The Use of Coatings and Polyethylene for Corrosion Protection

• There must be two dissimilar metals, an anode and a cathode
• They must be in electrical contact with each other
• A common electrolyte has to be present.

Coatings
One way to separate metal from an electrolyte is to cover the metal with a tightly adhering coating. Examples of these are: coal tar epoxy, fusion bonded epoxy, ceramic-based materials, and fluorocarbon resins. There are also a variety of polymeric and other organic-based coatings available. Each one performs essentially the same function: it prevents the metal from contacting the electrolyte. In addition to insulating a particular part from the electrolyte, the coatings also insulate the parts from each other.

In a study conducted by the Cast Iron Pipe Research Association (CIPRA), several mechanical joints were buried with various kinds of T-bolts and different types of coatings. The bolts that regularly suffered the least amount of weight loss due to corrosion were the coated or taped bolts. They were made from 0.5 percent copper-content gray-iron, and with various coatings, some applied at the factory and some field applied. Even bolts that were individually taped prior to installation were more stable than either un-coated bolts, or bolts in joints wrapped in polyethylene.

Polyethylene Wrap
The properties of the electrolyte are influenced by its oxygen content. The use of loose polyethylene wrap limits the effectiveness of the electrolyte even when small amounts of water are present between the wrap and the metal. As long as the water is stagnant and not replenished, the quantities of electrolyte and oxygen are limited. In this situation, the water's oxygen content will be depleted and corrosion will cease. In addition, the stagnant water will become saturated with bivalent iron (Fe⁺⁺) ions. Corrosive activity will be stopped once the electrolyte becomes saturated. Thus, as a result of the polyethylene wrapping, the stagnant water actually helps protect the pipeline from further corrosion.

Testing
In the early 1950's, E.F. Wagner conducted a test of polyethylene wrap in which a six-inch mechanical joint was encased in polyethylene and buried in highly corrosive cinder fill. Two years later, the test section was uncovered and the mechanical joint, the gland, nuts, bolts, and pipe inside the polyethylene wrap, were found to be in excellent condition. The unprotected section of pipe outside the wrap was pitted with corrosion. During a test conducted in the late 1950's and early 1960's, the Cast Iron Research Association (CIPRA) buried several polyethylene-wrapped mechanical joints in various locations.

In the highly corrosive soils found in Casper, Wyoming and Lombard, Arizona, the polyethylene performed in an exceptional manner. The bolts protected by the poly wrap lost an average of only 3 grams per year. It should be noted that the soil in each location was not continuously saturated. In another test, bolts from polyethylene-wrapped joints that were buried in the Atlantic City tidal marsh lost an average of 28.9 to 33.1 grams per year. This probably was due to the replenishing of the electrolyte by the tidal action. In each case, the polyethylene was described as "loose". However, in the test conducted at the Atlantic City site, the tidal action forced "new" water between the polyethylene and the pipe. The bolts in each case were made from 0.5% copper-content cast iron.

A paper, Corrosion Prevention With Loose Polyethylene Encasement by W.H. Smith in Water and Sewage Works (May 1972), reports: "After almost 20 years of experience, including research and application in the field, there has been no failure of pipe so protected." Since that time, however, there have been some reports of failures of polyethylene-wrapped pipe. They probably occurred in conditions similar to those encountered in the Atlantic City test where the electrolyte was replenished. Mr. Smith's paper also noted a test conducted by CIPRA in 1968 to evaluate "the performance of polyethylene encasement when protecting gray and ductile iron pipe." The test sections were installed in highly corrosive soils near strong stray direct electrical current. The results of that test were as follows:

• Loose polyethylene film wrap not only protects against severely corrosive soil but shields the encased pipe from stray direct current.
• The dielectric properties of polyethylene encasement prevent development of local corrosion cells.
• Polyethylene-encased gray and ductile cast iron pipe can be cathodically-protected.

• Electrical resistance normally achieved with rubber-gasketed, push-on joints is retained at a higher level in pipelines which have been encased in polyethylene film.
• Loose polyethylene film encasement is not affected by high voltage protection currents as would be adherent coatings. (A pipe-to-soil potential of -14.8 volts was maintained for six months with no effect on polyethylene.)

Coatings can provide total electrical insulation between different pipeline materials. However, if small holes develop in the coating, the adjacent area is susceptible to corrosion since a concentration cell may develop. It is possible to test for such small holes and to fix such inconsistencies in coatings. Coatings may be applied in a number of ways depending on the type applied. Coatings also provide protection from stray direct currents.

Polyethylene wrapping has been tested widely and has demonstrated good performance. It is covered in several national and international standards. It is the most popular method of corrosion protection of iron pipe in the water works industry's distribution systems. Coatings are good barrier materials that can separate pipeline parts from the electrolyte, and they also provide electrical insulation between the pipeline components. Both loose polyethylene wrap and tightly-adhering coatings should be considered whenever a pipeline's surroundings create a possible corrosion problem.

References:

A Michael Horton, "Protecting Pipe With Polyethylene Encasement, 1951-1988, Waterworld News, May/June 1988. Pp. 26-28.

AWWA Manual M27; External Corrosion-Introduction to Chemistry and Control, 1st ed., American Water Works Association, Denver, CO, 1987.

"Corrosion in Underground Restrained Piping Systems," EBAA Iron Inc. Technical Bulletin.

NACE, NACE Basic Corrosion Course, 11th Printing, National Association of Corrosion Engineers, Houston, TX, 1980.

W.H. Smith, P,E., "Corrosion Prevention With Loose Polyethylene Encasement," Water & Sewage Works, May 1972.

Editor's Note: This is a modified version of an article which appeared first in CONNECTIONS, a series of technical bulletins addressing design and application topics relating to such piping system components as joint restraints, flanges, flexible expansion joints, etc. They were published by EBAA Iron Sales Inc. of Eastland, Texas. Additional reports in the series will appear in Water Online in the future.

Edited by Ian Lisk