Inspection

Steel Pole Below Grade Coatings & Corrosion Failure and Remediation

November 30, 2020

For the last five decades, tubular steel monopoles have been a go-to and reliable product for the utility industry. Steel transmission poles have two primary foundation variations for these structure types. 

Steel pole after corrosion repair

These pole foundations are known as either “anchor based” or “direct embed”. While anchor based structures are bolted to a concrete pier foundation that extends above grade, direct embedded poles are buried a certain depth into the ground. These direct embedded poles require a protective coating applied at the factory that prevents corrosion on the steel shaft at or just below grade, where soil conditions are the most corrosive. 

Coatings Failure Factors

There are a number of different factors that can lead to premature coatings failure. And while steel poles are engineered for a minimum fifty year lifecycle, the coating systems rarely last that long. 

Contractor fixing corrosion on steel pole

Variables that may contribute to coatings failure consist of incorrect material specifications, fabrication defects, and installation or construction errors. The following is a case study outlining a recent coatings inspection and repair project in northern Iowa. 

What Happened?

This utility client had 160 weathering steel direct embed transmission poles built in 2007. Though still relatively young in age, these poles demonstrated visible coating degradation above grade. On nearly all of the poles, the coating was peeling and flaking aways from the surface of the steel pole.  The client’s primary concern was “wall loss” on the exposed pole shaft that could potentially lead to structural failure. 

Exo was engaged to perform a steel pole corrosion assessment which included the measurement and documentation of any wall loss damage caused by corrosion. We calculated the remaining strength capacity of the towers and also restored the coating system back to its original condition and fully protective capacity. 

To accomplish this, we excavated around the pole to determine the extent and depth of the failed coating.  After removing the degraded material, NDT was performed to measure wall loss via ultrasonic testing and a VWAC gauge. Through this inspection, we determined that the corrosion was discovered early enough such that no immediate structural risk was present for these poles based on their current loading and engineered design capacity. Our technicians applied a new coating and successfully filled the excavated hole. 

Corrosion on a steel pole

Results of the Project

From Exo’s inspection, we determined that several issues led to the premature failure of the steel pole coating system. The first was a common manufacturing mistake that we see quite often. When the factory applies coating to the pole surface, sometimes they will mask the boundaries of the coating area with tape to make a smooth, clean line or edge. When the tape is removed, a distinct ledge is left that can collect and hold moisture once the pole is installed in the field.  This trapped moisture will ultimately work its way behind the coating barrier, at which point corrosion can start to advance rapidly on the pole shaft. This mode of failure can happen to both galvanized and weathered steel poles. Our inspection also determined that the coating thickness measured roughly half of the engineered dry film thickness (DFT). 

Some installation errors also contributed to early failure. Poles have a very specific embedment depth: 10% plus two feet of the overall pole height. A protective sacrificial ground sleeve is welded to the pole to protect from corrosion at grade. The coating system is applied over the top of the ground sleeve. For this particular transmission line, the poles were set too deep, meaning the ground sleeve and coating system were buried and thus unable to protect the pole at grade. 

In these cases, especially farm land where earth and dirt is pushed up against the pole, the protective features are buried and are not able to protect the pole where it matters most, which can lead to corrosion and structural damage on the pole shaft.   

Exo Has Your Back!

In this case, the client acted quickly and we repaired the coatings and stopped further corrosion damage before the poles were weakened to the point of needing structural repair or replacement, saving the asset owner enormous sums of money. 

To avoid the risks associated with a coatings failure and saving yourself critical time and money, a full life cycle approach to steel pole corrosion assessment and remediation is essential. Build a robust quality assurance plan that prevents defects like the one above from impacting your project. Verify the construction teams are installing the assets properly and to your specification. Proactively monitor the poles with NDT inspection in case certain conditions change. And catch and repair issues early while they can be fixed cost effectively. 

This is certainly not something you want to leave to chance. You could end up with a safety risk and a big bill to pay. With services like Exo’s Ex-Tend asset management program, we strategize the best ways to approach lifecycle management and to protect your assets while saving you money. 

Whether it is through lifecycle management, field service and repair, or vendor audits and surveillance, Exo is uniquely prepared to serve your company’s needs and continue providing the asset management services  and utility services vital to all of your operations.

At Exo, we don’t wait for problems to arise before we take action. Stop problems like the ones in this photo before they occur by calling today to talk with the Exo experts. Phone us at 281-259-7000 or contact us online for a free consultation to see how Exo can protect your assets.

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