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Buried steel items are generally exposed to a wide range of corrosive forces that are quite different from those experienced in atmospheric exposure settings. Moreover, the performance of steel as well as galvanized steel in-ground applications is not as properly understood as is the durability of these kinds of materials in above-ground applications.
A great deal of research with regards to the management of underground corrosion has been carried out, specifically with pipelines and associated services.
Corrosion Factors In-Ground
When zinc and steel are exposed to soil, they both react in different ways. Therefore, a better insight into the performance of both these materials when they are exposed to soil makes it possible to accurately determine the service life of the structure.
Moisture, oxygen, and the presence of dissolved salts will cause corrosion in steel. Absence of any of these factors will either stop the corrosion reaction or allow it to proceed quite slowly. In acidic environments, steel will corrode rapidly but as alkalinity is increased, it will corrode gradually or not at all.
To make zinc resistant to corrosion, stable oxide films have to be present on its surface. Zinc performs optimally in neutral pH environments, even though it can withstand exposures in the pH range of 5.5 to 12. When air is absent, the stable oxide films will not form on the surface of zinc but when moisture is present under these conditions, corrosion can be expedited.
Due to this reason, galvanized steel offers the best combination in which structures are partly exposed to the atmosphere and partly buried, as steel performs unexpectedly in-ground, while zinc offers the durability above-ground.
Soil Types and Corrosion
In soil, metal corrosion is highly variable and although the soil environment is complicated, some generalizations can be made about the type of soil and corrosion. Any specified soil is a highly heterogeneous material comprising of three phases as described in the following sections.
The solid phase is composed of soil particles that will differ in chemical composition, size, and the level of entrained organic material. The aqueous phase is the soil moisture—the vehicle that promotes sustained corrosion. The gaseous phase contains air entrained in the pores of the soil. Some part of this air may get dissolved in the aqueous phase.
The Solid Phase
Soils are divided based on their chemistry and their average particle size. As per the convention, particles measuring more than 0.07 mm to about 2 mm are classified as sands, particles from 0.005 to 0.07 mm are classified as silts, and particles from 0.005 mm and smaller are labeled as clays. Soils seldom exist in the presence of just one of these components. Clay soils are defined by their potential to absorb water quickly. Due to this reason, clay soils pose a considerably higher risk of corrosion compared to sandy soils.
The Aqueous Phase
Soil moisture can be divided into three types, namely, free groundwater, gravitational water, and capillary water.
Free groundwater is governed by the water table, which can possibly range from ground level in swampy regions to several meters underneath the surface. This is the least significant factor in establishing corrosion since the majority of the buried structures are above the water table. If there are high water tables, they will cause the buried structures to act as if they were in a submerged environment.
This results from condensation, irrigation, or rainfall, soaking within the soil at a rate established by its permeability. The period of wetness of the surface of the metal will be determined by the frequency of contact. In regions of regular heavy rainfall, most of the soluble salts could have been leached from the soil. In desert regions with low rainfall, there may be extremely high salt levels, and hence, these regions can be more corrosive to buried metals when compared to tropical environments.
Capillary water is the water entrained in the pores and on the soil particle surfaces. Although the potential of soil to retain moisture is important to plant growth, capillary water is the main source of moisture in establishing the corrosion rates of metals in soil.
The Gaseous Phase
The permeability of soil controls the access of gas, or air, into the soil. Coarser grained soils or drier soils will enable more oxygen access to the sub-surface and boost the rate of steel corrosion in relation to the oxygen-deficient regions.
Corrosion Rates and Australian Standards
The AS/NZS 2041-1998 standard for buried corrugated metal structures includes a great deal of valuable information on tables and makes it possible to determine the product life in-ground. These tables consider soil resistivity, which factors in associated problems like levels of dissolved salts, soil characteristics, and pH. The resulting information is subsequently related to in-ground corrosion rates for both steel and zinc.
Steel’s corrosion rate in the soil can vary from less than 20 µm per year in favorable conditions to 200 µm per year or above in highly aggressive soils. In a similar way, galvanized coatings may corrode at below 5 µm per year in mild conditions to 25 µm per year or above in unfavorable soils. When the range of these corrosion rates is defined in a specific application, buried metal structures could be designed with a barrier coating, offering conditioned soil in contact, or with a corrosion allowance on the steel to achieve the preferred design life. In moderate soil conditions, an additional 1 mm of steel thickness can offer an extra five decades of service life.