Carbonation cathodic protection

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. 

The cathodic protection of plain carbon and low-alloy steels can be achieved with galvanic anodes of zinc, aluminum or magnesium. For materials with relatively more positive protection potentials (e.g., stainless steels, copper, nickel or tin alloys), galvanic anodes of iron or of activated lead can be used. 

There are different concrete replacement systems available for renovating reinforced concrete structures. They range from sprayed concrete without polymer additions to systems containing conducting polymers (PCC-mortar). Since with the latter alkalinity is lower, more rapid carbonization occurs on weathering [59] and the increased electrical resistivity has to be taken into account, so that with cathodic protection only sprayed concrete should be used as a repair mortar. 

A tank with a fixed cover of plain carbon steel for storing 60°C warm, softened boiler feed water that had a tar-pitch epoxy resin coating showed pits up to 2.5 mm deep after 10 years of service without cathodic protection. Two separate protection systems were built into the tank because the water level varied as a result of service conditions. A ring anode attached to plastic supports was installed near the bottom of the tank and was connected to a potential-controlled protection rectifier. The side walls were protected by three vertical anodes with fixed adjustable protection current equipment. 

In Europe, the first internal cathodic protection installation was put into operation in 1965 for 24 water-powered Kaplan turbines with a propeller diameter of 7.6 m. These were in the tidal power station at La Ranee in France. The protected object consisted of plain carbon and high-alloy stainless steels. Each turbine was... 

Stress corrosion can arise in plain carbon and low-alloy steels if critical conditions of temperature, concentration and potential in hot alkali solutions are present (see Section 2.3.3). The critical potential range for stress corrosion is shown in Fig. 2-18. This potential range corresponds to the active/passive transition. Theoretically, anodic protection as well as cathodic protection would be possible (see Section 2.4) however, in the active condition, noticeable negligible dissolution of the steel occurs due to the formation of FeO ions. Therefore, the anodic protection method was chosen for protecting a water electrolysis plant operating with caustic potash solution against stress corrosion [30]. The protection current was provided by the electrolytic cells of the plant. 

In principle, cathodic protection can be applied to all the so-called engineering metals. In practice, it is most commonly used to protect ferrous materials and predominantly carbon steel. It is possible to apply cathodic protection in most aqueous corrosive environments, although its use is largely restricted to natural near-neutral environments (soils, sands and waters, each with air access). Thus, although the general principles outlined here apply to virtually all metals in aqueous environments, it is appropriate that the emphasis, and the illustrations, relate to steel in aerated natural environments. 

The higher the pH of the seawater the greater will be the proportion of carbonate ions present (since as the pH increases, so the H concentration decreases, thereby moving the equilibrium to the right). It follows that at a surface under cathodic protection, the hydroxyl ion produced has the effect of increasing the local carbonate ion concentration. 

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

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. 

High-alloy pipeline steels (e.g. austenitic-ferritic or duplex) have been used where the product stream demands materials with better corrosion resistance than carbon steel. In practice the external corrosion resistance of these materials cannot be guaranteed, so cathodic protection is employed to protect areas which may be subject to corrosion. 

A continuous polymer anode system has been developed specifically for the cathodic protection of buried pipelines and tanks. The anode, marketed under the trade name Anodeflex , consists of a continuous stranded copper conductor (6AWG) which is encased in a thick jacket of carbon-loaded polymer, overall diameter 12-5 mm. To prevent unintentional short circuits an insulating braid is sometimes applied to the outer surface of the conductive polymer. 

A petroleum coke with round grains is available specifically for borehole cathodic protection applications The round grains ensure high porosity and enable gas to escape, allowing the coke to sink to the base of the borehole. This material hjis a higher bulk density than petroleum coke (1 185 kgm" ) which enables it to sink to the bottom of the borehole, yet a lower fixed carbon content (93%), with higher ash (2-06%) and sulphur (5-3%) contents. The resistivity of this material is quoted as 0-1 ohmm. 

A conductive polymer electrode has been designed specifically for the cathodic protection of steel reinforcing bars in concrete and is marketed under the trade name Ferex . The anode consists of a 16 AWG stranded copper conductor surrounded by a carbon-loaded polymeric coating similar to that used on the Anodeflex system ) to provide a nominal anode diameter of 8 mm The manufacturer claims that at the maximum recommended current density of 0 08 Am the anode life in concrete will be 32 years with a proportionately longer life at lower current densities. 

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. 

Cemented and Welded Cans. Beer and carbonated beverage cans, made by the now familiar cementing (22) and welding (20) processes, are shown in Figure 8. These processes could also be used for sanitary processed food cans. Enameled TFS materials are used for these cans. Corrosion performance of the enameled, cemented, and welded cans is similar to that of enameled soldered cans for products which do not require the cathodic protection usually supplied by the tin coating. 

Cathodic protection is a useful supplement to other forms of water treatment, as a general corrosion inhibiting device in HW boilers, or where specific design configurations can lead to inadequately protected localized metal in steam boilers. Where BW makeup demands are minimal and boiler output is fairly constant, cathodic protection devices can also provide some measure of protection against hardness scales. Calcium carbonate salt is formed as a floc-culant or soft sludge rather than a hard scale, due to the peptizing effects of a zinc hydroxide complex formed from zinc ions in alkaline BW. 

See also Carbon entries in cathodic protection, 72 759 as a refractory raw material, 27 491... 

In the case of electrode B the oonductlve carbon black was used with the purpose to Influence the cathodic protection period. The modification C was made with zinc phosphate to Influence the second step In protective action of zinc pigmented coatings. 

The National Association of Corrosion Engineers (NACE International) has developed the following to protect the soil side of bottoms of on-grade carbon-steel storage tanks NACE RP0193-01, Standard Recommended Practice—External Cathodic Protection of On-Grade MetaUic Storage Tank Bottoms. 

Conductive polymer material used for the positive temperature coefficient resistor (PTC) overcurrent protection device Activated carbon cathode and anode... 

The principle of the cathodic protection may be elucidated for the case of carbon steel. The Pourbaix diagram for iron in water consisting of the plot E (potential) vs pH is shown in Figure 1.68. The regions of passivity, immunity and corrosion are seen in the figure. 

Protection of amino groups as carboxamides is generally not useful however, ben-zyloxycarbonyl derivatives of primary and secondary amines may be reduced at vitreous carbon cathodes in DMF at very negative potentials, with cleavage to toluene and free amine in good yield (>80%) [174,175]. However, 4-nitrobenzyloxycarbonyl derivatives may be deprotected at 1.2 V (SCE) the cleavage of the initially formed radical anion is slow on the time scale of CV but sufficiently fast for a preparative-scale reduction. 

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