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:
SELF-WEATHERING STEEL LATTICE TOWERS
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.
SELF-WEATHERING STEEL POLES
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.
CONCRETE ENCASED STRUCTURES WITH OVERBURDEN SOIL
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 WITHIN A SHARED RIGHT-OF-WAY
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:
- 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: