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

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.