What is ASTM B117?
ASTM B117 ("Standard Practice for Operating Salt Spray (Fog) Apparatus") defines a controlled corrosive exposure: coated or bare test panels sit in a chamber filled with a continuous fog of 5 % sodium chloride solution at 35 °C. Panels are inspected for rust creep from a scribe, blistering, and other degradation after a stated number of hours. The standard itself is careful about what this means — it standardizes the chamber, not the interpretation. B117 produces comparable exposures; it does not promise that chamber hours correspond to field years.
That restraint is written into the standard and routinely ignored by the way datasheets get read. So the question worth answering is not "how many hours is good?" but "what can a B117 number legitimately tell me?"
What the test does well
Used as designed, B117 is a genuinely useful tool:
- Ranking under identical conditions. Two coating systems, prepared the same way and exposed side by side, can be meaningfully compared. If system A shows 2 mm of scribe creep when system B shows 8 mm, that difference is real information.
- Batch and process control. A product that has historically reached a given performance level and suddenly fails early is telling you something about the batch, the surface preparation or the application — B117 is a sensitive alarm bell.
- Detecting gross formulation problems. A coating that blisters or delaminates in a few hundred hours has a defect no field exposure needed to reveal.
- Qualification gatekeeping. Many specifications use a B117 threshold as an entry requirement — a filter that screens out weak candidates before more representative testing.
Why hours do not convert to years
The chamber differs from almost every real service environment in ways that change the corrosion mechanism, not just its speed:
| Chamber condition (B117) | Real atmospheric service | Consequence |
|---|---|---|
| Continuous wetness, no drying | Wet-dry cycling — rain, dew, sun | Drying cycles concentrate salts and drive different degradation; some coatings that excel wet-only perform worse in cycling, and vice versa |
| Constant 35 °C | Daily and seasonal temperature swings | No thermal stress, no condensation cycling in the chamber |
| Neutral 5 % NaCl only | SO₂, industrial pollutants, UV, abrasion | Entire degradation mechanisms absent from the test |
| No UV exposure | Sunlight degrades many binders | A UV-sensitive coating can post excellent B117 numbers and chalk away outdoors |
The consequence is well established in the industry: correlation between B117 hours and field life is poor and inconsistent across coating chemistries. Zinc-rich systems, for example, are notoriously mis-ranked by continuous salt fog relative to their excellent field performance. This is exactly why modern specification frameworks — including ISO 12944, which qualifies systems by corrosivity category and durability range — rely on cyclic test protocols (alternating salt spray, drying and UV or condensation phases) rather than raw B117 hours for durability claims.
How to read "hours" on a datasheet
A practical reading grid for the number every datasheet advertises:
| What you see | What to ask |
|---|---|
| "3,000 h salt spray (ASTM B117)" | At what result? Hours mean nothing without the acceptance criterion — scribe creep in mm, blistering rating, rust grade |
| A bigger number than a competitor | Same substrate, same surface preparation, same DFT, same scribe, same criterion? If not, the comparison is void |
| Hours presented as service life | Decline the conversion. Ask instead for cyclic test results (e.g. ISO 12944-6/9 protocols) or documented field history in a comparable environment |
| An impressive number on a UV-exposed application | B117 has no UV. Ask for weathering data separately |
For an actual purchasing decision, the hierarchy of evidence runs: documented field history in your environment, then cyclic accelerated testing under a recognized protocol, then B117 — as a comparator and screen, never as the verdict. Coating service life is ultimately governed by the full system: surface preparation grade, film thickness, application quality and exposure — the subject of our article on epoxy coating service life.
FAQ
How many B117 hours equal one year of service? There is no defensible conversion factor. The correlation varies by coating chemistry, exposure type and failure mode — any single number offered is marketing, not engineering.
Is a coating with 5,000 h salt spray better than one with 3,000 h? Only if both numbers come from comparable tests: same substrate, preparation, thickness, scribe and acceptance criterion. Under those conditions the ranking is meaningful for wet, chloride-driven exposure — with the usual caution for chemistries the test mis-ranks.
Why do specifications still require B117 if it predicts so little? Because it is cheap, universally available, reproducible, and effective as a screen and a quality-control alarm. Those are real virtues — the error is only in promoting it from filter to forecast.
What should I require instead for durability claims? Cyclic corrosion protocols aligned with your corrosivity category (the ISO 12944 framework), and field references in service conditions resembling yours. Keep B117 as an entry gate.
Does B117 apply to bare metals and platings too? Yes — the practice covers coated and uncoated specimens, and is widely used for plated fasteners and hardware. The interpretation limits are the same.
Selecting a coating on evidence, not hours
Induscoat Canada selects and applies coating systems against the documented exposure of your equipment — corrosivity category, immersion or atmospheric service, abrasion — with surface preparation and QC records that make the datasheet promises mean something. Tell us about your environment through our corrosion protection services or request a quote; we respond within 24 business hours.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat Canada. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.