Introduction
Soil has been defined in many ways, often depending upon the particular interests of the person proposing the definition. In discussion of the soil as an environmental factor in corrosion, no strict definitions or limitations will be applied; rather, the complex interaction of all earthen materials will come within the scope of the discussion. It is obvious only a general approach to the topic can be given, and no attempt will be made to give full and detailed information on any single facet of the topic.
Soil is distinguished by the complex nature of its composition and of its interaction with other environmental factors. No two soils are exactly alike, and extremes of structure, composition and corrosive activity are found in different soils. Climatic factors of rainfall, temperature, air movement and sunlight can cause marked alterations in soil properties which relate directly to the rates at which corrosion will take place on metals buried in these soils.
Soil Genesis
The condition of any soil represents a stage in the changing process of soil evolution. Soils develop, mature and change with the passage of time.
Whereas the time required for a true soil to develop from the parent rock of the earth may be thousands of years, rapid changes can result in a few years when soils are cultivated, irrigated, or otherwise subjected to man’s manipulation. The type of soil that develops from the parent material will depend upon the various physical, chemical and biological factors of the environment.
The weathering process which eventually reduces the rock of the parent material to the inorganic constituents of soil comprises both physical and chemical changes. Size reduction from rocks to the colloidal state depends not only upon the mechanical action of natural forces but also on chemical solubilisation of certain minerals, action of plant roots, and the effects of organic substances formed by biological activity.
Interrelated with change in particle size and changes in type and kind of soil minerals present, organic matter is formed and accumulates as an integral part of the soil. Organic-matter content varies from practically none in sands to almost 100%, as exemplified by peat formations. The amount of organic matter present thus reflects the interaction of all environmental
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factors influencing chemical and biological activity. Whether the percent- age of organic matter increases or decreases depends then upon the relation between the rate at which it is being formed by growth, death and accumula- tion of plant material, and that at which the microbiological activities within the soil are causing the decomposition of the complex organic molecules.
Moisture must be considered of primary importance in soil formation, in weathering, and in all of the changes taking place within the soil. The types of soil that form depend to a great extent upon the rainfall situation. Too little rainfall will prevent development of plant and animal life with their soil-building action. Too much moisture has a similar effect in preventing normal soil formation.
Closely associated with rainfall and climate is the acid or basic reaction which develops as a soil matures. When rainfall is high, water percolates through the soil, dissolving the soluble components, and leaching out alkaline minerals of the weathering rock. This happens whether a soil is developing from a naturally acidic or a naturally alkaline parent material.
The end result is a shift in reaction to an acid condition. The degree to which this acidity develops depends upon many factors such as the parent minerals, biological activity, and temperature, related to the moisture situation.
Should the loss of water from a soil be mainly by surface evaporation (as in arid regions), the dissolved salts tend to accumulate near the surface and alkaline conditions usually develop.
Although conditions of high rainfall and moderate to warm temperatures usually lead to an overall decrease in organic matter (particularly in cultivated soils), exceptions occur when the amount of water is great enough to prevent the adequate aeration necessary for maximum microbial activity.
Swampy areas with peat and muck soils are the result. In a parallel manner, low temperatures of sub-polar regions slow down decomposition of organic materials and again highly organic soils develop.
The Corrosion Process in Soil
Although the soil as a corrosive environment is probably of greater com- plexity than any other environment, it is possible to make some generalisa- tions regarding soil types and corrosion. It is necessary to emphasise that corrosion in soils is extremely variable and can range from the rapid to the negligible. This can be illustrated by the fact that buried pipes have become perforated within one year, while archaeological specimens of ancient iron have probably remained in the soil for hundreds of years without significant attack.
Corrosion in soil is aqueous, and the mechanism is electrochemical (see Section 1.4), but the conditions in the soil can range from ‘atmospheric’ to completely immersed (Sections 2.2 and 2.3). Which conditions prevail depends on the compactness of the soil and the water or moisture content.
