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Buried steel items are exposed to a wide range of corrosive forces that are quite different from those experienced in the atmosphere. The durability of steel, as well as galvanized steel, in in-ground applications is not as well understood as that experienced above-ground.
A great deal of research into the management of underground corrosion has been carried out, specifically in pipelines and associated services.
Corrosion Factors In-Ground
When zinc and steel are exposed to soil they react in different ways. Therefore, a better insight into the performance of both these materials when 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. The absence of any of these factors will either slow the corrosion reaction or stop it completely. In acidic environments steel will corrode rapidly but as alkalinity increases corrosion will reduce.
To make zinc resistant to corrosion, stable oxide films must 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.
As a result, galvanized steel is preferable for structures 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. All soils are highly heterogeneous and comprise of three phases.
The solid phase is composed of soil particles that will differ in chemical composition, size and level of entrained organic material. The aqueous phase is the soil moisture which 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 0.07 mm to about 2 mm are classified as sands, 0.005 to 0.07 mm as silts, and 0.005 mm and smaller 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. As a result, clay soils pose a considerably higher risk of corrosion compared to sandy soils.
The Aqueous Phase
Soil moisture can be divided into three types: free groundwater, gravitational water and capillary water.
Free groundwater is governed by the water table and can range from ground level in swampy regions to several meters below the surface. This is the least significant factor in establishing corrosion since the majority of buried structures are above the water table. If there are high water tables they will cause buried structures to act as if they were in a submerged environment.
This results from condensation, irrigation or rainfall soaking into 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 therefore these regions can be more corrosive to buried metals compared to tropical environments.
Capillary water is the water engrained in the pores and on the soil particle surfaces. Although the potential of soil to retain moisture is important for plant growth, capillary water is the main cause of metal corrosion in soil.
The Gaseous Phase
The permeability of soil determines the amount of gas present in the soil. Coarser grained soils or drier soils will allow more oxygen into 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 for a specific application, buried metal structures could be designed with a barrier coating, conditioned soil or corrosion allowance 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.