Soils factors affecting corrosion

Factors affecting the corrosion of pipe include soil resistivity, moisture content of the soil, pH, permeability of soil to moisture and air, soluble ion content of the soil, oxygen content, and the presence of corrosion-activating bacteria. 

Factors Affecting Corrosion in Soils Since corrosion is an electrochemical process, it requires a potential difference between two points electrically connected and immersed in an electrolyte. Electrons flow from the anodic area through the metallic path to the cathodic area to complete the circuit. The anodic area is the most negative in potential sind is the area that corrodes through the loss of metal ions to the electrolyte. 

The factors affecting corrosion in soils are aeration, electrolyte, stray d-c currents, and miscellaneous ... 

Half-cell potential measurements Simple to conduct May act as a screening mechanism for more expensive methods Well established as method to detect corrosion activity in buried objects Factors such as stray currents and soil conditions may affect readings Accuracy of results depends on distance between readings Small areas of localized corrosion can not be detected... 

The inherent instability of sulfur-infiltrated concrete in aqueous media illustrated in this study may be the most important factor in utilization, because it will affect long-term durability of the concrete in many natural settings. The Ca(OH)2 produced by the hydration of portland cement is a principal reactant in the leaching process, and while it remains sulfur could be extracted, leaving the matrix vulnerable to other destructive processes. The removal rate of sulfur will vary greatly, depending mostly upon the pH of the immersion medium thus, the concrete deteriorates in alkaline sulfatic soils but is relatively stable in the corrosive neutral sulfatic solutions from the sodium sulfate plant. 

The corrosion process of bronze metal which is buried in soil is described in a manner similar to the weathering of mineral ores. The factors that affect the corrosion include the composition of the metal. 

The environment on which an oil and gas facility is exposed to is also a significant factor that affects corrosion. For example, pipelines are laid in the soil, above the soil, and across the sea. The nature of seawater is a contributor to corrosion depending on whether it is a saltwater or freshwater since either will definitely have a different effect on a facility. Also, above the sea or across or in the soil, environmental activities such as snow, dew, soil type, and temperature also contribute to the corrosion of an oil and gas facility over time, notwithstanding the possibility of acid rain in a heavily industrialized area. 

The danger of corrosion on buried installations in industrial plants is increased by various soils and by cell formation with cathodes of steel in concrete. The rest potentials of these foreign cathodes are between t/cu-cuS04 = and -0.5 V [4-6]. Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than 10 m ... 

Section V on Testing in Environments (H. Hack, Section Editor) includes chapters on outdoor and indoor atmospheres, seawater, fresh water, soils, concrete, industrial waters, industrial chemical, petroleum, high-temperature gases, organic liquids, molten salts, liquid metals, corrosion inhibitors, in-vivo, and microbiological effects. Each chapter provides a descriptive overview of the environment and factors and variables affecting corrosion rates and mechanisms. 

The metal corrosion in soils is determined primarily by such factors as moisture content md its level of electrical (ionic) conductivity, aeration and oxygen content, relative acidity or alkalinity, and amount of dissolved salts. The two conditions necessary to initiate metal corrosion in soil are water (moisture) and oxygen. After these factors, a number of variables can affect the corrosion process. 

As mentioned before, soil is a physically, chemically, and biologically complex system. Factors that affect corrosion in soil, in addition to specific ions, are resistivity of soil, oxygen content, and acidity. Field measurements of soil resistivity are covered in ASTM G 57, Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method, which is the most widely used test, and using the proper meter produces accurate and reproducible results. Conducting field measurements of soil pH is covered in ASTM G 51, Test Method for pH of SoU for Use in Corrosion Testing. The corrosion resistance of lead and its alloys depends mainly upon the presence of silicate, carbonate, and to a lesser extent sulfates, in contributing to the passive film formation. 

Soils will pit steels, which obviously affects buried pipelines. In one study of 10 carbon and low-alloy carbon steels containing Cr, Ni, Cu, and Mo and exposed to a variety of soils for 13 years, the conclusion was that factors such as soil pH, resistivity and degree of aeration have more influence on the severity of corrosion than the alloy content of the steel. In any case, protective coatings and cathodic protection are the best means of reducing corrosion in buried pipelines. 

System reliability is of the utmost importance to water suppliers and their customers. However, corrosion problems can vary greatly within a single system because many variables affect corrosion, for example, pipe material, pipe age, pipe wall thickness, water additives, corrosion inhibitor treatment, soil chemistry, soil moisture content and/or local groundwater level, and stray currents [2]. Table 8.2 summarizes some of the physical, environmental, and operational factors that can affect the deterioration rate of water distribution systems and lead to their failrue [4]. 

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