Protection Anodic

In contrast to cathodic protection, anodic protection is relatively new. The feasibility of anodic protection was first demonstrated in 1954 and tested on a small-scale stainless steel boiler designed to handle sulfuric acid [23]. Anodic protection refers to the corrosion protection achieved by maintaining an active-passive metal or alloy in its passive state by applying an external anodic current. The basic principle for this type of protection is explained by the behavior shown in Fig. 5.40. 

When the potential of a metallic component is controlled and shifted in the anodic (positive) direction, the current required to cause that shift will vary. If the current required for the shift has the general polarization behavior illustrated in Fig. 5.40, the metal has an active-passive transition and can be anodically protected. Only a few systems exhibit this behavior in an appreciable and usable way. The corrosion rate of a metal with an active-passive behavior can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range shown in Fig. 5.40. 

The current required to shift the potential in the anodic direction from the corrosion potential can be several orders of magnitude greater than the current necessary to maintain the potential at a passive value. The current will peak at the passivation potential value shown as (Fig. 5.40). To produce passivation the critical current density (z), ) must be exceeded. The anodic potential must then be maintained in the passive region without allowing it to fall back in the active region or getting into the transpassive region, where the 

Fioure 5.40 Generalized polarization diagram showing various potential regions of a passivable metal. 

Anodic protection possesses unique features. For example, the applied current is usually equal to the corrosion rate of the protected system. Thus, anodic protection not only protects but also offers a direct means for monitoring the corrosion rate of a system. The main advantages of anodic protection are (1) low current requirements (2) large reductions in corrosion rate (typically 10,000-fold or more) and (3) applicability to certain strong, hot acids and other highly corrosive media. It is important to emphasize that anodic protection can only be applied to metals and alloys possessing active-passive characteristics such as titanium, stainless steels, steel, and nickel-base alloys. 

In some cases, passivity should be maintained. Anodic protection can be used to shift from the range of active dissolution to the passive range of the polarization curve. The necessary anodic current densities to maintain passivity are usually very small, in the range of a few pA cm. If however pitting is a problem in the presence of halides, the potential has to be maintained below Ep j. The presence of oxygen may cause a positive potential shift of a passive metal within the passive or even the transpassive range. 

The reference electrode is sometimes located at a position remote from the pipeline, recommended because currents do not penetrate remote areas, and hence, IR drop effects are avoided. Actually, the potential measured at a remote position is a compromise potential at some value between that of the polarized structure and the polarized auxiliary or sacrificial anode. These potentials differ by the IR drop through the soil and through coatings. The potential measured at a remote location, therefore, tends to be more active than the true potential of the structure, resulting in a structure that may be underprotected. 

For buried pipelines, the cost of cathodic protection is far less than for any other means offering equal assurance of protection. The guarantee that no leaks will develop on the soil side of a cathodically protected buried pipeline makes it economically feasible, for example, to transport oil and high-pressure natural gas across entire continents. 

As mentioned in Section 6.4, some metals, such as iron and stainless steels, can also be protected by making them anodic and shifting their potential into the passive region of the anodic polarization curve (see Fig. 6.1, Section 6.2). The passive potential is automatically maintained, usually electronically, by an instrument called the potentiostat. Practical application of anodic protection and use of the potentiostat for this purpose were first suggested by Edeleanu [21]. 

Steel against uniform corrosion in NH4NO3 fertilizer mixtures [25], carbon steel in 86% spent sulfuric add at temperatures up to 60 C (140 F) [26], and carbon steel in 0.1-0.7M oxalic add at temperatures up to SO C (HO F) [27], 

Anodic protection is apphcable only to metals and alloys (mostly transition metals) which are readily passivated when anodically polarized and for which /passive is very low. It is not apphcable, for example, to zinc, magnesium, cadmium, silver, copper, or copper-base aUoys. Anodic protection of aluminum exposed to high-temperature water has been shown to be feasible (see Section 21.1.2). 

It is important to note that passive behavior of the metal also depends upon the electrolytic environment. As an example mild steel is passivated in pure nitric acid, but not in dilute aqueous nitric acid. The current density required to passivate pass can be high and the current density to maintain the film r fiim, may be small. 

Hackerman, Corrosion 93, Plenary and Keynote Lectures, R.D. Gundry (ed.), NACE International, Houston, Texas, 1993, pp. 1-5. 

Bennett, J. Kruger, R.I. Parker, E. Passaglia, C. Reimann, A.W. Ruff, H. Yakowitz and E.B. Berman, Economic Effects of Metallic Corrosion in the United States - A Three Part Study for 

Congress a) Part INBS Special Publication 511-1, SD Stock No. SN-003-003-01926-7 b) Part II NBS Special Publication 511-2 Appendix B. A report to NBS by Battelle Columbus Laboratories, SD Stock No. SN-003-003-01927-5, US Govt. Printing Office, Washington, DC, 1978 c) Part III, Appendix C, Battelle Columbus Input/Output Tables, NBs GCR 78-122, PB-279 430, National Technical Information Service, Springfield, VA, 1978. 

P Hoar, Report of the Committee on Corrosion and Protection, Department of Trade and Industry, H.M.S.O., London, UK, 1971. 

These methods will be discussed in the sections below. Care will be taken not to go through all the details of each technique but rather link the technique with the theoretical aspects of electrochemistry, as covered in Chapter 1. 

The main benefit of using coatings and linings is that they prevent the electrolyte from coming into contact with the electrodes. In this way, there will not be an 

Anodic protection is achieved by applying an external cathode and a counter electrode in a manner similar to cathodic protection (see the next section), except that the current direction is in the opposite sense. 

Anodic protection has found some applications in the fertiliser production industry to control corrosion of mild steel in contact with ammonia-ammonium nitrate solutions. It has also been used in vessels containing sulphuric add. 

The main concern in CP is that there will be an increase in the alkalinity of the environment produced by the cathodic reaction (see Equation 1.3). This is important because many metals such as iron, aluminium, and zinc are affected under high pH conditions. If paints have been used with a CP system, they must also withstand the alkalinity of the medium The basic criteria for CP, using an Ag/AgCl seawater reference electrode for the potential measurement, is a negative voltage of at least -0.80 V between the reference electrode and the structure.  

12) The bursts of current result from meta-stable pits, where breakdown of the film, followed by a rapid repassivation, occurs. As the potential is increased, breakdown is more likely and repassivation is slower, until the potential is reached, beyond which breakdown is the predominant process. 

Since anodic passivation is performed potentiostatically, and since the currents involved are very small, uniform current distribution is easier to maintain and overprotection is not likely to occur. 

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Riggs O L and Locke C E 1981 Anodic Protection Theory and Practice in the Prevention of Corrosion (New York Plenum)... 

Anodic passivation and its appHcation to sulfuric acid equipment such as stainless steel acid coolers and carbon steel storage tanks has been weU studied (102—104). More recently, sheU and tube coolers made from Sandvik SX or Saramet have been installed in several acid plants. These materials do not requHe anodic protection. 

Plate and frame coolers using HasteUoy C-276 plates have been used successfuUy. Anodically protected plate coolers are available as weU as plate coolers with plates welded together to minimize gasketing. Another promising development is the introduction of plate coolers made of HasteUoy D205 (105). This aUoy has considerably better corrosion resistance to concentrated sulfuric acid at higher temperatures than does C-276. Because of the close clearance between plates, cooling water for plate coolers must be relatively clean. 

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

Anodic Protection This electrochemical method relies on an external potential control system (potentiostat) to maintain the metal or alloy in a noncorroding (passive) condition. Practical applications include acid coolers in sulfuric acid plants and storage tanks for sulfuric acid. 

Anodic Protection On the reverse anodic scan there will be a low current region (LCB) in the passive range. The passive potential range of the LCB is generally much narrower than the passive region seen on a forward slow scan. In anodic protection (AP) work the midpoint of the LCB potential is the preferred design range. This factor was verified for sulfuric acid in our laboratory and field studies. 

Note that zinc anodes are often used to protect steel and other relatively noble metals cathodically. In this case, the fasteners were acting as unintentional sacrificial anodes, protecting the stainless steel. Simple solutions to the problem would be to insulate the fasteners from the stainless steel electrically or to use stainless steel fasteners. 

These three passive systems are important in the technique of anodic protection (see Chapter 21). The kinetics of the cathodic partial reaction and therefore curves of type I, II or III depend on the material and the particular medium. Case III can be achieved by alloying additions of cathodically acting elements such as Pt, Pd, Ag, and Cu. In principle, this is a case of galvanic anodic protection by cathodic constituents of the microstructure [50]. 

Fatigue life can be slightly lengthened by anodic protection or by passivation. In acids even passive stainless CrNi steels suffer corrosion fatigue [104]. Resistance can occur if the passive film itself has a fatigue strength (e.g., in neutral waters [105]). 

There are limits to the use of anodic protection in the following cases ... 

In this section a survey is given of the critical protection potentials as well as the critical potential ranges for a possible application of electrochemical protection. The compilation is divided into four groups for both cathodic and anodic protection with and without a limitation of the protection range to more negative or more positive potentials respectively. 

Number Tank Volume (m ) Surface area (m ) No. of anodes Protection current (A) Voltage (V) Soil resistivity (Q m) [after Eq. (3-44)]... 

