Salt Spray vs Cyclic Corrosion Testing
Key facts
| Aspect | Continuous salt spray | Cyclic corrosion test |
|---|---|---|
| Exposure | Constant salt fog | Salt, dry and humid phases in a repeating cycle |
| Examples | ASTM B117, ISO 9227 NSS, GB/T 10125 | ASTM G85 A5, ISO 11997, SAE J2334, ISO 12944-9 |
| Field correlation | Weak — rankings can change outdoors | Better, especially for wet–dry sites |
| Duration and cost | Simple and widely available | Longer cycles, fewer laboratories |
What continuous fog misses
Outdoors, a coil is wet at night and dry at midday, salty after an onshore wind and rinsed after rain. Corrosion products form, dry, crack and re-wet. A continuous fog never dries, so corrosion develops differently, and coatings that do well in it can rank differently in the field.
How cyclic tests work
A cyclic test runs a fixed sequence — for example salt fog, then a dry-off at raised temperature, then a humid phase — and repeats it for a set number of cycles. Some tests add UV or low-temperature steps to reproduce offshore or automotive conditions.
Reading the result
Results are reported in cycles or total hours, with the method named. Compare only results from the same method, substrate, film thickness and assessment criterion.
Limits
Cyclic tests take longer, cost more and are offered by fewer laboratories. There is no single cyclic test for HVAC coils, so specifications have to name the one they want.
Should a coil specification require a cyclic test?
For critical projects in C5 or CX sites it is worth asking for. For most projects, neutral salt spray to a named standard plus a heat-transfer result is the practical minimum.
Do PG systems have cyclic test data?
No. Current PG data is from continuous salt spray tests, listed in the test data library with the standard behind each result.
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