Pinholing and porosity detection methods

Pinhole and Porosity Detection

Premature corrosion of the substrate is usually caused by the destruction of the coating. A major cause of coating failure is some cracks in the overlying coating. These cracks are collectively called pores, and the main types are:

Sag & Sag:

The coating flows away by gravity, leaving a thin dry coating.

Sag:

It is caused by the lack of paint backflow to fill the gap after the bubbles on the coating surface burst.

Pit shape:

The substrate is wet or the coating has poor fluidity, resulting in voids.

Pinhole:

Caused by trapped air bubbles bursting from the surface, or by particles (dust, sand, etc.) trapped in the coating falling off.

Coating too thick:

If too thick a coating is applied, the internal stress of the cured coating can cause cracking.

Insufficient coating:

Areas that are not coated, or where paint has flowed away from the substrate or the corner of a weld. Insufficient coating on rough surfaces can also expose profile peaks.

There are essentially three methods of crack detection:

wet sponge technique

Suitable for detecting insulating coatings within 500μm (20mils) on conductive substrates, wet sponge technology is suitable for powder coatings and thinner coatings that do not want to be damaged.

Wet sponge plus low voltage. The sponge moves across the coating, and when it encounters a defect, the liquid seeps into the substrate, creating a complete electrical circuit that sets off an alarm.

UV pinhole detection

UV inspection can be used as an inexpensive and quick method to detect pinholes in coatings. Apply a base coat with UV fluorescent additive. When a UV flashlight shines on the coating, you see areas of the base coating that are not covered by fluorescence, which are where the pinholes are located.

high pressure technology

High pressure or porosity technology, can be used to detect coatings up to 7.5mm (300mils) thick, suitable for inspection of pipelines and other protective coatings. This method also inspects coatings on concrete.

The power supply generates high voltage direct current to the probe. When the probe passes through the crack, a spark is generated at the point of contact, sounding the alarm. This technique is suitable for locating all types of cracks mentioned above, and requires care in thin coatings.