Moisture retained within a soil under field dry conditions is largely held within the capillaries and pores of the soil. Soil moisture is extremely signi- ficant in this connection, and a dry sandy soil will, in general, be less cor- rosive than a wet clay.
SOIL IN THE CORROSION PROCESS 2:75 Although the mechanism will be essentially electrochemical, there are many characteristic features of soif as a corrosive environment which will be considered subsequently; it can, however, be stated here that the actual cor- rosiveness of a soil will depend upon an interaction between rainfall, climate and soil reaction.
A characteristic feature of the soil is its heterogeneity. Thus variation in soil composition or structure can result in different environments acting on different parts of the same metal surface, and this can give rise to differing electrical potentials at the metal/soil interface. This will result in the estab- lishment of predominantly cathodic or predominantly anodic areas, and the consequent passage of charge through the metal and through the soil.
Differences in oxygen concentration (differential aeration), or differences in acidity or salt concentrations may thus give rise to corrosion cells. The distance of the separation of the anodic and cathodic areas can range from very small to miles (‘long-line’ corrosion).
The conductivity of the soil is important as it is evident from the elec- trochemical mechanism of corrosion that this can be rate-controlling; a high conductivity will be conducive of a high corrosion rate. In addition, the con- ductivity of the soil is important for ‘stray-current corrosion’ (see Section
] O S ) , and for cathodic protection (Chapter 10).
Properties of Soils Related to Corrosion Soil Texture and Structure
Soils are commonly named and classified according to the general size range of their particulate matter. Thus sandy, silt and clay types derive their names from the predominant size range of inorganic constituents. Particles between 0-07 and about 2 m m are classed as sands. Silt particles range from 0.005mm to 0-07, and clay particle size ranges from 0-005mm mean diameter down to colloidal matter.
The proportion of the three size groups will determine many of the pro- perties of the soil. Although a number of systems have been used to classify soils as to texture, the one shown in Fig. 2.17 represents commonly used ter- minology for various proportions of sand, silt and clay.
Since soils contain organic matter, moisture, gases and living organisms as well as mineral particles, it is apparent that the relative size range does not determine the whole nature of the soil structure. In fact most soils consist of aggregates of particles within a matrix of organic and inorganic colloidal matter rather than separate individual particles. This aggregation gives a crumb-like structure t o the soil, and leads to friability, more ready penetra- tion of moisture, greater aeration, less erosion by water and wind, and generally greater biological activity. The loss of the aggregated structure can occur as the result of mechanical action, or by chemical alteration such as excess alkali accumulation. Destruction of the structure or ‘puddling’ greatly alters the physical nature of the soil.
Mention should be made of the soil profile (section through soil showing various layers) because it is important to recognise that the soil’s surface
100 BO 6 0 4 0 20 PER CENT SAND
Fig. 2.17 Proportions of sand, silt and clay making up the various groups of soils classified on the basis of particle size.
gives a very poor indication of the underlying strata. Pipe-lines are buried several feet below surface soils and corrosion surveys based on surface obser- vations give little information as to the actual environs of the pipe when buried.
The Clay Fraction
Clays make up the most important inorganic constituents of soil. They consist of various minerals depending on the mineral composition of the parent material, and on the type and degree of weathering. Often clays may be grouped in a family series, depending upon the weathered condition, as, for example, montmorillonite + illite -, kaolinite. Weathering of mont- morillonite causes loss of potassium and magnesium which alters the crystal- line structure, and eventually kaolinite results. In this example (and also for other clay mineral groups) marked changes occur in the physical properties of a soil as clay minerals undergo the weathering process.
Montmorillonite clays absorb water readily, swell greatly and confer highly plastic properties to a soil. Thus soil stress (Section 14.8) occurs most frequently in these soils and less commonly in predominantly kaolinitic types. Similarly, a soil high in bentonite will show more aggressive corrosion than a soil with a comparable percentage of kaolinite. A chalky soil usually shows low corrosion rates. Clay mineralogy and the relation of clays to cor- rosion deserves attention from corrosion engineers. Many important rela- tionships are not fully understood and there is need for extensive research in this area.
