Protection cathodic

Cathodic protection is widely used on small to extremely large structures to protect metals and particularly steel against corrosion. This can often be accomplished by using a protective current that is either generated by a power supply in what is called impressed current cathodic protection (ICCP) or by using another metal that corrodes more readily than the metal being protected and therefore is sacrificed is the process. Chapter 13 provides a much detailed discussion of this important technique. 

Cathodic protection is a technique used to control the corrosion of a metal or alloy surface by making it the cathode of an electrochemical or galvanic cell. Cathodic protection was first described by Sir Humphry Davy in a series of papers presented to the Royal Society in 1824 [8] and developed by his pupil Michael Faraday [9]. Common application are steel water or oil and gas pipeline, storage tanks, pier piles, ships and boats, offshore oil wells and platforms. 

Magnesium is one of the most suitable metals to be used since it appears to be one of the most active metals, see Table 13.3. At variance with air, a marine environment is enough corrosive to destroy the oxide film that otherwise would passive magnesium avoiding its use as sacrificial anode. Soil may be enough acidic to allow cathodic protection to be used. 

Typically, the lower the resistivity of a soil, the higher will be its corrosivity. Generally, soil resistivity decreases with increasing water content and ionic species concentration. Sandy soils, for example, have high resistivity and appear to be the least corrosive while clay soils are on the opposite end of the corrosive spectrum since they are excellent in retaining water. Therefore, before using any cathodic protection the electrical resistance of the soil must be assessed by 

The best way to overcome the limitations ofcathodic protection related to changes in the environment is to monitor the potential and adjust the cathodic currents accordingly. This cannot be readily done with sacrificial anodes, and the method ofimpressed-current cathodic protection is sometimes preferred, in spite of its higher cost 

It is well to remember that using the impressed current cathodic protection does not alleviate the need to calculate the current distribution and locate the anodes in a way that ensures the best uniformity of current distribution on the protected structure. 

Cathodic protection is usually not used by itself, A pipeline buried in the ground is painted or coated for added protection against corrosion. Ideally, such coatings 

Impressed-current cathodic protection requires a little more sophistication than the use of sacrificial anodes, but it also lends itself to periodic adjustment and provides higher flexibility, particularly when structures having rather intricate shapes are considered. 

It is possible to prevent the corrosion of a metal by connecting it to a more active metal. This active metal becomes anodic and tends to corrode, whereas the cathodic metal is preserved. Iron pipes in soil or water will not corrode if they are connected to a sacrificial anode such as aluminum, zinc, or magnesium. Steel pipes for water and gas are usually protected in this manner. Galvanized iron pipes for hot water lines have a limited life which can be extended by introducing a magnesium rod to act as a sacrificial anode 

The potential needed to protect iron in seawater is —0.62 V with respect to the SHE or —0.86 V relative to SCE. Aluminum can provide this potential, —0.95 V relative to the SCE, and its use has been extended to offshore oil platforms, ship s huUs, ballast tanks, and jetty piles with life expectancies ranging from 3 to 10 years, depending on the mass of aluminum employed. 

An alternate approach is to apply a potential onto the steel, making it cathodic relative to an inert anode such as Pb, C, or Ni. A potential of 0.86 V is suitable for the protection of iron. 

Though more negative potentials, such as 1.0 V, can be used, it should be avoided in order to prevent hydrogen evolution and hydrogen embrittlement. 

Show how different oxygen concentrations in a cell for a single metal can result in corrosion. 

Corrosion resistance of metallic coatings is dependent on the composition and nature of the electrolyte, oxygen concentration, polarization characteristics, ratio of cathodic to anodic area and the surface contaminants. If the corrosion potentials of two metals such as iron and aluminum are close to each other in a particular environment there may be reversal of the galvanic couple. 

This method of corrosion protection consists of (i) the sacrificial anode method and (ii) impressed current cathodic protection. The other related impressed current protection method is anodic protection. 

The corrosion potential, Ecoa for iron in aerated water is in the range of —600 to —700 mV at pH 7 against the silver-silver chloride reference electrode (Point 0). By decreasing the pH below 7.0 the system is unaffected and corrosion persists. Increase in pH moves the system into a passive region, but this type of perturbation of the system is not the focus of the present discussion. By applying a more negative potential it is possible to move the system into the region of immunity which means the corrosion susceptibility is reduced. 

The more negative the potential, the greater the cathodic reaction and the smaller the anodic reaction the metal is more cathodic, which is the basis of cathodic protection of metals. By applying more positive potentials the system moves into the passive region where the corrosion rate may be reduced. This is particularly the case for some steels in particular environments and other metals, which forms the basis of anodic protection of metals. Thus, it is seen that changing the potential of a system in an environment which cannot be altered leads to effective corrosion control by cathodic or anodic protection as the case may be. 

Assuming corr in seawater is —650 mV, then the corrosion rate at —850 mV is only 

A reduction in metal to electrolyte potential of -0.850 V (saturated copper sulfate electrode as reference) is specified as the necessary potential that must be obtained for either optimum or absolute protection of ferrous structures in soil or water. Cathodic protection is applied by one of two methods, power impressed current or sacrificial anodes. 

The mechanism of corrosion involves metal dissolution due to an electrochemical phenomenon. Thus, corrosion is associated with current flow over finite distances from the corroding metal and the amount of corrosion that can be accounted for is quantitatively determined by the amount of current passing through the metal. The electrochemical phenomenon occurs because of differences in potential between areas of the corroding metal surface. Therefore, the driving force of corrosion is the decrease in free energy associated with the formation of corrosion product on the metal surface, hi contrast, preventing corrosion leads to cathodic protection under steady-state conditions. 

