Polymer coated steel pipe has been around for several decades, but we are just learning the full potential of the service life that this material can provide. The recent field study conducted by the National Corrugated Steel Pipe Association (NCSPA), in partnership with the American Iron and Steel Institute (AISI), is an effort to provide more information to engineers and specifiers about the expected service life of polymer coated steel pipe.
TRENCHCOAT Protective Film was developed by the Dow Chemical Company and is a tough and durable polymer film that is bonded to both sides of galvanized steel sheet coil prior to the corrugation forming process. The full product specification can be found in ASTM A742, Polymer Precoated Sheet for Sewers and Drains and AASHTO Materials Specification M245. This coating has been subjected to a battery of lab testing as well as comprehensive field studies conducted throughout the country for almost 40 years.
The research in this report covers inspections of more than 50 individual pipe sites in five states with the age of inspected pipes ranging from 16 to 38 years. The environmental conditions encountered spanned a range of severe abrasion with moderate corrosivity to severe corrosivity with moderate abrasion.
In August 2011, seven polymer coated corrugated steel pipes were inspected in the state of Wisconsin. The pipes ranged in age from 28 to 38 years old. The Wisconsin DOT representatives who visited during the inspection were pleased with the pipe performance. The same pipes were previously inspected in August 2001. At that time the coating was intact, well adhered and pliable. There was minor damage in the form of scratches in the polymer coating that was likely due to handling damage. Where the galvanized substrate was exposed as a result of this damage, no corrosion was evident. At the cut ends of the pipe there was very minor steel corrosion and nominally ¼ in. of coating undercutting (polymer delamination at a cut edge). The polymer film was tightly adhered at the edges of the undercutting.
During the ten years that elapsed between inspections, the polymer coated CSP has continued to provide excellent corrosion protection in these aggressive environments. There was no significant progression of steel corrosion from the cut ends or the damaged areas. The maximum undercutting, which was found in only isolated areas around the cut ends of the pipe entry, was 1 ¾ inches after 30 years of service.
Compiling the information from more than 50 other pipe sites, this report concludes that the majority of the polymer degradation occurred due to incidental external damage and there was no indication of a breakdown of the coating over the length of the pipe. The coating performance observed in this study further validates a significant extension of service life for these pipes and support industry guidance of an “add-on” life of 80 years to the underlying galvanized pipe, even in aggressive environmental and abrasive conditions. The NCSPA will continue to study this material, but the indication from this research is that polymer coated corrugated steel pipe is an economical solution for engineers, providing a material for cross drains, storm sewer and other drainage applications that will last more than 100 years and can exceed 125 years in normal environmental conditions.
The study was conducted in partnership with the AISI’s Steel Market Development Institute’s (SMDI) Construction Market Council (CMC).