Once Upon a Time … A Corrosion Fairy Tale
This issue of Insulation Outlook is focusing on corrosion under insulation (please feel free to check the front cover to see if I should have stopped to ask for directions). Fortunately, corrosion only occurs under insulation once every 3 or 4 years, because it can be a considerable threat to structural integrity. I just happen to be a corrosion engineer. (And that merely acknowledges the sequence of chance events that brought this about—relatively few of my colleagues always dreamed of being corrosion engineers. I wonder how many of you, when you were a 5-year-old future firefighter or cowboy, would have punctuated career day at school with, “Insulation is where it’s at!”)
I’m also a longtime member of The National Association of Corrosion Engineers (NACE) International—The Corrosion Society, and have contributed more or less monthly columns to the society’s principal periodical for more than 5 years now. That is, until I tested positive for steroids. (I never knowingly took steroids—honestly, I thought it was morphine—though I have experienced some conspicuous muscular development that has unexpectedly improved my competitive eyebrow-raising skills).
This would seem an appropriate time to mention that nothing contained in this article should be construed as an official statement on behalf of NACE, NIA, my company, or your company—I’m not even sure I’m speaking for myself.
It is noteworthy that NACE’s technical committees are readopting Publication 10A392, “Effectiveness of Cathodic Protection on Thermally Insulated Underground Metallic Structures.” It is not necessarily a coincidence that this roughly corresponds to the
25th anniversary of the most significant advance in the prevailing approach to corrosion control for these structures: the use of barrier coatings on structure surfaces prior to the placement of insulation.
Paraphrased in a nutshell (how fitting), here are the four
general approaches to corrosion control:
- Material selection—especially the use of components that resist corrosion in specific environments
- Inhibitors that reduce the corrosiveness of the environment, typically inside a tank or piping system
- Coatings and linings, typically with dielectric properties intended to isolate surfaces from corrosive environments
- Cathodic protection (CP)
CP has been variously defined, but one view is that metal loss (the main reason for all the fuss about corrosion) happens when current leaves a buried or submerged surface at anodic sites in the form of electrically charged metal ions; CP attempts to minimize this discharge by applying an external current to the entire structure surface. This makes all sites cathodic. Reduced current discharge means reduced corrosion.
Once Upon a Time
Once upon a time (before some of today’s best practices were in common use), in the magical land of the northern Midwest, there was a direct-buried steam-piping system with fiberglass thermal insulation in half shells and a thin outer moisture barrier of galvanized steel—hardly the best choice for direct burial. The real problem, though, was underestimating the effectiveness of the insulation in preventing heat loss to the environment, which might have avoided winter soil stresses. Instead, the ground froze (and expanded), the outer shell was crushed, and the spring thaw brought oxygenated water into contact with the pipe steel at joints and voids.
An external CP system had been installed, but current distribution is essentially linear. And CP current actually promotes moisture migration. When the water migrated along under the insulation, adequate protective current could not reach all of the corroding sites. The same conditions limited the ability to identify active corrosion with traditional test methods for CP evaluation. The resulting failures were basically consistent with the fundamental conclusion of 10A392: “Generally, the application of external CP to thermally insulated metallic surfaces has been ineffective.”
Surface coatings are much more successful in conjunction with a nonmetallic external jacket as a moisture barrier. The established developments in coatings and linings, material selection, and construction practices, along with evaluation techniques like internal pipeline inspection tools, make it possible to use the advantages of thermal insulation for underground metallic structures. And that greatly reduces the risk of corrosion failures, so we can all live happily ever after.