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Part G The Effect of Other Environments Chapter G.1 Corrosion in Soil 1.1. Types of Soil 551 1.1.1. Constituents of Soil 551 1.1.2. Physical Chemistry of Soil 552 1.2. The Influence of the Nature of Soil on the Corrosion Behaviour of Aluminium 552 1.3. Form of Aluminium Corrosion in Soils 554 1.4. Aluminium Corrosion Resistance in Soils 554 1.5. Protection Against Corrosion in Soil 555 555 References 555 549 Chapter G.1 Corrosion
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  Part GThe Effect of Other Environments  Chapter G.1 Corrosion in Soil 1.1. Types of Soil  551 1.1.1. Constituents of Soil  551 1.1.2. Physical Chemistry of Soil  552 1.2. The Influence of the Nature of Soil on the CorrosionBehaviour of Aluminium  552 1.3. Form of Aluminium Corrosion in Soils  554 1.4. Aluminium Corrosion Resistance in Soils  554 1.5. Protection Against Corrosion in Soil  555555 References  555 549  Chapter G.1 Corrosion in Soil The corrosion behaviour of aluminium in soil is not only a complex issue, but also animportant one because of many relevant applications: cables for electricity andtelecommunications, water and gas distribution grids, embeddings of street signs, streetlamps and various supporting structures, etc.Predicting the corrosion resistance of a metal and assessing the aggressiveness of agiven soil are very difficult. 1.1. TYPES OF SOIL The concept of soil is different for a geologist, an agronomist, a civil engineer and acorrosion expert. Indeed, soil can be very different, even at two locations only a fewhundred meters apart, including natural soil in rural areas. The nature of a soil varies asa function of depth, while the nature of successive layers depends on thelocal geology. Theuppermost layer is generally constituted by humus. Differences can be even more apparentin urban and industrial areas, where urbanisation often has deeply transformed the soil.Soil can partially consist in backfill.For soil, as for waters, the corrosion expert needs to know its physicochemicalcharacteristics in order to be able to establish a relationship between these characteristicsand the level of aggressiveness towards a metal or an alloy. 1.1.1. Constituents of soil  Natural soil results from the crumbling of rocks over geological time. There are severaltypes of constituents with specific physicochemical properties: clay, marl, limestone, sand,gravel, etc.Artificial soils are constituted from backfill, industrial slag, mining residues, etc. Thecomposition and structure of artificial soils often have nothing in common with that of natural soils, and they can vary substantially from one point to another, depending on theirsrcin. Besides inorganic constituents, all organic constituents of plant and animal srcin,which are very important in arable soil, need to be taken into account. Bacterialdecomposition of organic matter forms humus.551  1.1.2. Physical chemistry of soil  Soil is a very heterogeneous, more or less humid medium. The humidity level depends onthe soil’s nature and on the volume of precipitation, and thus on the local climate. Water isretained mainly by capillary action.The main parameters of a soil are–  Physical parameters : the shape and size of inorganic constituents and their plasticity,on which both water drainage and aeration depend. Clays or silty soils have a finetexture that retains water; they are poorly drained and aerated. On the other hand,sandy soils are more aerated, and water is easily evacuated.–  Chemical parameters : mainly the composition of the water that soaks the soil, whichdepends on the climatic conditions and on the layers crossed by the water. The maininorganic constituents in water are: †  cations: Na þ , K  þ , Ca 2 þ , and Mg 2 þ †  anions: Cl 2 , NO 3 2 , SO 42 2 .The mineral content of water in soil varies between 0.5 and 1.5 g·l 2 1 .On industrial sites, even on former industrial sites, other inorganic elements can befound, the nature of which is related to the industrial activity.Soil also contains gases: oxygen, nitrogen, and carbon dioxide, srcinating mainly fromthe decomposition of organic matter, etc.–  Organic and bacteriological parameters : natural soils can exhibit intense biologicalactivity, mainly in the uppermost layer constituted from arable earth and humus,which is acidic. This layer contains organic acids, the nature of which is more or lesswell known [1].A soil is characterised by a pH value and an electrical resistivity that is closely related tothe nature of salts dissolved in the humidity. The pH also depends on the quantity of inorganic and organic acids, and on the carbon dioxide (CO 2 ) level, as well as on possiblecontamination by industrial or household wastewater. In general, soils have an acidic pH,between 3.5 and 4.5. The pH increases with depth (Figure G.1.1). 1.2. THE INFLUENCE OF THE NATURE OF SOIL ON THE CORROSIONBEHAVIOUR OF ALUMINIUM The corrosion resistance of a metal depends on several more or less related parameters:– the water content,– the structure of the soil, Corrosion of Aluminium 552

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