What are the principles of holiday testing?

DeFelsko Corporation

Published: 10/17/2023

Pipelines are a vital piece of modern infrastructure, transporting all manner of gasses and liquids that power today’s world—whether it is natural gas moving from flowlines, biofuels through feeder lines, or gasoline being moved from a refinery through a transmission line to storage or a distribution center. As a critical piece of infrastructure, pipelines undergo many quality control checks to ensure their structural integrity and efficiency. One aspect of protecting a pipeline is the coating used to guard against corrosion. Verifying that the coating system is performing to specifications is paramount to the integrity of the pipeline.

There are a myriad of tests, procedures, and instruments used to check the quality of protective coatings before, during, and after the application of paint:

• Dry film thickness measurement—arguably the single most important measurement made during the application and inspection of protective coatings.
• Surface preparation—the surface of the pipe must be properly prepared before the coating is applied. This includes identifying and removing any rust, dirt, dust, or other contaminants such as salt. Additionally, creating a proper surface profile or anchor pattern via abrasive blasting helps the coating adhere to and bond with the substrate.
• Environmental conditions—optimal environmental conditions are essential for surface preparation, application, and curing of coatings and linings to maximize successful performance.
• Adhesion Testing—the ability for a coating to adhere to a substrate termed as ‘substrate adhesion’ or the ability for a coating to adhere to multiple coats in the system which is termed ‘inter-coat adhesion’.
• Holiday/Pinhole detection—after a protective coating has been applied, it is important to ensure that there are no defects or discontinuities present that expose the substrate beneath. Small areas of thin or missing coating, called 'pinholes' or 'holidays', can become foci for corrosion and drastically reduce the life of a protective coating system.

This article will focus on holiday detection on coatings applied to pipelines according to the test methods and procedures described in ASTM G62—“Standard Test Methods for Holiday Detection in Pipeline Coatings”.

What is Holiday Detection?

Holiday Detection is a non-destructive test performed on non-conductive coatings applied to conductive substrates, such as pipelines. Outlined by test methods like ASTM G62, a holiday detector uses a charged electrode passed over a coating. When a holiday is encountered, current flows from the electrode into the conductive substrate below. The current returns to the detector through a ground wire, completing the circuit and triggering an alarm, alerting the inspector to the defect.

Additional information:

“How to Locate Pinholes / Holidays in Protective Coatings”

What is a holiday? Why is holiday detection necessary? How are holidays detected?

How are holidays formed?

What is ASTM G62—“Standard Test Methods for Holiday Detection in Pipeline Coatings”?

ASTM G62—“Standard Test Methods for Holiday Detection in Pipeline Coatings” describes two test methods used to locate pinholes and holidays in pipeline coatings:

• Method A is designed to detect pinholes and holidays in thin-film coatings (25 µm to 0.254 mm or 1 to 10 mils) using a wet-sponge exploring electrode and an applied voltage of less than 100 V DC. If using a wetting agent in addition to water, it is effective up to 0.508 mm (20 mils).

See: PosiTest LPD Low voltage Pinhole Detector

• Method B is used to detect pinholes and holidays in thick-film coatings (≥ 250 µm or ≥ 10 mils) using a variety of flat wire brush and rolling spring exploring electrodes. The applied voltage range for these instruments is recommended to be between 4 and 35 kV DC.

See: PosiTest HHD and PosiTest HHD C High voltage Holiday Detectors

Both methods rely on similar principles of operation. A charged electrode is passed over a coating. When a holiday is encountered, current flows from the electrode into the conductive substrate below. The current returns to the detector through a ground wire, completing the circuit and triggering an alarm, alerting the inspector to the defect.

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ASTM G62 Test Procedure, Calibration, and Verification of Operation:

The following describes the ASTM G62 test procedure for each test method using the PosiTest LPD (method A) and PosiTest HHD Series holiday detectors (method B).

Note: These instructions provide summaries of procedures used to perform the holiday detection tests in accordance with ASTM G62. Consult and follow the ASTM G62 test method prior to testing.

Calibration:

ASTM G62 recommends yearly calibration performed by the equipment manufacturer, an authorized agent, or an accredited calibration laboratory approved by the manufacturer. A Certificate of Calibration traceable to NIST shall be retained with the instrument.

