Verification, by the high voltage method, of the continuity of a coating applied to a cement-based substrate.

Find below the practical sheet number 25 of AFICPAR (French-speaking Academy of Certified Inspectors in Anticorrosion Protection by Coatings):

Area of use

The high voltage dielectric test method makes it possible to verify and detect the possible existence of porosities, perforations, microcracks or conductive inclusions in a paint coating system.

Principle :

The coating, which acts as an insulator, is subjected to a direct electric voltage emitted by the electrode of the high-voltage “electric brush” controller. The presence of discontinuities or inclusions allow the passage of electric current to the support. An electric arc (white / yellow) therefore occurs at the location of these faults. The device can emit an audible alarm (as well as a visual alarm for certain models of apparatus), it is advisable to be vigilant as for this setting of the audible alarm, which if it is cut, does not prevent the appearance of 'arcs in case of porosities.

Before the test:

• The test can only be carried out after polymerization / crosslinking of the paint coating system used (see technical data sheet or the supplier directly).

• In the case of a laminate coating, this test must be carried out directly on the laminate layer of the coating, ie before the application of the topcoat!

• The test voltage is a function of the coating thickness.

It must be defined beforehand either:

- in the specifications,

- via supplier data

- or in conjunction with table n ° 1 of standard NF EN ISO 29601 (standard applicable for a steel substrate whose values are recognized by the Anticorrosion Sector).

The table below summarizes some values extracted from table n ° 1:

Carrying out the test:

1. Make sure that the surface to be checked is dry and clean,

2. Connect the earth cable to the wall on a metal part via the controller's “alligator clip”.

3. Switch on the device.

4. Define the "tare" of the cement-based substrate: After having set the device to 0 kV, place the electrode on a part of the bare support then gradually increase the voltage until an electric arc is obtained ( white / yellow) and the sound signal.

5. Set the device to the selected test voltage + the “tare” of the cement-based substrate

6. Carry out the check by applying the electrode to the surface at an average speed of 3 to 5 meters per minute.

7. If an electric arc appears (white / yellow) and an audible signal, locate the porosity for later repair.

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