Soils corrosion process

SOIL IN THE CORROSION PROCESS Aeration and Oxygen Diffusion... 

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 role in weathering and soil genesis has already been mentioned. 

The development of soil corrosivity assessment techniques has largely been due to the pipeline industry s requirements for better corrosion risk assessment and the reduction of pipeline failures. Corrosion in soil is a complex process and over the years several parameters have been identified as having a significant effect on the corrosion rate in a given soil. 

Water, whether as a liquid, moisture in the soil, or water vapor in the atmosphere, is essential for corrosion processes to take place. Under dry environmental conditions most metals and alloys are resistant to corrosion. The more humidity there is at a site, the more active are the corrosion processes. Some metals and alloys that are resistant to corrosion under dry conditions rapidly corrode under humid or wet conditions, particularly, in the presence of pollutants. Depending on their susceptibility to corrosion processes, the metals and alloys can be divided into three groups ... 

Most corrosion processes in copper and copper alloys generally start at the surface layer of the metal or alloy. When exposed to the atmosphere at ambient temperature, the surface reacts with oxygen, water, carbon dioxide, and air pollutants in buried objects the surface layer reacts with the components of the soil and with soil pollutants. In either case it gradually acquires a more or less thick patina under which the metallic core of an object may remain substantially unchanged. At particular sites, however, the corrosion processes may penetrate beyond the surface, and buried objects in particular may become severely corroded. At times, only extremely small remains of the original metal or alloy may be left underneath the corrosion layers. Very small amounts of active ions in the soil, such as chloride and nitrate under moist conditions, for example, may result, first in the corrosion of the surface layer and eventually, of the entire object. The process usually starts when surface atoms of the metal react with, say, chloride ions in the groundwater and form compounds of copper and chlorine, mainly cuprous chloride, cupric chloride, and/or hydrated cupric chloride. 

A characteristic of the iron oxide system is the variety of possible interconversions between the different phases. Under the appropriate conditions, almost every iron oxide can be converted into at least two others. Under oxic conditions, goethite and hematite are thermodynamically the most stable compounds in this system and are, therefore, the end members of many transformation routes. The transformations which take place between the iron oxides are summarized in Table 14.1. These interconversions have an important role in corrosion processes and in processes occurring in various natural environments including rocks, soils, lakes and biota. In the latter environments, they often modify the availability and environmental impact of adsorbed or occluded elements, for example, heavy metals. Interconversions are also utilized in industry, e.g. in the blast furnace and in pigment production, and in laboratory syntheses. 

Cathodic protection is an electrochemical method of corrosion control that has found widespread application in the protection of carbon steel underground structures such as pipelines and tanks from soil corrosion. The process equipment metal surface is made as the cathode in an electrolytic circuit to prevent metal wastage. 

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. 

By their designation, inhibited films can be subdivided into those protecting metals against electrochemical or microbiological corrosion. As a rule, under service conditions, corrosion processes, whether atmospheric, soil or water, go hand in hand. A number of Cl used as modifiers of polymer films combine the properties of inhibitors of both electrochemical and microbiological types of corrosion [7,54]. 

Many other issues are involved in the application of cathodic protection. For example, consider the case of cathodic protection of underground structures in which the corrosivity of soil is likely to play a major role, as does the degree of aeration and the resistivity. Bacterial effects also can change the corrosion potential. AU these factors influence the corrosion process so that along a pipeline there can be varying cathodic control requirements that have to be estimated from potential measurements, experience, and so forth. 

One of the primary causes of external corrosion is exposure to corrosive soils. The electrical and chemical characteristics of soil and water are closely related to corrosivity. Variations in soil characteristics because of soil type, fill compaction, amount of moisture, bacteria, chloride concentration help establish corrosion cells. Over a period of time, if untreated, the corrosion process can result in wall thickness reduction and can lead to leaks. The 6 o clock position of the USTs is one of the most critical locations because that is the rest point where the tank bottom touches the bottom of the hole dug for the tank. At such a location, the layer of backfill is relatively thin therefore, the soil characteristics can be different from the adjacent soil, setting up conditions for macrocell corrosion. 

Pipelines are exposed to aggressive soil environments, varying climate conditions, microorganisms, and stray currents that initiate corrosion processes. Research carried out in the last several decades indicated cathodic protection is the most promising protection method for pipelines [1,2]. Basic information on cathodic protection is well documented in several textbooks and handbooks [3-9]. 

In soils with low conductivity, a corrosive process that is initially fast corresponds to a high concentration of metal ions in the anodic areas, with the formation of protective films, and a strong alkalinization of the cathodic areas, with the formation of calcareous deposits for which the corrosion process tends to be negligible over time. Low-conductivity soils with a content of carbonates greater than 1% may be protective, and to them, we owe the good state of preservation of many... 

The process has been used for over 25 years and this section will document one of the older pipes that was evaluated after 20 years service. The service environment that the pipehnes experience are sewer water and corrosive soils. The process is used to restore corroded steel, cast iron and concrete pipes. Steel and cast iron pipe are susceptible to corrosion by sewer water through normal acid and salt attack, and by the galvanic... 

Underground corrosion is particulariy insidious in that, generally, we are not aware of its progress untQ a failure occurs. However, there are physical and electrochemical techniques available that will provide information on corrosion processes. Physical measurements furnish cumulative corrosion information obtadned at the final removal stage, while electrochemical measurements supply data during the period of exposure to soil. 

Soil as a corrosive environment is probably of greater complexity than any other environment. The corrosion process of buried metal structures is extremely variable and can range from rapid to negligible. In fact, pipes in soil can be perforated within one year, presenting very localized or uniform corrosion attack (Figs. 1-2). With background knowledge of the principal soil specifics and their influence on metal corrosion, the most serious corrosion problems can be prevented. 

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. 

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