Resistance to Corrosion by Soil

Natural soils vary too widely in composition to permit very specific advice to be given, but sandy, well-aerated soils with a neutral or slightly alkaline pH are likely to cause only limited corrosion of zinc and its alloys. Zinc coatings will prevent pitting of steel in the soil and, even where the zinc coating is destroyed, the coated steel corrodes much less than do bare specimens. Control of backfill in earth reinforcement ensures that zinc or its alloys can be used satisfactorily. 

Tests by the British Iron and Steel Research Association (Hudson and Acock, 1952) covered five types of soil in the United Kingdom and exposures of 5 years. The galvanized samples failed in cinders they had an expected total life of about 10 years in the salty marshland and 25 or more years in the other soils tested (Table 4.3). 

Average metal loss ( xm/year) and maximum pit depth (mm) after burial for years stated  

Soil type Metal years depth years depth years depth years depth 

Organic reducing acid soils Carlisle muck P 14 0.15 18 0.25 22 0.56 15 0.46 

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Lead coatings are mainly applied by cladding and find principal use in the chemical industry for resistance to sulphuric acid, for cable sheathing resistant to attack by soils and in architectural applications where resistance to industrial atmospheres is particularly good. They rely for their protective action on the formation of insoluble corrosion products which stifle the corrosion reaction and lead to very long service lives, but the corrosion resistance is impaired when chlorides are present. 

Titanium has an unusually high ratio of strength to weight. It is considerably stronger than either aluminum or steel, two metals with which it competes (for special purposes). Its density (4.5 g/cm3) is intermediate between that of Al (2.7 g/cm3) and that of Fe (7.9 g/cm3). Titanium is extremely resistant to corrosion by air, soil, seawater, and even such reactive chemicals as nitric acid and chlorine gas. Like aluminum, it forms a thin, tightly adherent oxide layer that protects the metal from further attack. 

Copper is generally considered to have good resistance to corrosion in soils. Corrosion concerns are mainly related to highly acidic soils and the presence of carbonaceous contaminants such as cinder. Sulfides, often produced by SRBs, also greatly increase the risk of corrosion damage. 

The factors influencing the corrosion of metals in soil are more numerous than those prevailing in air or water, and the electrochemical effects are more pronounced. Moreover, soils vary widely in their composition and behaviour even over very short distances. It is difficult therefore to obtain reliable data. It is evident, however, that zinc has considerable resistance to corrosion when buried, and the greatest attack is caused by soils which are acid or contain large amounts of soluble salts. 

Because of the excellent resistance of copper to uniform corrosion by soil and ground water, especially in oxygen-free environments, copper is a candidate material for fabrication of nuclear-waste containers in Canada (Shoesmith et al., 1996). 

With some important exceptions, gray-iron castings generally have corrosion resistance similar to that of carbon steel. They do resist atmospheric corrosion as well as attack by natural or neutral waters and neutral soils. However, dilute acids and acid-salt solutions will attack this material. 

Saponification Paints are most commonly used to protect steel from corrosion by seawater in marine applications and soil in the case of buried structures. Additional protection is often supplied by the application of cathodic protection to the steel. Any paint coating used in conjunction with cathodic protection must be resistant to the alkali which is produced on the steel at defect sites in the coating. The amount of alkali generated depends on the potential to which the steel is polarized. Some paint binders such as alkyds and vinyl ester are very susceptible to saponification, and should not be used on cathodically protected structures. Cathodic disbondment testing should be undertaken if the relevant information is not available. 

Soil resistivity The role of soil in the electrical circuitry of corrosion is now apparent. Thus the conductivity of the soil represents an important parameter. Soil resistivity has probably been more widely used than any other test procedure. Opinions of experts vary somewhat as to the actual values in terms of ohm centimetres which relate to metal-loss rates. The extended study of the US Bureau of Standards presents a mass of data with soil-resistivity values given. A weakness of the resistivity procedure is that it neither indicates variations in aeration and pH of the soil, nor microbial activity in terms of coating deterioration or corrosion under anaerobic conditions. Furthermore, as shown by Costanzo rainfall fluctuations markedly affect readings. Despite its short comings, however, this procedure represents a valuable survey method. Scott points out the value of multiple data and the statistical nature of the resistivity readings as related to corrosion rates (see also Chapter 10). 

Other tests to determine bacterial-notably sulphate reducing-activity, soil resistivity, pH, redox potential, etc., will provide valuable data to supplement the results obtained with test specimens. A useful account of some of these was given in Reference 336 and they are also discussed in Sections 2.6 and 10.7. A scheme for assessment of corrosivity of soils based on some of the above parameters has been given by Tiller . 

The physical properties of lead and several of its compounds are listed in Table 3-2. Lead readily tarnishes in the atmosphere but is one of the most stable fabricated metals because of its corrosive resistance to air, water, and soil (Howe 1981). A waste that contains lead or lead compounds may (or may not) be characterized a hazardous waste following testing by the Toxicity Characteristic Leaching Procedure (TCLP) as prescribed by the Resource Conservation and Recovery Act (RCRA) regulations. 

Standards require that today s underground tanks must last thirty or more years without undue maintenance. To meet these criteria, they must be able to maintain structural integrity and resist the corrosive effects of soil and gasoline, including gasoline that has been contaminated by moisture and soil. The tank just mentioned that was removed in 1991 met these requirements, but two steel tanks unearthed from the same site at that time failed to meet them. One was dusted with white metal oxide and the other showed signs of corrosion at the weld line. Rust had weakened this joint so much that it could be scraped away with a pocketknife. Tests and evaluations were conducted on the RP tank that had been in the ground for 25 years tests were also conducted on similarly constructed tanks unearthed at 51 and 71 years that showed the RP tanks could more than meet the service requirements. Table 6.3 provides factual, useftil data from these tests. 

Isophthalic polyesters provide corrosion resistance in a wide variety of end uses. Recent excavations of 25-year-old FRP underground gasoline storage tanks offer evidence to the resin s resistance to internal chemical attack or external attack by water in the siuroimding soil. Isopolyester pipes and containment vaults also provide protection against failme from corrosion by acid media. Reference [2] provides information on FRP piping systems. 

From studies at the National Physical Laboratory in the United Kingdom, it was concluded that the corrosivity of soil towards ferrous metals could be estimated by measuring the resistivity of the soil and the potential of a platinum electrode in the soil with respect to a saturated calomel reference electrode [7]. Soils of low resistivity (<2000 Q-cm) were considered to be aggressive. Soils in which the potential at pH 7 was low [<0.40V (S.H.E.), or <0.43V if the soil is clay] were considered to provide a suitable environment for sulfate-reducing bacteria and to be aggressive. Borderline cases by these two criteria were resolved by considering that a water content of more than 20% causes a soil to be aggressive. 

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