Testing of the Zinc Coating Itself on Galvanized Rigid Conduit per UL6 Preece Test
Evaluating corrosion protection of PVC coated conduit Galvanized conduit underneath the PVC coating Preece Test With so much riding on the integrity of their electrical conduit systems, facilities need the superior protection offered by the Thomas & Betts Ocal PVC Coated Conduit Systems. The simple fact is that Ocal is the only PVC coated conduit system to comply fully with the design and performance standards for PVC Coated Conduit set forth by UL6, NEMA RN-1 and ANSI C80.1. ANSI C80.1, UL6 and NEMA RN-1 have determined the appropriate ASTM standards and test methods that apply, and the Preece test is one test that must be passed in order to be in full compliance. Why is the Preece test relevant to PVC coated conduit? In cases where the PVC protection is accidentally breached, resulting from cuts, scrapes, etc, it is important to have a second line of defense a zinc or galvanized coating. The zinc coating will significantly slow the corrosion and give more time for repairs. Conduit systems without adequate zinc protection underneath the PVC coating are most likely to suffer catastrophic corrosion damage in such cases. This is why NEMA RN-1, section 3.1.1 requires the proper and correct treatment of galvanized conduit before it is PVC coated. It states The surface shall be cleaned in such a manner that the galvanized surface of the conduit is not harmed or eroded. The purpose of the Preece test is to evaluate the zinc coating on galvanized rigid conduit to ensure adequate protection from corrosion per UL 6.2.2. The test will also determine if the surface of the conduit has been damaged as a result of the preparation for PVC coating. In evaluating the test results, the conduit receives a passing grade when the sample does not show a bright, adherent deposit of copper after four 60-second immersions in the Copper Sulfate solution. The conduit showing the bright, firmly adhering copper has failed to provide adequate zinc protection against corrosion. Disturbed zinc coating not adequate for corrosion protection The Preece test follows procedures set forth by UL 6.2.2 and ASTM A239 and is the test recognized by UL6, NEMA RN-1 and ANSI C80.1 to adequately assess zinc protection for rigid steel conduit. Only the Ocal line of PVC coated conduit systems, manufactured by Thomas & Betts, complies with UL6, NEMA RN-1 and ANSI C80.1 without exception. Zinc coating surpasses requirement for corrosion resistance 2
Testing of the zinc coating itself on galvanized rigid conduit per UL6 (Preece Test) 6.2.2.1 A solution of copper sulfate for this test, shall be made from distilled water and a reagent grade of cupric sulfate (CuSO4). In a copper container or in a glass, polyethylene, or other chemically nonreactive container in which a bright piece of copper is present, a quantity of cupric sulfate shall be dissolved in hot distilled water. The purpose is to obtain a solution that has a specific gravity slightly higher than 1.186 after the solution is cooled to a temperature of 18.3 C (65.0 F). Any free acid that might be present shall be neutralized by the addition of approximately 1 gram of cupric oxide (CuO) or 1 gram of cupric hydroxide (Cu(OH)2) per liter of solution. The solution shall then be diluted with distilled water to obtain a specific gravity of exactly 1.186 at a temperature of 18.3 C (65.0 F). The solution shall then be filtered. 3
6.2.2.2 Several 150-mm (6-in) specimens shall be cut from a sample length of the finished zinc-coated conduit. With prudent attention to the risks to health and to the risk of fire, the specimens shall be cleaned with a suitable organic solvent. Each specimen shall then be examined for evidence of damage to the zinc coating, and only specimens that are not damaged shall be selected for use in the test. When a zinc coating is used on the inside and outside of the conduit, half of the specimens shall be longitudinally cut in half to expose the inside surface. If a zinc coating is used only on the outside of the conduit, none of the specimens shall be cut longitudinally. In order to test the zinc coating of PVC coated conduit, the PVC coating must be removed. Acetone is a suitable organic solvent for cleaning the zinc coating prior to testing since it does not attack the zinc coating or leave a greasy or waxy deposit. Acetone is also very effective for removing the PVC jacket. The PVC jacket should be cut around the circumference of the conduit with a knife. The cut should be made approximately 3" from the end of the conduit. To aid the removal process, the PVC jacket should be cut several times down the long axis of the conduit. This will allow the acetone to attack a greater surface area of the PVC, and reduce the time it takes to remove the PVC jacket. 4
The color of the interior urethane coating assists in sample identification. Placing a cork stopper in the end of the sample protects the interior urethane coating from the acetone. The samples are placed in the beaker with the cork end down. The beaker is then filled with acetone up to the level where the PVC jacket is cut around the circumference. After about 1 2 hours, the PVC jacket can be removed. The acetone will also clean the zinc surface. The samples are not ready to be tested. 6.2.2.3 The selected specimens shall be rinsed in water, and all surfaces shall be dried with clean cheesecloth. As much of the water as possible shall be removed in the drying operation because water slows the reaction between the zinc and the solution, thereby adversely affecting the test results. The surface of the zinc shall be dry and clean before a specimen is immersed in the solution of copper sulfate. The specimens shall not be touched by the hands or anything else that can contaminate or damage the surfaces. 5
6.2.2.4 A glass, polyethylene, or other chemically nonreactive beaker having a diameter equal to approximately twice the diameter measured over the specimen shall be filled with the solution of copper sulfate to a depth of not less than 76 mm (3 in). The temperature of the solution shall be maintained at 18.3 ±1.1 C (65.0 ±2.0 F). The specimen shall be immersed in the solution and supported on end in the center of the beaker so that not less than 64 mm (2-1 2 in) of its length are immersed. The specimen shall remain in the solution for 60 s, during which time it shall not be moved nor shall the solution be stirred. 6.2.2.5 At the end of the 60 s period, the specimen shall be removed from the beaker, rinsed immediately in running tap water, rubbed with clean cheesecloth until any loosely adhering deposits of copper are removed, and shall then be dried with clean cheesecloth. Again, the hands and other damaging and contaminating objects and substances shall not touch the surfaces that were immersed. The part of the specimen that was immersed shall be examined, considering each broad surface separately and disregarding any threaded area and the portions of the specimen within 13 mm ( 1 2 in) of the cut ends on sizes 12 53 ( 3 8 2) and 25 mm (1 in) for sizes 63 155 (2-1 2 6) and within 3 mm ( 1 8 in) of any longitudinal edges cut in the process of preparing the specimen. 6
6.2.2.6 A record shall be made when the immersed part of the specimen has any deposit of bright, firmly adhering copper, exclusive of any threaded area, and the 13 mm ( 1 2 in) cut-end portions and any 3 mm ( 1 8 in) longitudinal cut-edge portions. Deposit of bright, firmly adhering copper 6.2.2.7 When bright adhering copper is not found, the process of immersing, washing, rubbing, drying, examining, and recording shall be repeated up to the required number of immersions, or until the presence of copper is noted, whichever comes first, using the same specimen and beaker of solution. After the dips are completed on any one specimen, the portion of the solution of copper sulfate used shall be discarded. A fresh portion of the solution shall be employed for each of any succeeding specimens. Zinc only darkened by attack from copper sulfate 6.2.2.8 A protective zinc coating that provides the sole means of primary corrosion resistance on the exterior of the conduit shall be such that a specimen of the finished conduit does not show a bright, adherent deposit of copper after four 60-s immersions in a copper sulfate solution. Disturbed zinc coating not adequate for corrosion protection Zinc coating surpasses requirement for corrosion resistance 7
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