ASTM B117 Neutral Salt Spray: What the Hours Mean
Key facts
| Parameter | Value |
|---|---|
| Salt solution | 5% NaCl by mass |
| Chamber temperature | 35 °C (95 °F) |
| Solution pH | 6.5–7.2 |
| Fog collection rate | 1.0–2.0 mL/h per 80 cm² |
| Exposure | Continuous, no drying phase |
| Result | Hours until the failure criterion in the specification |
What the test is
ASTM B117 is the most widely quoted accelerated corrosion test. Panels sit in a closed chamber filled with a fine neutral salt fog, and are inspected at set intervals until they reach the failure criterion agreed in the specification — for example, the first appearance of red rust or a set amount of blistering.
How it is run
Coated panels, usually scribed through to the substrate, are placed at an angle in the chamber. The salt solution is atomised continuously, the chamber is held at 35 °C, and the fog collection rate and pH are checked to keep the conditions inside the standard.
Reading the result
A result is only meaningful with four details attached: the standard, the substrate, the film thickness and the failure criterion. “≥ 2,000 h, no red rust on the flat surface” and “1,000 h, rating 9 at the scribe” describe different things even if the hours look comparable.
Limits
Continuous salt fog does not reproduce wet–dry cycling, UV, dust or pollutant gases. Coatings that perform well in B117 can rank differently in the field, which is why cyclic corrosion tests are increasingly used alongside it.
Reading a supplier report
Check which standard was used. GB/T 10125 and ISO 9227 neutral salt spray are closely aligned with B117 in principle, but a report is only comparable with another if the method, substrate, film thickness and failure criterion all match. PG system results on this site always state the standard they were tested to.
How many salt spray hours equal one year outdoors?
There is no reliable conversion. Continuous fog never dries, while real sites cycle between wet and dry, so the ranking of coatings can change in the field.
Is 2,000 hours better than 1,200 hours?
Only if both were tested to the same standard, on the same substrate and film thickness, and judged by the same failure criterion.
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