Ground Level/In Soil
At ground level, corrosion mechanics of HDG are less defined than in atmospheric exposure. Soil may be piled up against the lattice tower legs or poles and alternately removed and/or grass and weeds may grow up around the steel, abrade it, decay around the base, and remain moist. The corrosive elements in this environment can be very unpredictable and it doesnt really follow the rules of exposure in the atmosphere or in soil; its somewhere in between.
One power structure often exposed to all three environments (atmosphere, in soil, ground level) is transmission and distribution poles. They require a coating able to provide consistent corrosion protection in each of these varying conditions. Galvanizings abrasion resistance and impervious barrier protection ensure it can perform in this transitionary environment.
Longevity in Soil
Hot-dip galvanizings performance in soil is varied and hard to predict. In power transmission and distribution, it is common for hot-dip galvanized steel elements to be buried in the soil. The main factors that dictate the corrosivity of the soil are moisture content, pH level, and chlorides. These soil conditions are affected by additional characteristics of the soil such as aeration, temperature, resistivity, and texture or particle size. A general rule of thumb is galvanizing performs well in brown sandy soils, and not as well as gray, clay-like soils. This is because soil with larger particles wick moisture away from the surface more quickly so the galvanized piece has less exposure to moisture.
In order to predict the performance of hot-dip galvanized steel in soil, you must first classify the soil. And as the corrosion rate of steel in soil can range from less than 0.2 microns per year in favorable conditions, to 20 microns per year or more in very aggressive soils, misclassifying the soil can lead to unpredicted performance.
The AGA has developed a new chart for estimating HDGs performance in soil based on real-world corrosion data. Service life as defined by the chart is total consumption of the coating plus 25%, and is an indication of when the structure should be replaced. There are four different charts based on the characteristics of the soil.
To use the chart, you must first classify the soil by chloride content Charts 1 and 2 are used for soils with high chlorides (>20 PPM) and Charts 3 and 4 are used for soils with low chlorides (<20 PPM). Once you have identified the chloride content, there is a second classification to determine the correct chart to use. For soils with high chlorides, the second determination would be moisture content. Soils with low moisture (<17.5%) fall on Chart 1, while soils with high moisture (>17.5%) fall on Chart 2. For low chlorides, the second determination is the pH level. Soils with high pH levels (>7.0) fall on Chart 3, while soils with low pH (<7.0) fall on Chart 4.
The blue line on all four charts represents the average for soils surveyed in that characteristic group. The green line represents the best soil in the category sampled, and the red line represents the worst soil in the category from the study. The shaded areas show how the changes in pH and moisture content affect the estimated service life. Assuming 3.5 mils as a minimum thickness for HDG buried in soil, the chart shows the average life in the harshest soils (uncommon) would be approximately 30 years and in the best soils exceed 120 years.
Longevity Case Study
Bass River Crossing - Stafford Township, NJ 2011
The project was installed in a salty and corrosive environment. The customer required 5.1 mils of galvanizing and the base sections needed to be corrocoated after galvanizing. The cost to galvanize instead of paint was much more cost-effective. With the poles being galvanized they will require very minimal repairs to the coating. Our galvanizer had the capabilities to turn the material in 24 hour turn time. This order had delivery penalties if it was delayed. The galvanizer helped meet and exceed the customer's expectations for delivery and quality. Our customer specified the material be hot-dip galvanized with 5.1 mills. They also required the base sections to be corrocoated after galvanizing. These poles were embedded into the ground between 25-30 feet deep for stability of the sections.
The six structures were oversized and extremely heavy. Four of the base section caisson were corrocoated inside and out after galvanizing. The base sections weighed nine tons per pole with a total of four sections. The project required the poles to be embedded into the ground next to the Bass River. The soil was soft and sandy which required the poles to be embedded 25 to 30 feet into the ground. The total project was designed, detailed, fabricated, galvanized and delivered 2,500 miles in five weeks.
The customer was extremely pleased with the performance of the entire team on this project, from the quote and design phase to the installation. They were very pleased the project was turned so quickly from the fabricator and the galvanizer. They also commented at the ease of installation of the poles and caissons due to the fabrication and galvanizing cleanup. The 5.1 mills of the galvanized coating was exactly what the customer required for the coastal and salty environment. The project was designed, detailed, and fabricated by a company in Muskogee, Oklahoma.