Salt Spray Testing Explained: What 96 Hours in the Fog Actually Proves
Nearly every corrosion spec quotes salt-spray hours. Used correctly it is a sharp QC instrument; used naively it becomes marketing fiction.
The test
Parts hang in a chamber at 35°C filled with atomized 5% sodium-chloride solution (neutral pH — hence "NSS"). Inspectors record hours until:
- White rust: zinc corrosion products (the sacrificial layer working)
- Red rust: base steel attacked (the coating defeated)
Common gates: zinc clear 72–120 h to white rust; zinc-nickel 500–1,000 h to red rust; zinc flake systems similar or better.
What it is good for
- Comparing coatings and process control lot-to-lot — its real strength
- Catching process failures: thin plating, missed sealing, bad chromate cure show up fast
- Contractual acceptance: an unambiguous, cheap, universally available referee
What it cannot do
Salt spray is continuous wet salt at constant temperature — no UV, no drying cycles, no thermal shock, no road grime. Correlation to field life is loose and environment-specific: a coating lasting 2× longer in the chamber is *probably* better outdoors, but "480 hours = 5 years" style equations are fiction. Automotive specs increasingly use cyclic corrosion tests (CCT) that alternate salt, humidity, and drying for closer realism.
Specifying sensibly
"Salt spray per ISO 9227 NSS: ≥ 96 h to white rust, ≥ 240 h to red rust, sample 3 pcs per lot" — attached to a finish spec, tested on real parts (edges and coil gaps matter, panels flatter). For springs, ask that tested parts be *stressed or deflected* if your application holds them deflected — stress accelerates coating breakdown at bends.
We arrange salt-spray verification through accredited Taiwanese labs, with reports in English or Japanese — request it in your RFQ.
Frequently Asked Questions
How many salt-spray hours equal one year outdoors?
No defensible constant exists — outdoor environments vary 100× in corrosivity (ISO 9223 categories C1–C5). Use salt spray to compare and control, cyclic tests to approximate reality, and field data to know.
Why did plated springs pass salt spray but rust at the customer?
Common causes: coil-contact points shielding fog during test (but trapping condensation in service), hydrogen-bake omitted turning cracks into rust starters, packaging condensation, or a marine/pool environment beyond zinc's class.
Should stainless springs be salt-spray tested?
Occasionally, to verify passivation quality — 304 should survive NSS hundreds of hours unstained. It is a cleanliness check, not a coating-life ranking.
Is "salt fog testing" the same as salt spray testing?
Yes — "salt fog" and "salt spray" refer to the same test (ASTM B117 / ISO 9227): parts exposed to atomized 5% NaCl at 35°C, with results reported as hours to white or red rust. The names are used interchangeably across specs and regions.