Applications cathodic protection

Cathodic protection applications in fresh water include use of ferrite-coated niobium , and the more usual platinum-coated niobium . Platinised niobium anodes have been used in seawater, underground and in deep wells " and niobium connectors have been used for joining current leads Excellent service has been reported in open-seawater, where anodic potentials of up to 120V are not deleterious, but crevice corrosion can occur at 20 to 40V due to local surface damage, impurities such as copper and iron, and under deposits or in mud ... 

Sintered and sprayed ceramic anodes have been developed for cathodic protection applications. The ceramic anodes are composed of a group of materials classified as ferrites with iron oxide as the principal component. The electrochemical properties of divalent metal oxide ferrites in the composition range 0- lA/O-0-9Fe2O3 where M represents a divalent metal, e.g. Mg, Zn, Mn, Co or Ni, have been examined by Wakabayashi and Akoi" . They found that nickel ferrite exhibited the lowest consumption rate in 3% NaCl (of 1 56 g A y at 500 Am and that an increase in the NiO content to 40mol 7o, i.e. O NiO-O-bFejO, reduced the dissolution rate to 0-4gA y at the expense of an increase in the material resistivity from 0-02 to 0-3 ohm cm. 

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. 

This Is another vital part of an impressed current system. The T/R must be rugged and reliable with minimal maintenance requirements. It should be easy to maintain with good instruction manuals, circuit diagrams for maintenance and easy access to fuses and other replaceable components. Compared with pipeline or marine cathodic protection applications (steel piles, etc.) the power demand is modest. Steel in concrete needs less than 20 mA m - to provide protection, usually at less than 10 V, The power for a 100 W light bulb will protect 5000 m, This means that a single-phase, air cooled T/R vill usually protect even the largest structure and power consumption is rarely an economic concern. 

Cathodic protection application. Hydrogen bubbles produced by cathodic protection mitigate the effect of imploding cavitation bubbles. Sacrifidal anodes, such as zinc or magnesium, can also be used. 

A typical analysis of high silicon cast iron anode is shown below in Table 5.8. It is generally used for onshore cathodic protection applications. 

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. 

Judicious use of chemical inhibitors or cathodic protection may directly and indirectly reduce crevice attack. However, if conditions are very severe, economics may make such applications impractical. 

If the corrodent cannot be removed or reduced, the addition of an appropriate inhibitor may prevent cracking. However, complete, effective coverage by an inhibitor may not be achieved or economically feasible. Application of cathodic protection can stifle SCC in some metal... 

The basic standard for cathodic protection was laid down for the first time in DIN 30676 to which all the application areas of the different branches of protection can be referred. In this the most important point is the technique for accurately measuring the object/soil potential [58]. The usual off-potential measurement method for underground installations has been slowly implemented and enforced in Europe since the 1960s [59]. 

Enamels have very varied properties where their chemical stability is concerned. Relevant stability testing must be carried out for the different areas of application. Enamel coatings for hot water heaters, their requirements and combination with cathodic protection are described in Section 20.4.1. 

Lloyds Register of Shipping, Guidance Notes on Application of Cathodic Protection, London 1966. 

Aluminum-sheathed cables should not be connected to other cables because aluminum has the most negative rest potential of all applicable cable sheathing materials. Every defect in the protective sheath is therefore anodically endangered (see Fig. 2-5). The very high surface ratio SJS leads to rapid destruction of the aluminum sheathing according to Eq. (2-44). Aluminum can also suffer cathodic corrosion (see Fig. 2-11). The cathodic protection of aluminum is therefore a problem. Care must be taken that the protection criterion of Eq. (2-48) with the data in Section 2.4 is fulfilled (see also Table 13-1). Aluminum-sheathed cables are used only in exceptional cases. They should not be laid in stray current areas or in soils with a high concentration of salt. 

Application of Cathodic Protection to Reinforced Concrete Structures... 

Internal cathodic protection of water tanks and boilers is most economical if it is taken care of at the design stage. It can, however, be installed at a later stage as a rehabilitation measure to halt the progress of corrosion. Tanks and boilers in ships were described in Section 17.4. Further applications of internal protection are dealt with in Chapter 21. 

Measures a and c in Section 2.2 are directly relevant for internal electrochemical protection. In the previous chapter examples of the application of not only cathodic protection but also anodic protection were dealt with in this connection see the basic explanation in Sections 2.2 and 2.3 and particularly in Section 2.3.1.2. 

The use of corrosion-resistant materials and the application of corrosion protection measures are in many cases the reason that industrial plants and structures can be built at all. This is particularly so in pipeline technology. Without cathodic protection and without suitable coating as a precondition for the efficiency of cathodic protection, long-distance transport of oil and gas under high pressures would not be possible. Furthermore, anodic protection was the only protective measure to make possible the safe operation of alkali solution evaporators (see Section 21.5). 

Fig. 22-1 Economic application range for cathodic protection with magnesium anodes or with impressed current.

Considerable alterations have been made in the chapters concerned with technical applications which are the result of advances in electrochemical corrosion protection in general practice. Here also, abbreviation and omission of less relevant parts of the older editions have had to be made to create space for more recent information. Recent applications in the chemical industry have necessitated a complete rewriting of the industrial chapter. A new chapter is included on the cathodic protection of steel reinforcement in concrete. 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

This handbook deals mainly with the practice of cathodic protection, but the discussion includes fundamentals and related fields as far as these are necessary for a complete review of the subject. We thought it appropriate to include a historical introduction in order to explain the technological development of corrosion protection. The second chapter explains the theoretical basis of metal corrosion and corrosion protection. We have deliberately given practical examples of combinations of various materials and media in order to exemplify the numerous fields of application of electrochemical protection. 

At present cathodic protection is only generally applied for materials in contact with natural waters and soil, but future applications are envisaged for industrial plants and containers. For this reason we have included a chapter on anodic... 

Further chapters cover in detail the characteristics and applications of galvanic anodes and of cathodic protection rectifiers, including specialized instruments for stray current protection and impressed current anodes. The fields of application discussed are buried pipelines storage tanks tank farms telephone, power and gas-pressurized cables ships harbor installations and the internal protection of water tanks and industrial plants. A separate chapter deals with the problems of high-tension effects on pipelines and cables. A study of costs and economic factors concludes the discussion. The appendix contains those tables and mathematical derivations which appeared appropriate for practical purposes and for rounding off the subject. 

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