coating inspection | Holidays
appropriate voltage range and set the correct voltage on the equipment so as not to cause further damage to the coating. This is a crucial point, as is the speed of testing. When using a DC high-voltage spark tester, the inspector must not move the probe too slowly across the coating in a single pass as this can in itself create holidays (the minimum speed must be at least 0.3m/s, according to NACE SP0188). With a low-voltage holiday detector, the user will have
Above: High-voltage spark testing being applied
identified using both high and low-voltage instruments. However, each must be matched specifically with the application and this is dependent upon several vari- ables. For example, the environmental conditions must be suitable for effectively testing the surface, especially with regard to surface conditions. Typically, either the low-voltage wet-sponge test or the high-voltage spark test is used, with each possess- ing distinctly different capabilities in terms of function- ality. Obviously, the significant difference concerns the voltage of the two pieces of equipment; however, this does impact on the type of coating each can be used for. Manufacturers’ guidelines typically state that low-volt- age wet-sponge testers are recommended for coatings with a maximum thickness of 500 microns. For coatings which are applied at thicknesses exceeding 500 microns high-voltage equipment should be used (assuming, of course, that the coating is compatible). Another significant difference concerns how these
two techniques detect the presence of holidays. The low-voltage equipment uses water as a pathway for the electricity to connect with the areas of exposed substrate. In contrast, the high-voltage equipment is able to bridge the gap between the tester and conduc- tive material simply through air. There are also considerable safety differences. The
wet sponge system, which operates at a maximum of 90V, cannot harm the operator or the coating, while the high voltage version operating up to 60,000V can. If not correctly operated, high voltage equipment can seriously damage both the operator and the coating being tested.
One of the biggest problems surrounding holiday
detection revolves around voltage settings and dielectric strength. Every coating will display a unique dielectric strength, which determines the specific voltage that a coating can withstand before it begins to break down. This means that the user has to select a tester with an
22 PIPELINE COATING | December 2016
to make sure that full contact with the coating is continuous and that the probe is neither moved too quickly or too slowly. Moreover, it is important to take care not to under or over-saturate the sponge as this can lead to erroneous results (the recommendation for the wetting solution is one part wetting agent to 128 parts water). Grounding of the equipment can be checked by making contact on a bare spot on the substrate with the sponge and repeating periodically to verify. Sometimes the pinholes in a coating can be so small that the passage of water from a low-voltage tester through to the conductive surface underneath can be restricted. In these situations, applicators will use a surfactant in order to lower the surface tension of the water. This allows it to penetrate the pinhole and effectively recognise the coating defect. Additives and fillers are added to coating materials
to provide different characteristics and additional properties. In some cases these may comprise of metallic fillers and this must be acknowledged before any testing is conducted. When confronted with metallic-filled coatings, which may be semi-conductive, a low-voltage testing method technique such as the wet-sponge technique is preferable. High-voltage testing equipment can be used to discern whether a non-metallic filled coating has a holiday.
Restoring the armour The risk of holidays due to poor application technique can best be eliminated by following specific coating guidelines and procedures. In terms of ensuring that the best coating possible has been achieved – meaning one void of any potential defects such as holidays and pinholes - it is essential to follow NACE standard SP0188 testing procedures. This will ensure not only that the coating is correctly tested but also that the testing is carried out safely – of paramount concern when using equipment of this nature. NACE guidelines include the electrical safety checks
required when conducting high-voltage tests and explain the assessment of the testing environment to ensure that there are no potentially explosive elements. Moreover, the guidelines cover ensuring complete electrical grounding of the substrate, which is vitally
www.pipeline-coating.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34