Below-Grade Corrosion 

What You Can't See, Can Hurt You

Although once thought to be maintenance-free, steel has proven that it does indeed require inspection and maintenance. Steel structures degrade over time due to corrosion activity and, to a lesser degree, from mechanical damage. According to a recent article in Electricity Today, the associated cost of corrosion for electric power generation and delivery is well in excess of 6.9 billion dollars annually.

A review of 25,000 steel T&D structures (35-45 years of age) inspected by Osmose Utilities Services, Inc. over the last nine years revealed the following:

  • 12% of structures had 10% - 24.9" cross section loss and required remediation or repair
  • 8% of structures were in need of significant repair with an average cross section loss of 25% or greater.  Of these structures, 2% were priority structures in need of immediate attention (with an average cross section loss of 50% or greater)

Failure How & Why

Original protection systems, including factory-applied galvanizing and coatings, help to reduce or eliminate the effects of corrosion by creating a barrier between the steel and the soil in its environment. As structures age, the initial protection systems begin to deteriorate. Deterioration of the original protection allows environmental influences to directly impact the steel itself. In many cases failed factory-applied coatings have actually become detrimental to structures because they not only allow moisture to come in contact with the structure, they encapsulate the moisture, holding it against the structure and accelerating the corrosion process.
Once the steel is exposed the natural process of corrosion activity sets in and begins to thin and weaken the steel. This typically begins at the ground line and proceeds downward along the surface of the structure below grade. The speed at which this process occurs can be accelerated by several contributing factors including:
  • Soil type
  • Moisture
  • Agricultural activity
Other factors that aid in the corrosion process are typically not associated with the environment but are directly related to the structure. These include:
  • Design features
  • Construction materials
  • Dissipation of static current

At Risk Structures

Structures most at risk for corrosion activity are those structures whose initial protective coatings have begun to deteriorate, as well as those structures placed in environments that can contribute to accelerated corrosion. Other structures at risk include:
These towers are susceptible to a phenomenon referred to as "pack rust" or "pack out." Pack rust occurs when water enters into a bolted joint and does not dry out. As the water permeates the original corrosion layer the un-activated steel beneath it reacts to the water and creates another layer of corrosion. This process sacrifices a small layer of good steel in order to create the layer of corrosion. As this occurs, the steel in the area of the pack rust activity thins, eventually weakening the steel. Pack rust will continue to create more and more layers as it remains wet and will result in a wedge of rust or "pack out" in the joints causing strain within the bolt group.
These poles are subject to pack rust primarily at the ground line, especially in areas where the factory-applied coating has failed. In these instances, the pack rust continues to build layer upon layer until it sloughs off, thinning out the pole in a manner similar to lattice towers which can create perforations in the pole wall.
Oberburden soil occurs when migrating soil from water, wind, or agricultural activity builds up on top of the concrete foundations, directly contacting the steel. This is especially destructive on structures where either the galvanizing or coatings have deteriorated. It typically results in a concentrated band of corrosion extending from the top of the concrete foundation to the top of the soil.
Anchors on both wood and steel structures are at risk for corrosion, especially those found to conduct current to ground. In areas where current is dissipating from the anchor, as much as one pound of steel can be lost for every one amp of direct current (DC) annually.  Typically, this current is measured in milliamps of current so the loss of steel occurs more slowly, but is significant nonetheless.
Structures that share a right-of-way with other utilities can be subject to additional influences that contribute to corrosion activity. A gas pipe line is a prime example. In some instances, cathodically protected (CP) gas pipe lines can indirectly impact the corrosion activity on electric utility structures including steel towers, poles and anchors. In such cases, current from the CP system (mostly from rectifiers) finds its way onto the steel structure through the soil and then discharges back into the ground. This process is typically referred to as "DC interference" or "DC uptake." In these situations, damage does not occur where the current is drawn onto the structure, but rather where the current discharges back into the ground.
In many cases, one or more of these issues can exist at the same site. Through thorough inspection and by taking environmental condition measurements at the site, technicians can help determine the extent of corrosion activity currently taking place and help identify contributing influences.

Challenges for Utilities

Many utilities currently don't have a sustainable program to address corrosion and corrosion-related issues. Without identifying the level of need most cannot create a business case to acquire the funding necessary to support a full-fledged program including inspection, repairs and mitigation.
The few utilities that do have programs in place typically do not have the necessary resources in personnel and expertise to manage and support it appropriately.

Developing and Inspection Program

Program drivers are similar, but vary by level of importance to the specific utility. These include, but are not limited to:
  • Age
  • Structure type (including foundation)
  • Material type
  • Geographic location
  • Line importance
  • Failure and maintenance history
  • Previously installed corrosion control system
  • Previous inspection history
  • Grounding system

Most often the first step in determining whether an inspection program is necessary is to determine the most critical items from the list of program drivers and then weight them accordingly. By utilizing this approach, a prioritized list of lines can be developed to focus on those structures most important to the utility. Once developed, a sampling of structures from several of the most critical lines can be selected to initiate a pilot project.

Components of a pilot project usually include:
  • Line Structure Selection - This is a critical aspect of the project as it will define its results. Selecting a sampling of lines and structures that are representative of the utility's system as a whole will provide more representative results of the entire system. This will allow for a more clear assessment of the system condition and help to determine guidelines for the development of a larger system-wide corrosion program.
  • Inspections - The inspection process involves two primary types of evaluations: 1) structural assessment of each structure to help determine existing corrosion and its effect on the integrity of the structure 2) determination of key predictive environmental indicators present at each site which influence the rate of corrosion activity. Structural members encased in concrete receive rudimentary concrete evaluation only. In addition, all structures receive a visual overhead inspection primarily for safety purposes.
  • Data Analysis - The data from the inspection process is reviewed and evaluated by Corrosion Engineers and the structures are categorized.
  • Summary Report - A summary report of the pilot project findings is written and reviewed by engineering staff to convey the comprehensive findings in a consolidated report.
  • Repair Recommendations - Structures that have deteriorated beyond the protective capabilities of coatings and anodes are usually significantly weakened by section loss of their supporting structural members or foundations. In most cases, in-place repairs can be individually engineered for these structures in order to avoid the high cost of replacing the entire structure.
Based on the results of the pilot, assessment, repair, and remediation options can be evaluated for inclusion into a more comprehensive program approach.


Safety First


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