Definition
The paint film leakage detector is a special instrument used to evaluate the continuity of coatings and detect small defects on the surface of coatings. It helps determine whether the coating's protective performance on the substrate meets the standards by identifying pinholes, cracks, or areas with insufficient thickness in the coating, and is widely used in industrial anti-corrosion, automobile manufacturing, ship coating, and construction engineering.
Principle
The instrument is mainly based on the principle of conductivity detection. When the coating is complete, it acts as an insulator to block the current flow path; If there is a leak, the conductive substrate is exposed, and the detection voltage applied by the instrument will form a current loop there. The specific implementation methods are divided into wet sponge method and DC high-voltage method: the wet sponge method moves on the coating surface through the wet sponge electrode, and the electrolyte conducts to trigger an alarm when it encounters a leakage point; The DC high voltage method uses the electric field change to detect leakage points through the high-voltage probe scanning. The choice of both methods depends on the coating thickness and the application environment.
Measurement method
The standard measurement process includes surface pre-treatment, parameter setting, system calibration and scanning inspection. Before operation, the coating surface needs to be cleaned, and the appropriate voltage should be set according to the coating thickness (usually follow the empirical relationship of V=1000×√d, where d is the coating thickness, in millimeters). After calibration, the electrode moves along the surface at a steady speed, and the instrument emits an audible and visual prompt when a leak is found. The test results are usually recorded in the form of the number of leaks per unit area or distribution map, and are evaluated with reference to ISO 2746, ASTM D5162 and other standards.
Influencing factors
Measurement accuracy is affected by multiple factors. Ambient humidity will affect the conductivity of the electrolyte of the wet sponge method. Coating surface cleanliness may cause false alarms; Uneven conductivity of the substrate may lead to detection blind spots; The speed of the operator's movement and the electrode pressure also affect the sensitivity. In addition, the voltage selection of the instrument needs to match the coating characteristics, too high voltage may break through the intact coating, and too low voltage will not be able to identify minor defects.
Applications
This instrument is of great value in the field of industrial protection. In pipeline anti-corrosion engineering, it is used to detect the integrity of epoxy asphalt coatings; In the inspection of the inner wall of the tank, the potential leakage point of the FRP coating can be found; Used in the automotive industry to verify the quality of electrophoretic paint films; Detection of the impermeability of deck coatings in shipbuilding. At the same time, it also plays a quality control role in the fields of steel structure bridges, underground pipe networks and container manufacturing.
Selection reference
Testing needs and environmental conditions should be comprehensively considered when selecting. For thin coatings (less than 500 microns) or humid environments, wet sponge instruments are more suitable; For thick coatings or high-temperature surfaces, the DC high-voltage type has an advantage. It is necessary to pay attention to whether the voltage regulation range of the instrument covers the thickness of the coating to be measured and whether the protection level is suitable for the site environment. At the same time, the probe design of the instrument should be suitable for detecting surface shape, and the reliability and data recording function of the alarm system are also aspects to consider. It is recommended to refer to the technical guidelines in industry standards such as GB/T 18593 and NACE SP0188 for selection.