AUTO TECHNOLOGY SALT FOG CHAMBERS: ISO 9227 vs ASTM B117
A Brief History of International Salt Fog Standards
Since the beginning of the Industrial Revolution, when iron ships narrowed the oceans and railroads connected distant lands, corrosion prevention has been a global priority. The great wars that followed only deepened that urgency: fleets, aircraft, and armies could not afford to have rust undo what industry had built.
For much of the twentieth century, corrosion testing rested on a scattered framework of national standards. Manufacturers leaned on industry best practices, but there was no universal agreement on how to measure durability. In 1939 the American Society for Testing and Materials introduced ASTM B117, the first formal salt fog method. By the 1950s other nations had followed: the United Kingdom issued BS 1224, Germany published DIN 50021, Japan introduced JIS Z 2371, France established its AFNOR standards, and the Soviet Union created its GOST system. Each one gave structure to its own industries, yet across borders there was still no common ground.
The first real attempt at international standardization came in 1926 with the International Federation of the National Standardizing Associations, which was disbanded during the Second World War. In 1944, the United Nations formed the Standards Coordinating Committee to prepare for postwar cooperation. Two years later, delegates from 25 countries met in London to create a permanent solution. Out of those meetings came the International Organization for Standardization, formally established in Geneva in 1947. Its name was chosen not from English initials but from the Greek word isos, meaning “equal,” so that ISO would be the same in every language.
In the decades that followed, the need for uniform testing only grew. The postwar boom in automobiles, aircraft, and ships demanded common measures of durability. By the 1970s ISO issued its first corrosion test standards: ISO 3768, 3769, and 3770. These were the first steps toward true international alignment. In 1990, ISO withdrew those early documents and replaced them with the first unified edition of ISO 9227.
Through the 1990s, as globalization reshaped supply chains, ISO 9227 became the reference point for salt fog testing outside North America. Today it stands as the international standard, much like ASTM B117 before it, giving manufacturers, laboratories, and industries across the world a single benchmark for evaluating corrosion resistance.
ISO 9227 Overview
ISO 9227 sets out how to create corrosive atmospheres used to evaluate coatings, platings, and materials, but it does not set pass/fail criteria, prescribe exposure times, or dictate how results are judged. Those details are left to the governing product standards and OEM specifications.
ISO 9227 is broader and less prescriptive than ASTM B117. Where B117 includes detail on chamber construction and operation, ISO 9227 emphasizes the achievement of three environments: Neutral Salt Spray (NSS), Acetic Acid Salt Spray (AASS), and Copper-accelerated Acetic Acid Salt Spray (CASS). Together, these environments form the backbone of international salt spray testing.
The standard defines the work simply:
“Accelerated corrosion tests by exposing specimens to a spray (fog) of a salt solution in specified conditions.”
In practice, ISO 9227 creates harsh, continuous exposures that reveal pores, defects, and coating weaknesses long before they would show up in real-world service. The results do not predict outdoor life or rank different materials, but they provide a powerful yardstick for comparison, quality control, and qualification.
Where ASTM divides its practices into separate documents, B117 for NSS, G85 for acidified spray, and B368 for copper-accelerated testing, ISO gathered them into one unified framework. That decision gave labs and manufacturers around the world a single international reference.
NSS is the baseline: a neutral salt fog used for metals, coatings, conversion coatings, anodic oxide coatings, and organic coatings on metals.
AASS introduces acetic acid to the salt fog, making it especially useful for decorative copper-nickel-chromium and nickel-chromium coatings, as well as aluminum coatings and finishes.
CASS builds on the AASS solution with the addition of copper(II) chloride dihydrate at 0.26 g/L ± 0.02 g/L. The collected spray must have a pH between 3.1 and 3.3 at 25 °C ± 2 °C.
ISO 9227 made these three tests the common language of corrosion labs worldwide.
Equipment and Configuration
Chamber Construction
ISO 9227 does not tell you how to build a chamber from the ground up. Instead, it sets non-negotiables: the chamber must be made of inert materials that will not alter the fog, the spray cannot strike specimens directly, and the ceiling must be shaped so condensate never drips onto panels.
“The materials used in the construction of the test cabinet shall be such that they will not affect the corrosivity of the sprayed solutions. The solution shall not be sprayed directly onto the test specimens. The upper parts of the cabinet shall be so shaped as to prevent drops of solution falling onto the test specimens.”
