Components with large surface areas are often cleaned with a high-pressure washer. Paints and other coatings need to be able to withstand these pressurised water jets.
The degree of resistance of a coating on a component to such a cleaning process can be simulated with a pressure water-jetting test. In this the damage that a pressurised water jet can cause is not only dependent on how well the coating adheres. The water pressure, water temperature and duration of jetting influence the degree of damage, as does the distance of the high-pressure nozzle to the component tested, the geometry of the nozzle and the surface area impacted.
ISO standard 16925 of the International Organization for Standardization (ISO) exists to ensure the worldwide comparability of the results of such tests. The German version is DIN EN ISO 16925 coatings – testing of resistance of coatings to water jetting.
A number of examples for the uniform procedure of the standard:
- Two straight cuts are etched into the coating of the component. The length of the first cut is at least 100 mm. The second cut is at least 20 mm long. Both cuts stand at an angle of around 30° to each other.
- The component is secured in the test chamber of the pressure water-jetting device. The distance between the high-pressure nozzle and this sample is 100 or 130 mm. If the shorter distance is chosen, the pressure is higher. The geometry of the nozzle is defined.
- The water jet impacts the component vertically at a right angle (90°). It is aimed centrally at the interface of the two cuts.
- The sample is subjected to a jet of 60 °C demineralised water for 30 or 60 seconds. If the longer time is selected, the coated part is subjected to greater stress.
- The flow quantity is 11.3 l/min.
- Water pressure at nozzle outlet is 68 bar.
- Immediately after the test the degree of separation or cracking is assessed with the naked eye.
Experts compare the test results with images from the ISO 16925 standard and assign the separated surface areas or cracks co-efficients of from 0 to 5 for the test report. A co-efficient of 0 corresponds to very good adhesion with water jet impact, a co-efficient of 5 a very poor one.
Co-efficients from 1 to 5 are supplemented with a letter specifying the type of damage: the letter “a” is added where the coating breaks off along the longer cut. The letter “b” is selected where the damage is local, at one or more points. Experts use the letter “c” to indicate that the coating has not flaked off, but that cracks are formed.
If the resistance of a multi-layered coating is tested in this way, it is also investigated whether only the top coat flakes off, or also the base coat.