Specifying Products for Highly Corrosive Environments
The stakes are high when specifying products for highly corrosive environments. Beyond costly product failures, catastrophes can ensue, some even resulting in human harm.
The total cost of corrosion in the United States has been estimated at approximately $276 billion/yr,1 a staggering figure that underscores the inherent economic dangers caused by improper product specification and resulting product failure. The corrosion costs for drinking water and sewer systems alone amounts to approximately $36 billion/yr (see Figure 1).
Those who study corrosion and the dramatic implications of the damage it causes concur that:
There is a misconception that nothing can be done about corrosion; to the contrary, the specification of optimum anticorrosive products provides opportunities for tremendous cost savings.
Sound corrosion management, built upon proper product specification, depends on a comprehensive understanding of regulations and standards.
Product life predictions and performance assessment methods are essential for determining what products will perform well over time.
ALL PRODUCTS ARE NOT CREATED EQUAL
For years, all available brands of PVC-coated galvanized conduit met exactly the same UL 6 standards and carried an identical UL Mark relating to safety conformance. Yet, it was apparent in the marketplace that not all brands performed the same.
In 2005, the ETL SEMKO division of Intertek, an independent testing, inspection, and certification organization, evaluated PVC-coated galvanized conduit brands not for safety issues, but for product performance and longevity as tested under conditions consistent with highly corrosive environments. The results established that some of these products, previously viewed as equal by way of UL certification, were in fact far from equal in terms of meeting ETL-Verified performance standards.
The testing began in December 2004 and ended in July 2005; specimens from four sources were tested. Performance was assessed based on how the products performed when:
Immersed in boiling water (according to the intent of ASTM D 870, Standard Practice for Testing Water Resistance of Coatings Using Water Immersion)
Exposed to heat and humidity (according to the intent of ASTM D 1151, Standard Practice for Effect of Moisture and Temperature on Adhesive Bonds; and ASTM D 4585, Standard Practice for Testing Water Resistance of Coatings Using Controlled Condensation).
Heat and humidity testing was selected for several reasons:
Heat and humidity are universally accepted in the coating industry as standard parameters for evaluating how well a coating protects against corrosion. Standard test procedures are documented and recognized
Heat and humidity are widely acknowledged as corrosion-accelerating agents
Heat and humidity are typical conditions in coated conduit application environments
Heat and humidity chambers are available as standard equipment for testing
Heat and humidity testing in the laboratory provides a method that, when combined with real-time exposure, can be used to predict the product's lifecycle in corrosive environments.
Adhesion was evaluated in accordance with the procedures outlined in Section 3.8, Adhesion, of NEMA Standards Publication RN 1, PVC Externally Coated Galvanized Rigid Steel Conduit and Intermediate Metal Conduit. Section 3.8 describes how to conduct the adhesion test; however, it has no associated performance requirement other than that the PVC tear before separation from the metal substrate.
The results of both tests confirmed significant differences in adhesion performance of the four brands examined, and help explain why only certain brands carry the ETL Verified Mark.
ENSURE SUCCESSFUL PERFORMANCE IN THE FIELD
There are no performance requirements for coated conduit in existing standards; however, committees that develop conduit standards are currently considering performance testing. In the meantime, the best course of action while specifying products for highly corrosive environments is to select those that carry an independent product testing label such as the ETL Verified Mark. The reason is built upon solid empirical facts: First, all available brands of PVC-coated galvanized conduit meet exactly the same UL 6 standard and carry the same UL Mark relating to safety conformance. However, only the brands that also have passed the ETL testing requirements are authorized to carry the ETL Verified Mark as well.
To obtain the ETL Verified Mark, a manufacturer provides an initial qualification sample to Intertek. The sample is then independently tested to the specifications of the appropriate standard. If the sample meets the requirements, an Intertek field representative will visit the manufacturer's facility to independently select a final qualification sample for further testing. If the second sample meets performance requirements, the manufacturer may use the ETL Verified Mark for the product. The manufacturing facility is then subject to quarterly audits to ensure ongoing compliance.
ETL testing, and consequently ETL verification, is based on actual product performance as a predictor of reliable service life. Brands that carry the ETL Verified Mark have a documented, proven ability to perform in a corrosive environment over an extended period of time—a fact that has been evaluated and confirmed by a world-recognized, independent, third-party source. Specifying tested and verified products will go a long way toward ensuring successful performance in the field and avoiding the high cost—and sometimes disastrous effects—of product failure.
The ability to successfully combat the high costs of corrosion demands an awareness of how varying products resist corrosion damage, which are most effective, and why. Proper specification requires that all applicable standards for evaluating anti-corrosion performance are thoroughly understood. Knowledge-based decision-makers require comprehensive, quantitative methods to compare the relative performance of products. Objective evaluations, such as those required to achieve ETL-Verification, provide an important foundation for effective specifications, i.e., reliable product-life projections and performance assessment.
Martin is principal of Henry Martin Consulting LLC, Metairie, La. He has more than 40 years of engineering experience in both the private and public sectors, including the design and inspection of electrical systems for commercial spaces, institutions, industrial and municipal plants, and utilities.
Choosing the right material
Selecting the best anticorrosion products and applying them in the most effective ways begins with an in-depth conversation with the client, who can help you determine how important reliability is to the production and profitability of his or her operation. After the mechanisms of corrosion in the environment are defined, consider the material as much as the design itself. Choosing the wrong material can result in frustrating, or even dangerous, situations.
Defining the corroding agents and determining the concentration can be a complex process. Several corrosive elements usually are present, and interactions are not always well documented. Water is the most common corrosive element and usually is present to some extent in every enclosure application. Adjacent processing operations or other intermittent activities, such as industrial cleaning and the general plant environment, may expose the enclosure to a variety of corrosive agents and temperatures. Each environment is unique, so identify all possible corrosive agents for the intended application.
Metal corrosion is influenced by surface finish, surface treatment (such as painting or galvanizing), and use of materials such as stainless steel or composite fiberglass materials, which are naturally corrosion-resistant. Aluminum, for example, should not be used in high-mineral acid environments. Stainless steel should not be used in applications exposed to high levels of salts. Some of the preferred methods of controlling corrosion are by material selection, alteration of the environment, design changes, and coatings. The most common and cost-effective corrosion protection method for metal used throughout the world is coatings.