Understanding Salt Spray Testing for Plated Fasteners
Key Takeaways
- Neutral salt spray (NSS) testing per ISO 9227 / ASTM B117 is the most common corrosion benchmark for plated fasteners - Results are judged in two stages: white rust (coating oxidation) and red rust (substrate corrosion) - Corrosion life varies enormously by process — from tens of hours for plain zinc to 2,000 hours for Zinc-Nickel
How Salt Spray Testing Works
Neutral salt spray testing exposes parts to a continuous 5% sodium chloride fog at 35°C, accelerating atmospheric corrosion. It follows ISO 9227 (NSS) and ASTM B117, and OEM specifications typically reference these standards with required hours.
Keep in mind that salt-spray hours are an accelerated comparison metric for judging processes and batch quality — they do not translate directly into service life.
White Rust vs. Red Rust
Two failure stages are judged separately: white rust is the white corrosion product of the zinc coating itself — the passivate has failed but the substrate is still protected; red rust means the steel substrate is corroding and the coating has been breached.
Specifications usually set both, e.g. "96 hours to white rust, 720 hours to red rust." White-rust hours reflect passivation and sealing quality; red-rust hours reflect coating type and thickness.
Corrosion Hours by Plating Process
At well-passivated production quality, typical salt-spray performance runs roughly:
- Trivalent zinc: 24-96 hours to white rust, 72-240 hours to red rust
- Zinc-Iron alloy: about 240-480 hours to red rust, with a uniform black passivate available
- Zinc-Nickel alloy: 96-480 hours to white rust, 720-2,000 hours to red rust
Actual hours depend on coating thickness, the passivation system, and sealing — always confirm the test standard and the failure-area criteria when comparing specs.
FAQ
- No direct conversion exists. Salt spray is an accelerated comparative test for ranking processes and batches; real-world life depends heavily on the service environment — temperature, humidity, salt, and chemicals.
- Thickness distribution in barrel plating, passivate uniformity, and sealing quality all contribute. Equipment setup (such as one rectifier per barrel) and process control are key to tightening within-batch variation.
- Specifications usually define the allowable corroded-area percentage (for example 5%) and the surfaces to be judged when calling white or red rust, and definitions differ between standards. When comparing supplier reports, confirm the test basis (ISO 9227 or ASTM B117), the judged area and whether thread edges are excluded.
