Coulometry

Coulometric coating thickness measurement according to DIN EN ISO 2177.

The COULOSCOPE instrument series operates according to the coulometric test method as per DIN EN ISO 2177. This involves dissolving a metal coating off of its metallic or non-metallic substrate by means of an electrolyte under controlled electric current, i.e. reversing the electroplating process (de-plating). The current required for de-plating is directly proportional to the mass of the metal to be removed. A clear correlation between the de-plating time and the coating thickness arises when the test current and area are both kept constant.

Schematic diagram of the Coulometric method.
Schematic diagram of the Coulometric method.
Schematic diagram of the Coulometric method.

The following formula applies for determining coating thickness via the coulometric de-plating process: De-plating takes place in a measuring cell – a kind of miniature electrolytic bath. The measuring area is defined by a plastic gasket attached to the measuring cell. The electrolytes used are formulated for the various coating materials such that de-plating does not start until current flows. The de-plating process is initiated by the instrument’s electronics. A pump circulates the liquid within the measuring cell, continually refreshing the electrolyte in contact with the de-plating area, thereby ensuring uniform de-plating. Depending on the size of the area to be measured, gaskets with various diameters are available for the measuring cell.

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