How long does a salt spray test need to run to pass?

Separate the two kinds of "duration" first

Test duration: a technical parameter specified by the reference standard, part of the test conditions, set by the product specification or the customer requirement. It decides how long the sample stays in the salt spray cabinet.

Turnaround time: the actual time from sample receipt to report issue, arranged according to the project. The two are not the same thing, and it is worth being clear which one you are asking about.

Common duration bands

Different standards and product specifications give different bands. The following is only a common reference range; the standard your customer specifies takes precedence:

Durations cannot be compared across types. A hundred-odd hours of CASS and several hundred hours of NSS cannot simply be ranked — the corrosion mechanism and acceleration factor differ.

What the pass criterion actually looks at

"How long until it passes" is the wrong question in itself: passing is not decided by time, it is decided by the condition after the test. Common ways of judging:

• Appearance rating: protection and appearance ratings scored per the GB/T 6461 system, looking at blistering, flaking and rust spot area
• Base metal corrosion: whether the substrate is exposed — a knock-out criterion for most projects
• Unilateral creep from a scribe: measured where a scribe is required
• Functional status: for electronic parts, whether electrical parameters have drifted

Acceptance limits come from the standard, or from an agreement the customer makes in addition to it. The same test result may be judged differently by different customers.

Why "running it longer is safer" is wrong

The duration of a salt spray test is fixed by the product specification. Extending it unilaterally means, first, that the result no longer matches the customer standard, and second, that it may introduce corrosion modes outside the standard, which makes the judgement harder rather than easier.

If you are worried the sample will not pass, the right approach is to run one round to the customer requirement, then improve specifically with the data in hand — not to "check it" by stretching the duration.

Controllable factors that affect the result

• Salt solution concentration and pH (prepared to the standard and verified periodically)
• Cabinet temperature and spray rate (the settling rate has a specified range)
• Sample orientation and spacing
• Whether the sample is cleaned and whether it is scribed
• Post-test cleaning and drying method (affects the appearance rating)

Lose control of these and the same batch of samples can yield two different conclusions on two cabinets.

Where we stand

TMC LAB holds both CMA and CNAS accreditation and covers neutral, acetic acid, copper-accelerated acetic acid and cyclic salt mist, executed to the customer's specified standard with rating photographs provided. Send us the material, the surface treatment and the customer standard, and we will confirm the salt mist type and the acceptance method first.