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How to Locate Pinholes / Holidays in Protective Coatings

Referenced products:

There are two types of non-destructive pinhole/holiday detection solutions for locating pinholes in a protective coating system: Low voltage Pinhole Detectors such as the PosiTest LPD and High voltage Holiday Detectors such as the PosiTest HHD.

Background on Holiday Detection

Protective coatings and linings are applied to metallic and concrete substrates to prevent corrosion. Areas of thin or missing coating, called pinholes or holidays, can drastically reduce the life of a protective coating system. These discontinuities can be caused by entrapped air bubbles or particulate contamination that leave small regions of the substrate with little or no coating coverage, and if not repaired can become foci for corrosion. Even in areas where the dry film thickness is sufficient, pinholes or holidays can form, and they can be invisible to the naked eye. Therefore, testing of a cured coating using an electrical porosity detection instrument to locate pinholes is crucial to preventing coating failure.

Pinhole/holiday detection can be performed on non-conductive coatings applied to both metallic and concrete substrates. Although concrete is significantly less conductive than metal, it is still slightly conductive due to moisture content, and can carry enough current to allow holiday detectors to function. Corrosion and pitting beneath protective coatings can result in substantial coating failure in industries such as storage tanks, pipelines, and general construction.

Application

Before testing is performed, pinhole/holiday detection instruments are grounded to an area of the bare metal substrate. High voltage detectors featuring pulse technology, such as the PosiTest HHD,  can also be grounded using a trailing ground wire, grounding collar, or grounding mat. On concrete substrates, the ground wire is attached to the rebar or another metallic ground permanently installed in the concrete. If metallic ground is not visible, the ground wire is placed directly against the uncoated concrete surface.

The instrument outputs a voltage, charging the electrode as it is moved over the coating. When a holiday is encountered, current flows from the electrode to the substrate below. The current returns to the detector through the ground wire, completing the circuit and triggering an alarm.

Whether to utilize a low voltage or high voltage porosity detection instrument depends on the thickness of the applied coating. For coating thickness up to 20 mils (500 μm), low voltage pinhole detectors (wet sponges) are typically used. High voltage holiday detectors (spark testers) are used on coatings with a total thickness of 8 mils (200 μm) or greater.

DeFelsko's Holiday Testing Solutions

DeFelsko offers low and high voltage porosity detectors for locating pinholes in non-conductive coatings and linings applied to electrically conductive substrates. These instruments conform to ASTM, NACE, ISO, and other standards for holiday detection. A long form Certificate of Calibration is included with each instrument at no additional cost.

Low Voltage Pinhole Detector
PosiTest LPD Low voltage Pinhole Detector is suitable for coating systems with a total thickness of up to 20 mils (500 μm).

High Voltage Holiday Detector
PosiTest HHD High voltage Holiday Detector is suitable for detecting defects in coating systems with a total thickness of 8 mils (200 μm) or greater.

Note: The low range of the PosiTest HHD varies with standards

PosiTest LPD Low voltage Pinhole Detector

The PosiTest LPD Low voltage Pinhole Detector is designed to detect defects in coating systems that are up to 20 mils (500 μm) thick. In use, an operator moves the moistened sponge over the coated surface. When a pinhole is encountered, current flows from the sponge into the substrate below. The current returns to the detector through the attached ground wire, completing the circuit and triggering audible and visible alarms.

A voltage selection button allows the user to cycle through four test settings (9-90 V) that comply with most standards and specifications.

The GroundSense™ feature visibly reassures the instrument is properly grounded with an indicator light for each selected voltage that will change from blinking to steady when a connection to ground is detected.

A wide range of adaptable sponge hardware and extension accessories are available for the PosiTest LPD. Roller sponge hardware for testing large surfaces, an inside-diameter sponge attachment and an 8-inch square customizable sponge that can be cut into any shape are available to suit a variety of test configurations.

For complete instrument and accessory options visit: PosiTest LPD Low voltage Pinhole Detector.

PosiTest HHD High voltage Holiday Detector

The PosiTest HHD High voltage Holiday Detector is used for detecting defects in coatings of 8 mils (200 μm) or greater in thickness, and features a charged conductive brush or spring electrode. Pulsed DC technology is used to detect flaws, emitted at a pulse of 30 times per second, making it safer and easier to ground. The principle of operation for high voltage holiday detectors is similar to low voltage detectors. The charged electrode is passed over the coating, and when a holiday is encountered current flows into the substrate below. The current returns to the detector through the ground wire, completing the circuit and triggering audible and visible alarms. The instrument can be used in a stick configuration out-of-the-box or in a shoulder-carried configuration using the optional wand attachment.

A user adjustable voltage output is maintained over the range of 0.5 to 35 kV. The advanced circuitry is capable of maintaining a precise voltage output so the instrument voltage will not drop under load. Using the voltage calculator in the menu, the operator can enter the dry film coating thickness and the standard they are conforming to, and the correct voltage will automatically be calculated.

Rolling spring electrodes for pipes and flat wire brushes are both available in various sizes. Electrode connectors are manufactured from stainless steel and aluminum to be corrosion resistant, and no tools are required to attach them. Adaptors for connecting other manufacturer’s spring and brush electrodes are available.

For complete instrument details and kit options visit: PosiTest HHD High voltage Holiday Detector.

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