⚡ Key takeaways
- Salt spray is an accelerated test, not a life prediction. 1,000 hours in a salt spray cabinet is not 1,000 hours outdoors.
- Three flavours: NSS (neutral, the workhorse), AASS (acetic acid) and CASS (copper-accelerated, roughly 8× faster).
- Judge on red rust area, not on the first rust spot. Five percent red rust is the usual fail line.
- Rough correlation to service: 24 h NSS is often compared with about a year indoors or half a year outdoors.
- Do not chase the longest salt spray number. Over-specifying wastes money; match the real environment.
- Coating damage collapses salt spray performance. Transport and installation scratches are real, so test scribed samples.
- Wet–dry cycling is harsher than continuous spray. Real weather alternates, and UV and temperature swings add to it.
1. The 720 h report that still rusted
2. The three salt spray tests
3. Salt spray hours versus service life
4. Pass and fail criteria
5. Coating choice by environment
6. Common misconceptions
7. Decision tree and FAQ
1. The 720 h report that still rusted
A customer held a report stating "720 h salt spray, no red rust". Three months after installation on the coast the parts had rusted. Why?
Three reasons:
- The cabinet sprays continuously, while the outdoors alternates wet and dry and adds UV and temperature cycling — a much harsher combination.
- Transport and installation scratched the coating, and a scratch is exactly what an unscribed cabinet test cannot see.
- The report was written on an unscribed sample. The real product was scribed.
This is why the automotive industry now asks for cyclic corrosion testing (CCT) rather than plain NSS: the sample is scribed and the chamber cycles between salt fog, dry, humidity and sometimes freeze.
2. The three salt spray tests
| Type | Solution | pH | Speed | Typical use |
|---|---|---|---|---|
| NSS — neutral salt spray | 5 % NaCl | 6.5–7.2 | 1× | The default for most fasteners |
| AASS — acetic acid salt spray | NaCl + acetic acid | 3.1–3.3 | 3× | Decorative plating |
| CASS — copper-accelerated | NaCl + CuCl₂ + acetic acid | 3.1–3.3 | 8× | Rapid screening |
Standard references are ASTM B117 and ISO 9227 (GB/T 10125 is the Chinese equivalent). Chamber temperature is 35 °C for NSS and AASS, and 50 °C for CASS. The salt concentration is 50 g/L ± 5 g/L and the collection rate is 1–2 mL/h per 80 cm².
3. Salt spray hours versus service life
| Environment | Recommended NSS | Rough outdoor equivalent |
|---|---|---|
| Indoor, dry | 24 h | Several years without red rust |
| General outdoor | 240 h | 2–3 years |
| Coastal or chemical plant | 720 h and above | 5–10 years |
| Buried or submerged | 1,000 h and above | 10 years and beyond |
Treat the right-hand column as a rule of thumb for comparing specifications, never as a warranty. The conversion factor depends on climate, sheltering, pollutant load and how often the part gets wet. A part under a car bonnet and a part on a seaside handrail can both be "outdoor" and differ by an order of magnitude.
4. Pass and fail criteria
Inspect after the test:
- Red rust (iron oxide): the substrate itself is corroding — a failure for a coated steel part.
- White rust (zinc corrosion product): the coating is being consumed but the substrate is intact — generally acceptable, though it indicates the coating is working through its life.
- Red rust area of 5 %: the usual failure threshold in purchase specifications.
- Blistering, cracking or flaking of the coating is a failure regardless of rust area.
Write the criterion into the drawing. "Salt spray 480 h" alone is ambiguous: 480 h to the first red spot and 480 h with less than 5 % red rust are very different requirements, and one of them costs roughly twice as much to meet.
5. Coating choice by environment
| Environment | Recommended coating | Salt spray expectation |
|---|---|---|
| Indoor cabinets and electronics | Blue-white zinc plating | 24–48 h |
| General outdoor | Zinc plating with trivalent passivation, or zinc flake | 240 h |
| Coastal | Zinc flake (Geomet/Dacromet type) or 316 stainless | 720 h and above |
| Automotive underbody | Zinc flake with topcoat, chromium-free | 720 h and above, plus cyclic test |
| Aluminium parts | Anodising, sealed | 240–500 h depending on seal |
Note that chromium-free zinc flake has largely replaced hexavalent chromium systems under ELV and RoHS. It costs more per kilogram but delivers the same or better corrosion resistance at a much thinner film, which matters for thread fit.
6. Common misconceptions
7. Decision tree and FAQ
Step 1: where does it go?
Indoor and dry → 24 h. General outdoor → 240 h. Coastal, chemical or underbody → 720 h and above, and test to a cyclic standard.
Step 2: what is the substrate?
Steel → zinc plating or zinc flake. Aluminium → anodising with a sealed film. Stainless → passivation, then verify with a salt spray or blue spot test.
Step 3: does it need a scribed test?
Safety-relevant or outdoor parts → yes, scribed CCT. Non-critical indoor parts → unscribed NSS is enough.
FAQ
Q: How long does 1,000 h of salt spray actually take? NSS at 1,000 h is 42 days of continuous running. CASS at the same corrosion effect takes about five days, which is why it is used for screening rather than qualification.
Q: Why did my part pass salt spray but rust in service? Scratches from transport, installation stress, galvanic coupling to a dissimilar metal, and a service environment with more wet–dry cycling than the cabinet all push real corrosion beyond the test result.
Q: How long should 316 be tested? 316 usually runs 2,000 h and beyond in NSS without red rust, but pitting is the real risk and NSS is poor at ranking it. Use ASTM G48 ferric chloride testing for pitting and crevice resistance.
Q: Does passivation change the salt spray result? Yes, substantially. Removing free iron from the surface can take a 304 part from failing at 96 h to passing 480 h, with no change in material or geometry.
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