Cathodic protection methods are useful for designing against corrosion, but theses methods require knowledge of electrochemical polarization. The main objective in protecting a metallic stmcture is to eliminate or reduce corrosion rate by supplying an electron flow to a stmcture to reduce or eliminate metal dissolution (oxidation). This implies that the anodic reactions is suppressed on the surface of the stmcture. This can be accomplished using secondary materials and appropriate instrumentation to supply electrons to the stmcture. 

Cathodic protection is an electrochemical technique in which a cathodic potential is pUed to a stmcture in order to prevent corrosion from taking place. This implies that Ohm s law, E = IRx, can be used to control the potential so that E Eearr and implicitly the current must be J /  

A second possibility is the generation of a local element by combination of the part to be protected with a less noble metal, preferentially Mg. The magnesium dissolves and protects the iron. 

As a consequence, Cu corrosion with hydrogen evolution should not occur. However, Cu dissolution will occur if Cu is exposed to air with the much more positive potential of the O2/H2O electrode. For ( 2) = 0.2 bar, one obtains according to Equation 1.169 a pH-dependent potential expressed by Equation 1.170. 

As a consequence, Cu will corrode in solutions in contact with air. This might be prevented by cathodic protection with E Ep, = 0.166 V. 

A comparable effect to the sacrificial electrodes is provided by the direct supply of a cathodic current to the dissolving metal. A proper cathodic current applied to the metal structure sets the potential to a value where corrosion is prevented. A disadvantage is the requirement of a permanent cormection to a current supply (or even a potentiostat). However, with this approach, the metal construction may be tuned in for complicated situations. Metallic structures often consist of several metals with different corrosion properties. The environmental conditions may be very difficult. For example, passivation should be maintained but the potential should not become more positive than the critical pitting potential. In these complicated cases, a potentiostat with a CE and a RE is useful. The WE of this circuit is the metal construction. Protection by a current source is applicable to chemical reaction vessels or constructions with permanent location but not always to mobile devices like cars, ships, etc. Therefore both methods are useful, and the choice depends on the specific requirements for the construction in service conditions. In well-conducting electrolytes, one has to take care of the equilibrium potentials of the involved electrodes (metal/metal ion and redox electrode). If the environment has a low conductivity (wet soil), ohmic drops have to be taken into account in order to establish an appropriate protecting 

In the presence of aggressive anions like chloride, a cathodic shift to E Ep is required to avoid localized corrosion. This is achieved by a cathodic current from an external source, which takes over the above-mentioned current density of diffusion-limited oxygen reduction of in = 0.15 mA cm and thereby shifts the potential within the passive range below Epif This is another important example for cathodic protection. 

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See also rusting, cathodic protection, cortisol See hydrocortisone, cortisone, CjjHjaOs. M.p. 215 C. A steroid. 

Unprotected steel corrodes at a rate which is generally assumed to be 0.1 to 0.2mm per annum. Factors that influence the actual rate of corrosion include the maintenance program applied by the owner - particularly preservation of protective coatings, efficiency of cathodic protection systems in ballast tanks, corrosive properties of the cargo carried and environmental factors such as temperature and humidity. Under extreme conditions it has been known for the annual rate of corrosion on unprotected steel exposed on both surfaces to approach 1mm. 

A process resulting in a decrease in touglmess or ductility of a metal due to absorjDtion of hydrogen. This atomic hydrogen can result, for instance, in the cathodic corrosion reaction or from cathodic protection. 

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

Ashworth V and Booker C J L (eds) 1985 Cathodic Protection Theory and Practice (New York Ellis Norwood)... 

Titanium has potential use in desalination plants for converting sea water into fresh water. The metal has excellent resistance to sea water and is used for propeller shafts, rigging, and other parts of ships exposed to salt water. A titanium anode coated with platinum has been used to provide cathodic protection from corrosion by salt water. 

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

A Russian ammonia pipeline of nearly 2400 km extends from Togliatti on the Volga River to the Port of Odessa on the Black Sea, and a 2200-km, 250-mm dia branch line extends from Godovka in the Ukraine to Panioutino. The pipeline is constmcted of electric-resistance welded steel pipe with 7.9-mm thick walls but uses seamless pipe with 12.7-mm thick walls for river crossings. The pipeline is primed and taped with two layers of polyethylene tape and suppHed with a cathodic protection system for the entire pipeline. Mainline operating pressure is 8.15 MPa (1182 psi) and branch-line operating pressure is 9.7 MPa (1406 psi) (11). 

Corrosion. Anticorrosion measures have become standard ia pipeline desiga, coastmctioa, and maintenance ia the oil and gas iadustries the principal measures are appHcation of corrosion-preventive coatings and cathodic protection for exterior protection and chemical additives for iaterior protectioa. Pipe for pipelines may be bought with a variety of coatiags, such as tar, fiber glass, felt and heavy paper, epoxy, polyethylene, etc, either pre-apphed or coated and wrapped on the job with special machines as the pipe is lowered iato the treach. An electric detector is used to determine if a coatiag gap (hoHday) exists bare spots are coated before the pipe is laid (see Corrosion and corrosion control). 

Corrosion prevention (liners, coatings, cathodic protection)... 

Thermoelectric devices represent niche markets, but as economic and environmental conditions continue to change, they appear poised to advance into more common use. Thermoelectric power generators are in use in many areas, including sateUites, deep-space probes, remote-area weather stations, undersea navigational devices, military and remote-area communications, and cathodic protection. 

The low cost, light weight, and exceUent electrical conductivity of graphite anodes have made this impressed current protection system valuable for cathodic protection of pipelines, storage vessels, process equipment, and also for weU casings both on- and offshore. 

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