Dacromet Fasteners for Hydrogen-Free Corrosion Protection
Dual Failure: Corrosion and Hydrogen Embrittlement in Drive-Line Fasteners
A manufacturer of class 10.9 and 12.9 alloy steel suspension bolts and fasteners for heavy-duty truck applications was experiencing two simultaneous field failure modes. First, conventional electroplated zinc-nickel fasteners showed red rust on the hexagonal head and thread areas after 250–350 hours of salt spray exposure. Second and more critically, three field failures of suspension crossmember bolts were traced to hydrogen embrittlement delayed fracture—a catastrophic failure mode where atomic hydrogen, introduced during the electroplating process, diffuses into the steel lattice and causes intergranular cracking under sustained tensile stress.
The electroplating bath, even with optimized current density and post-plate baking at 190 °C for 8 hours per ASTM B850, could not completely eliminate the hydrogen embrittlement risk for the high-hardness 12.9 grade bolts (core hardness 38–42 HRC). Fractography of the failed bolts confirmed classic intergranular fracture surfaces with microvoid coalescence at the crack initiation site—the signature of hydrogen-induced delayed cracking.
Dacromet: A Hydrogen-Free Alternative
Dacromet is a water-based coating system containing zinc and aluminum flakes suspended in a chromate-containing binder. The coating is applied by dip-spin or spray, then cured at approximately 300 °C. The key advantage over electroplated zinc coatings is that Dacromet involves no cathodic electrolytic process, so absolutely no hydrogen is generated during application. Additionally, the zinc-aluminum flake structure provides sacrificial cathodic protection in a lamellar matrix that is inherently more corrosion-resistant than an equivalent thickness of electroplated zinc.
The fastener manufacturer selected Dacromet 320L, a lead-free formulation compliant with ELV and RoHS directives, applied via dip-spin at two layers with an intermediate bake.
| Parameter | Electroplated Zn-Ni (12–15 µm) | Dacromet 320L (8–12 µm) |
|---|---|---|
| Application method | Cathodic electrolytic | Dip-spin, non-electrolytic |
| Hydrogen generation | Yes — inherent in process | None — no electrolysis |
| Post-bake required? | Yes (190 °C / 8 h for >1,000 MPa) | No (curing at 300 °C / 20 min) |
| Neutral salt spray to red rust | 250–400 h | 720–1,000 h |
| Coating thickness tolerance | ±3–5 µm | ±2–3 µm |
| Friction coefficient (clean/dry) | 0.12–0.18 | 0.10–0.14 (with top coat) |
| Maximum service temperature | 120 °C (Zn-Ni degrades) | 300 °C |
Qualification Testing Program
A comprehensive test program was conducted on M16 × 1.5 class 10.9 bolts used in heavy-duty truck suspension crossmembers. The testing covered corrosion resistance, mechanical properties, and hydrogen embrittlement verification.
| Test | Method | Requirement | Dacromet Result | Electroplated Zn-Ni Result |
|---|---|---|---|---|
| Neutral salt spray to red rust | ASTM B117, unscribed | ≥480 h (OEM spec) | 840 h to first red rust | 280 h to first red rust |
| Cyclic corrosion test | SAE J2334 | 60 cycles, no red rust on head | Pass — 80 cycles, no red rust | Fail — red rust at cycle 45 |
| Hydrogen embrittlement | ISO 7539 / sustained load at 80% UTS | No failure in 200 h | Pass — no failure in 500 h | 1 of 10 failed at 120 h |
| Torque-tension consistency | ISO 16047, n=50, M16-10.9 | K-factor range ≤0.04 | K = 0.14 ± 0.015 | K = 0.16 ± 0.025 |
| Thermal cycling | −40 °C to 150 °C, 100 cycles | No coating cracking or peeling | Pass — no visible change | Minor blistering at cycle 80 |
Production Implementation and Field Tracking
The Dacromet line was implemented at the fastener supplier's facility with a four-basket dip-spin system capable of processing 400 kg of fasteners per hour. The coating process sequence was: alkaline pre-clean → rinse → Dacromet first coat → flash dry at 120 °C → Dacromet second coat → cure at 300 °C for 20 minutes → cool → optional topcoat for friction control.
The per-kg coating cost for Dacromet was $1.85, compared to $1.40 for electroplated zinc-nickel. However, the elimination of the 8-hour hydrogen embrittlement bake cycle reduced total processing time and energy costs by 47%, narrowing the total cost difference to just $0.12 per kg in favor of electroplating—a negligible premium for eliminating the embrittlement risk.
Over 36 months of field deployment, more than 8 million Dacromet-coated fasteners were installed on the truck assembly line without a single reported corrosion failure or fastener fracture. The three historic electroplated fastener failures that triggered the change had each resulted in warranty costs exceeding $15,000 per incident, including vehicle downtime and repair labor. The total elimination of these failures gave an effective net negative coating cost when the warranty reduction was factored in.
Dacromet coating offers automotive and heavy equipment engineers a proven, hydrogen-free corrosion protection solution for high-strength fasteners. The zinc-aluminum flake system delivers salt spray resistance far exceeding conventional electroplating, eliminates the hydrogen embrittlement risk inherent in electrolytic processes, and provides consistent torque-tension characteristics essential for critical joint assembly.
