THE DEFINITIVE GUIDE TO ASTM B117 TESTING
Introduction to The Salt Spray (Fog) Test
ASTM B117 has been the backbone of accelerated corrosion testing since 1939 (and it’s the predecessor to today’s ISO 9227). Known sometimes as the salt spray test or the salt fog test, it remains the most widely cited corrosion test in history.
Why the two names? Because the practice calls for a spray of salt solution to create a fine fog. The spray is the mechanism; the fog is the atmosphere.
It is important to understand that ASTM B117 is not really a “test” at all. As the title states, it is a “Standard Practice for Operating Salt Spray (Fog) Apparatus.” Without the right chamber, built the right way, you cannot run B117.
Purpose of the Practice
The official scope of ASTM B117 explains:
“This practice provides a controlled corrosive environment which has been utilized to produce relative corrosion resistance information for specimens of metals and coated metals exposed in a given test chamber.”
In other words, B117 provides a common yardstick. It creates a continuous, aggressive fog so that coatings, platings, or materials can be compared under the same conditions.
The practice is also clear about its limits:
“The results obtained from the practice are not intended to be taken as a direct guide to the corrosion resistance of the tested materials in all environments where these materials might be used.”
And just as important:
“This practice does not prescribe the type of test specimen, the exposure periods, or the interpretation of results.”
These three statements are the backbone of B117. It does not predict outdoor service life, and it does not define pass or fail. It defines the environment. Other standards and OEMs decide how long specimens are exposed and how results are interpreted.
ASTM B117 Chamber Construction
Running ASTM B117 correctly depends on the right chamber, built to the exact requirements of the standard. The practice does not simply say “create a fog”; it specifies how the apparatus must be constructed and operated.
ASTM B117 requires:
“The materials used in the construction of the salt spray exposure chamber shall be such that they will not affect the corrosiveness of the test specimen.”
The chamber must be made of inert, corrosion-resistant materials. The ceiling must be sloped so condensate cannot drip on the specimens.
Auto Technology chambers are built this way, with robust construction and a sloped lid that prevents drips from spoiling results.
Fog Generation: Nozzles and Air Handling
B117 specifies:
“The atomization of the salt solution shall be accomplished by means of one or more atomizers located at the bottom of the exposure zone of the cabinet.”
And:
“Compressed air introduced into the atomizers shall be passed through a saturator tower containing heated water to ensure the air is saturated with moisture.”
This “saturator tower,” often called the bubble tower, is a column where compressed air is forced through heated water before reaching the nozzle. The bubbling process ensures the air is nearly 100% humidified, so when it meets the salt solution, the result is a fine, uniform fog instead of a coarse spray.
Without the bubble tower, fog quality breaks down. Droplets become too large, wetting is uneven, and the test loses validity. With it, B117 conditions are maintained exactly as written.
Auto Technology chambers use a precision-engineered bubble tower with controlled heating, reliable level management, and durable construction. Our design maintains fog consistency for weeks or months of continuous operation, so your results on day 500 look just like they did on day 5.
Heating and Temperature Control
According to the practice:
“The temperature within the exposure zone of the cabinet shall be maintained at 35 ± 2 °C (95 ± 3 °F).”
The practice also notes in its construction appendix:
“The maintenance of temperature within the salt chamber can be accomplished by … surrounding the basic chamber by a jacket containing water…”
The water jacket has been the trusted method for decades. By surrounding the chamber in a heated water envelope, temperature remains uniform and stable. Auto Technology chambers continue this approach, the same one described in the practice, because it delivers reliable performance across long test runs.
The Salt Solution
ASTM B117 defines the test solution as:
“A solution of 5 ± 1 mass % of sodium chloride in water, with the pH adjusted to be between 6.5 and 7.2 when measured at 25 °C (77 °F).”
On paper, this sounds simple. In practice, it is one of the hardest parts of the test. Evaporation, contamination, and operator error can all push the concentration or pH out of range, and once that happens, the test is no longer valid.
Auto Technology chambers solve this with a sparging and filtering system. The sparger keeps the solution mixed, while the filtration removes impurities before they reach the nozzle. The result is a clean, stable salt solution delivered hour after hour without drift.
With the Right Chamber, Running B117 is Straightforward
- Prepare your specimens. Clean them per ASTM D609 or equivalent, scribe coated panels if required.
- Load them. Mount specimens at the correct angle (15–30°).
- Start the fog. A steady stream of humidified salt solution at 35 °C fills the chamber.
- Monitor conditions daily. Check fallout rate, pH, and concentration.
- Run for the required duration. The time is determined by the governing specification, not B117 itself.
- Remove and evaluate. Rinse, dry, and inspect according to the applicable coating or material standard.
A chamber running B117 has a rhythm of its own. The bubble tower gurgles, the nozzle whispers, and a fine mist drifts across the panels. It is the same sound laboratories have relied on for more than 80 years.
Conclusion
ASTM B117 endures because it is simple, repeatable, and universally understood. But it only works when the chamber is built to the letter of the standard.
That is why Auto Technology has spent decades refining salt fog chambers. From corrosion-proof construction to precision nozzles, from water jacket heating to sparging and filtration, our chambers are the backbone of reliable corrosion testing.
If you need to run B117, whether for internal QC, customer qualification, or compliance, start with a Salt Spray (Fog) chamber built to the standard. Because at the end of the day, the chamber is the standard.
If you’d like to learn more about selecting the right ASTM B117 chamber or explore options for having testing done in our accredited ATC Test Lab, our team is available to guide you through the process.