— ISO 9227:2022
Auto Technology chambers are built exactly this way, with corrosion-proof interiors and carefully sloped lids that keep the fog uniform and uncontaminated. These design details are why our Salt Fog & Humidity chambers remain the trusted backbone of labs that need ISO 9227 compliance.
Fog Generation and Air Handling
Unlike ASTM B117, which specifies saturator towers and other hardware, ISO 9227 cares only about the result: a fine, uniform fog created by atomizing salt solution with clean, oil-free, humidified air. The standard provides guiding values for saturator temperatures and atomizer pressures, and it requires distilled or deionized water with conductivity ≤ 20 µS/cm both for solution preparation and for humidification.
Auto Technology uses a proven bubble tower design, visible, reliable, and time-tested, to keep the air stream clean and fully humidified. The result is a fog that remains stable not just for a day, but for months of continuous testing.
Temperature Control
The exposure zone must be kept at:
35 ± 2 °C for NSS and AASS
50 ± 2 °C for CASS
ISO does not prescribe how you achieve those temperatures. Auto Technology relies on water-jacket heating, the very method these standards were originally designed around. For over 75 years, it has proven the most stable and uniform approach, and with our high-temperature package, chambers can reliably maintain setpoints above 40 °C for long runs.
Collection
Spray homogeneity is checked with collection funnels. During a test, at least two collectors must be placed in the specimen zone. For verifying a cabinet, ISO requires at least one collector per 400 × 400 mm area of floor space, ensuring uniformity across the zone.
Environmental Discharge
ISO 9227 also states:
“Preference shall be given to apparatus that has a means for properly dealing with fog after the test, prior to releasing it from the building for environmental conservation, and for diluting salt solution prior to discharging it to the drainage system.”
Auto Technology chambers are equipped with multiple solutions to meet this requirement, including ventilation and purging systems as well as a scrubber / jet exhaust dilution system that neutralizes fog before discharge. These features ensure compliance while protecting both operators and the environment.
Testing Solutions
All ISO 9227 tests begin with the same base: a 5 percent sodium chloride solution prepared with high-purity water. The collected spray in the chamber must contain 50 g/L ± 5 g/L of NaCl. ISO does not require filtration, but Auto Technology chambers include it as standard to prevent nozzle blockage and keep fog conditions consistent.
Neutral Salt Spray (NSS): Uses the base solution directly, creating a neutral fog that serves as the foundation of salt spray testing.
Acetic Acid Salt Spray (AASS): The sodium chloride solution is adjusted with acetic acid until the fog reaches a pH of 3.1–3.3. ISO allows either glacial acetic acid, which is nearly pure, or pre-diluted acetic acid, provided it remains stronger than 10 percent by mass.
Copper-accelerated Acetic Acid Salt Spray (CASS): Builds on the AASS solution with the addition of copper(II) chloride dihydrate at 0.26 g/L ± 0.02 g/L. The collected spray must have a pH between 3.1 and 3.3 at 25 °C ± 2 °C.
How the Practice is Run
Prepare specimens according to the governing product or coating standard.
Load and arrange them in the cabinet at the correct angle, with collectors placed as required.
Run the fog with NSS, AASS, or CASS solution at the prescribed temperature.
Monitor daily for pH, NaCl concentration or density, collection rate, and temperature.
Continue exposure for the duration set by the product standard or OEM requirement, from 24 hours to well over 1,000.
Evaluate specimens in line with the method specified by the relevant product or OEM specification.
With the right chamber, running ISO 9227 is straightforward
ISO 9227 is focused on producing three salt spray environments, NSS, AASS, and CASS, under clearly specified conditions. It does not dictate chamber construction in detail, but it does demand that the fog, chemistry, temperature, and collection rates be maintained with precision. Achieving those conditions test after test is what separates a compliant chamber from an unreliable one.
For more than 75 years, Auto Technology has refined its Salt Fog & Humidity chambers to do exactly that. Our systems deliver the exact and repeatable environments ISO 9227 calls for, while also providing safe and compliant ways to handle exhaust and waste. That gives you confidence that your results are valid and defensible, whether for internal quality checks, customer approvals, or regulatory compliance.
If you need to run ISO 9227, start with a Salt Spray (Fog) chamber designed to the standard. Because at the end of the day, Auto Technology corrosion chambers are the standard.
If you’d like to learn more about selecting the right chamber or explore options for having testing done in our accredited ATC Test Lab, our team is available to guide you through the process.