Corrosion testing: When the fog is salty

The functional principle of the salt spray test

In the salt spray test, the resistance of a coated component to specified corrosive conditions is tested. A modern salt spray corrosion test chamber is used for this. For the test, the samples are placed in the test device and are continuously fogged with a 5% saline solution at a temperature of 35 °C. The fog is deposited on the samples and covers the surface with a corrosive salt water film.

During the exposure time of e.g. 96, 240, 480 or even 720 hours, the samples are checked regularly and surface changes are documented by photo. For example, the occurrence of white rust (i.e. corrosion of the zinc layer protecting the steel) is normally first observed and documented at a usual electroplated coating on a steel component. In the course of the test, after failure of the zinc coating, red rust (i.e. base metal corrosion of the steel) can also occur. Depending on how long the component has withstood the test conditions without the respective corrosion, it can be evaluated according to the required minimum durations without corrosion. However, it should be noted that this is purely a laboratory test under controlled conditions. Under real conditions during the use of the component, many different factors influence the corrosion behavior, so that there is no direct correlation with the laboratory test regarding the duration until the start of rusting and no such prediction can be made. The salt spray test is only used for quality control under defined conditions, e.g. for batch comparison.