Neutral salt spray test to ISO 9227 and ASTM B117

The neutral salt spray test is the widely used accelerated test for the corrosion protection of coatings. In a closed chamber, the parts are continuously sprayed with a fine mist of 5% sodium chloride solution at 35 °C; at fixed intervals, they are checked to see whether and where corrosion occurs. ISO 9227 describes the neutral salt spray (NSS) test; ASTM B117 is the American equivalent. The result is the number of hours until the first appearance of white rust, the corrosion of the zinc, or of red rust, the corrosion of the underlying steel, known as base metal corrosion.

What the hours mean

For zinc flake coatings to ISO 10683, 480, 720 and 1,000 hours without base metal corrosion are the usual levels; electroplated zinc coatings with passivation achieve 72 to 400 hours depending on thickness, zinc-nickel considerably more; manganese phosphate with PTFE anti-friction coating is often specified at 1,000 hours. The hours are a benchmark for comparing coatings under identical conditions. They do not indicate service life: a part rated at 1,000 hours will not last 1,000 hours outdoors, nor ten times as long, but will perform better relative to a part rated at 480 hours. Results can only be transferred to practice within the same type of coating.

What to consider with threads

Thread roots, edges and hexagon sockets are the weak points of any coating because the coating is thinner there. Specifications that require a high number of hours on threads therefore presuppose uniform application; with coatings applied by spray gun, compliance on threads can only be promised with reservations. RS Randack states this in the quotation so that it does not only come to light during the third-party inspection.

How compliance is verified

The coating partner supplies the certificate for its system; testing on the component is carried out by an accredited partner laboratory, on request as a first article inspection with test report. Coating thickness and adhesion are also checked.

What you should specify

Test standard, required hours and assessment criterion, whether verification is to be carried out on the component or on a test panel, coating system.