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coating must be applied as a con-tinuous film to perform its intend-

ed function. Early failure will occur ifthere is a discontinuitya holiday, as itis commonly calledsuch as a pinhole,void, crack, thin spot, foreign inclusion,or contaminant in the coating film. Many of these defectsare not readily visible, but they can be located using holi-day detection equipment.

Holiday detection is typically performed on coating sys-tems designed for critical service such as immersion orchemical storage. It is also conducted on coating systemsapplied to steel that is in contact with soil and/or con-stantly wet. Examples include buried pipelines, the under-sides of tanks, and sheet pilings.

This months Applicator Training Bulletin discusses howholiday detectors work, the operation of low-voltage andhigh-voltage detectors on coated steel, holiday detection ofcoatings on concrete substrates, and repair of holidays.

Principles of OperationHoliday detectors are instruments that use electricity to lo-cate film discontinuities. Most coatings are poor electricalconductors, and so they act as insulators. On the otherhand, a metallic substrate such as steel is conductive toelectricity.

A holiday detector consists of a power source, a groundwire, a probing electrode, and an indicator. (Note: theterms ground and grounding in this article are syn-onymous with earth and earthing.) Current wouldflow if the leads of the ground wire and the probing elec-trode were attached to the power source and their tipswere touched. If the ground wire was connected to acoated piece of steel and the probing electrode wasplaced on the coating surface, the coating would act as aninsulator, and no current would flow. However, if a holi-day was present in the coating, there would be a pathwayfor the current to flow. Holiday detectors have an indica-tor such as a sound or a light to tell when current is flow-ing. So, when the indicator shows current flow, it meansa holiday is present.

Some coatings, especially zinc-rich primers, are excellentelectrical conductors because of the conductive nature ofthe zinc particles, and so holiday testing cannot be per-formed on them. However, if a topcoat of a non-conductivecoating such as an epoxy or urethane is applied over theconductive primer, the holiday test would be valid. In this

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Fundamentals of Holiday Detection

This months column was written by Lloyd M. Smith,Ph.D., of Corrosion Control Consultants and Labs, Inc.,Herndon, Virginia, USA.

A

case, the holiday test would not distinguish between a dis-continuity that reaches only to the primer and one thatreaches to the substrate.

Low-Voltage Holiday DetectorsLow-voltage holiday detectors (Fig. 1) are used on coatingsthat are thinner than 500 microns (20 mils). They are pow-ered by a self-contained battery with a voltage that rangesfrom 5 to 90 volts direct current, depending on the manu-facturer. Low-voltage holiday detectors have either abuzzer or a light to show when current is flowing. Theprobing electrode consists of an open-cell sponge, similar

to one used on a household sponge mop.The coating must be sufficiently dried or cured prior to

running the holiday test because retained solvents in thecoating can give false results. Check with the coating man-ufacturer about when a holiday test can be performed on afresh coating. Also, measure the dry film thickness to makesure it is less than 500 microns (20 mils). If holiday detec-tion is required for coatings thicker than 500 microns (20mils), then the high-voltage method is recommended.

Follow the manufacturers instructions for assembling theunit. Differences do exist among commercial instruments, butthey all require attaching the ground wire to one terminal onthe power supply and the probing electrode to the other.

The sponge must be wetted with tap water. Dont usedistilled water because it must be able to support a cur-

Continued

Fig. 1: Low-voltage holiday detector (Photos courtesy of the author)

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rent, and that requires some conductive impurities, whichare removed when water is distilled. There is no need toadd salt to the water to improve conductivity. Doing sowould only spread a corrosive contaminant on the surface.However, the water must be able to flow into the smallestof pinholes, so a low-sudsing wetting agent (surfactant) isoften added to the water. (Some people believe that a wet-ting agent is not needed if the coating thickness is lessthan 250 microns [10 mils]. Others do not recommend theuse of detergent as a wetting agent because excessive de-tergent can block pinholes after testing, giving the ap-pearance that the coating has healed itself.) Soak thesponge until it is soft, pliable, and sat-urated. Then squeeze the sponge toremove any extra water. The watershould barely drip when the sponge ismoved over the coated surface.

Turn on the power supply after theunit has been assembled, and checkthe instrument by touching the spongeto the clamp on the ground wire. Thisaction should complete the electricalcircuit and cause the light or buzzer toturn on. If it does not, check the con-nections, the wires, the battery, and thewetness of the sponge. There really arenot many things that can go wrong.

To perform the holiday test, attachthe clamp on the ground wire to the structure. It must bein direct contact with the metal. The easiest place to attachthe clamp is a nut, a bolt, or an edge. Scrape or sand thepaint away so the clamp has good contact with the metal.Be sure to repair these areas after the testing is done.

