Metal structures, underground, corrosion

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

Apart from corrosion due to differential aeration, corrosion of underground metal structures and pipelines may also arise from stray currents. How this comes about can be seen in the accompanying diagram (Fig. 12.32). The presence of a current-carrying cable in conducting soil results in stray currents passing through the soil. These stray currents may set up a potential difference between two portions of a pipeline, which then develops electron-source (cathodic) and -sink (anodic) areas. Thus, pipelines tend to corrode when they pass near electric lines. 

The evaluation of field of current density is essential in problems of galvanic corrosion. In many cases the direct measurement of current density is not feasible, while the electric potential can be obtained from experimental measurements. This is particularly true in case of cathodic protection systems in general, where many surveying techniques (for example DCVG and CIS for underground structures) rely in potential measurements at different points at the electrolyte in order to identify the current distribution along the metallic structures. 

W.B. Russel, Cathodic isolation/protection technologies applied to underground metallic structures, Proc. Appalachian Underground Corrosion Short Course 46 (2001) 117—125. 

Iverson WP. An overview of die anaerobic corrosion of underground metallic structures. Evidence for a new mechanism, In Escalante E. editor. Underground Corrosion. ASTM STP 741. American Society for Testing and... 

Iverson, W. P., "An Overview of the Anaerobic Corrosion of Underground Metallic Structures, Evidence for a New Mechanism, Underground Corrosion, ASTM STP 741, ASTM International, West Conshohocken, PA, 1981. 

Investigating the presence of stray currents to prevent or explain corrosion problems is not a new field in corrosion engineering. In fact, as mentioned in App. A, such activities were carried out by probably the first corrosion engineers in North America when the American Committee on Electrolysis was established at the turn of the twentieth century to combat the serious effects of railcar stray currents to underground metal structures (Figs. 7.4 and 7.5). 

Among pioneers in studying the effects of corrosion was the American Committee on Electrolysis, which noted in 1921 that its preliminary report had been published in October, 1916. This committee, composed of representatives of the American Institute of Electrical Engineers, American Electric Railway Association, American Railway Engineering Association, National Bureau of Standards, and others, concerned itself with the then serious problem of stray current damage to underground metal structures, especially the protection of communication cable from electrified street and interurban railways. 

Cathodic protection is a proven corrosion control method for protection of underground and undersea metallic structures, such as oil and gas pipelines, cables, utility lines and structural foundations. Cathodic protection is now widely applied in the protection of oil drilling platforms, dockyards, jetties, ships, submarines, condenser tubes in heat exchangers, bridges and decks, civil and military aircraft and ground transportation systems. 

Factors Leading TO Corrosion of Underground Metallic Structures... 

Any underground metallic structure would corrode at the point of exit of Fe" " " ions. To prevent this undesirable stray current corrosion a metallic bond, such as a bond cable between the pipeline and the negative bus of the DC substation, is installed as shown in Fig. 5.46. The current is then drained off by the metallic bond and all the surface of the secondary pipes becomes completely cathodic. The situation here is rather over-simplified, as there may be hundreds of substations serving the system depending on the traffic load and the load may vary during the 24 hours period. In certain instances, a bond connected may not be useful as the direction of flow of current in the bond may reverse and the current may flow to the pipelines rather than to the negative return. In order to handle this problem, rectifier discs may be inserted in the circuit so as to prevent the reversal of the current flow. 

Stray current corrosion differs from other forms in that the source of the current causing the corrosion is external to the affected equipment. This cause of metal deterioration is frequently misdiagnosed. Stray-current corrosion can cause local metal loss in huried or submerged metal structures, but it occurs much less frequently in underwater transporting equipment than in underground structures. Stray-current corrosion is almost always associated with direct current. At the anodic areas, metal goes into solution and the electrolyte tends to become acidic. It is most commonly encountered in soils containing water. 

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. 

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. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

The corrosivity of soils also depends upon the oxidation-reduction potential as classified by Booth et al.15 The classification scheme of the corrosivity of soils is given in Table 4.4b. Macrogalvanic cells are formed in underground pipelines due to foreign structure the combination of new and old pipe dissimilar metals (stainless steel and carbon steel) differential aeration dissimilar soils and stray currents. All these lead to localized corrosion of underground pipelines. 

The scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

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. 

Electric current flows in the soil from the buried anode to the underground structure to be protected. Therefore, the anode must be connected to the positive pole of the rectifier, and the structure to the negative pole. All cables from the rectifier to the anode and to the structure must be electrically insulated. If not, those from the rectifier to the anode will act as an anode and deteriorate rapidly, while those from the rectifier to the structure may pick up some of the current, which would then be lost for protection. The specific metal and environment will determine the current density required for complete protection. The applied current density must always exceed the current density equivalent to the measured corrosion rate under the same conditions. Therefore, as the corrosion rate increases, the impressed current density must be increased to provide protection. 

The electrolyte is usually water - present as moisture, rain or sea water - which may also contain elements of dust and gases which accelerate the corrosion process. The metal may be in constant contact with the electrolyte, e.g. underground structures and liquids in pipes, tanks and various vessels alternatively, the metal may be indoors subjected to differing degrees of humidity or dampness or outdoors in all weather conditions. The rate of corrosion is influenced by the electrical conductivity of the electrolyte i.e. high rate in salt solutions, low rate in high-purity water. 

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