Distribution Line Anchor Coatings and Galvanizing

| RUS has received some requests from manufacturers of    anchors to consider allowing RUS borrowers to use bare steel (ungalvanized or uncoated)    anchors in distribution line construction. Manufacturers advised that use of bare steel    anchors would allow them to achieve production economies that could be passed on to    borrowers. These requests prompted RUS to review anchors and RUS' requirements related to    coatings.

At the present time, RUS requires non-power-installed anchors to be:
- Galvanized in accordance with American Society for Testing and Materials (ASTM) Standard        A153, "Specification for Zinc Coating (Hot Dip) on Iron and Steel Hardware", or
- Coated with a coal tar asphalt.

There are a number of reasons for galvanized or asphalt coated anchors.    These manufacturing processes help to preserve the anchors and prevent users from having    to deal with messy, rusting, stain-producing pieces of steel in warehouses, storage yards,    and the back of line-crew trucks. However, the more important reason for the RUS    requirement is to help minimize corrosion of the anchor rods to which the anchors are    attached while in service. Corrosion of anchor rods can cause the anchor rods to fail and    can result in possible catastrophic structure failure of the power line in the anchor's    vicinity. Such failures cannot be tolerated, especially in today's utility market when    customers are expecting and demanding highly reliable electric service.

Galvanizing, which is essentially the application of zinc on the    surface of steel, is a form of cathodic corrosion protection. When placed in an    electrolyte such as soil and interconnected with steel, zinc forms a galvanic corrosion    cell. Electrochemically, zinc is more reactive than steel and, thus, the zinc becomes the    anode of the cell and corrodes. Steel, being less reactive electrochemically, becomes the    cathode of the cell and is protected from corrosion. If there is much less zinc exposed to    the electrolyte than steel, the zinc will be completely consumed and the steel would no    longer receive protection and eventually becomes susceptible to corrosion. (Galvanic    corrosion cells will form between various locations on the steel and the anodic locations    will corrode, eating away the metal.) If there is much more zinc exposed in the    electrolyte than steel, zinc corrodes rapidly at first but the two metals gradually    polarize and the galvanic corrosion cell process slows down providing long term corrosion    protection with minimal loss of zinc. Hot dip galvanizing of steel anchors and anchor rods    completely encases the two steel products and provides a high zinc-to-steel ratio exposure    resulting in efficient corrosion protection for the anchor and anchor rod.

Asphalt coatings provide corrosion protection in a different manner.    Coatings act as a barrier between a steel anchor and the soil, preventing the steel from    coming in contact with soil. Connected to a galvanized steel anchor rod, an asphalt coated    steel anchor causes less galvanic stress to the anchor rod than would be the case if the    anchor was bare steel. In the latter case, a bare steel anchor would create a very    reactive galvanic cell with a galvanized steel anchor rod. The anchor rod would soon loose    all its zinc coating and expose the steel under it to eventual corrosion and possible loss    of the rod's holding power. With a good asphalt covering on an anchor there would be much    less galvanic stress on a connected anchor rod and the anchor rod would be protected from    corrosion.

A perfect coating would not let any anchor steel contact soil and there    would be no corrosion cell created between the anchor material and the anchor rod.    Unfortunately, no coating is perfect as there are always voids or small holes in the    coating which expose very small amounts of steel to the soil. During installation of an    anchor there is always some degradation of the anchor's coating as well. However, the    overall coverage of a properly coating anchor is still effective in providing adequate    corrosion protection to the anchor rod and the anchor.

Anchor coatings need to be of the highest quality to provide the most    reliable service possible. Coatings should have a high permeability to effectively    insolate the anchor steel from contacting the soil. Coatings should also be relatively    inert with respect to the soil to ensure they do not deteriorate and otherwise will    provide long service life. As can be imagined, coatings must also have good impact    resistance to minimize damage during shipping, handling, and installation. A coating must    also have excellent adherence characteristics so that it makes and maintains an effective    bond to the anchor's steel. A coating should also have low water absorption properties to    help keep moisture from coming into contact with the steel surface of the anchor.

RUS also requires that coatings be electrically conductive to minimize    radio and television interference problems. The normal electrical field stresses existing    near the high voltage phase conductors electrostatically couple to the anchor guy and    anchor rod seeking a pathway to ground. With a galvanized anchor or an anchor with a    conductive coating, these stresses are easily grounded and alleviate the stresses. The    grounded anchor assembly also is much safer for line crews and the public. If an anchor is    coated with a material that is electrically insulating, the coating and steel under it    acts like a capacitor. The phase conductors' electrical stresses still couple onto the    anchor guy and flow down through the anchor rod and then flow to ground through the    coating's "capacitor" causing a voltage drop to appear across the anchor-soil    interface. The anchor assembly now acts like a small low-power radio station with a 30 to    45 foot (9 to 14 meter) antenna which can generate a broad spectrum of the    powerline's harmonic frequencies which include commercial radio and television    frequencies. Commercial radio and television signals are usually weaker in rural areas    making reception troublesome anyway. A noisy anchor assembly near a consumer's residence    can wreck havoc with reception and make listening or watching or both impossible.

RUS' review of anchor manufacturer's request to use paint instead of    asphalt coatings involved Electric Staff Division (ESD) visits to several anchor    manufacturers and RUS borrowers. During visits to borrowers we learned that borrowers are    quite satisfied with asphalt coated anchors and that there was no aversion to handling and    installing them. During visits to anchor manufacturing plants, ESD discovered that various    coating application methods and differing coating thickness are resulting on finished    anchors.

RUS is currently in the process of developing an anchor coating    specification which will be used to evaluate anchors for RUS acceptance. Such a    specification will help to ensure that anchor coatings are consistently satisfactory.

If you would like more information or have any questions, please call    George Keel, Equipment Specialist, Distribution Branch at (202) 690-0551.
 

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