Salt Spray Testing (ASTM B117) Explained for Plated Fasteners

"Minimum 96 hours salt spray" is one of the most common notes on a plating drawing. It is also one of the most misused. Salt spray testing is a valuable comparative tool, but it does not translate directly into years of service life. This article explains what the test does, how zinc plated parts behave in it and how to write a specification that a plater can actually meet.

What ASTM B117 actually is

ASTM B117 (and the equivalent ISO 9227 NSS) describes a neutral salt spray, or salt fog, test. Parts are hung in a closed chamber at 35 °C while a 5% sodium chloride solution is atomised continuously. Samples are inspected at intervals and the time to a defined failure is recorded.

The standard defines the apparatus and conditions. It does not define pass or fail. Those come from the product specification, for example ASTM B633 service condition classes or an OEM standard.

White rust vs red rust

For zinc plated steel, two failure points matter:

• White rust is zinc corrosion product, a white, powdery deposit. It shows the passivation has broken down and the zinc is now corroding. The steel is still protected.
• Red rust is iron oxide, meaning the zinc has been consumed locally and the base steel is corroding.

Passivation type mostly controls hours to white rust. Zinc thickness mostly controls hours to red rust. A specification should state which one it means; "96 hours" without qualification is ambiguous.

Typical guidance for zinc finishes

These are indicative ranges. Results depend on deposit thickness, part geometry, how parts are racked or barrelled, and the exact chemistry used. Edges, threads and recesses fail first.

Why the numbers do not equal service life

Salt spray is a continuous wet, high-chloride environment that does not exist in the field. A part that survives 200 hours in the chamber is not guaranteed a set number of years on a vehicle, and a part with 48 hours is not doomed if it lives inside a dry housing. Use salt spray to compare finishes and control process consistency, and use cyclic corrosion tests or field data when you need to predict life.

How to write a useful specification

1. Name the standard: "Neutral salt spray per ASTM B117".
2. State the criterion: "no white rust before 96 h" and, if needed, "no red rust before 240 h".
3. Define the sample: production parts, quantity, and whether threads or edges are excluded.
4. Reference the plating standard and class so thickness is defined too, for example "ASTM B633 Fe/Zn 8, Type III".
5. Agree who tests and how often: every lot, periodic, or on request.

How we support testing

KVEE METALS controls plating thickness and passivation batch-wise and can supply witness samples from your lot for your own or a third-party laboratory. If your drawing asks for hours we cannot reasonably meet with the specified finish, we will say so and propose the finish that can, before production starts.