When most people hear the word corrosion, they often think of metals and instances, such as a corroded battery or rusty nail. Corrosion is an electrochemical reaction that occurs in metals, even buried metals, such as helical piles, that gradually degrades the material causing material loss including reduction in thickness and area. The reaction takes place when differences in the surface potential also changes with time and the anode can move to another location. The difference between two points that are electrically connected and immersed in an electrolyte, such as moist soil, cause a small current to flow from the anode area through the electrolyte to the cathode area and then through the metal to complete the circuit. The anode area has the most negative potential and is the area that becomes corroded through loss of metal ions to the electrolyte. Steel helical piles are susceptible to metallic corrosion even when driven beneath the ground as soil can pose as a corrosive environment.
The rate of corrosion in soil is determined by the soil resistivity or the measure of soil characteristics, such as pH, moisture content, oxygen availability and anaerobic bacteria, that can enhance or restrict electrochemical corrosion reactions. For instance, the more conductive and acidic the soil, the more corrosive the soil is. Moisture content of soil has the most profound effect when considering corrosion potential. Sandy soils that easily drain water away are typically non-corrosive, while clay soils that hold water are characteristically corrosive. Generally, backfill material will be more corrosive than native earth because the backfill soil has a higher moisture content.
Based on environmental conditions corrosion rates may vary. It is important to take into consideration the ratio of surface area to weight in order to understand the predicted time it takes for corrosion in a pile to reach the weight limit included in the design. There are many options for minimizing the effects of corrosive soils, such as using zinc coatings on helical products, which is one of the most common passive control methods for increasing pile life in corrosive environments.
Zinc coatings are applied using several methods, including hot-dipped galvanizing and plating. Zinc provides a sacrificial anode and the cathode is removed. As a result, the anode/ cathode system is disrupted resulting in reduced corrosion.
Zinc is extremely durable that even when damaged, the zinc corrodes before the steel is attacked because it is higher on the galvanic scale. Hot-dipped galvanized steel can be “double dipped”, doubling the thickness of the galvanizing and generally doubles the life of the coating. Galvanizing is a common protection option because it is effective against atmospheric corrosion and is also cost conscious. The hot-dip galvanizing process creates a metallurgical bond to the steel, which forms zinc-iron alloy layers. The iron/zinc alloy layers become harder with increasing iron content to where the inner alloy layers are harder than that of the base steel. This is critical for helical anchors and piles since the coating must be both flexible and hard to stand up to the abrasion during installation. A hot-dip galvanized pile is coated on all surfaces including the interior of the shaft and along the edges.
The passive control of using hot-dipped galvanizing steel with a zinc coating has a metal loss allowance of 1/8”. The service life is defined as the estimated length of time required for 1/8” of material loss to occur on the helical pile/anchor. When hot-dipped galvanized steel is used, the total service life should be increased by the time it takes the zinc coating to be lost due to corrosion. For example, in moderately corrosive soil, such as slightly moist clays, dry silts and loams, sand and gravel, a bare steel helical pile has a service life of 55 years, while a hot-dipped galvanized helical pile has significantly longer service life at 140 years.
At Carolina Foundation Solutions (CFS), we use only the best and most proven products, equipment and material in the foundation repair industry. We are a premier certified installer of CHANCE helical products. We use CHANCE helical products that solely consist of hot-dipped galvanized steel, material which has been tested and proven reliable over decades in all types of soil conditions, such as expansive soils, high water tables, fill areas and other areas where unstable soils require loads to be transferred to a stable bearing soil layer. Trust CFS with your foundation repair and deep foundation system needs, and get products that stand the test of time and leave you with a permanent solution.
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