Coulometric method

Coating thickness measurement using coulometry.

The coulometric method is one of the simplest and oldest methods for the determination of coating thickness and is one of the electrochemical analysis methods. Coulometry uses Faraday's law to determine coating thickness. It is suitable for many metallic coatings on any base material. Especially for multilayer systems, the coulometry method often offers the cost effective alternative to X-ray fluorescence.

This is how the coulometric method works.

Coulometry is the reverse of the galvanization process. In coulometric coating thickness measurement, a metal layer is dissolved by a constant electric current.

Measuring is carried out using a measuring cell filled with an electrolyte. This miniature bath is placed on the coating with the opening having a defined area. Due to the action of the electrolyte and the direct current, the metal atoms from the coating enter the solution as cations and migrate to the cathode of the measuring cell.

When the entire coating is detached and the electrolyte reaches the underlying material (base material or another layer), there is an increase in electrical resistance and thus a measurable voltage jump, which leads to an automatic end to the measurement.

From the time that the dissolution process has taken, the coating thickness can be calculated via Faraday's Law.

Full control with STEP – simultaneous thickness and electrochemical potential determination

Complex coatings with several nickel layers on top of each other are frequently used in automotive construction. These coating systems are intended, on the one hand, to provide the necessary high gloss for decorative elements and, on the other hand, to increase corrosion resistance.

The STEP test method has long been standardized in accordance with ASTM B764 and DIN EN 16866 as a measuring method for checking the individual layers of such a nickel coating system.

In contrast to the conventional coulometric method, the course of the electrode potential is recorded with an additional silver electrode. This electrode is sensitive enough to detect the small potential differences between the individual nickel layers. Here, the layer thickness and the potential differences are determined manually from the course of the electrode potential (voltage-layer thickness curves). The instrument does not switch off after a potential jump to allow measuring of several layers in one run.

Where is this process used?

  • Metallic coatings (aluminum, copper, nickel, gold, silver) on different base materials
  • Electroplating coatings
  • Multilayer systems such as Cr/Ni/Cu on iron or plastic substrates (ABS)
  • Multiple nickel coatings (porous/bright/semi-bright) by means of step test

What factors can influence the measurement?

As with all methods, there are factors in coulometry that can affect the measuring.

  • Electrolyte composition

      On the one hand, the composition of the electrolyte must match the base material and the coating to be dissolved. Secondly, the electrolyte can be consumed in the case of very thick coatings or during repeat measurements.

  • Measuring point

      To keep error quantities to a minimum, contamination of the measuring point should be avoided. It is a good idea to lightly etch over the site before measuring to remove any oxide layers. If the measuring point is too close to the edge of the sample, edge effects can falsify the results. After measuring, it should also always be checked whether the layer has come off cleanly.

      The measuring point should also be checked for leaks. If the measuring point is not flat or the measuring cell is used near some bends or curves, the electrolyte could leak out (even some drops), resulting in a larger resolution area and thus incorrect results.

      Changes of the measuring point due to wear of the ring seal, changes of the contact pressure and other influences on the measuring point can cause measuring errors.

Which standard is applied here?

Coulometric method according to DIN EN ISO 2177