Nondestructive testing is the only viable option for inspecting coatings systems that are already in service and are intended to be kept in service. As you may have guessed, NDT is a subset of testing procedures that do not compromise the integrity of the system or material while being conducted. In much of manufacturing, measuring things like hardness, ductility and tensile strength involves stressing parts or materials to their breaking points. The only way to really tell where that breaking point is, with most of these methods, is to break them.
NDT is different. As applied to coatings, these procedures can include using electromagnetic waves to gauge the thickness of a coating, measuring heat distribution (thermography) to determine how well a coating is binding to its substrate, or the use of lasers to measure surface profile without physically contacting the substrate. While some NDT methods sound incredibly complex (and some are), checking a tank for leaks by filling it with water is technically an NDT method. To satisfy the definition, the test only needs to not render the subject it’s testing unworkable.
One example of NDT in the coatings industry involves the use of a permanent base coating that fluoresces under a black light. This allows inspectors to check for any imperfections in the base coating without in any way damaging it. It is a relatively quick and inexpensive NDT method sometimes used for inspections covering a large surface area.
NDT methods are numerous and varied, at least as much as the applications and the industries which they affect. An entire professional organization, The American Society for Nondestructive Testing, has even sprung up to provide standards and best practices for the field.
NDT gauges and industrial coatings
Probably the most significant contribution to NDT to industrial coatings, though, concerns electronic gauges. These devices are now more or less standard inspection equipment for quality control and quality assurance in the industry. Made by manufacturers such as Elcometer or DeFelsko and others, these tools are capable of performing such functions as using ultrasonic waves to measure the dry film thickness (DFT) of an applied coating.
Because coatings are manufactured to perform at a specific DFT, accurately measuring it is one of the most important things coating inspectors can do for an asset owner. Recording DFT precisely, down to microns of thickness, helps the inspector to know that a coating was applied according to specifications and should perform as intended.
Thanks to coating thickness gauges, measuring DFT doesn’t have to be a destructive practice. This and other NDT techniques like it make smarter, more economical and less labor-intensive inspection processes possible.
There are old NDT techniques, newer ones and more will surely be developed in the future. Which are relevant for a particular inspection will vary, but one thing is certain: your coating inspector should have a bag of NDT tricks at the ready to verify that a coating application has been carried out according to the specifications provided.