Corrosion protection and finish are important considerations when designing a line using steel structures. The most commonly used corrosion protection and finish options used for steel poles include (1) paint and coating systems, (2) galvanizing, (3) weathering steel, and (4) metalizing.
For comprehensive information regarding these protections, refer to ASCE/SEI 48-11 Design of Steel Transmission Pole Structures.
1. Paint and Coating Systems
A large variety of paint and coating systems are available that not only provide corrosion protection for the underlying steel but also offer a choice of color.
- Above-Grade Protection
- For above-grade corrosion protection of steel poles, superior paint system use zinc-rich primer. These types of primers offer cathodic protection.
- Below-Grade Protection
- For the embedded portion of steel structures, a thick coating of polyurethane or epoxy material provides significant protection.
- Additional protection us a welded steel ground sleeve that should extend 2-4ft above and below the grade line.
Galvanizing, in the context of transmission structures, generally refers to the hot-dip process. This process involves the immersion of the section into a bath of molten zinc.
Hot-dip galvanizing has been an economical means of corrosion protection for steel structures. It has been used extensively because of its dynamic protective nature consisting of a barrier coating and zinc’s sacrificial action.
Double-dipping is not recommended for tubular structures because corrosion can occur in the overlap portion.
3. Weathering Steel
Weathering steel develops a tight oxide coating that protects against continuing corrosion of the substrate. This type of steel develops a finish that is relatively maintenance-free.
Weathering steels should not be used in bare conditions in atmospheres with high concentration of corrosive chemicals, industrial fumes or severe continuous salt fog or spray environments. It should not be buried without a method of protection against corrosion.
The structure may be protected against corrosion by thermal spraying a coating over the base metal. Thermal spraying includes flame spraying, electric arc spraying, and plasma spraying.
Hot-Dip Galvanizing Process
The typical step-by-step process of hot-dip galvanizing is shown below. Images are courtesy of PHILGALV INDUSTRIAL COATING INC during our visit last October 2019.
1. LOADING AND JIGGING
Proper loading of steel items for galvanizing to ensure proper venting and draining.
Use a hot alkali solution to remove organic contaminants such as dirt, paint markings, grease, and oil from the steel surface.
3. HYDROCHLORIC ACID-PICKLING
Uses a diluted solution of hot sulfuric acid or ambient temperature hydrochloric acid to remove scale and rust from the steel surface.
4. WATER RINSE
Removal of acid residues.
Pre-fluxing in zinc ammonium chloride solution for surface conditioning.
Pre-heating accelerates the whole HDG process by 20% compared to the conventional type. It also reduces the violent reaction during the immersion process.
7. ZINC BATH
Immersion of steel to molten zinc at 450-455 degrees centigrade. The product is immersed until it reaches the temperature of the kettle. The zinc then reacts with the steel to form a zinc-iron intermediate alloy at the steel surface.
Passivation of the zinc surface to prevent early oxidation.
9. DRESSING AND QUALITY CONTROL
Removal of excess zinc, sharp ends, and drippings.
10 Major Measurable Benefits of Hot-Dip Galvanizing
- Lowest First Cost.
- In countries where HDG is widely used, galvanizing is lower in first cost than other commonly specified protective coatings for steel due to increasing labor and material cost.
- Less Maintenance / Lowest Long Term Cost.
- It is cheapest in the long term.
- Long Life
- The life expectancy of galvanized coating on typical structural members is far in excess of 50 years in most rural environments, 20-25 years in severe urban and coastal exposure.
- Galvanizing is carried out to internationally accepted standards. Coating life and performance are reliable and predictable.
- Toughest Coating
- A galvanized coating has a unique metallurgical structure which gives outstanding resistance to mechanical damages in transport, erection and service.
- Automatic Protection for Damaged Areas
- Galvanized coatings corrode preferentially to steel, providing cathodic or sacrificial protection to small areas of steel exposed to damaged.
- Complete Protection
- Every part of the item is protected even in recesses, sharp corners, and inaccessible areas.
- Ease of Inspection
- Galvanized Coatings are assessed readily by eyes and simple non-destructive thickness testing methods.
- Faster Erection Time
- As galvanized steel members are delivered, they are ready for use. No time is lost on-site in surface preparation, painting, and inspection.
- A Full Protective Coating can be Applied in Minutes.
- A 4-coat paint system requires a week. The galvanizing process is not dependent on weather conditions.