In a salt spray test, samples are placed in a temperature-controlled cabinet and continuously misted by a 35°C salt solution. One records the total number of hours required to reach a set level of surface rust. This includes, for example, the time lag required to observe the first red rust or until 5% of the surface is covered with red rust.

The salt spray or salt spray test, as described in ISO 9227 (see
possibly also ASTM B117), looks at the results of an x-number of hours (between 24 hours and 1000 hours) test of a preserved product and compares the results between different measurement times/batches. By this we mean a quality control of a specific coating or product. Thus, the test is not devised to make comparisons with other types of coatings. The results can therefore in no way be translated into an actual life expectancy or other conclusions in terms of corrosion resistance.

There is quite a difference between properties of organic coatings (including paint and powder coatings) and those of metallic coatings (including galvanizing, nickel plating or chrome plating). Even between metallic coatings there is much difference. Indeed, even between zinc coatings and zinc alloy coatings, there is a big difference in the results of the salt spray test. Magizinc/Magnelis (a zinc-aluminum-magnesium alloy) is superior to almost pure zinc in a salt spray test. In practice, however, there is no evidence of this at all. Therefore, based on the results of the salt spray test, the choice can easily be made for an unsuitable system resulting in large repair and maintenance costs or, in other words, a large loss of value.

As mentioned earlier, zinc is known for its excellent corrosion resistance. That this property results from the so-called zinc patina that forms on the zinc surface exposed to the atmosphere is less well known.

The zinc patina is formed by zinc corrosion products and consists almost mainly of zinc hydroxycarbonate formed by the reaction of zinc with the CO2 and moisture of the atmosphere. The zinc patina is a very dense and impermeable layer with high chemical stability (i.e., low solubility). Only after this patina has formed does hot-dip galvanized steel have maximum corrosion resistance. For this, in a temperate climate such as in the Benelux, several weeks are required. In the salt spray test, the wet-dry cycles necessary for good patina formation are not present. Worse, it is never dry. In addition, particularly high chloride levels of 5% NaCl are used. The North Sea has a NaCl content of about 2.4%. This unrealistic laboratory environment therefore misrepresents the excellent corrosion resistant properties of hot-dip galvanized steel.

A hot-dip galvanized coating that is resistant for many decades in a normal atmosphere completely breaks down the salt spray test in a few days. The best way to determine the behavior of a hot-dip galvanized coating in a specific environment is to compare this behavior with historical data or determine the corrosion category of the area (see also Technical Data Sheet 10: Corrosion Resistance of Hot-dip Galvanized Steel). Using the Zinc Coating Life Predictor at www.zinkinfobenelux.com, you can perform a calculation of the period until first maintenance for a given location.

So why is the test requested anyway? For one thing, it is because of the familiarity of the test method. The predecessor of the ISO 9227 dates back to 1976 and a salt spray cabinet is a fairly standard product available in many materials labs and the costs for the test are not at all prohibitive. In the case of continuously galvanized or electrogalvanized serial products (in short, products with very thin zinc layers), for example, it is possible to test whether the various suppliers provide equivalent products in terms of preservation. There is also a linear relationship between layer thickness and time to formation of 5% red rust. Layer thickness differences on the surface of the product can be detected quite easily by performing the test. In addition, it is often included in a job specification that there is a minimum number of hours to red rust as a condition. Thus, none of this is intended for discontinuous hot-dip galvanized steel.