If a coating shows pinholes, the resistance to diffusion at these locations is not optimal. Pinholes are often difficult to detect visually. DCC tests the coating or lining on the presence of pinholes using a pinhole detector. It is an important additional test to a visual inspection.
Ensuring that a fully closed and continuous coating film has been applied is of particular importance in the case of linings inside tanks and vessels. Pinholes in the coating system will significantly reduce the resistance of the coating film to permeation and will lead to absorption of water or the contents of a tank or vessel, possibly resulting in corrosion damage. The coating industry uses a variety of methods and tools to detect pinholes in a coating film. So called low voltage “sponge testers” or high voltage “spark testers” may be used. DCC has the necessary equipment and more importantly, the experience and knowledge of the relevant standards and procedures such as NACE SP0188. Performing a pinhole test without the knowledge and experience may result in irreversible damage to the coating system. Click here for an overview of the most common methods for the detection of pinholes.
Common methods ‘pinhole detection’
The durability of a coating system is significantly influenced by the presence or absence of pinholes. Essentially, a pinhole is an opening in the coating film that allows penetration of moisture or cargo into the coating film. Since the primary purpose of the coating system (lining) is to separate the substrate from the environment this is to be considered a coating defect. The presence of pinholes or pores in a coating film can be detected using a pinhole detector. Pinhole detectors can be divided into two main types, high voltage and low voltage detectors. With the use of high voltage detection full or partial pinholes can be detected. When using low voltage pinhole detectors only full pinholes (actual exposure of substrate) can be found. If executed correctly both high and low voltage pinhole testing are considered to be a non-destructive test method,
The incorrect execution of a high voltage pinhole test will however result in irreversible damage to the coating system. A voltage setting that is too high, an incorrect detection speed as well as repeatedly testing the same area, will damage the coating system. On a regular basis and with the best of intentions, old coating are tested for the presence of pinholes using high voltage detectors to decide if the system has any remaining service life. Operators however, fail to consider the fact that an old existing coating film has over time absorbed moisture (or cargo). This significantly adds to the electrical conductivity of the coating film. Use of a high voltage detector may ultimately irreversibly damage the coating system. Testing of coatings for the presence of pinholes is best left to experts and should preferable be performed in strict conformance with agreed standards such as NACE SP-0188. However, many other standards are available in the industry. Call or Email DCC in case you have questions regarding testing of coatings for the presence of pinholes.