ISO 9227 and ASTM B117 — neutral salt spray (NSS) testing

Scope — and what it does not cover

ISO 9227 and ASTM B117 describe how to run a salt-spray cabinet: solution concentration, pH, chamber temperature, spray collection rate, specimen orientation and reporting. ISO 9227 also defines the acetic acid (AASS) and copper-accelerated (CASS) variants. Both documents define an ENVIRONMENT. Neither sets a pass/fail duration for your coating, and neither claims a correlation between cabinet hours and years in service — the acceptance rule has to come from your drawing, the customer specification or the control plan.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1The test method and the acceptance criterion are two different sentences. "ISO 9227 NSS, 240 h, no red rust on significant surfaces" is a specification; "salt spray 240 hours" is not.
• 2White corrosion (the zinc corroding) and red rust (the steel corroding) are different events at very different times. State which one is being judged.
• 3Specimen preparation changes the result: as-received or degreased, threads masked or exposed, cut faces and edges, scribed or unscribed.
• 4The two documents differ in small but real ways — collection rate and specimen angle among them — so hours from a B117 cabinet and an ISO 9227 cabinet are not automatically interchangeable.
• 5Results are comparative quality control. Two lots run in the same cabinet to the same criterion can be compared; a number quoted from another laboratory generally cannot.
• 6Passivation and sealer dominate time to white corrosion; zinc thickness dominates time to red rust. If the requirement is red rust, thickness is the lever.

What to write on the drawing or PO

• Method and variant: ISO 9227 NSS (or AASS / CASS), or ASTM B117.
• Total exposure hours and the inspection interval.
• The acceptance criterion in words: white corrosion or red rust, on which surfaces, and how much is allowed.
• Specimen count, preparation, masking, and whether a scribe is required.
• Reporting: photographs, the time of first corrosion, and whether a report is issued per lot.

What goes wrong when it is cited loosely

• Writing "salt spray 480 h" with no criterion, then arguing after the test about what counted as failure.
• Converting cabinet hours into years of outdoor service life.
• Comparing an unsealed clear passivate against a sealed one on hours alone.
• Testing threaded parts without deciding whether threads and cut ends are significant surfaces — they are where corrosion starts.