The coulometric method is one of the simplest and oldest procedures for measuring coating thickness. According to DIN EN ISO 2177, it’s suitable for many metallic coatings on any kind of substrate material. Particularly in multi-layer systems, coulometry is often a cost-effective alternative to X-ray fluorescence.
As an electrochemical analysis method, coulometry is basically the electroplating in reverse: A metal layer is dissolved by an electric current.
For this purpose, a measuring cell is filled with an electrolyte. This miniature bath, the opening of which has a pre-defined area, is placed onto the coating. Under the influence of a DC voltage, the metal atoms from the coating go into solution as cations and migrate to the cathode of the measuring cell.
As long as the coating is still being dissolved, a constant current will flow. But once the entire layer is detached and the electrolyte reaches the underlying material (the base material or another layer), there’s a measurable jump in voltage; the device switches off automatically.
Using Faraday's law, the layer thickness can be calculated from the intensity of the current and the time taken to dissolve the coating.
In automotive engineering, one often finds complex coatings consisting of several nickel layers on top of each other. These coating systems are used to ensure the desired high gloss for decorative elements while simultaneously increasing corrosion resistance.
The STEP test is a measuring method for checking the individual layers of such nickel multilayer systems; it has long been standardized according to ASTM B764 and DIN EN 16866.
In contrast to the conventional coulometric method, it records the curve of the potential with an additional silver electrode, which is sensitive enough to detect the small differences in potential between the individual nickel layers. Plus, the device continues to run after a jump in potential, so that several layers can be measured in one go.
As with all methods, there are factors that can influence the measurement, for example the composition of the electrolyte.
First, the electrolyte must fit the composition of the coating to be dissolved. Second, it can get completely used up by very thick layers or repeat measurements. Please contact us! We’d be happy to help you choose the right measurement strategy.