This article comes from the industrial galvanizers specifiers manual
It is particularly relevant to the reinforced concrete structure used in my major assignment
INTRODUCTION
The use of steel underground is not new. There are many applications where steel is used in the ground, from simple applications like sign posts and fence posts, to engineered applications like piling and foundations. Over the past five years, new applications have been developed for steel foundation products. These products offer significant performance and cost advantages over traditional masonry and timber alternatives.
Alternative methods of installing steel utility poles for lighting and power distribution have been developed using direct embedded poles to reduce the installation costs and environmental impact of installation.
It is not practical to install expensive corrosion management technologies on many of these embedded steel products, as is the case for more critical infrastructure such as pipelines and tunnels. An understanding of the mechanism of corrosion will allow a predictable life to be designed into utility steel products that are to be used in-ground for new piers, piling and pole applications. This article has consolidated information from a number of authoritative sources to assist in evaluating the life of steel in-ground products.
STEEL CORROSION IN-GROUND
In the atmosphere, most materials have predictable modes of corrosion that are largely dependent on pollution levels, temperature and relative humidity. Once the important parameters are identified, the mechanism of metallic corrosion will then be common to all the products that are within that climatic zone.
In-ground situations are vastly different because of the wide local variations in soil chemistry, moisture content and conductivity that will affect the way coated or uncoated steel will perform in the ground.
Research into steel corrosion in soil started in the early years of this century, when Melvin Romanoff began a study for the National Bureau of Standards that continued for over 40 years. Many other corrosion-in- soil research projects were undertaken concurrently or subsequently.
Much of this activity has taken place in Australia sponsored by various road authorities and private enterprise companies such as BlueScope Steel and Ingal Civil Products, in evaluating in-ground corrosion performance on a range of products from culverts to piling.
Corrosion of metals in soil is extremely variable and while the soil environment is a complex one, it is possible to draw some conclusions about soil types and corrosion.
Any given soil will appear as a very heterogeneous electrolyte which consists of three phases:
- The solid phase made up of the soil particles which will vary in size and will vary in
chemical composition and level of entrained organic matter. - The aqueous phase which is the soil moisture - the vehicle which will allow corrosion to take place.
- The gaseous phase which consists of air contained in the soil’s pores. Some of this air may dissolve in the aqueous phase.
THE SOLID PHASE
Soils are commonly classified according to the general size range of their particulate component. Sandy, silty and clay soils are thus identified from the predominant size range of their inorganic particles. Convention classifies particles over 0.07mm to around 2mm as sands, particles from 0.005mm to 0.07mm as silts and 0.005mm smaller as clays. Soils rarely exist with only one of these components present.
The various groups of sand, silt and clay make up the soil classifications on the basis of their particle size.
Clay soils are characterised by their ability to absorb water readily, the level of which is determined by the nature of the clay. For this reason, clay soils present a significantly higher corrosion risk than sandy soils. For this reason also, the nature of the soil on the surface may not reflect its nature below the ground.
THE AQUEOUS PHASE
Corrosion will only occur in the presence of moisture that contains ions that will transmit the electric current maintaining corrosion activity. There are several types of soil moisture. These are:
- free ground water
- gravitational water
- capillary water.
The free ground water is determined by the water table, which may range from near ground level to many metres below the surface. This is the least important factor in determining corrosion of buried steel as most installations are above normal water tables. Where high water tables bring ground water in contact with embedded steel, corrosion will progress as if the steel were in an immersed environment.
Gravitational water arises from rainfall or man-made irrigation and will soak into the soil at a rate determined by its permeability. This will increase the period of wetness of the steel’s surface and this in turn will impact on the soil’s corrosive effects, depending on the conductivity of the gravitational water. Where regular rainfall occurs, most soluble salts may be leached from the soil over time, which will reduce the corrosive effects of gravitational water. Gravitational water will ultimately end up in the water table.
Capillary water is water that is entrained in the pores and on the surfaces of the soil particles. The ability of soil to retain moisture is obviously important to plant growth. It is the capillary water that is the prime source of moisture in determining corrosion rates of steel in soil.
The fluctuations in water content in soil due to precipitation and evaporation cause a variation in oxygen content, as drier soils allow more oxygen access and oxygen concentration cell formation may be enhanced.
SOIL CHEMISTRY
Acid or alkaline conditions develop in the soils depending in their parent rock and the geological or man-made activity that may impact on them over time. Most soils are in the pH range of pH 5.0 to pH 8.0. Highly acidic soils are relatively rare, and generally occur in swamp soils or areas subjected to high accumulations of acidic plant material such as pine needles.
Soluble salts are essential to plant growth and are a major factor in corrosion. These salts may include salts of potassium, sodium, calcium and magnesium. Salts such as calcium and magnesium, while initially promoting corrosion, frequently act beneficially as their insoluble oxides and carbonates become corrosion inhibitors over time.
Bacteria in soil is another factor that is important in corrosion activity. Sulfates can promote rapid bacteriological corrosion of steel because of sulphate reducing bacteria. Hydrocarbon-using bacteria can accelerate failure of organic coatings used underground also.
Soil has to be able to conduct electricity to participate in the corrosion of buried steel. The resistivity of the soil is used as an important measure of soil corrosivity. The higher the resistivity, the more the resistance to current flow moving between anodic and cathodic regions of the steel.
Regions of moderate or high rainfall will commonly have low levels of soluble salts in the soil, while desert soils may have very high salt levels. Some of the most aggressive soils in Australia are located in desert areas like the Simpson Desert clay pans have higher corrosion rates for galvanized coatings than many surf-side environments.
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