72, 96, 240, 720 Hours of Salt Spray: What the Numbers Actually Mean
Salt-spray hours are quoted as if they described a coating. They do not. They describe the outcome of a specific test on a specific part, judged against a specific criterion. The same coating can report 240 h and 48 h without anyone lying — the substrate, zinc thickness, passivation, film weight and judgement rule were different.
The tiers you will meet in quotations
| Hours | Where it usually appears | What it implies |
|---|---|---|
| 24–48 h | Indoor parts, low-cost hardware, decorative items | Basic zinc with a thin passivation; little or no sealer |
| 72 h | General industrial fasteners, furniture hardware | Zinc 8–10 µm with a trivalent passivation |
| 96 h | Construction fasteners, automotive under-body brackets | Zinc 8–12 µm plus passivation and sealer, process under control |
| 240 h | Automotive engine bay, export hardware, some appliance parts | Zinc-nickel or thicker zinc, controlled passivation, validated sealer |
| 480–720 h | Marine, heavy equipment, specific OEM standards | Zinc-nickel plus sealer, or a different coating system entirely |
These are common associations, not rules. Always work back from the customer's written specification, including which standard and which judgement criterion.
White rust versus red rust
Almost every dispute about salt spray comes from this distinction:
- White rust is zinc corrosion product. It indicates the passivation and sealer barrier is being consumed. Its appearance time is sensitive to film quality.
- Red rust means the steel is corroding. Its appearance time is driven mainly by zinc thickness and distribution.
A specification that says "96 h salt spray" without saying which of the two is being judged is incomplete. Many OEM standards express it as "no white rust after 96 h, no red rust after 240 h", or percentage-area limits at each inspection interval.
Why the same part gives different numbers
- Zinc thickness. A 2 µm difference changes time-to-red-rust substantially.
- Passivation type and age. A freshly passivated part and one that has been sitting for two weeks behave differently.
- Sealer film weight. Thin films protect for hours, thick films for longer — until the film cracks and lets water in.
- Part geometry. Thread roots, blind holes and contact areas are always the weak points.
- Test standard. GB/T 10125, ISO 9227 and ASTM B117 are similar but not identical; chambers differ in uniformity and recovery after opening.
- Judgement. Visual assessment by two people can differ by a whole inspection interval unless photographs and area limits are agreed up front.
Writing a specification that can be tested
- Name the standard: GB/T 10125, ISO 9227, ASTM B117, or the customer's own.
- State the substrate and zinc coating mass or thickness range.
- State the passivation type and the sealer, including film weight if it is controlled.
- State the judgement rule: white rust / red rust, area percentage, inspection intervals.
- State the sample preparation: unscribed or scribed, edges protected or not, how parts are supported.
- Name the acceptance photos or the reference sample.