What is the difference between ASTM B117 and ISO 9227?

When salt spray testing is requested, one of two names usually appears in the specification: ASTM B117 or ISO 9227. Both define chamber tests examining the accelerated corrosion behaviour of coated and metallic surfaces in a salt-laden mist. Not knowing the difference leads to a correctly performed test being rejected by the customer.

What do both do?

Both standards define exposing the specimen to a mist of salt solution in a closed chamber, controlling the chamber conditions and inspecting at defined intervals. The result is not a life prediction; it is for comparing different coating systems under the same conditions and monitoring production consistency.

The fundamental distinction

Which one should I ask for?

• If the customer specification names a standard, there is no debate: that standard is applied.
• If the target market is the United States and the supply chain works in the ASTM language, ASTM B117 is expected.
• If conformity to a product standard of EU origin is to be shown, ISO 9227 is more often required.
• If an acetic acid or copper-accelerated variant is needed, ISO 9227 covers those methods directly.
• In automotive, neither is often sufficient: cyclic corrosion specifications such as VDA 233-102 or SAE J2334 are required.

A common mistake: comparing results

The neutral salt spray methods of ASTM B117 and ISO 9227 are close to one another but differ in the details of solution preparation, pH control and reporting. Comparing two results obtained under different standards directly is misleading. If a comparison is to be made on a change of supplier, the same standard, the same duration and the same specimen preparation method must be used.

The difference between constant salt spray and cyclic corrosion

Both standards define a constant-condition exposure: the chamber produces mist continuously. In the real environment, wetting and drying alternate. For this reason, particularly in automotive, cyclic corrosion specifications including wet-dry cycles are preferred over constant salt spray. The constant test is still valuable but its relationship to the field is weaker.

Why doesn't the report say "pass/fail"?

A salt spray report is not a conformity decision but a RECORD OF OBSERVATION, and not knowing this leads to the report being read wrongly. The laboratory writes the chamber conditions, the exposure duration and the findings of the inspections made at the defined intervals: is there blistering on the surface, has corrosion advanced from the scribe, is red rust visible on the base metal. The text that decides whether those findings are acceptable is the product specification.

For this reason, three pieces of information must be written together when placing a salt spray request: the standard to be applied, the exposure duration and the acceptance criterion. When one of the three is missing, the laboratory performs the test but the report cannot be assessed by the customer; the process returns with a second round of correspondence.

On what basis is the assessment made?

Rating corrosion findings is the subject of separate standards and does not appear within the salt spray standard. A separate series of standards is used for rating blistering, cracking, flaking and rusting on coated surfaces; measuring creep from a scribe on paint systems has its own method. If the specification says "480 hours to ISO 9227" but does not name the rating standard, the two parties can look at the same specimen and write different results.

Specimen preparation directly changes the result

• Cut edges are the first place where corrosion starts; whether the edge is to be masked must be written in the specification, otherwise two laboratories will give different results.
• The angle of the specimen in the chamber is defined in the standard and affects mist accumulation; a different angle means a different exposure.
• If the coating thickness is not measured and reported before the test, it cannot be distinguished whether a poor result comes from insufficient coating or from the system itself.
• How many specimens are taken from the same panel and how the results are to be combined must be established; a decision made from a single specimen is statistically weak.
• The surface cleaning method before the test affects the result; oil residue can behave like a protective film and improve the result.

Can the result be converted into field life?

It cannot, and this is the most widespread false belief in the field. Conversion tables of the form "1,000 hours of salt spray = 10 years of field life" come from no standard. A constant salt spray environment does not imitate the real atmosphere: in the real environment wetting and drying alternate, the composition of contaminants differs, and there is ultraviolet exposure and temperature fluctuation. The valid use of the test is COMPARISON: it shows which of two coating systems is more resistant and whether production stays consistent from lot to lot.