Anodic protection theory

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

Klinghoffer, O. and Linder, B., A New High Performance Aluminium Anode Alloy with High Iron Content , Paper No. 59, Corrosion/87, San Francisco, USA, March (1987) Crundwell, R. F., Sacrificial Anodes — Old and New . In Cathodic Protection Theory and Practice, 2nd International Conference, Stratford upon Avon, June (1989)... 

Shreir, L. L. and Mayfield, P, C. S., Impressed Current Anodes , Conference on Cathodic Protection Theory and Practice —The Present Status, Coventry, 28-30 April (1982)... 

Anodic protection was developed using the principles of electrode kinetics and is difficult to understand without introducing advanced concepts of electrochemical theory. Briefly, anodic protection is controlled by the formation of protective passive film on metals and alloys using an externally applied potential. Anodic protection is used to a lesser degree because of the limitations on metal-environment systems for which anodic protection is viable. In addition, it is possible to accelerate corrosion if proper controls are not implemented during anodic protection. 

Chapter 4 presents the fundamentals of passivity the film and adsorption theories of passivity criterion for passivation methods for spontaneous passivation factors affecting passivation, such as the effect of solution velocity and acid concentration alloy evaluation anodic protection systems and design requirements. A fuU discussion on stainless steel composition and crystalline structure, oxidizer concentration, and alloy evaluation is included. The chapter also considers anodic protection to establish a basis for anodic... 

Corrosion current density is a particularly suitable measure of corrosion rate when treating corrosion theory and in connection with electrochemical corrosion testing. Current density is also directly applicable for cathodic and anodic protection (Sections 10.4 and 10.5). In corrosion testing the unit pA/cm is most often used. When dealing with cathodic protection the units mA/m and A/m are used for the cathode (sfructure to be protected) and the anode, respectively. 

Up to now, no complete and uniform theory of anodic protection and its design principles has been developed, especially for structures of complex geometric shape. Errors made can lead to catastrophic corrosion and breakdown. However, empirical principles of its application have been developed. Anodic protection can be applied when ... 

In contrast to cathodic protection, anodic protection is relatively new. Edeleanu first demonstrated the feasibihty of anodic protection in 1954 and tested it on small-scale stainless steel boilers used for sulfimc acid solutions. This was probably the first industrial apphcation, although other experimental work had been carried out elsewhere. This technique was developed using electrode kinetics principles and is somewhat difficult to describe without introducing advanced concepts of electrochemical theory. Simply, anodic protection is based on the formation of a protective film on metals by externally applied anodic currents. Anodic protection possesses unique advantages. 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. As an... 

The development of acidity within an occluded cell is by no means a new concept, and it was used by Hoar s as early as 1947 in his Acid Theory of Pitting to explain the pitting of passive metals in solutions containing Cl ions. According to Hoar the Cl ions migrate to the anodic sites and the metal ions at these sites hydrolyse with the formation of HCl, a strong acid that inhibits the formation of a protective film of oxide or hydroxide. Edeleanu and Evans followed the pH changes when aluminium was made anodic in Cl solutions and found that the pH decreased from 8-8 to 5-3. 

Civilization depends on the protection of metals, for most of them are unstable in normal environments unless they are protected by some kind of oxide film. The basic idea about the theory of corrosion is that the metal gets involved in a kind of local fuel cell in which it consumes itself. The partner to most of this self-dissolution is the deposition of hydrogen (favored in acid solutions) or the reduction of oxygen (favored in alkaline). Corrosion is measured in many ways, but the quick way in the laboratory is to move the potential a little bit away ( 5 mV) from the corrosion potential in both anodic and cathodic directions and measure the corresponding current. A simple equation takes the data from this type of measurement and produces the corrosion rate. 

Halide ions, according to the adsorption theory of passivity, tend to break down passivity by competing with the passivator for adsorption sites on the metal surface. Should a halide ion find a vacant site and closely approach the surface, hydration and dissolution of metal ions are favored, and the anodic reaction can proceed with low activation energy, in contrast to the high activation energy required when a passivator is adsorbed. The anode reaction, if it persists, is confined to localized areas where the competitive process first succeeds, because surrounding metal immediately becomes cathode of an electrolytic cell, and is protected by flow of current from further anode activity, a process called cathodic protection. This attack at specific sites leads to corrosion pitting typical of metals otherwise passive that are actually corroded by their environment. 

The precise mechanism resppnsible for the passivity conferred on metals by anodic inhibitors, such as chromate, is not known. While some early workers thought that a protective salt film (e.g., chromate) was formed, this view is not generally applicable, since passivity can occur in a system where the salt film would be freely soluble (e.g., iron in nitric acid). It is, however, generally accepted that passivity is associated with the formation of a protective film, and current views ascribe the action of anodic inhibitors either to adsorption at anodic sites or to continuous repair of the protective film. The former view has received attention in recent publications by Cartledge ), while the latter is favored by Evans (2). However, work on aluminum has suggested that true passivity is associated with the crystal structure of the film, which in turn determines its stability. This principle has recently been introduced by one of the authors (3) and is developed below into a general theory of passivity. 

Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode by means of impressed current or sacrificial anode (usually magnesimn, aluminum, or zinc) [11]. This method uses cathodic polarization to control electrode kinetics occurring on the metal-electrolyte interface. The principle of cathodic protection can be explained by the Wagner-Traud mixed potential theory [12]. 

L.I. Freiman, A theory and design of an insulating insert in a cathodicaUy protected underground water pipeline. II. The corrosion rate of anodic zones and effectiveness of the insert, Prot. Met. 38 (2002) 277-283 (Translation of Zashchita MetaUov). 

In line with the theory of passivators just described, transition metals are those expected and found to be inhibited best by passivators their anodic polarization curves have the shape shown in Fig. 17.1, allowing passivity to be established and then maintained at low current densities. A lesser degree of inhibition can be obtained with the nontransition metals, such as Mg, Cu, Zn, and Pb, using, for example, chromates. Protection of these metals apparently results largely from formation of relatively thick diffusion-barrier films of insoluble metal... 

Both the protective-film-rupture theory and the high-index-plane theory may be valid for one and the same system, depending on conditions. This has been demonstrated by the behavior of an iron wire strained in an aerated nitrate solution.When straining was done in the active region of potentials, the observed increase of anodic current was of the magnitude expected on the basis of the high-index-plane theory (within a power of 10). However, in the passive region, a 1500-2000-fold increase was observed which can be explained only in terms of the film-rupture mechanism. 

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