How it works
ASTM B117, the Standard Practice for Operating Salt Spray (Fog) Apparatus, defines a chamber in which a 5 % sodium chloride solution is atomized into a continuous fog at 35 °C. Specimens are supported at an angle in the fog, often with a scribe cut through the coating to the steel. After a stated number of hours, they are rated for blistering (ASTM D714), rusting (ASTM D610) and creep from the scribe (ASTM D1654), or under the ISO 4628 series.
B117 standardizes the apparatus and the exposure conditions. It does not set the specimen preparation, the test duration or the pass criteria: those come from the product specification or the purchaser. A report that quotes a number of B117 hours without stating the substrate, surface preparation, film thickness, scribe and rating method says little.
ISO 9227
ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests, defines three variants: neutral salt spray (NSS), acetic acid salt spray (AASS) and copper-accelerated acetic acid salt spray (CASS). The acidified variants are mainly used for decorative and metallic finishes; NSS is the one paint specifications cite and is the counterpart of B117. ISO 12944-6 uses ISO 9227 NSS among its laboratory tests for protective paint systems.
Limits of correlation with the field
The chamber is wet all the time, at constant temperature, with no UV, no drying phase and no pollutants other than neutral chloride. Real atmospheres alternate wetting and drying, concentrate salts as surfaces dry, cycle in temperature and degrade binders with sunlight. Different coatings respond differently to those missing mechanisms, so B117 can rank them in an order the field does not confirm; zinc-rich primers are a known example. ASTM B117 itself states that correlation of results with performance in natural environments can seldom be predicted.
The practical use is therefore narrow: quality control, comparison between closely similar products, and screening. Converting chamber hours into years of service has no defensible basis. For durability claims, specifications rely on cyclic corrosion tests matched to the corrosivity category and on field history. The ASTM B117 article covers how to read a salt spray result in a submittal.