Zinc-Aluminium Dacromet Coating: The Preferred Solution for Long-Term Corrosion Protection of High-Strength Bolts
Executive Summary
The reason zinc-aluminium (Dacromet) coating has become the first choice for high-strength bolt corrosion protection is that it solves three problems at once: no hydrogen embrittlement, over 1000 hours of salt spray, and a designable friction coefficient. This article breaks down its film-forming mechanism, the technical difference between chromated and chromium-free systems, the quantitative relationship between coating thickness and salt spray, and the process red lines and typical failures in high-strength applications.
Table of Contents
Why Grade 10.9 Cannot Be Electroplated
A grade 10.9 M16 bolt, with tensile strength of 1040 MPa and hardness around 33 HRC, is a typical high-strength fastener. If such a part is electroplated with zinc by the conventional process, even with post-plate hydrogen baking, the risk of delayed fracture remains.
The cause is cathodic hydrogen evolution during plating: both pickling and plating generate atomic hydrogen on the steel surface. The atomic hydrogen diffuses into the lattice and is trapped by dislocations and inclusions, inducing cracks under the combined action of stress and hydrogen. The fracture surface usually shows intergranular brittle characteristics, appearing hours to days after assembly; this is why it is called delayed fracture.
Zinc-aluminium coating (commonly known as Dacromet in the industry) fundamentally avoids this problem: its film formation is dip/spin or spray + high-temperature sintering, with no electrolytic hydrogen evolution at any stage, so it is clearly classified as a hydrogen-embrittlement-free process. This is the first and hardest reason it is the preferred choice for high-strength bolts.
Film-Forming Mechanism: The Flakes Are the Key
The composition of a zinc-aluminium coating can be summarized as: flake zinc powder + flake aluminium powder + inorganic binder + organic additives, dispersed in a solvent to form the coating liquid.
Zinc and aluminium powders are ball-milled into flakes with an aspect ratio of 50–100. After application, these flaky metals overlap and lie parallel like roof tiles, forming two parallel protection paths:
- Physical barrier: the overlapping layers lengthen the penetration path of corrosive media by tens of times, creating a labyrinth effect.
- Sacrificial anode: good electrical contact between layers means zinc corrodes first at gaps and scratches, protecting the steel substrate.
The contribution of aluminium is especially critical: after oxidation it forms a dense Al₂O₃ passivation film that lowers the dissolution rate of corrosion products. This is the mechanistic root of why zinc-aluminium coatings outperform pure zinc coatings in corrosion resistance.
Chromated vs Chromium-Free
Early Dacromet used hexavalent chromium as binder; corrosion protection was excellent, but it caused serious environmental and occupational-health problems (Cr̿̿̿ is carcinogenic). The EU ELV and RoHS directives restrict it, and the automotive industry has almost entirely switched over.
| System | Binder | Neutral salt spray | Compliance |
|---|---|---|---|
| Traditional Dacromet (chromated) | Hexavalent chromium | 1000–1500 h | Non-compliant |
| Chromium-free zinc-aluminium coating | Trivalent chromium / silane | 720–1200 h | Compliant |
| Titanium/zirconium salt + silane | Silane system | 600–1000 h | Compliant |
A reminder: the chromium-free and chromated systems are not simply a matter of swapping one chemical; their curing mechanism, film hardness and alkali resistance all differ. When switching systems, salt-spray validation and torque-clamp-force tests must be repeated.
Coating Thickness and Friction Coefficient
Coating thickness is the most basic control parameter and directly determines service life. The table below shows the typical relationship for chromium-free systems in C4 environments.
| Thickness | Application method | NSS first red rust | Thread impact |
|---|---|---|---|
| 4–6 µm | Dip-spin (one coat) | 240–480 h | Minimal |
| 6–8 µm | Dip-spin (high solids) | 480–720 h | Machine to 6e |
| 8–12 µm | Dip-spin (two coats) | 720–1200 h | Machine to 6e/6f |
| 12–20 µm | Spray + two dip coats | 1000–1500 h | Re-tap after coating |
The value of two coats is not just added thickness: the dense layer formed after the first sinter seals the channels through which the second coating liquid would penetrate into the thread root, so two coats give clearly better uniformity and sealing than simply thickening one coat.
Tuning experience: rather than chasing thickness in a single coat, switch to two coats. This is the most cost-effective way to improve salt-spray performance and usually raises salt-spray hours by 40%–80%.
Friction Coefficient and Assembly Performance
| Coating type | Friction coefficient μ | Torque coefficient K | K spread |
|---|---|---|---|
| Zinc-aluminium (no lubrication) | 0.18–0.30 | 0.24–0.34 | ±20%–±30% |
| Zinc-aluminium + wax lubricant | 0.12–0.18 | 0.16–0.22 | ±12%–±18% |
| Zinc-aluminium + PTFE composite | 0.10–0.15 | 0.13–0.19 | ±8%–±14% |
| With integrated washer assembly | 0.08–0.13 | 0.11–0.16 | ±8%–±12% |
Adding lubrication cuts the K spread by more than half, which directly translates into assembly benefits: under the same torque tolerance, preload scatter decreases, allowing a higher design preload utilization factor.
Typical Failures and Process Control Points
| Failure symptom | Likely root cause | Control point |
|---|---|---|
| Peeling, flaking | Incomplete degreasing / shot blasting | Cleanliness after blasting; roughness Ra 3–6 µm |
| Local bare spots | Coating liquid not drained from thread roots / blind holes | Spin parameters, loading method, coating viscosity |
| Early red rust in salt spray | Inadequate sintering; flakes not cross-linked | Sinter at 300–330 °C for 20–30 min |
| Brittle film shedding powder | Over-sintering or too thick a coating | Temperature control ±5 °C; cap thickness upper limit |
| Hydrogen-embrittlement fracture | Pickling in pretreatment evolves hydrogen | Never acid-pickle; use shot blasting instead |
The last row deserves special emphasis. Although the zinc-aluminium coating itself has no electroplating step, if pretreatment uses hydrochloric acid pickling for rust removal, hydrogen is still introduced. The correct practice is to replace pickling with shot blasting; this is a non-negotiable red line in high-strength bolt processing.
Putting the Corrosion Class on the Drawing
The reason zinc-aluminium coating can be the first choice for long-term protection of high-strength bolts is essentially that it gives answers on three dimensions at once: a hydrogen-embrittlement-free process, over 1000 hours of salt spray, and a designable friction coefficient. These three points correspond exactly to what high-strength joints care about most—safety, service life and assembly consistency.
The key to using this process well is managing it as a complete system: shot-blast pretreatment, coating flakiness, two-coat application, sintering curve, lubrication system and thread tolerance—every link affects the final result. Relax any one link, and the coating degrades from "preferred solution" to "ordinary plating."
