Coatings Clinic: holidays and holiday detection.

I will take a month off from discussing thermal analysis and turn to a group of paint application problems that often are lumped together under the label of holidays, namely coating discontinuities, thin spots, voids, pinholes, and porosity. Such defects can hurt the appearance of the coating, but even more important is the loss of integrity so that the coating no longer protects the substrate against corrosion or other damage. In the case of coatings on the interiors of food and beverage cans, holidays can lead to contamination of the contents of the can leading to color and taste changes. Holidays testing should be part of quality control on the job or in a customer's plant, but it also can be a laboratory technique for evaluating application processes, new formulations, new additives, or other changes that might affect paint coverage.

With many coatings, it is obvious if the painter has missed a spot or the paint is very thin. However, if a topcoat is the same color as the undercoat or if the topcoat is clear, it can be very difficult to evaluate whether coverage is complete, partial, or not at all. In addition, coatings inside cans or pipes cannot be inspected properly. Therefore, it usually is necessary to employ one of the several holiday detection tests that exist. The choice of test depends on the application and the thickness of the coating. Electrical tests are commonly used for new coatings on steel structures and pipes. Standard practices NACE SP0188 and ASTM D 5162 each include a low voltage wet sponge test and a high voltage spark test. The former technique consists of an open-cell sponge electrode saturated with water and connected through a buzzer or light and a battery to the back of the part or structure. The sponge may be flat and rectangular in shape or cylindrical and act as a roller. The wet sponge is moved back and forth across the coated surface. When it encounters a holiday, the circuit is completed and the buzzer or light is activated. The sponge test is used for coatings less than 500 [mu]m (20 mils) in thickness. The high voltage spark test may be used on conductive concrete substrates as well as metal ones. The apparatus consists of power supply, ground wire, probing electrode, and an indicator. The probe may be a metal or plastic filament brush, a wire, or a wire spring. The ground wire is attached to the conductive substrate (in the case of concrete, to the rebars) and the exploring electrode is moved over the surface of the coating. A pinhole or other discontinuity will cause a spark to jump from the electrode to the specimen. A brush type spark tester is shown in Figure 1. The spark test rarely is used on coatings less than 500 pm (20 mils) in thickness because it has a tendency to blow holes in thin films. Voltage may be as high as 30,000 V. ASTM Standard Test Method G 62 gives details of the wet sponge and spark tests as applied to pipeline coatings. A spark probe for exterior pipe coatings may be a full or half circle spring electrode that goes around the pipe. Automated equipment can mark the discontinuity as well as detect it. A search on the Internet under "holiday detectors" will turn up a number of different instruments.

[FIGURE 1 OMITTED]

Another type of electrical test, called the enamel rater, is used for interior can coatings. A low voltage (4-10 V) is applied between an electrode immersed in a can filled with electrolyte (usually a dilute NaCl solution) and the can body. Current flow indicates that metal is exposed and the current level (in milliamps) indicates how much metal is exposed. When polarity is reversed, the electrolyte disassociates and hydrogen gas forms where the metal is exposed. These sites can be detected by noting the gas bubbles. Other DC voltage-current tests such as those used for studying corrosion should have possibilities as holiday tests. However, electrical tests are not effective for identifying thin spots unless you are using a spark test and do not mind making a hole where the coating is thin.

There is a copper spot test that has been used to test for holidays in coatings and, occasionally, for porosity and discontinuities in pretreatments. The surface of interest is treated with an acidified copper sulfate solution, usually by immersion. The solution reacts with an unprotected steel surface, leaving reddish copper deposits that may be visible to the eye and definitely may be seen with a low power microscope. Microscopes often are used for examination of painted surfaces for porosity, pinholes, and other discontinuities, as a holiday test in itself, for preliminary inspection or to confirm or explain what was found by other testing. Hand lenses and small low power shop microscopes are useful for such examinations in the field and at customer plants. A new technique for identifying holidays in topcoats involves the use of a primer coat containing fluorescing pigment. After the topcoat has been applied, illumination with light of a specific wavelength will cause pinholes and other discontinuities to show up as fluorescing spots.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o " Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].

By Clifford K. Schoff

Schoff Associates