Holiday Testing of Coatings

Holiday testing, also known as holiday detection or voltage indication testing, is a type of quality control procedure used to identify defects in protective coatings on pipelines, tanks, and other structures. These defects, known as "holidays," are areas where the coating is not present and the underlying substrate is exposed to the environment. Holiday testing helps ensure that coatings are applied consistently and effectively, and can detect holidays that may not be visible to the naked eye.

Holiday Testing Procedure

There are several different methods for holiday testing, but the most common method involves the use of a holiday detector, a device that uses electrical current to detect the presence or absence of coating. The holiday detector consists of a high-voltage power supply, a probe, and a ground rod. The operator holds the probe and moves it over the surface of the coating, while the ground rod is placed in contact with the substrate. If the coating is continuous and the probe is in contact with it, the electrical current will flow from the probe to the ground rod. If there is a holiday, or area where the coating is not present, the electrical current will not be able to flow and the holiday detector will indicate the presence of a defect.


There are a few different types of holiday detectors, including AC holiday detectors and DC holiday detectors. AC holiday detectors use alternating current, which is more sensitive and can detect smaller holidays, but may also produce false positives due to stray currents. DC holiday detectors use direct current, which is less sensitive but more reliable and less prone to false positives. Both types of holiday detectors are commonly used in the industry, and the choice of which one to use depends on the specific application and requirements.

Holiday Testing for Painting/Coating

Holiday testing is particularly important for painting and coating applications, as the coating is the primary means of protecting the substrate from corrosion and other forms of degradation. The procedure for holiday testing of painting and coating is similar to that for other types of coatings, but there are a few additional considerations to keep in mind. First, it is important to ensure that the coating is dry and has reached its full cure before holiday testing, as wet or partially cured coatings may not be fully bonded to the substrate and may produce false negatives. Second, it is important to follow the manufacturer's recommendations for holiday testing, as different coatings may have different voltage and current requirements for holiday testing.

Holiday Testing for Wrapping Coating

Holiday testing is also important for wrapping coatings, such as tapes and liners used for corrosion protection of pipelines and other structures. These coatings are applied in a continuous, wrapped layer, and holiday testing is used to ensure that the wrapping is complete and free of defects. The procedure for holiday testing of wrapping coatings is similar to that for other types of coatings, but it is important to pay attention to the specific requirements of the wrapping material, as different materials may have different voltage and current requirements for holiday testing.


In conclusion, holiday testing is a crucial quality control procedure for ensuring the effectiveness of protective coatings on pipelines, tanks, and other structures.