All DeFelsko pinhole and holiday testing instruments include a long form Certificate of Calibration traceable to NIST.

Verification:

Verification of the output voltage and overall function shall be performed prior to and after each period of use, if the detector is dropped, or if the coating thickness or electrode changes.

ASTM G62 Verification Procedure:

Verification of Output Voltage:

Note: PosiTest LPD, PosiTest HHD, and PosiTest HHD C testers contain built-in certified voltmeters that verify the voltage at the electrode. Unless otherwise required, the procedure below is not necessary for DeFelsko pinhole and holiday detectors. If required for QA protocols, DeFelsko offers a PosiTest LPD Verifier as well as PosiTest HHD Series Verifiers for either pulsed (PosiTest HHD) or continuous DC (PosiTest HHD C) models.

1. Ensure proper grounding of the holiday detector and that the electrode is connected to the unit and in contact with the coating.
2. Connect the ground (earth) cable from the high voltage voltmeter to the ground connecter of the holiday detector.
3. Turn the holiday detector on and set the voltage based upon the expected/measured low coating thickness (minimum thickness) and activate the voltage output. Refer to the voltage lookup table below. Make contact between the electrode and substrate.
4. Place the voltmeter’s probe tip against the holiday detector’s electrode. The voltmeter measurement shall be within 10% of the desired test voltage.
5. If required, adjust the holiday detector's voltage setting and reverify.

Functional Verification:

1. Locate a known holiday in the coating that is 40mils (1mm) in diameter that reaches to the substrate.
2. Turn on the holiday detector and activate the voltage output. Sweep the electrode across the known holiday according to the test procedure (found below or in Section 11 of ASTM G62).
3. Ensure the alarm activates when the electrode passes over the known holiday.

Test Procedure:

ASTM G62 test procedures are divided into method A (low voltage pinhole detectors) and method B (high voltage holiday detectors).

Method A (Low Voltage Holiday Detector):

1. Assemble the low voltage detector according to the manufacturer’s instructions. (PosiTest LPD instruction manual)

• Dampen the sponge electrode with clean tap water and secure the sponge into the wand
• Squeeze excess water from the sponge until it no longer drips

*The PosiTest LPD is capable of measuring on DFT ranges up to 20 mils (500µm). If testing on DFT > 250 µm (10 mils), adding a wetting agent or other low-sudsing surfactant to the water will reduce the surface tension of the water and promote better flow into discontinuities.

2. Connect the ground (earth) clip to an uncoated point of the part to be tested. Plug the ground wire into the ground wire jack (at the bottom of the PosiTest LPD).

3. Power the unit on.

4. Select the test voltage. The PosiTest LPD will automatically verify the set voltage using the built-in voltmeter.

5. Verify operation by placing the electrode on a bare surface. The LED and audible alarms should  trigger.

6. Place the sponge electrode on the coated part. PosiTest LPD Low voltage Pinhole Detectors feature GroundSense™—visibly reassuring the user that the instrument is properly grounded. Verify that the GroundSense™ LED indicator remains illuminated and does not blink.

• If the LED blinks, check the ground wire connections at the ground clamp and detector
• If all connections are secure and the LED is still blinking, attach the ground clip to a different uncoated area on the part

7. The low voltage pinhole detector is now ready to test. Move the sponge over the coated surface at a maximum rate of 0.3 m/s (1ft/s). If the PosiTest LPD sponge contacts a pinhole, all display LEDs will illuminate and an audible alarm tone will sound.

8. Use the corner of the sponge to precisely locate the pinhole. Then, use either chalk or painter’s tape to locate the pinhole for repair.

Method B (High voltage Holiday Detector):

1. Assemble the high voltage holiday detector according to the manufacturer’s instructions. (PosiTest HHD instruction manual, PosiTest HHD C instruction manual)

• Attach the electrode rod and electrode, as well as the ground cable. Install the battery.

2. Measure the coating thickness of the area to be tested in accordance with ASTM D7091. The PosiTector 6000 Coating Thickness Gage for All Metal Substrates is ideally suited for this task.

