ASTM B117 Salt Spray Testing
ASTM B117 Salt Spray (Fog) Testing: The Standard Corrosion Test
For as long as people have worked with metal, corrosion has been a constant enemy. From bridges to ships, boilers to automobiles, rust has been behind some of history’s worst disasters. By the 1930s, industry had learned the hard way that something had to be done. Out of that need came ASTM B117, the world’s most recognized salt spray (fog) test.
Why Corrosion Testing Became Essential: Lessons Written in Rust
The Quebec Bridge Collapses (1907 & 1916)
The Quebec Bridge collapsed twice, killing 88 workers. While poor design played a role, corrosion of bracing and steel members was a contributing factor both times.
The USS Maine Explosion (1898
When the USS Maine exploded in Havana Harbor, 260 sailors were killed and a war began. One theory was corrosion of the coal bunker bulkhead, which may have allowed heat to ignite nearby powder magazines.
The Boston Molasses Flood (1919)
A massive storage tank burst in Boston, flooding the streets with 2.3 million gallons of molasses. Twenty-one people were killed. Later analysis pointed to corrosion in the thin steel plates as a key factor in the failure.
The Steamboat Sultana Explosion (1865)
On the Mississippi River, the Sultana’s corroded boilers exploded, killing nearly 1,800 passengers in the deadliest maritime disaster in U.S. history.
From Trains to Farms: How Transportation Learned the Hard Way
Corrosion wasn’t just a problem for bridges and ships. As America entered the machine age, it showed up everywhere.
Train Boiler Explosions
Railroads suffered repeated boiler failures, many tied to hidden corrosion and thinning riveted seams.
Farm Machinery and Fertilizer Corrosion
Fertilizer ate away at tractor frames and gear housings, leaving farmers frustrated when their new equipment failed too soon.
Road Salt and Cars Rusting Out in the North
When cities began spreading salt on winter roads in the 1920s, cars in northern states sometimes rusted out in just a few winters. Fenders, frames, and suspensions all suffered, fueling lawsuits and public distrust. By the 1930s, automakers had a vested interest in finding reliable ways to test corrosion resistance.
The Origin of ASTM B117 and the Push for a Standard Test
In 1898, a group of railroad engineers and steel makers formed what became the American Society for Testing and Materials (ASTM). Their first mission was to stop brittle rail failures by standardizing steel quality.
By the late 1930s, ASTM had become the nation’s trusted authority for testing standards. Shipbuilders and the U.S. Navy needed accelerated tests for coatings. Automakers needed a way to compare paints and platings before mass production. Industrial manufacturers needed a fair benchmark to prove their machines could hold up.
In 1939, ASTM B117 was published—a simple but powerful salt spray (fog) test that would become the foundation of modern corrosion testing.
Inside the Salt Spray Test: How ASTM B117 Is Performed
The idea is simple: expose test panels to a continuous fog of saltwater in a controlled, heated chamber. The details, however, are exacting.
🔧 Equipment
- A sealed fog chamber built for even fog dispersion
- Salt solution tank connected to atomizing nozzles
- Compressed air, humidified and heated (114–121 °F / 46–49 °C)
🧂 Solution
- 5% sodium chloride (table salt) mixed with ASTM Type IV purified water
- Neutral pH (6.5–7.2), adjusted only with precise lab chemicals
🌫️ Application
- Saltwater solution applied as a fine fog, evenly distributed
- Chamber held at 95 °F (35 °C) during testing
- Samples protected from drips
Why 5% Salt, Type IV Water, and Precise Temperatures Matter
Why 5% Sodium Chloride?
Salt fog chambers had already been tried with different chemicals, but sodium chloride was cheap, abundant, and predictable. At 5% concentration—slightly higher than seawater—it accelerates corrosion fast enough for testing, but not so fast that every coating fails immediately.
Why Type IV Water?
Tap water contains minerals and impurities that either slow down or speed up rust in unpredictable ways. Type IV purified water removes those variables, ensuring the only corrosive force at work is the salt.
Why 35 °C Chamber Temperature?
This “sweet spot” produces visible corrosion in days without instantly destroying every coating. It’s also easy for labs to maintain steadily.
Why 35 °C Chamber Temperature? Why the Humidifier/Bubble Tower at 46–49 °C?
Compressed air cools when sprayed. If humidified at room temperature, the fog would dry out and salt crystals would form. Pre-heating ensures the fog stays wet, fine, and consistent across the chamber.
ASTM B117 Today: Why This Test Still Sets the Standard
While more advanced tests (like cyclic corrosion) have been developed, ASTM B117 remains the baseline. It’s still the first test many coatings, paints, and platings must pass before going into cars, ships, pipelines, or aircraft.
It may be nearly a century old, but B117 endures because it is simple, reliable, and repeatable. It represents an important turning point—when industry and government came together to fight corrosion with science.
Learn More About Salt Fog Testing
Want to see the equipment up close? Explore Auto Technology’s Salt Fog Chamber.
If you need ASTM B117 corrosion testing services, contact us today and we’ll help you prove your materials can stand the test of time.
