Zinc plating, Dacromet, electrophoresis, black oxide — spring suppliers offer a confusing menu of coatings, and the wrong choice shows up months later as rust, fatigue cracks, or failed validation. This guide compares the most common spring surface treatments by the number that actually matters: salt-spray hours.
The short answer: for most indoor applications, zinc plating is enough. For automotive, outdoor, or high-humidity environments, zinc-flake coatings (Dacromet) deliver 7–10× the corrosion resistance with no hydrogen embrittlement risk. Electrophoresis adds uniform coverage and a finished appearance, usually as a topcoat over zinc.
A spring under load is uniquely vulnerable to corrosion. Corrosion pits act as stress concentrators — a pit a fraction of a millimeter deep can cut a spring's fatigue life dramatically, because cracks start at the surface where stress is highest. For a spring, corrosion protection isn't cosmetic; it's fatigue-life protection.
There's a second trap: some coating processes themselves can damage the spring. Acid pickling and electroplating introduce hydrogen into high-strength steel, causing hydrogen embrittlement — springs that pass inspection and then crack days later in service. Any plating on high-strength spring steel requires a de-embrittlement bake, or a coating process that avoids the problem entirely.
| Coating | Typical Salt-Spray Hours* | Hydrogen Embrittlement Risk | Best For |
|---|---|---|---|
| Zinc plating (clear/blue) | 24–72 h | Yes — bake required | Indoor, dry, cost-sensitive parts |
| Zinc plating (yellow chromate) | 96–120 h | Yes — bake required | General industrial use |
| Black oxide | Minimal (needs oil) | No | Appearance, anti-glare, oiled assemblies |
| Electrophoresis (e-coat) | 200–500 h (over zinc) | Low | Uniform coverage, consumer-facing parts |
| Dacromet / zinc-flake | 480–1,000+ h | None | Automotive, outdoor, marine, safety-critical |
| Powder coating | 500–1,000 h (system-dependent) | No | Thick protective layer, colored finishes |
| Passivation (stainless only) | N/A — restores natural oxide layer | No | Medical, food-grade, marine stainless springs |
*Salt-spray hours to first red rust under ASTM B117 / ISO 9227 are indicative ranges — actual results depend on coating thickness, chromate/passivate type, and test conditions. Always specify the requirement, not just the coating name.
Electro-galvanized zinc is the workhorse spring coating: cheap, bright, and adequate for dry indoor environments. Yellow chromate versions roughly double the protection of clear passivation.
The condition: on high-strength spring steel (roughly above 1,700 MPa tensile), electroplating must be followed by a hydrogen de-embrittlement bake within hours of plating. If your supplier doesn't mention this step for a high-strength spring, ask why. Skipping it is how springs crack in the field after passing inspection.
Dacromet is a water-based dispersion of zinc and aluminum flakes, dip-spin applied and cured at temperature. Because no acid pickling or electrolysis is involved, there is no hydrogen embrittlement risk at all — which is why the automotive industry made it the default for safety-relevant fasteners and springs.
In one of our own automotive programs, Dacromet-coated door-checker springs pass 720 hours of salt spray while running at 2,000,000+ pieces per month — see the door-checker spring case study for the full specification.
E-coating deposits a uniform epoxy film by electrodeposition, reaching recesses and coil gaps that spray processes miss. It's typically applied over zinc plating as a duplex system — the zinc provides sacrificial protection, the e-coat seals it and adds a clean black or colored finish. Common on consumer-visible springs and automotive interior components.
For medical, food-contact, and marine applications, the better answer is often stainless steel with passivation — no coating to flake, chip, or outgas. Our medical spring case study used medical-grade stainless with a burr-free finish for exactly this reason. For a broader overview of all options, see our guide to spring surface treatments and when to use each.
Q: Is Dacromet RoHS compliant?
A: Yes. Modern zinc-flake coatings (Dacromet and equivalents such as Geomet) are chromium-VI-free and meet RoHS, REACH.
Q: Does coating affect spring rate or dimensions?
A: Zinc-flake and e-coat films are thin (8–25 μm) and rarely matter. Thick powder coating can affect fit in tight bores — flag it on the drawing if your spring works in a close-tolerance housing.
Q: Can I get colored springs for identification?
A: Yes — powder coating and dyed e-coats handle color coding. Die springs, for example, are color-coded by load class under ISO 10243.
Send us your operating environment, salt-spray requirement, or just the drawing — our engineers will recommend the right coating and quote within 24 hours.
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