ASTM B117 is the most widely referenced salt spray standard in the world — but it only defines the apparatus and procedure, not the pass/fail criteria. Understanding what the standard does and does not specify is essential before running or commissioning a test.
ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus, defines the construction and operation of the salt spray chamber — the concentration of the salt solution, the test temperature, the fog collection rate, and the nozzle and baffle geometry. It does not define how long to test or what constitutes a failure.
Pass/fail criteria are always set by a separate material standard, product specification, or OEM requirement that references ASTM B117. When a customer asks for "500 hours salt spray," the 500h and the acceptance criterion come from their spec — ASTM B117 only defines the test environment.
Key Parameters — ASTM B117
NaCl concentration5 ± 0.1 wt%
Solution pH (at 25°C)6.5 – 7.2
Fog zone temperature35 ± 1°C
Reservoir temperature (max)47°C
Collection rate1.0 – 2.0 mL/80cm²/hr
Exposure typeContinuous
Specimen angle15 – 30° from vertical
Water purityDistilled or deionised
Equipment Requirements
What the Standard Requires of the Chamber
ASTM B117 places specific requirements on the salt spray apparatus itself, not just the test conditions. The chamber interior and all wetted components must be constructed of materials that do not affect the corrosivity of the salt solution — typically fibreglass, PVC, or polypropylene.
A baffle must deflect the spray nozzle output so that fog falls uniformly across the specimen zone without direct impingement. Collection funnels must be positioned near each specimen grouping to allow periodic verification of the collection rate (1.0–2.0 mL/80cm²/hr).
Important distinction
ASTM B117 is the apparatus and procedure standard. Individual product standards (ASTM B633, B545, A123, OEM specs) specify the duration and the acceptance criteria — they reference ASTM B117 for how to run the test.
Specimen Preparation
Preparation and Placement Requirements
Specimen preparation is critical to test validity. All surfaces must be free of oil, grease, and temporary protective media prior to test commencement. Cleaning should use a non-reactive solvent — acetone or isopropyl alcohol for metal substrates — and specimens must be handled with clean gloves after cleaning.
Flat panels must be mounted at 15–30° from vertical, with the longer axis parallel to the horizontal.
Specimens must not touch each other or the chamber walls — minimum 30mm clearance.
Cut edges and drilled holes that are not part of the evaluation area must be masked with an inert sealant.
Scribing (cross-cut through coating to bare metal) is required by some product standards — scribe location and tool must match the referencing standard.
Multiple specimens of the same material must be tested simultaneously and receive the same exposure — do not mix different substrates in the same test run if cross-contamination is a concern.
Results
Reading ASTM B117 Test Results
Inspection intervals and the metrics to record are set by the referencing product standard. Common measurements include: time to first rust (TTR), percentage of specimen area showing corrosion, type of corrosion (white rust, red rust, blistering, undercutting from scribe), and creep distance from scribe line.
Representative product standards that specify ASTM B117 as their test method:
ASTM B633Electrodeposited zinc — 96h to 240h by service class
ASTM B545Electrodeposited tin — 6h to 24h
ASTM A123Hot-dip galvanised steel — acceptance criteria vary by grade
ASTM A153Zinc coating on hardware — bolt and fastener specification
Ford FLTM BI 106-01Ford exterior paint scribe/creep test
Known Limitations
Known Limitations of ASTM B117
—Does not correlate reliably with real-world outdoor service life — duration-to-failure in the chamber does not predict years of field life.
—No wet/dry cycling or UV exposure — both of which accelerate real-world corrosion. Cyclic corrosion tests (ISO 11997, SAE J2334) are more predictive for automotive bodywork.
—Limited relevance for aluminium alloys — CASS (ISO 9227) is preferred for anodised aluminium and aluminium castings.
—Does not simulate crevice corrosion geometry — specimens on open racks do not replicate joint or fastener stack-up conditions.
—Pass/fail criteria are not defined within ASTM B117 — the duration and acceptance criteria must come from the material or product standard that references it.
