Key takeaways
- A salt spray result is a time to a defined appearance change under one set of conditions, not a prediction of years in service.
- Specify the method, the duration, the acceptance criterion and the coating thickness, or the hours have no context.
- The failure mode matters as much as the time: white corrosion, red corrosion, blistering and tarnish are different results.
- Trend results by lot and by plating supplier; a sudden drop at the same thickness points to a process change.
What the test actually does
A salt spray test places a plated or coated sample in a closed chamber with a continuous fog of salt solution. It accelerates corrosion so that two finishes, or two processes, can be compared in a short time. It does not reproduce the conditions of a real installation.
The result is a time to a defined appearance change under one set of conditions. That makes it useful for process control and for ranking. It is weak as a prediction of years in service, because the chamber holds one aggressive condition while a real part sees wet and dry cycles, pollution, heat and mechanical damage.
Neutral salt spray in the cabinet
Neutral salt spray per ASTM B117 uses a sodium chloride solution, a controlled pH, a chamber temperature around 35 degrees Celsius and a continuous fog. ISO 9227 contains an equivalent neutral salt spray method. The test report should name the method, the duration and the acceptance criteria.
The chamber has to be qualified: solution concentration, pH, temperature, salt fog deposition rate and the collection of the fall-out. If those drift, the hours mean nothing. Ask for the chamber log, not only the certificate.
What B117 does not tell you
B117 does not model UV, humidity cycling, vibration, temperature cycling, chemical exposure or galvanic coupling. A part that lasts 500 hours in the cabinet can fail quickly at a coastal site, and a part that fails at 200 hours can survive indoors for years.
The test also says little about the base metal until the coating is breached. Two finishes with the same hours can fail in different ways: one blistering, one pitting, one staining. The failure mode matters as much as the time, and the report should say which one ended the test.
Writing the hours into a specification
Write the requirement as a method, a duration and an acceptance criterion. Method: ASTM B117 or ISO 9227 neutral salt spray. Duration: hours. Acceptance: what may appear and what may not, at what point on the part, with or without a scribe.
Do not write salt spray hours as a warranty. A 480 hour requirement is a process control, not a promise of five years. The table is an illustrative ranking; it is not a specification and not a service life model.
| Finish | Illustrative hours | First sign usually seen | Note |
|---|---|---|---|
| Zinc, clear trivalent passivate | 96 to 200 | White corrosion on the zinc | Depends on thickness and passivate quality |
| Zinc-nickel | 480 to 1,000 | White corrosion, then red | Chosen where the base metal must be protected longer |
| Tin | 96 to 240 | Discolouration and white corrosion | Often chosen for solderability, not for corrosion |
| Electroless nickel | 240 to 500 | Pitting at pores | Porosity drives the result more than thickness |
| Silver | 96 to 240 | Tarnish and discolouration | Tarnish is not the same as base metal corrosion |
| Passivated stainless steel | 200 to 500 | Rust staining at inclusions | Passivation quality and surface finish matter |
Illustrative ranges for comparison only. They are not acceptance values. Set the requirement from the drawing, the standard and the service environment.
Specimen preparation and orientation
Prepare the specimen as the part will be used: cleaned, handled by the edges, and with any cut edge or scribe specified. The way a sample is cut can create a corrosion path that the production part does not have.
Orientation matters. ASTM B117 sets the specimen angle so the fog condenses and runs consistently. Parts that trap water or sit flat can collect solution and fail differently. Support the sample so it does not touch other samples or the rack.
Record the coating thickness on the same sample or on a witness coupon. A salt spray result without a thickness is a result for an unknown finish, and it cannot be compared with the next lot.
Reading the result
Name the failure, not just the time. The same hour count can hide very different behaviour.
- White corrosion on zinc or tin: the coating is reacting. Note the time and the location.
- Red corrosion: base steel is exposed. This is usually the end point for a steel part.
- Blistering: loss of adhesion or a pretreatment problem, often before the coating is breached.
- Discolouration: may be tarnish rather than corrosion. Say which one the report records.
- Creep from a scribe: measure the distance from the scribe, not only its presence.
- Edge and hole corrosion: often the first place a coating is thin or damaged.
Reporting and trending
Report the method, the duration, the solution, the pH, the temperature, the deposition rate, the sample orientation, the coating thickness and the result. Also report the number of samples and whether any were removed early.
Trend the results by lot and by plating supplier. A sudden drop in hours at the same thickness is a process change, even if the certificate still names the same specification. That trend is often more useful than a single pass, because it shows when the process moved.
Where salt spray fits in a qualification
Salt spray is one test in a qualification, not the qualification. Use it with adhesion, thickness, hardness, conductivity where relevant, thermal cycling and a review of the service environment.
For a new finish, run a comparison against a finish with known field history. That gives the hours a reference point, which is more useful than an absolute number with no context.
The hours are a comparison, not a calendar
A 500 hour result does not mean 500 hours, five years or five winters. It means the sample reached the agreed failure point later than the reference under the same test.
Frequently asked questions
What does a 480 hour salt spray result mean?
It means the sample reached the defined failure point after 480 hours under the specified test. It does not mean the part will last 480 hours, or any number of years, in service. The result is only comparable with another sample tested the same way.
Is ASTM B117 the same as ISO 9227?
They describe equivalent neutral salt spray methods with similar apparatus and conditions, but they are separate standards with their own details. State which one the requirement uses and report the method with the result so the two are not mixed.
Should the part be scribed before the test?
Scribe the sample when the specification needs to assess damage tolerance, such as a coated part that will be scratched in service. For a plain plated part, the drawing may require an unscribed sample. State the choice in the requirement, because the two give different results.
Why do two suppliers get different hours for the same finish?
Thickness, pretreatment, passivation, bath chemistry, porosity and sample preparation can all change the result. Compare the coating thickness and the process route before treating the difference as a material fault.
Sources and further reading
- ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus
- ISO 9227, Corrosion tests in artificial atmospheres, Salt spray tests
- ASTM D1654, Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments
Qumek Quality Desk
Quality team
The quality team responsible for first article inspection, PPAP packs, supplier audits and the records shipped with production parts.
Writing a salt spray requirement?
Send the finish, the service environment and the current test report. The quality desk will check the method, the hours and the acceptance criterion.
Send the finish details