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2. Authors: M. Zamanzadeh, Ph.D., FASM
MATCO Associates, Inc.,
NACE Certified Corrosion, Coatings,
Materials Selection/Design, Cathodic Protection Specialist
MATCO Associates, Inc.,
Manager, Sales & Marketing Division
NACE Certified Corrosion
Dick Aichinger, PE
Manager of Engineering, Valmont/Newmark
Manager of Division Quality Systems
NACE Certified Corrosion, Coatings
3. Field Corrosion Investigation “In Service Utility Poles” Study was conducted on in-service galvanized utility transmission poles.
Included soil excavation to determine below ground conditions.
Soil data collected.
Pole conditions collected.
Collected data provided information on corrosion protection.
4. On-Site Study
5. Field Observation: Mechanical Damage
6. Field Observation: Varying Water Tables
7. Pole Exhibiting Red and White Rust Above & Below Ground Line
8. First . . . A Little About Galvanizing Pole life expectancy is determined by how well we control corrosion.
Galvanized structures are protected from corrosion attack due to both the barrier effect and the galvanic (sacrificial) action of zinc.
Zinc does a fine job of protecting a steel pole in moderately corrosive and oxidizing soils.
Organic coating systems are used over galvanizing as additional protection against corrosive environments.
9. Where Do We See Corrosion? Corrosive Soils (below 5000 ohm cm)
Soils with less than 4.0 pH
High Water Tables
11. Corrosion Protection of the Coated Portion of a Pole by Galvanic Action
In service, galvanic action will protect the pole in non corrosive environments due to cathodic protection by zinc in the lower portion of the galvanized pole.
The bottom portion of the pole is anodic to the coated area of the pole.
This C.P. will help protect low thickness, pin holes and mechanically damaged areas in the coating from corrosion attack in non-corrosive to mildly corrosive soils.
12. Corrosion Control Considerations Non-corrosive oxidizing soil conditions: Organic coatings are not needed.
Reducing corrosive wet soil conditions with high water tables:
Organic coatings, concrete or corrosion resistant backfills or sealing can be used to increase life expectancy.
13. Methods to Enhance Service Life Select a coating system that maximizes corrosion protection for the environment.
In corrosive soils, coat the entire length of embedded pole.
In high water tables, seal, coat or backfill the pole interior.
Utilize corrosion inspection and monitoring.
14. Coating System Selection Select coatings that can provide adequate barrier and corrosion protection in soil and atmospheric exposures.
Cathodic delamination resistance.
15. Corrosion System Selection Should Consider Soil Conditions & Water Tables
Mapping of information on soil resistivity, pH and water tables should be developed
Determine soil corrosivity and “hot spots”
Determine type of pole corrosion protection that should be installed:
In mildly corrosive soils, (resistivity above 5000 ohm-cm), zinc alone will perform well and provide a long life. In these type of soils, no organic coating is necessary. Corrosion monitoring should be performed in long term increments (5 to 10 years) depending on soil corrosivity.
In reducing corrosive soils with soil resistivities less than 5000 ohm-cm and high water tables, galvanized poles should be coated by protective organic coatings to prevent accelerated corrosion of galvanized surfaces both internally and externally. Soil permeability will play a role in both the rate of penetration of corrosive water and the rate of removal.
16. Consider Using Corrosion Resistant Backfills Select backfills can be used in highly corrosive soils to reduce the corrosive conditions.
Select backfills provide corrosion rates comparable to typical atmospheric conditions.
Backfills can increase the life expectancy depending on soil conditions.
18. Inspect and Maintain Poles There are no maintenance free poles.
Poles in service should be inspected and maintained on a routine basis. The frequency of inspections and maintenance will depend on the corrosivity of the soil.
Also, inspect poles in storage prior to installation.
19. Poles in Storage Experience a Different Type of Corrosion Process
20. Corrosion Process During Storage Poles held in storage for an extended period of time experience a different type of corrosion mechanism than poles in service.
Moisture accumulates in pinholes, voids and mechanically damaged areas in the coating.
Corrosion cells form in these areas.
21. Corrosion Process During Storage
Corrosion of zinc takes place which can result in blistering and cathodic delamination of the organic coating.
Ultra violet (UV) sensitive coatings may also be susceptible to this type of failure.
22. In storage, there is no sacrificial action of zinc in low milage or mechanical damaged areas that exhibit corrosion attack.
23. Considerations for Poles in Extended Storage
Formation of corrosion cells in pin holes, defects, voids, and mechanically damaged areas can take place.
Additional thickness of an organic coating is recommended with a UV inhibitor additive or top coat.
Stock rotation should be implemented using the “first in, first out” method.
Areas damaged in storage need to be repaired prior to installation.
24. Final Words
In general, a galvanized coating, corrosion resistant backfills, maintenance and corrosion monitoring will greatly increase the life of utility poles.
In environments with corrosive reducing soils, stray currents and high water tables, additional protective measures to extend life expectancy are required.
Environmental information such as soil resistivity, pH, chemistry, and water tables should be utilized to determine the “hot corrosion spots”, the type of utility pole that should be installed for that particular environment and the frequency of inspection needed.
In environments where the soil is moderately corrosive, galvanic action will protect the painted portion of a pole due to cathodic protection by zinc.