Buried systems soils corrosion

It has long been observed that a piece of iron buried in a dry soil suffers much less corrosion than when it is buried in a wet soil. However, soils are commonly wet due to rain, natural springs, and rivers soils. Corrosion in soils is a major concern, especially as much of the buried infrastructure is aging. Increasingly stringent environmental protection requirements are also bringing corrosion issues to the forefront for many systems either buried or aboveground and possibly in contact... 

When a buried system that has been in corrosive soil without adequate protection for some time is examined, it is usually foimd that by far the greater part of the area is unaffected. Where corrosion has taken place, it is in the form of pits, which are relatively small areas where the attack has been deep (Fig. 10.3). 

The primary form of corrosion protection for steel buried in soil is the application of coatings. When such coatings represent a physical barrier to the environment, cathodic protection in the form of sacrificial anodes or impressed current systems is usually apphed as an additional precaution. This additional measure is required because coating defects and discontinuities will inevitably be present in protective coatings. 

Stray currents are currents flowing in the electrolyte from external sources, not directly associated with the cathodic protection system. Any metallic structure, for example, a pipeline, buried in soil represents a low-resistance current path and is therefore fundamentally vulnerable to the effects of stray currents. Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion can be expected. Corrosion damage induced by stray current effects has also been referred to as electrolysis or interference. For the study and understanding of stray current effects it is important to bear in mind that current flow in a system will not only be restricted to the lowest-resistance path but will be distributed between paths of varying resistance, as predicted by elementary circuit theory. Naturally, the current levels will tend to be highest in the paths of least resistance. 

Depending on the bacteria and soil conditions which can be described appropriately as an extremely heterogeneous system, these transformations may be assimilatory or dissimilatory metabolic functions. Based on the recent field surveys and laboratory studies, the bacterial-environmental interactions, with reference to the cycles of sulfur and other elements, in corrosion on buried pipes are shown as Figure 1. This Figure demonstrates that microbiologically influenced corrosion (MIC) results from the activities of a microbial community. 

The size of the surfaces acting as anode and cathode (and thus r, which is their ratio) also depend on the resistivity of the concrete and the geometry of the system. For reinforced - concrete structures exposed to the atmosphere, usually concrete has a high resistivity and thus only those areas near the site of active corrosion act as cathode (e. g. r can approach unity). In the case of structures immersed in seawater or buried, the concrete is wet and has a low resistivity and, moreover, the sea or the soil also act as electrolytes. Therefore, even areas very far from the anodic areas can fimction as cathodes, so that ratio r can be very small. 

In the case of buried exposure, fadhties and structures are subject to corrosion primarily due to the interaction between the facility or structure and the soil environment.The properties of the soil are therefore very important when planning a pipeline or other buried facility or structure. Based on the experience of corrosion engineers, it appears that soil resistivity is the one soil property that is the best indicator of corrosion potential for buried metal-based structures and fadhties. The lower the soil resistivity, the greater is the tendency for corrosion. Another key factor in buried exposure is the amount of chloride present in the soil. Levels of 500 ppm or more in a soil wiU cause corrosion of metal-based systems. 

External corrosion system that includes corrosion problems such as those occurring in the soil surrounding the buried pipe, the coating, the cathodic protection system,. .. 

Offshore platforms are, in essence, similar to buried pipelines because in both, external and internal surfaces are exposed to corroding environments in buried pipelines the external surface of the pipe is exposed to the soil (which is a corrosive environment), and its internal surface is under the corrosive impact of the fluid that is going through, either water, oil or the like. In case of offshore platforms, the whole immersed stmcture is exposed to seawater (a corrosive medium), and the internal surfaces of the systems such as seawater injection systems or oil storage facilities can be considered locations at which corrosion is occurring internally. 

In the discussion of cathodic protection monitoring, two important distinct areas can be identified. The first domain lies in monitoring the condition and performance of the CP system hardware. Monitoring of rectifier output, pipe-to-soil potential and current measurements at buried sacrificial anodes, inspection of bonds, fuses, insulators, test posts, and permanent reference electrodes are relevant to this area. The second domain concerns the condition of the pipeline (or buried structure) itself and largely deals with surveys along the length of the pipeline to assess its condition and to identify high corrosion-risk areas. 

In soils, stray current corrosion can be caused by close proximity to other buried metal systems that are being protected by an impressed current cathodic protection system. These stray currents can leak onto a buried aluminum structure at one point, then off at another (where corrosion occurs), taking a low-resistance path between the driven buried aiwde and the nearby structure being protected. (Totmnon bonding of all buried metal systems in close proximity is the usual way to avoid such attack (Ref 39 and 40). 

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