SOIL IN THE CORROSION PROCESS Aeration and Oxygen Diffusion
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The pore space of a soil may contain either water or a gaseous atmosphere.
Thus the aeration of a soil is directly related to the amount of pore space pre- sent and to the water content. Soils of fine texture due to a high clay content contain more closely packed particles and have less pore capacity for gaseous diffusion than an open-type soil such as sand.
Oxygen content of soil atmosphere is of special interest in corrosion. It is generally assumed that the gases of the upper layers of soil are similar in composition to the atmosphere above the soil, except for a higher carbon dioxide content. Relatively few data are available showing oxygen content of soils at depths of interest to the corrosion engineer. Judging by the fact that plant roots require oxygen to penetrate a soil, however, it may be assumed that soil gases at depths of 6 m or more contain significant amounts of oxygen.
Diffusion of gases into soil is enhanced by a number of climatic factors.
Temperature changes from day to night conditions cause expansion and con- traction of the surface-soil gases. Variation in barometric pressure has a bellows-like effect on gaseous diffusion. To illustrate the magnitude of this diffusion rate on a large scale, it may be recalled that air within the more than 43 km of underground passages of the Carlsbad Caverns in New Mexico undergoes a complete change each day, despite the fact that the single open- ing of these caverns to the surface is only a metre or so in diameter.
Biological activity within the soil tends to decrease the oxygen content and replace the oxygen with gases from metabolic activity, such as carbon dioxide. Most biological activity occurs in the upper 150 mm of soil, and it is in this region that diffusion would be most rapid. Factors which tend to increase microbial respiration, such as the addition of large amounts of readily decomposed organic matter, or factors which decrease diffusion rates (water saturation) will lead to development of anaerobic conditions within the soil. The significant microbiological relationships to corrosion under both aerobic and anaerobic situations are discussed in Section 2.6.
Water Relations
No corrosion occurs in a completely dry environment. In soil, water is needed for ionisation of the oxidised state at the metal surface. Water is also needed for ionisation of soil electrolytes, thus completing the circuit for flow of a current maintaining corrosive activity. Apart from its participation in the fundamental corrosion process, water markedly influences most of the other factors relating to corrosion in soils. Its r61e in weathering and soil genesis has already been mentioned.
Types of Soil Moisture
1. Free ground water. At some depth below the surface, water is con- stantly present. This distance to the water table may vary from a few metres to hundreds of metres, depending upon the geological formations present.
Only a small amount of the metal used in underground service is present in the ground water zone. Such structures as well casings and under-river pipelines are surrounded by ground water. The corrosion conditions in such a situation are essentially those of an aqueous environment.
2. Gravitational water. Water entering soil at the surface from rainfall or some other source moves downward. This gravitational water will flow at a rate governed largely by the physical structure regulating the pore space at various zones in the soil profile. An impervious layer of clay, a ‘puddled’ soil, or other layers of material resistant to water passage may act as an effective barrier to the gravitational water and cause zones of water accumulation and saturation. This is often the situation in highland swamp and bog formation.
Usually gravitational water percolates rapidly to the level of the permanent ground water.
3. Capillaty wafer. Most soils contain considerable amounts of water held in the capillary spaces of the silt and clay particles. The actual amount pre- sent depends upon the soil type and weather conditions. Capillary moisture represents the important reservoir of water in soil which supplies the needs of plants and animals living in or on the soil. Only a portion of capillary water is available to plants. ‘Moisture-holding capacity’ of a soil is a term applied to the ability of a soil to hold water present in the form of capillary water. It is obvious that the moisture-holding capacity of a clay is much greater than that of a sandy type soil. Likewise, the degree of corrosion occurring in soil will be related to its moisture-holding capacity, although the complexities of the relationships do not allow any quantitative or predictive applications of the present state of knowledge.