Also, make sure the instrument is properly grounded bytouching a bare spot on the metal with the probing elec-trode. The bare spot can be anywhere, including the backsurface of the structure. If there is no access to bare metal,make (and mark) a small holiday in the coating. Thisshould be above the fill line of a tank or in an area wherea discontinuity will have the fewest consequences. Theeasiest place is near the ground clamp, as that area willhave to be repaired anyway. Periodically check that theequipment is still grounded properly.

Place the sponge flat on the surface and move it acrossat a moderate rate (i.e., 0.3 m/sec [1 ft/sec]). Apply suffi-cient pressure so that the surface appears wet just behindthe sponge. Remoisten the sponge if it dries out. The indi-cator light or buzzer will activate when a holiday is found.Turn the sponge on end when this occurs and relocate theholiday. Mark the area with chalk or an easily removablematerial that is compatible with the repair coating.

Resume the holiday testing after the area has beenmarked. It will be necessary to dry the area near the holi-

day or to leave a small space so the current does not trav-el back through the water to previously found defects.

The holiday test must be performed over the entire sur-face. Therefore, it is best to perform the test in a regularpattern, similar to a pattern used for spray painting. Payspecial attention to nuts, bolts, washers, rivets, etc. Thesponge will not conform to the entire surface whendragged over a bolt or a nut and threaded shaft, for exam-ple. In this case, it is easier to use one of the ends of thesponge. It may be necessary to reposition the groundclamp due to the length of the ground wire. Dont forget tomark the areas where the clamps were attached so they

can be repaired.Some people prefer to perform low-

voltage holiday testing before the finalcoat is applied. The reason is that if aholiday is found after the final coat isapplied, the repair coating over theholiday will be thin. Testing for holi-days and then repairing them beforeapplication of the final coat will achieveadditional coating thickness over theholiday. However, for holiday testingbetween coating layers, use plain waterwithout a wetting agent so as not toleave a thin layer of contaminant on thesurface that might reduce adhesion.

High-Voltage Holiday DetectorsHigh-voltage holiday detection is used when a coating isthicker than 500 microns (20 mils). The basic componentsof high-voltage detectors (Fig. 2) still include a power sup-ply, ground wire, probing electrode, and indicator. Howev-er, the power supply for these units will provide thousandsor tens of thousands of volts, and the probing electrodewill be made of copper wires or carbon-embedded rubber.

For personal safety, extreme care is needed when usinghigh-voltage holiday detectors. These units generate rela-tively low current while in use, which is good because highcurrent can be quite dangerous. However, when you use ahigh-voltage probe in an electrically isolated environment,you can become charged to the test voltage, which cancause you quite a shock from a build-up of static electrici-ty. The shock itself is not particularly dangerous, but how

Fig. 2: High-voltage holiday detector

Before conducting holiday testing in a confined space or otherarea where a flammable gas or vapour may be present, be surethat proper procedures are followed to thoroughly check (andif necessary purge) the atmosphere of the work area to preventthe holiday detector from becoming an ignition source.

Safety Tip

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it causes you to react can be quite dangerous, especially ifit prompts you to jump or fall. Always keep the workingend of the probing electrode away from your body and donot touch it when the instrument is activated. Read the op-erating directions carefully.

Not all high-voltage detectors have batteries, but most ofthem do these days. Some are powered by plugging theminto an electrical outlet; some use rechargeable batteries; andsome use dry cells. If there is a battery, check it for propervoltage output before running any tests. (Refer to the manu-facturers instructions.) Then connect the probing electrodeand grounding cable to the terminals of the detector andswitch the instrument on. Touch theprobing electrode to the alligator clip onthe grounding cable and make sure theinstrument signal activates.

A high-voltage holiday detector causesthe air between the probe and the sub-strate to conduct electricity, or break theair gap in the holiday, as it sometimes iscalled. The size of the air gap will varywith coating thickness, and breakdownof the air gap will depend on atmospher-ic conditions such as relative humidity. Anon-conductive spacer such as a thinpiece of plastic that is the same thicknessas the coating layer can be placed on abare steel surface to test whether the de-tector will cause a spark to jump through the spacer. If it does,the test voltage is too high.

Setting the proper voltage is critical because too high avoltage may actually produce a holiday in the coating filmrather than test for it. The first step, therefore, is to deter-mine the voltage to use. This is based on the thickness ofthe coating and its insulation properties. Set the instrumentto the proper voltage according to the manufacturers in-structions for the coating thickness being tested, becauseusing a high-voltage holiday detector on a coating that istoo thin also can produce holidays in the coating film. Like-wise, verify the voltage calibration in accordance with themanufacturers instructions and calibrate the instrument ifnecessary.

