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Nondestructive Test Instruments Page 1

Coating Inspector Program Level 1

NACE International, 2003 January 2007

Overview Once the applied coating is dried and cured, it should be inspected to ensure that it meets the required standards. Typical inspection tests check for adequate film thickness, pinholes, and other conditions. In this section of the program, we will discuss the instruments used for measuring dry-film thickness (DFT) and for detecting the presence of holidays. Much discussion has been focused on the importance of applying coatings at the thickness required by the specification. Coatings that are too thick or too thin often result in early coating failures and costly repairs. Holidays or pinholes (i.e., discontinuities, voids, inclusions, areas of low film thickness, or breaks in the coating) should be detected and remedied to ensure an effective coating system. Any defect, even as small as a single holiday, may not be acceptable for critical coating performance situations, such as coatings in immersion service in a tank or pressure vessel, and may cause the rejection of the entire coating system.

Nondestructive Test Instruments Nondestructive test instruments, used properly, do not destroy the coating on which they are used. Most types of DFT gauges are nondestructive. In this section, well discuss essential quality-control instruments, including:

Wet-film thickness (WFT) gauges

DFT gauges, including:

Pull-off magnetic gauges Fixed probe magnetic gauges

Holiday detectors, including:

Low-voltage DC (wet sponge) detector High-voltage DC detector High-voltage AC detector




Wet-Film Thickness Gauge

An essential companion to any instrument used to measure DFT is the wet-film thickness (WFT) gauge. Using knowledge of the volume-solids content of the coating, the applicator can calculate the WFT required to produce the desired DFT.

Figure 1 Wet-Film Thickness Gauges

The most common instrument for measuring WFT is the comb gauge. It consists of a series of graduated teeth lying between two outer teeth. WFT combs with different scales (e.g., m, mils) are available. The gauge is pushed firmly into the wet paint film so that the outermost teeth make contact with the substrate or previously coated surface. The gauge must be at a right angle to the surface. The gauge is removed and the teeth examined. Some of the heads of the teeth will be coated with paint while the remainder will remain clean. The true WFT lies between the last tooth that is coated and the next (higher) tooth that is uncoated. The reported WFT is that of the last wet or coated tooth on the gauge.

Magnetic DFT Gauges

Common types of simple DFT measurement instruments include:

Pull-off magnetic DFT gauges, which use a calibrated spring to pull a small permanent magnet from the coated surface, including:




Dial-type gauge

Figure 2 Dial-Type Magnetic Pull-Off DFT Gauge

Pencil pull-off gauge

Figure 3 Pencil-Type Magnetic Pull-Off DFT Gauge

Magnetic constant-pressure DFT gauges, which depend on changes in magnetic flux within the probe and include:

Dual-probe gauge Single-probe gauge




Figure 4 Magnetic Constant-Pressure DFT Gauge

Magnetic Pull-Off DFT Gauge

The magnetic pull-off DFT gauge is a simple mechanical tool that works by magnetic attraction to a ferrous surface. The force of attraction is reduced by the presence of a nonmagnetic barrier between the permanent magnet and the surface. The measurement of the force required to pull the magnet away from the surface is equated with the thickness of the film.

Figure 5 Magnetic Pull-Off DFT Gauge Calibration on NIST Shims

The gauge is used for nondestructive measurement of the DFT of nonmagnetic coatings on a ferrous metal substrate.




Magnetic pull-off gauges do not depend on batteries or any other source of power and are therefore considered by many users to be more reliable in the hands of inexperienced users. They are also used in hazardous environments when non-spark instruments are required. They are widely used despite the frequent appearance of more sophisticated and more accurate electronic gauges. They are portable, simple to operate, and inexpensive. Measurements are taken in accordance with the specification or referenced standard. If no specific method is mentioned in the contract documents, this issue should be discussed at the pre-job meeting and a suitable method agreed. It is always better to follow an industry standard whenever possible.

Pencil Pull-Off (SSPC-PA 2 Type 1B)

The pencil pull-off gauge, Type 1B is another type of magnetic pull-off gauge. The instrument is a hollow tube, similar in size to a large pencil, with an internal magnet and spring. The extension spring is attached to the magnet and to the top of the instrument housing. The instrument is held perpendicular to the surface, and the magnet is placed in contact with the surface. As the instrument is lifted, the magnet remains attracted to the surface until the spring tension overcomes the magnets pull and lifts it from the surface. The tension on the spring necessary to lift the magnet is read from the scale, which may be marked in mils, m, or both. This type of instrument is less accurate and therefore less frequently used than other types of magnetic instruments. The inspector should check with the manufacturer or with his or her supervisor regarding the use and limitations of this instrument.

Constant-Pressure Probe DFT Gauge Constant-pressure probe DFT gauges are nondestructive instruments that measure DFT of nonmagnetic coatings over ferrous metal substrates. They are categorized by SSPC-PA 2 as Type-II instruments. They determine coating thickness by measuring changes in the magnetic flux within the instrument probe or in the instrument circuitry. The instrument probe must remain in contact with the coating at all times during measurement.