3. Select the test voltage—

Note: The PosiTest HHD features a built in voltage calculator. It prompts the user to enter the coating thickness on the part to be tested and the standard the user is conforming to and automatically calculates the correct test voltage.

The test voltage shall be 1.5 times the dielectric strength of air at the measured Dry Film Thickness as calculated using Paschen’s law, plus 1500V. For air at standard atmospheric temperature and pressure, that voltage can be calculated as:

V=1500+1.5[170+2.48d+58√d] (microns)

V=1500+1.5[170+63.0d+293√d] (mils)

Where V=the test voltage and d=the coating thickness

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Find the Voltage Lookup table for coating thickness values based on the above formula below in Appendix A.

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ASTM G62 recommends that any coating that varies substantially in thickness should use multiple voltage settings appropriate to the DFT on the area being tested.

Warning: The set voltage shall not exceed the dielectric strength of the coating or damage may occur. Check the manufacturer’s product data sheet for the correct dielectric strength.

4. Ground the test specimen

5. Ensure that the coated surface is dry and keep the electrode at least ½” (12.7 mm) away from any bare sheared or slit edge.

6. Turn on the high voltage holiday detector.

Note: Thoroughly read and understand the applicable Instruction Manual before activating Detection Mode.

PosiTest HHD Instruction Manual

PosiTest HHD C Instruction Manual

Warning: DO NOT touch the electrode when the instrument is turned on.

7. Move the electrode over the test specimen at a rate less than 0.3 m/s (1 ft/s). Ensure that the electrode contacts the entire coated area.

8. The audio and visual alarms of the holiday detector will trigger if it passes over any holiday, void, pinhole, or area of thinner coating. Use a “pinhole locater” (Conductive Rubber Paddle) to precisely locate the defect.

9. Mark the location of the holiday/pinhole using chalk or painter’s tape.

10. Repair the coating

11. Allow sufficient time for the repair to cure before retesting. Curing conditions are generally specified by the coating manufacturer. Only repaired areas need to be retested until no defects are detected.

Reporting:

ASTM G62 test reports shall contain:

• Complete identification of the test specimen, (names/code number of the coating, pipe diameter, source, production data, and production run number). For polymeric precoated corrugated steel pipe, use ASTM A742/A742M reporting requirements.

• Name and type of instrument used, the method of verifying operation, and whether method A or B was used.

• If method B was used, list the test voltage/s and the minimum reported dielectric strength of the coating (V/mm [V/mil]), if known.

Conclusion

Holiday detection on pipelines is critically important to quality assurance programs and ergo, to the integrity of the coating systems that protect pipelines from the effects of corrosion. ASTM G62 establishes two test methods for the proper detection of various defects such as pinholes, holidays, and other deformities.

The PosiTest LPD Low voltage Pinhole Detector and PosiTest HHD Series of High voltage Holiday Detectors are ideally suited for performing the tests detailed in ASTM G62. Call 315-393-4450, complete a sales inquiry form, or request a quote direct from the product page.

Appendix A: ASTM G62 Voltage Lookup Table

What Does Holiday Test Mean?

A holiday test is an inspection method used to detect discontinuities in painted/coated surfaces using specialized tools and equipment. These tools, called holiday detectors, are portable devices that are swept across the coated surface.

Holidays tests work on the concept of electrical conductivity. Metal substrates are excellent conductors of electricity, and therefore allow current to flow through them. On the other hand, many coatings are poor conductors of electricity and resist the flow of electricity. Using this principle, holiday tests use instruments to locate flaws in anticorrosive paints and coatings.

During holiday testing, a ground wire and probing electrode are attached to the same power source. The ground wire is clamped to the specimen being tested while the probe is swept across the surface of the metal substrate. If the probe comes into contact with a coating discontinuity, the exposed metal completes the electrical circuit between the electrode and the grounding wire, resulting in a flow of electricity. This electricity shows up on an indicator, alerting the equipment operator of the defect.

While holiday tests are effective, they do possess limitations. Because these tests depend on the coating being non-conductive, they are not effective on conductive metallic coatings, such as zinc-rich primers.

A holiday test is also known as a continuity test.

Free Download: All About Holiday Testing