Cathodic protection metallic structure

The calcareous deposit formed on cathodically protected metal surfaces will also be affected by the presence of micro-organisms and macro-fouling. Physical disruption and alteration of structure, composition and crystal form will influence the response to cathodic protection. The consequent economic implications of changes in current density requirement for protection, and the engineering design implications and associated economics may be significant [Maines 1993]. Work is therefore required to improve the understanding of the relationship between... 

The protected metal structure in a cathodic protection system. 

An alloy of 2% Ag-Pb is used as a corrosion-resistant anode in impressed current systems for cathodic protection of structures in seawater (see Section 13.6) [4]. Alloying with 6-12% Sb increases strength [only at temperatures <120 °C (<250°F)] of the otherwise weak metal, but corrosion resistance of the alloy in some media is below that of pure lead. 

Adoption of the concept of kinetic criteria allows a simple optimization of protective installation working parameters. In many cases it is unnecessary to fully retard the corrosion of a protected metal structure. Deep cathodic polarization is an expensive process. It requires the application of high power equipment, developed anodic systems, and is connected with high operation costs. For practical reasons, partial protection is sufficient in most cases, ensuring a decrease of the corrosion rate to such a level at which the assumed lifetime of the structure is attained. Evaluation of the effectiveness of cathodic protection based on a criterion taking into account the degree of decrease of corrosion process rates can become in the near future a new, important application element in the anticorrosion protection technology. 

Cathodic protection of structures, where the cathodic polarization of metal is secured by electric currents emitted from an independent source. 

The cost and economics of cathodic protection depend on a variety of parameters so that general statements on costs are not really possible. In particular, the protection current requirement and the specific electrical resistance of the electrolyte in the surroundings of the object to be protected and the anodes can vary considerably and thus affect the costs. Usually electrochemical protection is particularly economical if the structure can be ensured a long service life, maintained in continuous operation, and if repair costs are very high. As a rough estimate, the installation costs of cathodic protection of uncoated metal structures are about 1 to 2% of the construction costs of the structure, and are 0.1 to 0.2% for coated surfaces. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

The Al-HjO diagram does show, however, the danger that may arise due to an increase in pH when the metal is cathodically protected in near-neutral solutions indeed, the possibility of alkaline corrosion has seriously limited the use of cathodic protection for aluminium structures. 

The salts content of soils may be markedly altered by man s activities. The effect of cathodic protection will be discussed later in this section. Fertiliser use, particularly the heavy doses used in lawn care, introduces many chemicals into the soil. Industrial wastes, salt brines from petroleum production, thawing salts on walks and roads, weed-killing salts at the base of metal structures, and many other situations could be cited as examples of alteration of the soil solution. In tidal areas or in soils near extensive salt deposits, depletion of fresh ground-water supplies has resulted in a flow of brackish or salty sea water into these soils, causing increased 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). 

Effect of cathodic protection on soils Long-term application of an electrical potential to the metal structure with resulting flow of electrical current through the soil has two noticeable effects, the magnitude of which will be in proportion to the time and amount of current passing through the soil. 

Some metals are amphoteric. That is, they form simple cations (in acid solutions) and soluble oxyanions (in alkaline solution) only in the mid-pH range is a protective film stable. Since cathodic protection produces alkali at the structure s surface, it is important to restrict the polarisation, and thereby the amount of hydroxyl ion produced, in these cases. Thus both lead and aluminium will suffer cathodic corrosion under cathodic protection if the potential is made excessively electro negative. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Many structures are coated. Thus the presented area far exceeds the area of steel to be protected, which is restricted to uncoated areas and holidays in the coating. It is therefore practice to assume an arbitrary level of coating breakdown for coated areas to obtain the area of metal requiring cathodic protection ... 

The anode is fixed to the concrete using non-metallic fixings and may be supplied as a prefabricated mesh or more often as a continuous anode strand which is laid over the surface of the structure to be protected. The spacing between the anode strands may be adjusted to give the required current distribution and current density per unit area of concrete necessary to provide cathodic protection to a particular structure. 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Scrap steel In some fortunate instances a disused pipeline or other metal structure in close proximity to the project requiring cathodic protection may be used. However, it is essential in cases of scrap steel or iron groundbeds to ensure that the steelwork is completely electrically continuous, and multiple cable connections to various parts of the groundbed must be used to ensure a sufficient life. Preferential corrosion can take place in the vicinity of cable connections resulting in early electrical disconnection, hence the necessity for multiple connections. 

When cathodic protection is applied to an underground metal structure the greatest effect on the pipe to soil potential is at the drainage point. This effect decreases, or attenuates, as the distance from the drainage point increases. 

If a continuous metallic structure is immersed in an electrolyte, e.g. placed in the sea or sea-bed or buried in the soil, stray direct currents from nearby electric installations of which parts are not insulated from the soil may flow to and from the structure. At points where the stray current enters the immersed structure the potential will be lowered and electrical protection (cathodic protection) or partial electrical protection will occur. At points where the stray current leaves the immersed structure the potential will become more positive and corrosion may occur with serious consequences. 

Often it is necessary in designing a cathodic-protection system to know the conductivity of a protective coating (e.g. bitumen enamel) on a structure. This measurement is usually carried out by finding the resistance between an electrode of known area placed in contact with the coating and the structure itself. The electrode placed on the structure can be either of thin metal foil or, preferably, of material such as flannel soaked in weak acidic solution. The resistance between the pad and the metal is measured by means of either a resistivity meter, as previously described, or a battery with a voltmeter and an ammeter or microammeter. Generally speaking, in field work where such measurements have to be made, a resistivity meter is preferable. 

High-voltage coating-testing equipment When cathodic protection is applied to a structure which has a protective coating, the current required is proportional to the bare metal area on the structure. Thus whenever a protective coating is applied it should be of good quality, with very few failures or pin holes in it, so that the cathodic-protection system may be economic. 

Groundbed in cathodic protection of underground structures, a buried mass of inert material (e.g. carbon), or scrap metal connected to the positive terminal of a source of e.m.f. to a structure. 

Protection Current current made to flow into a metallic structure in order to effect cathodic protection. 

In addition to the careful selection of structural metals, the cathodic protection of water-wetted parts may also be specified. For most boiler plant systems, however, because of the tortuous and extended waterside surfaces involved, the use of cathodic protection is only a partial solution to controlling corrosion and should never be the sole secondary protocol. Rather, cathodic protection functions well when employed as part of a more comprehensive program that includes appropriate internal chemical treatments. 

Cathodic Protection method where a more active metal is connected to a metal structure such as a tank or a ship protecting the structure because the active metal is oxidized rather than the structure Cation a positively charged ion Cellulase a group of enzymes that hydrolyze cellulose... 

Saccharides carbohydrates Sacrificial Anode in cathodic protection, the metal connected to the structure to be protected that is more readily oxidized than the structure... 

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. 

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