Salt Spray Test (ASTM B117) — How to Read It

The salt spray test (ASTM B117) places parts in a cabinet fogged with 5% salt solution at a controlled ~35°C to accelerate corrosion, recording the time until rust appears.

It is the international standard for comparing the corrosion performance of different coatings.

Two values are usually reported: time to white rust (the zinc layer) and time to red rust (the base steel). Red rust is the real indicator of protective life.

E.g. “Cr3+ yellow, 200h to red rust” means red rust appeared after 200 hours — higher is better.

As a rough guide for our trivalent (Cr3+) zinc plating on small parts, typical hours to white rust in an ASTM B117 chamber are:

• Clear / blue passivation — about 72–120 h to white rust
• Yellow / iridescent passivation — about 120–200 h to white rust
• Black passivation — about 72–96 h to white rust
• Olive or sealed systems — can reach 240 h and beyond to white rust

Thickness drives the result. ASTM B633 sets zinc service-condition classes: SC1 = 5 µm (mild), SC2 = 8 µm (moderate), SC3 = 13 µm (severe), SC4 = 25 µm (very severe) — thicker zinc buys more hours.

A common automotive benchmark for clear trivalent zinc is 72 h to white rust / 168 h to red rust.

These are typical accelerated test results, not outdoor service life — confirm the exact number against your applicable specification and a trial lot.

Salt spray is a comparative lab test; it does not convert directly into years of real service, which is more complex. It remains an excellent way to set specs and accept work.

How to use it: decide the salt-spray rating your application needs, specify it up front, and ask for the test report with your order. The plater then selects the zinc thickness, chromate colour and sealer to meet that target — and proves it.

V.S. Heat Treatment runs ASTM B117 salt-spray testing in-house and can issue a report per lot.