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Electrical conduit and fittings in heavy industrial and hazardous environments sometimes require additional protection from corrosive elements such as solvents, fuels, plating solutions, salts, alkali, acids and oxidizing agents. Typical facilities that require superior corrosion protection include water and wastewater treatment, food and beverage processing, petrochemical, mining, pulp and paper, and transportation infrastructure, such as bridges and tunnels.
Corrosive elements can cause millions of dollars in damage through lost time, materials and labor. However, facilities significantly can reduce their damages by installing polyvinyl chloride (PVC) coated rigid steel conduit and fittings. This coating is known to provide effective protection from corrosion in most challenging environments.
Table 1 on page 21 provides a general guide so the end-user can choose the most suitable material for any corrosion protection needs. PVC-coated conduit and fittings are suitable for almost all applications.
Evaluating corrosion protection
There are applicable standards for PVC-coated rigid steel conduit. Three standards govern the design and performance of PVC-coated rigid steel conduit: American National Standards Institute (ANSI) C80.1, UL6 and National Electrical Manufacturers Association (NEMA) RN-1. All three of these standards conform to the appropriate American Society for Testing and Materials (ASTM) standards and test methods.
Determining the effectiveness of corrosion protection on products requires an evaluation of the zinc-galvanized coating beneath the PVC coating and on threaded surfaces, as well as the adhesion of the PVC coating to the zinc.
In cases where the PVC protection is accidentally breached, due to cuts, scrapes, or other damage, it is important to have a second line of defense—a zinc or "galvanized" coating. The zinc coating will significantly slow the corrosion and allow more time for repairs.
Conduit systems without adequate zinc protection underneath the PVC coating are more likely to suffer catastrophic corrosion damage. This is why NEMA RN-1, section 3.1.1 requires the proper and correct treatment of galvanized conduit before it is PVC coated. The standard says, "The surface shall be cleaned in such a manner that the galvanized surface of the conduit is not harmed or eroded."
While the PVC may be UL listed as the primary corrosion protection, if the zinc coating is not adequate, the conduit does not comply with NEMA RN-1. Figure 1 shows the difference between adequate and inadequate corrosion protection.
The Preece test, which follows procedures set forth by ASTM A239, is the test recognized by UL 6, NEMA RN-1 and ANSI C80.1 to assess the adequacy of zinc protection for rigid steel conduit. 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 by subjecting the coated product to four 60-second immersions in a copper sulfate solution. The test also determines 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 the immersion testing. Conduit showing the bright, firmly adhering copper has failed to provide adequate zinc protection against corrosion.
Hot-dipped galvanized threads
Since electrical conduit systems "breathe," the threads will be exposed to the corrosive environment for the duration of the installation. NEMA RN-1 Section 2.1 states, "Where unusually corrosive environments are encountered, it is recommended that threads be given additional protection suitable for the intended application."
Hot-dipped galvanizing is a plating method in which the component's steel shell is dipped in molten zinc, which alloys with the steel on the component's surface. Hot-dipped galvanizing has been proven to provide the most effective zinc protection for conduit threads.
A compelling demonstration of the protection that hot-dipped galvanizing provides is shown in Figure 2 using a common corrosive agent, salt, on hot-dipped galvanized threads versus threads that are not. UL 6, the standard for rigid metal conduit, references ASTM B117 for evaluating protective coatings. Figure 3 shows the results of a salt-fog test using the standard test method ASTM B 117.
Evaluating PVC coating
NEMA RN-1, Section 3.8, addresses the evaluation process for adhesion of PVC coating on conduit. It says, "The adhesion of the PVC coating to the conduit shall be greater than the strength of the coating itself."
In order for PVC-coated conduit to be UL listed with the PVC as the primary corrosion protection, UL 6 Section 6.2.4.9 (for adhesion), requires that "The adhesion of the alternate corrosion-resistant coating (PVC in this case) … shall be greater than the strength of the alternate corrosion-resistant coating itself."
This adhesion test, shown in Figure 4, is straightforward and simple. There are no special conditions necessary to perform this test. Conduit that passes this test demonstrates that the adhesion will provide years of trouble-free service.
- Step 1 consists of two cuts through the plastic to the substrate along the length of the conduit at approximately 1/2 inch apart, three to four inches in length. A third perpendicular cut crosses the lengthwise cuts.
- Step 2 requires the edge of the PVC that was cut on the perpendicular to be carefully lifted to form a plastic tab.
- Step 3, the tab is pulled perpendicular to the conduit with a pair of pliers. The plastic tab will tear off rather than peel away, with no coating separating from the substrate.
- Step 4 is the evaluation of the test, which in this case, results in a passing grade.
Installing PVC-coated conduit
PVC-coated conduit is installed in much the same manner as conventional rigid galvanized steel or aluminum conduit. Certain precautions must be taken, however, to assure satisfactory results.
Most manufacturers of PVC-coated conduit provide installation training and certification for electrical contractors. The most effective teaching method is hands-on training of the electrical contractors on-site with the tools they will be using for the project. Contractors who have been trained are able to reduce their installation costs through reductions in both material (through less scrap) and time, while providing the end-user with a quality installation that will ensure years of trouble-free service.
Standards are in place for the proper use and protection of electrical conduit in corrosive environments. If these standards are followed, along with proper installation, you should expect corrosion protection that will extend the life of your electrical raceway systems for years while saving time and money along the way.
George Dauberger is director — engineering support services, Thomas & Betts.
author: By George Dauberger