Si@nTxance of Fluctuations in Weter Content
Except for zones below the level of permanent ground water where the environment is water-saturated, and for zones of dry surface sand, continual variation may be expected to occur in the water content of soils. This is usually dependent on rainfall, snow, flooding and such climatic influences, though irrigation practices in many agricultural areas influence water con- tent and hence the corrosion rates.
Water losses from the soil represent the sum of downward movement of gravitational water and surface losses by evaporation. Man’s activities, other than drainage procedures or long-term water use from pumps in industrial areas, do not usually influence the downward movement of water. On the other hand, agricultural practices have a great effect on surface evaporation losses.
As mentioned earlier, there is an inverse relationship between water volumes and oxygen concentration in soil. As soils dry, conditions become more aerobic and oxygen diffusion rates become higher. The wet-dry or anaerobic-aerobic alternation, either temporal or spatial, leads to higher cor- rosion rates than would be obtained within a constant environment. Oxygen- concentration-cell formation is enhanced. This same fluctuation in water and air relations also leads to greater variation in biological activity within the soil.
SOIL IN THE CORROSION PROCESS 2 : 7 9 Chemical Properties of Soils
Soil reaction (pH) The relationship between the environment and develop- ment of acid or alkaline conditions in soil has been discussed with respect to formation of soils from the parent rock materials. Soil acidity comes in part by the formation of carbonic acid from carbon dioxide of biological origin and water. Other acidic development may come from acid residues of weathering, shifts in mineral types, loss of alkaline or basic earth elements by leaching, formation of organic or inorganic acids by microbial activity, plant root secretions, and man-made pollution of the soil, especially by industrial wastes.
As with other factors, no direct statements can be made relating the reac- tion of a soil to its corrosive properties. Extremely acid soils (pH 4.0 and lower) can cause rapid corrosion of bare metals of most types. This degree of acidity is not common, being limited to certain-bog soils and soils made acid by large accumulations of acidic plant materials such as needles in a con- iferous forest. Most soils range from p H 5 . 0 to pH8-0, and corrosion rates are apt to depend on many other environmental factors rather than soil reac- tion per se. The 45-year study of underground corrosion conducted by the United States Bureau of Standards' included study of the effect of soils of varying pH on different metals, and extensive data were reported.
Soluble salts of the soil Water in the soil should most properly be con- sidered as the solvent for salts of the soil; the result being the soil solution.
In temperate climates and moderate rainfall areas, the soil solution is rela- tively dilute, with total dissolved salts ranging from 80 to 1 500 p.p.m.*.
Regions of extensive rainfall show lower concentrations of soluble salts as the result of leaching action. Conversely, soils in arid regions are usually quite high in salts as these salts are carried to the surface layers of the soil by water movement due to surface evaporation.
Generally, the most common cations in the soil solution are potassium, sodium, magnesium and calcium. Alkali soils are high in sodium and potassium, while calcareous soils contain predominantly magnesium and calcium. Salts of all four of these elements tend to accelerate metallic corro- sion by the mechanisms mentioned. The alkaline earth elements, calcium and magnesium, however, tend to form insoluble oxides and carbonates in non- acid conditions. These insoluble precipitates may result in a protective layer on the metal surface and reduced corrosive activity.
The anionic portions of the soil solution play a rBle of equal importance to the cations. The anions function in the manner outlined for cations in con- ductivity and concentration-cell action, and have an additional action if they react with the metal cation and form insoluble salts. Thus, if the metal is lead and the predominant anion is sulphate, a layer of insoluble lead sulphate may precipitate on the metal surface and form an effective barrier against further loss of metal.
Another important relationship between the salts of the soil and corrosion has to do with biological activity. Since the growth of plants and micro- organisms depends upon the proper inorganic mineral nutrients, the action of these forms of life varies with the mineral content of the soil. While many of the possible indirect effects, such as the role of various nitrogenous