Steel soils, corrosion

Cathodic Protection This electrochemical method of corrosion control has found wide application in the protection of carbon steel underground structures such as pipe lines and tanks from external soil corrosion. It is also widely used in water systems to protect ship hulls, offshore structures, and water-storage tanks. 

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. 

Although certain conditions very often lead to a particular type of attack, attempts to categorise soil corrosion in this way cannot be made on a general basis and most corrosivity assessment techniques categorise the soil as reacting to bare steel or iron in one of four ways ... 

The coating is applied to protect the steel from corrosion due to the acid or alkaline condition of the soil surrounding the pipe in service. Usually, the process requires three layers. First, an epoxy powder is applied to achieve adhesion to the pretreated metal and therefore resistance to cathodic disbondment. Second, a tie layer of polyolefin copolymer is applied and third a thick layer of polyethylene is cascaded, which in effect protects the epoxy from physical damage. 

Soil corrosion does not lend itself readily to direct study in the laboratory. However, indirect methods involving the action of differential aeration cells have yielded valuable information in comparing the probable corrosivities of different soils towards steel. The details of this technique were described by Denison , Ewing , Schwerdtfeger, and by Logan, Ewing and Denison ... 

The level of natural versus man-made emissions to the environment are of a similar magnitude. Soil erosion is the major contributor of natural emissions with zinc mining, zinc production facilities, iron and steel production, corrosion of galvanized structures, coal and fuel combustion, waste disposal and incineration, and the use of zinc fertilizers and pesticides being the principal anthropogenic contributors. 

Use Chemical intermediate, corrosion inhibitor, lab reagent, solvent stabilizer, prevents hydrogen embrittlement of steel, soil fumigant. 

Steel W beam (Fig. 8-5soil corrosion a problem in underground flowing-water conditions. 

F. Kajiyama, K. Okamura. Evaluating cathodic protection reliability on steel pipes in microbially active soils. Corrosion, Vol. 55, No. 1, pp. 74—80, 1999. 

Cathodic Protection. In general, buried pipelines and tanks are protected by a combination of cathodic protection and an organic coating. This combination protects steel against corrosion in all soils, both effectively and economically, for as many years as cathodic protection is adequately maintained. 

In some of the more corrosive soils, corrosion of the steel (after the zinc has been completely destroyed) is considerably less than that observed on uncoated steel control specimens. Also, it appeared in some soils that the protection afforded to the steel was not due solely to sacrificial corrosion of the zinc but was supplemented by some other mechanism. A few tests indicated that a film, consisting primarily of zinc silicate formed by galvanic action between the zinc and the underlying zinc-iron alloy or bare steel, was responsible for the additional protection. 

Galvanized pipe and flat specimens were included in the soil corrosion tests (Table 4.3) conducted by the Subcommittee on Buried Metals of the Corrosion Committee of the British Iron and Steel Research Association (Hudson and Acock, 1952). The galvanized test pieces averaged 670 g/m coating (about 95 p,m). The specimens were buried for 5 years at a depth of 0.6 m in five British soils. With the exception of the specimens buried in cinders at Corby, all the test pieces were rated excellent (no sign of breakdown) on visual examination. However, the weight loss data show that... 

Table 10.4 Relationship between the resistivity of soil corrosion activity and estimated lifetime of buried steel pipe...

The electrons liberated in the anodic reaction (Eq. (5 a)) have to be consumed immediately by the cathodic reaction (Eq. (5 b)) (condition of electroneutrality), thus both reactions have to take place with the same reaction rate. In the atmosphere or in soil, the corrosion product Fe(OH)2 is further oxidized and forms rust in the highly alkaline pore solution in concrete the same reaction forms a protective, very thin oxide film on the steel and corrosion stops. 

Engineers concerned with soil corrosion and underground steel piping are aware that the maximum pit depth found on a buried structure is somehow related to the percentage of the structure inspected. Finding the deepest actual pit requires a detailed inspection of the whole structure. As the area of the structure inspected decreases, so does the probability of finding the deepest actual pit. 

Stainless steels are rarely used in soil applications, as their corrosion performance in soil is generally poor and not better than bare steel. Localized corrosion attack is a particularly serious concern. The presence of chloride ions and concentration cells developed on the surface of these alloys tends to induce localized corrosion damage. 

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