Feb 26 2002
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Buried steel products are open to a wide variety of corrosive forces that are rather different from those endured in the atmosphere. The durability of both steel and galvanized steel in in-ground applications is not properly understood, like that experienced above-ground.
A lot of research has been performed to handle underground corrosion, particularly in pipelines and related services.
Corrosion Factors In-Ground
When steel and zinc are exposed to soil, they react in various ways. Thus, a better knowledge of the performance of both these materials when exposed to soil makes it possible to precisely define the service life of the structure.
Oxygen, moisture, and the presence of dissolved salts will lead to corrosion of steel. The absence of any of these factors will either slow the corrosion reaction or prevent it altogether. In acidic surroundings, steel will corrode quickly but as alkalinity rises, corrosion will decrease.
To make zinc corrosion-resistant, stable oxide films should be present on its surface. Zinc behaves ideally in neutral pH environments, even though it can endure exposures in the pH range of 5.5 to 12. In the absence of air, the stable oxide films will not develop on the surface of zinc. But when moisture is present under these circumstances, corrosion can be accelerated.
Consequently, galvanized steel is preferred for structures that are partially exposed to the atmosphere and partially buried, since steel performs unpredictably in-ground while zinc provides the durability above-ground.
Soil Types and Corrosion
Metal corrosion is extremely variable in soil and although the soil environment is complex, some generalizations can be made regarding the type of soil and corrosion. All soils are extremely heterogeneous and include three phases.
The solid phase is made up of soil particles that will vary in size, chemical composition, and level of entrained organic material. The aqueous phase is the soil moisture which encourages sustained corrosion. The gaseous phase comprises air trapped in the soil pores. Some portion of this air could get dissolved in the aqueous phase.
The Solid Phase
Soils are categorized based on their chemistry and average particle size. According to the convention, particles measuring 0.005 to 0.07 mm are classified as silts, 0.07 mm to about 2 mm are classified as sands, and 0.005 mm and smaller are classified as clays. Soils rarely exist in the presence of merely one of these components.
Clay soils are determined by their potential to absorb water rapidly. Therefore, clay soils pose a significantly higher risk of corrosion than sandy soils.
The Aqueous Phase
Soil moisture can be separated into three types: gravitational water, free groundwater, and capillary water.
Free Groundwater
Free groundwater is controlled by the water table and can range from ground level in swampy areas to several meters under the surface. This is the least important factor in determining corrosion since most of the buried structures are above the water table. If there are high water tables, they will cause buried structures to behave as if they were in an inundated environment.
Gravitational Water
Gravitational water results from irrigation, condensation, or rainfall soaking into the soil at a rate determined by its permeability. The period of wetness of the metal surface will be defined by the frequency of contact. In areas of steady heavy rainfall, plenty of the soluble salts could have been leached from the soil.
In desert areas with low rainfall, there may be very high salt levels, and thus, these areas can be more corrosive to buried metals when compared to tropical atmospheres.
Capillary Water
Capillary water is the water that gets entrenched in the pores and on the surfaces of soil particles. Although the potential of soil to keep moisture is crucial for the growth of plants, capillary water is the leading cause of metal corrosion in soil.
The Gaseous Phase
Soil permeability establishes the amount of gas in the soil. Drier soils or coarser-grained soils will permit more oxygen into the sub-surface and increase the rate of steel corrosion in relation to the oxygen-deficient areas.
Corrosion Rates and Australian Standards
The AS/NZS 2041-1998 standard for buried corrugated metal structures comprises a significant amount of useful information on tables and makes it possible to establish the product life in-ground. These tables look into soil resistivity, which factors in related issues like levels of soil characteristics, dissolved salts, and pH. The information, thus obtained, is then related to in-ground corrosion rates for both zinc and steel.
Summary
Steel’s corrosion rate in the soil can differ from less than 20 µm annually in conducive conditions to 200 µm annually or above in very aggressive soils. Similarly, galvanized coatings may disintegrate at below 5 µm per year in mild environments to 25 µm per year or above in hostile soils.
When the range of these corrosion rates is established for a particular application, buried metal structures could be built with a barrier coating, corrosion allowance, or conditioned soil to accomplish the desired design life. In moderate soil environments, an extra 1 mm of steel thickness can offer an additional 50 years of service life.