For partial protection of the stem, 33% of the anodes used for complete protection should be installed instead of the usual 25%. Of these, 25% serve as actual protection for the stem and 8% as shielding for the stem area against the remainder of the current-consuming body of the ship. These anodes are known as gathering anodes and are fixed in front of the anodes protecting the stem. 

Test potential Cu-C S04(V) Medium at the anode Protection current density (mA m ) Anode 1 Anode 2 ... 

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. 

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

Anodic Protection of Chemical Plant 21.4.1 Special Features of Anodic Protection... 

The fundamentals of this method of protection are dealt with in Section 2.3 and illustrated in Fig. 2-15. Corrosion protection for the stable-passive state is unnecessary because the material is sufficiently corrosion resistant for free corrosion conditions. If activation occurs due to a temporary disturbance, the material immediately returns to the stable passive state. This does not apply to the metastable passive state. In this case anodic protection is necessary to impose the return to the passive state. Anodic protection is also effective in the unstable passive state of the material but it must be permanently switched on, in contrast to the metastable passive state. 

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

Anodic Protection with Impressed Current 21.4.2.1 Preparatory Investigations... 

A particular problem arises in the anodic protection of the gas space because the anodic protection does not act here and there is a danger of active corrosion. Thus these endangered areas have to be taken account of in the design of chemical... 

Anodic protection against acids has been used in a number of processes in the chemical industry, as well as during storage and transport. It is also successful in geometrically complicated containers and tubings [12], Carbon steel can be protected from nitric and sulfuric acids. In the latter case, temperature and concentration set application limits [17]. At temperatures of up to 120°C, efficient protection can only be achieved with concentrations over 90% [ 18]. At concentrations between 67 and 90%, anodic protection can be used at up to 140°C with CrNi steels [19]. 

In sulfuric acid production involving heat recovery and recovery of waste sulfuric acid, acids of various concentrations at high temperatures can be dealt with. Corrosion damage has been observed, for example, in sulfuric acid coolers, which seriously impairs the availability of such installations. The use of anodic protection can prevent such damage. 

Anodic protection allows the use of materials under unfavorable conditions if they are also passivatable in sulfuric acid. CrNi steels [material Nos. 1.4541 (AISI 321) and 1.4571 (AISI 316 Ti)] can be used in handling sulfuric acid of 93 to 99% at temperatures up to 160°C. This enables a temperature of 120 to 160°C to be reached, which is very suitable for heat recovery. 

Anodic protection today allows safe and efficient protection of air coolers and banks of tubes in sulfuric acid plants. In 1966 the air cooler in a sulfuric acid plant in Germany was anodically protected. Since then more than 10,000 m of cooling surfaces in air- and water-cooled sulfuric acid plants worldwide have been protected. The dc output supply of the potentiostats amounts to >25 kW, corresponding to an energy requirement of 2.5 W per m of protected surface. As an example. Fig. 21-9 shows two parallel-connected sulfuric acid smooth tube exchangers in a production plant in Spain. 

Fig. 21-10 Diagram for the internal anodic protection of a sulfonation plant.

Economic heat recovery could be achieved in certain cases by the further development of anodic protection at higher temperatures. High temperatures allow the production of steam and lead to a considerable increase in efficiency [20]. 

Anodic protection is particularly suitable for stainless steels in acids. Protection potential ranges are given in Section 2.4. Besides sulfuric acid, other media such as phosphoric acid can be considered [13,21-24]. These materials are usually stable-passive in nitric acid. On the other hand, they are not passivatable in hydrochloric acid. Titanium is also a suitable material for anodic protection due to its good passivatability. 

Carbon steels can be anodically protected in certain salt solutions. This involves mainly products of the fertilizer industry such as NH3, NH4NO3 and urea. Anodic protection is effective up to 90°C [26]. Corrosion in the gas space is suppressed by control of pH and maintenance of a surplus of NH3. 

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. 

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

The external heating boxes for the caustic soda evaporator with forced circulation must be anodically protected separately. Ring-shaped impressed current electrodes of carbon steel are mounted and insulated on supporting brackets (see Fig, 21-12). 

The safety, availability and capacity of production plants are predetermined by the quality of the materials and the corrosion protection measures in the essential areas both are major considerations in the initial planning. Even today, damage to equipment and tanks is often assumed to be unavoidable and the damaged components are routinely replaced. By carrying out damage analysis, which points the way to knowledge of prevention of damage, the availability and life of plants can be increased considerably. This particularly applies to the use of anodic protection. 

In recent years, a number of protective installations have come into operation, especially where new installations must be maintained, or where older and already damaged installations have to be saved and operating costs have to be lowered. Worldwide, equipment, tanks and evaporators in the aluminum industry and industries using caustic alkalis with a capacity of 60,000 m and a surface area of 47,000 m are being anodically protected. Equipment for electrochemical protection has been installed with a total rating of 125 kW and 12 kA. 

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