Constant pressure probe instruments may have fixed integrated probes or separate probes. In each case the probes are placed against the coated surface and held against the surface while a measurement is taken. Probes may be magnetic or electromagnetic. Coating thickness is displayed on the meter or instrument scale. Manufacturers of fixed-probe constant-pressure gauges recommend different methods of calibration or adjustment. Some provide built-in self-calibration routines or can revert to a factory standard calibration. Some modern gauges have many alternative methods for calibration, each of which is likely to lead to variation in results when measurements are made. All gauges should be calibrated according to the manufacturers instructions and/or in accordance with an agreed procedure.

Figure 6 Constant-Pressure Probe Gauge Calibration with Plastic Shims

Pinhole and Holiday Detection Holiday detectors are used to detect holidays or pinholes in the coating. General types of holiday detectors include:

Low-voltage DC

High-voltage DC

High-voltage AC

(Also Reference NACE Standard RP0188.)




Low Voltage DC ( Wet Sponge) High Voltage DC

High Voltage AC

Figure 7 Holiday Detectors Holiday testing is performed to find nicks, pinholes, and other defects or discontinuities in the film. Correction of coating defects is especially important for structures like tanks that are intended for immersion service, and for buried pipelines. The specifications should indicate the point in the job when holiday testing is done. The coating should be reasonably well cured (but not fully cured, for ease of repair) before testing is done. Coatings that are not cured may show false holidays. For example, solvent remaining in the coating may create weak spots (of low electrical resistance) that a high-voltage detector may disrupt, creating a holiday where none previously existed. Nevertheless, in some cases, such as with baked phenolics or glass-flake epoxies, the user may elect to test the coating before final cure so that any repair material would effectively bond to the underlying coat. Holidays in coatings should be repaired. The coating should then be tested again, after the repair, to ensure that repairs were successful.

Low-Voltage (Wet-Sponge) Holiday Detector

This holiday detector is a sensitive, low-voltage (wet-sponge) electronic device powered by a battery with output voltages ranging from 5 to 120 V DC, depending upon the equipment manufacturers circuit design.




The detector consists of:

portable battery-powered electronic instrument

nonconductive handle with clamps (to hold sponge)

open-cell sponge (cellulose)

ground wire.

The instrument is generally housed in a plastic case with an OFF/ON switch and a socket for headphones. Some low-voltage holiday detectors are fixed at a specific voltage, while other may have a test voltage selected. Some common voltages used are 9, 67.5, 90, and 120 V. Different results are obtained with each voltage, so selecting the proper voltage is important. Ideally the instrument to be used and its voltage should be specified. This type of instrument may be used to locate holidays in nonconductive coatings applied to a conductive substrate. According to NACE Standard RP0188, the low-voltage DC detector is generally used on coating films that are less than 500 m (20 mils) thick. The instrument will still locate defects in coatings thicker than 500 m (20 mils) and is preferred by some users because it cannot easily damage the coating film tested.

High-Voltage Pulse-Type DC Holiday Detector

High-voltage pulse-type holiday detectors generally have a voltage output range from about 900 to 15,000 V and in some cases may range as high as 40,000 V. They are designed for locating holidays in nonconductive coatings applied over a conductive substrate. Generally, these devices are used on protective coating films ranging in thickness from 300 to 4,000 m (12 to 160 mils). The detector consists of a source of electrical energy, such as a battery, or high-voltage coil; an exploring electrode; and a ground connection from the detector to the coated substrate.

High-Voltage AC Holiday Detector

The AC-type holiday tester is used for testing nonconductive linings on steel substrates, for example, rubber, glass, or sheet linings. An AC tester has a variety of voltages but typically is used for very thick coatings, with test voltages in the range of 25,000 to 60,000 V.




The AC detector is based on the principle of the Tesla coil and does not use a ground wire. The probe emits a blue corona, which is attracted to any ground. When it passes over a break in the lining or a foreign object included in the lining but in contact with the substrate, a spark will jump to the surface at the imperfection. Surface contaminants and dampness may also cause a spark, so the area should be cleaned and retested to verify a pinhole or holiday. AC holiday detectors (as opposed to pulsed-DC holiday detectors) are generally less frequently encountered. Incorrectly used, AC holiday detectors may present a much higher possibility of severe shock than DC high-voltage holiday detectors; thus, great care in operation is necessary. Standards commonly used to define holiday testing include:

NACE Standard RP0188 (latest revision), Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates

ASTM G 62 (latest revision), Standard Test Methods for Holiday Detection in Pipeline Coatings

NACE Standard RP0490 (latest revision), Holiday Detection of Fusion-Bonded Epoxy External Pipeline Coatings of 250 to 760 m (10 to 30 mils)

NACE Standard RP0274 (latest revision), High-Voltage Electrical Inspection of Pipeline Coatings

OverviewNondestructive Test InstrumentsWet-Film Thickness GaugeMagnetic DFT GaugesMagnetic Pull-Off DFT GaugePencil Pull-Off (SSPC-PA 2 Type 1B)Constant-Pressure Probe DFT Gauge

Pinhole and Holiday DetectionLow-Voltage (Wet-Sponge) Holiday DetectorHigh-Voltage Pulse-Type DC Holiday DetectorHigh-Voltage AC Holiday Detector

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Documents

dft gauge pencil

types of dft gauges

dryfilm thickness dft

applied coating

comb gauge

adequate film thickness

wet paint film

areas of low film thickness