In the protection of large structures such as bridges, storage tanks, and pipelines, coating systems act like a layer of "armor," effectively blocking external corrosion and extending the service life of the structure. However, if this "armor" has discontinuous defects such as pinholes or missed coating areas, it becomes a "hidden entry point" for structural degradation—external oxygen and electrolytes can penetrate through these gaps to the substrate, causing corrosion of the metal base material and seriously affecting the safety and durability of the structure. Today, let's talk about how to scientifically detect and identify these hidden coating defects.

What are coating holidays?

Coating holidays are mainly divided into two types:

Pinholes : These are tiny gaps or needle-like holes penetrating the coating, which may reach the substrate or may not fully penetrate. They are mostly caused by the coating's inability to fully wet the surface, such as issues like bubble rupture or residue of impurities during coating application.

Missed coating spots : These refer to areas within the coating that are not fully covered by the paint. This problem is usually caused by improper application operations and is more likely to occur in complex structures (such as hard-to-reach corners and gaps) or concealed parts (such as inside buried pipelines or inner walls of storage tanks).

These defects may be difficult to detect visually, but once put into use, they become a "hotbed" for corrosion. Therefore, it is crucial to conduct holiday detection and repair before the coating system is put into service.

Timing and precautions for detection

Holiday detection is not better the more it is done; it is important to grasp the key timing:

Best timing: Usually performed after the final layer of the coating system is completed. It is not recommended to test after each coating layer, as surface contamination may affect the adhesion of subsequent coatings.

Applicable scope: Mainly used for newly applied coatings on metal substrates systems. For old coatings that have been exposed to immersion conditions, detection may cause coating damage or false readings—permeability, moisture absorption, or surface deposits on old coatings may interfere with detection results, and sparks from high-voltage detection may even penetrate intact coatings.

Two mainstream detection methods

Depending on the coating thickness, holiday detection is mainly divided into two methods:

Wet sponge low-voltage detection

Applicable scenarios: Systems with coating thickness less than 500 microns.

Equipment composition: Metal clamp with sponge fibers (soaked with tap water), grounding wire (connected to exposed areas of the structure), and lead wire (connected to the detection device).

Working principle: When the sponge contacts the coating surface, if there are pinholes or holidays, current will form a circuit through the gaps, and the device will immediately sound an alarm.

Voltage setting: The voltage for coatings on steel is fixed at 67.5V, making operation simple and easy to handle.

Electric spark high-voltage detection

Applicable scenarios: Systems with coating thickness greater than 500 microns; coatings between 250-500 microns can also be tested but require precise calculation and voltage setting.

Equipment features: Uses metal brushes, neoprene rubber, or coil electrodes (instead of sponge), voltage is adjustable, and input to the device must be calculated before use.

Working principle: Lead wires connect the detector and the test structure to form a circuit. When a holiday is encountered, in addition to an audible alarm, sparks can be visually observed, making detection more accurate.

Summary

Both detection methods share one common premise:

The substrate must be conductive, and the coating must be non-conductive . Coatings containing conductive pigments such as zinc or aluminum flakes cannot be tested by these two methods.

Coating holiday detection is a key step in safeguarding structural safety. It can promptly identify hidden hazards that are difficult to see with the naked eye, preventing premature failure of the substrate due to corrosion. Choosing a detection method that matches the coating thickness (low voltage or high voltage) and correctly setting parameters (especially the voltage for high-voltage detection) ensures accurate results while avoiding damage to the coating.