1500 h | High | 0.12–0.20 | 2.0–2.6 |
| Hot-dip galvanizing | 40–70 µm | 500–1000 h | None | 0.20–0.35 | 1.6–2.2 |
| Zinc-aluminium coating (Geomet / Dacromet) | 6–12 µm | 720–1500 h | None | 0.10–0.18 | 2.2–3.0 |
| Phosphating (zinc / manganese) | 2–8 µm | 24–72 h | Medium | 0.10–0.20 | 0.6–0.9 |
| PTFE coating (Teflon) | 10–25 µm | 240–720 h | None | 0.06–0.12 | 2.0–3.0 |
| Graphite lubricating coating | 8–20 µm | 120–480 h | None | 0.08–0.14 | 1.5–2.2 |
| PVD (CrN / TiAlN) | 1–5 µm | 240–720 h | None | 0.15–0.30 | 3.5–6.0 |
| DLC (diamond-like carbon) | 1–4 µm | 240–720 h | None | 0.05–0.15 | 4.0–7.0 |
How the Three Technical Routes Differ
Route 1: Sacrificial anode
Electroplated zinc, zinc-nickel, hot-dip galvanizing and zinc-aluminium coatings all contain zinc; the mechanism is that the coating itself acts as the sacrificial anode and corrodes first. The higher the zinc content and the thicker the coating, the longer the protection. Zinc-aluminium coatings rely on aluminium to form a dense Al₂O₃ barrier, combining barrier and sacrificial mechanisms, and deliver the best salt-spray performance.
Route 2: Barrier isolation
Phosphating, PTFE and some graphite coatings rely on a physical barrier to block water, oxygen and chloride ions. These coatings contain no sacrificial metal, so once mechanically damaged, rust spots spread quickly along the defect; their pretreatment requirements are therefore far higher than sacrificial systems, and a sealant or primer is usually needed.
Route 3: Functional enhancement
The main purpose of PVD and DLC is not corrosion protection but wear and friction reduction. PVD hardness is 1500–3200 HV and DLC can reach 1500–4000 HV, 2–5 times that of high-speed steel. At only 1–5 µm thick, their corrosion protection is limited.
Four Steps from Working Conditions to Process
Step 1: Determine the environmental corrosion class
| Class | Typical environment | Minimum salt spray | Options |
|---|---|---|---|
| C1–C2 | Dry indoor equipment rooms | ≥ 48 h | Electroplated zinc, phosphating |
| C3 | Urban atmosphere, general workshops | ≥ 240 h | Yellow zinc, zinc-aluminium coating, PTFE |
| C4 | Industrial areas, light coastal salt spray | ≥ 720 h | Zinc-nickel, zinc-aluminium coating, hot-dip galvanizing |
| C5 | Coastal, de-icing-salt roads | ≥ 1000 h | Zinc-nickel + sealant, chromium-free zinc-aluminium coating |
Step 2: Confirm the strength class
Tensile strength ≥ 1000 MPa (grade 10.9 and above) should avoid electroplating processes; if plating is unavoidable, hydrogen relief baking must be completed within 4 hours after plating (190–230 °C, ≥ 4 h). Zinc-aluminium coatings, hot-dip galvanizing and PVD carry no hydrogen-embrittlement risk and are the first choice for high-strength parts.
Step 3: Confirm the friction-coefficient requirement
Zinc-aluminium coatings with lubricating additives (μ ≈ 0.10–0.18) and PTFE coatings (μ ≈ 0.06–0.12) hold the K factor in a narrow band and are the preferred choice for automated lines.
Step 4: Check thread accuracy
When plating exceeds 8 µm, a 6g external thread should be manufactured to 6e or 6f and returned to 6g after plating; otherwise it will not thread on or will gall. The thick layer of hot-dip galvanizing requires post-plate spin-off and thread re-tapping.
Deciding with the Cost Ledger
Comparing surface-treatment cost cannot look only at the price per kilogram; three hidden costs must also be considered:
- Scrap and rework: plating hydrogen embrittlement is a delayed failure that often occurs hours to days after assembly, and the rework cost far exceeds the coating price difference.
- Assembly cost: unstable friction coefficient causes frequent adjustment of tightening tools and failed preload spot checks.
- Life-cycle cost: the direct cost of replacing one rusted bolt can be more than 10 times its own price.
Practical criterion: in corrosion class C4 and above with strength class 10.9 and above, the weight of surface-treatment cost should exceed the weight of purchase unit price. The coating cost saved usually cannot cover a single warranty repair.
Putting the Conclusion on the Drawing
Salt-spray testing is a powerful comparison tool, but its value depends on whether the test conditions and acceptance criteria are clear. Neutral salt spray gives the relative magnitude of different processes; cyclic corrosion is closer to the real environment; specimen state determines whether data can be extrapolated. The most useful habit is to read the test conditions, acceptance criteria and data together; a "1000 hours" with no condition statement carries almost no information.