Operating a high-voltage holiday detector is very simi-lar to using a low-voltage holiday tester. The groundingcable is attached to the steel substrate and the detector ischecked by touching the probing electrode to bare steel.Move the probing electrode over the surface in a singlepass at a rate of approximately 0.3 m/sec (1 ft/sec).When the instrument signals a holiday, go over that areaslowly while watching the head of the probing electrode.You will be able to see a blue spark and can then markthe spot for repair.

Special accessories are available for performing high-voltage

holiday testing on pipes and pipelines. These include half- orfull-circle coiled spring electrodes (Fig. 3). A special groundwire to give the operator more mobility can be used if thepipeline segments are grounded to earth. The ground wire isabout 7 m (23 ft) long and is dragged along the ground ratherthan attached to the steel. If no holidays are found, occasion-al checks should be made by touching the probing electrodeto the steel to see if a spark and signal occur. If not, the groundwire may be dirty or the soil may have very high resistance(i.e., be very dry). If so, a ground cable with an alligator clipattached to the bare metal will have to be used instead.

The hazards of high-voltage holiday testing cannot beoverly stressed. Do not touch the headof the probing electrode, and keep theprobing electrode away from yourbody at all times when the instrumentis turned on. After the instrument hasbeen turned off, always ground theprobe before disassembling the unit toensure that any residual charge hasdissipated. Also, remember that theremay be an explosive environment inan enclosed area or a confined space ifthe coating being tested hasnt fullydried and solvents are still present. Donot use holiday detectors in these cir-cumstances.

Holiday Testing on Concrete SubstratesCoatings applied to concrete substrates can also be testedfor discontinuities using either low-voltage or high-voltageholiday detectors, depending on the coating thickness.Concrete can be conductive depending on its moisture con-tent, the type and density of concrete, and the location ofthe reinforcing steel (rebars or wire mesh).Before coating, the concrete must be tested to determine if it

is conductive. Set up the holiday detector by attaching theground wire to the reinforcing steel, making sure it is not coat-ed metal. If the reinforcing steel is not accessible, the groundwire can be placed on the bare concrete surface and weightedwith a damp cloth or paper and a sand-filled bag, or attachedto a metal rod or nail driven firmly into the concrete. Touch theprobing electrode to another bare spot on the concrete and seeif the instrument responds.

A conductive underlayment will be needed if the instru-ment does not respond. Most conductive underlayments arecoatings containing carbon or graphite fillers or conductivepolymers. The underlayment must be compatible with thelining material, as it will be part of the coating system. Theunderlayment can be a primer, if so formulated, or an inter-mediate coat, depending on how the coating manufacturerdesigns the system. The holiday test is performed as previ-

Continued

Fig. 3: Performing holiday detection on a pipelinewith a full-circle coiled spring electrode

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ously described for low-voltageand high-voltage instruments,depending on the coating thick-ness, except that ground is nowmade to the underlayment.

In general, if the concrete isnot conductive and a conduc-tive underlayment is not incor-porated into the system, thenhigh-voltage holiday detectionwould not be possible.

Because of expansion joints orconstruction joints in concretestructures, there may not beelectrical continuity between

sections. Therefore, it will be necessary to check the responseof the holiday detector when moving to a new panel. If elec-trical continuity does not exist, move the ground connectionto the panel being tested.

Repairing HolidaysAll holidays need to be repaired after testing is completed.Consult the coating manufacturer about how to perform therepair. Surface preparation for the repair may involve wash-

ing or it may include roughening the surface. The surfaceshould be washed with clean water if a low-sudsing wettingagent was used for performing a low-voltage holiday test.Sometimes holidays are the result of abrasive particlestrapped in the coating. In that case, they will have to be re-moved and the coating system reapplied to the areas wherethe holidays were found. It is important to repeat the holidaytest after the repair work to show that it has been successful.However, only the repaired areas need to be tested again.

SummaryHoliday testing is used to find coating film discontinuitiesthat are not readily visible. Holiday testing is usually per-formed on tank interiors, chemical storage vessels, andburied structures because of the importance of maintainingadequate coating protection in aggressive service environ-ments. Low-voltage holiday testing is used when the coatingsystem is less than 500 microns (20 mils) thick. High-voltageholiday testing is used when the coating system is thicker.High-voltage holiday testing requires special care not to dam-age the coating or cause personal injury to the operator.

Next month: Cleaning and Painting Galvanized Steel

Holiday detectors can find coating defects that may not be visible