Device and method for leak detection
Since most coating defects cannot be visibly found, special inspection equipment and techniques are used to locate coating anomalies.
Essentially, a coating is an insulating product that protects the underlying conductive metal. A holiday Detector uses electricity to locate film anomalies by passing a current through the coating to form a closed circuit under the substrate. Anomalies can be identified through the use of high-low pressure instruments. However, each variable needs to be specifically matched to the application, depending on several variables.
For example, environmental conditions need to be suitable for effective holiday testing of surfaces, especially in terms of surface condition. A dried and cured coating is required for a positive test result, otherwise, residual solvent or uncured areas still present in the coating may cause false readings.
Typically, either a low pressure wet sponge test or a high pressure spark test are used, each with very different capabilities in terms of functionality. (See Tools and Tips: Breakout Detection on Coated Steel and Concrete Substrates for more information.) Apparently, the apparent difference is related to the voltage of the two devices. However, this does affect each paint type.
The manufacturer's guidelines state that the low pressure wet sponge Tester is recommended only for coatings with a maximum dry film thickness of 500µ (microns). For coatings thicker than 500µ, high voltage Tester equipment should be used (assuming the coating is compatible).
Another significant difference concerns how these tools detect the presence of holidays. Low voltage equipment utilizes water as a conduit for electrical power to connect to exposed substrate areas. Instead, high voltage equipment bridges the gap between the Tester and the conductive material by simply passing air.
There is also a big safety difference, as the wet sponge system operates at a low voltage of up to 90V without harming the operator or the coating; whereas the high voltage version, up to 60,000V, can seriously damage the operator and the coating being tested if not done correctly .
Common problems in leak detection
One of the biggest challenges in holiday detection is voltage setting and dielectric strength. Every coating has a certain dielectric strength, which determines the specific voltage the coating can withstand before it begins to break down. In addition to setting the correct voltage, the importance of choosing between high-low voltage Testers cannot be overemphasized when using this type of testing equipment.
Inspection speed can also have a detrimental effect on coatings, says Ian Wade, an experienced NACE-certified paint inspector. "When using a DC high voltage spark Tester, the inspector must not move the probe too slowly over the coating in single steps, as this will create a holiday. The minimum speed needs to be at least 1 ft/s (0.3 m /s). Likewise, with a low-pressure holiday Detector, the user needs to ensure that full contact with the coating is continuous and that the probe cannot be moved too fast or too slow.
"Additionally, users need to ensure that they do not oversaturate the sponge, as this may lead to erroneous results," Wade further noted. "Similarly, if the sponge is undersaturated (1 part of wetting agent to 128 parts of water) , the same problem occurs. Check the grounding of the device by bringing an exposed spot of the substrate into contact with the sponge, and repeat periodically to confirm grounding."
As highlighted, sometimes the pinhole is so small that the passage of water from the low voltage Tester to the conductive surface below is restricted. In these cases, the applicator may use a surfactant to lower the surface tension of the water. With reduced surface tension, the solution is able to penetrate the pinholes and enable the electrical current to find coating defects.
Certain types of paint contain metallic fillers to enhance paint performance. The presence of metals needs to be confirmed before any testing is performed. When inspectors are working with metal-filled coatings that may be semi-conductive, it is better to use low-voltage testing methods such as the wet sponge technique.
If the coating is metal-free, high-voltage equipment can be used to determine whether the applied coating has holidays or other defects.
control risk
Application techniques can be controlled by following specific coating guidelines, procedures and requirements. In order to obtain a good coating - to avoid potential defects such as holidays and pinholes - NACE Standard SP0188 should be followed. (More suggestions are discussed in 5 Coating Pitfalls You Can Avoid by Following Coating Specifications.)
As always, the importance of safety cannot be overemphasized when using equipment of this nature. NACE guidelines include certain measures to protect the safety of paint application and inspection personnel. When performing high voltage testing, it is necessary to conduct a thorough assessment of the test environment, including electrical safety checks, to ensure that there are no potentially explosive elements or conditions. Furthermore, the importance of complete electrical grounding of the substrate to avoid electric shock cannot be overemphasized.
restore armor
The solution to repairing a holiday depends entirely on whether the particular coating can be renewed. If the liner is at maximum coat thickness, another coat cannot be applied on top of the existing holiday. In this case, first delineate the area around the coating, feathering the edges towards the defect. Once complete, the area can be sandblasted before a replacement coat is applied.
On the other hand, if the coating is less than the maximum thickness, an additional layer can be applied over the defective area. Cracks can be covered and successfully eliminated by simply roughening the surface prior to continuous application.
After the repair is complete, another leak test will be required to confirm the repair was successful. It is important to note that the repair area is where testing should be done to prevent undue stress on the integrity of the coating.
