Anodic protection limitations

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. 

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]. 

CV of solutions of lithium bis[ salicy-lato(2-)]borate in PC shows mainly the same oxidation behavior as with lithium bis[2,2 biphenyldiolato(2-)-0,0 ] borate, i.e., electrode (stainless steel or Au) passivation. The anodic oxidation limit is the highest of all borates investigated by us so far, namely 4.5 V versus Li. However, in contrast to lithium bis[2,2 -biphenyl-diolato(2-)-0,0 Jborate based solutions, lithium deposition and dissolution without previous protective film formation by oxidation of the anion is not possible, as the anion itself is probably reduced at potentials of 620-670 mV versus Li, where a... 

The terminology anodic and cathodic inhibitors is based on these functions. Anodic protection prevents or limits electron flow to the cathode area. Cathodic inhibitors generally reduce the corrosion rate by forming a barrier at the cathode thereby restricting the hydrogen ion or oxygen transport to the cathode surface. Tables 14.6 and 14.7 provide some information on common corrosion inhibitors. Specific corrosion control requirements are usually based on blends of two or more of the listed chemicals perhaps, in addition to chemicals to control scale formation and biological activity. 

The use of tannin is not commonly practised and nitrite and silicate treatments are less popular than orthophosphates for anodic protection because of their technical limitations, and the need for constant supervision to maintain effectiveness. Among the shortcomings are the potential for the formation of deposits, and the encouragement of microbial activity. 

Anodic protection relies on the formation of protective films on metal surfaces by means of externally applied anodic currents. It is a relatively new development in comparison to cathodic protection, with at present, limited practical application. 

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. 

Anodic protection against acidic solutions has been used in a number of chemical processes and in the transportation and storage of liquids. Unalloyed steels can be protected in this way in salt solutions with nitrates and sulfates and in nitric and sulfuric acid, although there are limits imposed in sulfuric acid by temperature and concentration. Stainless chromium aud chromium-nickel steels are particularly suited to anodic protection. It has so far been practiced with sulfuric acid (H2SO4), oleum, and phosphoric acid (H3PO4). 

A successful anodic protection design, not only requires a controlled poten-tial/current, but a high quality passive film which must be insoluble in the aggressive solution. In fact, the very low anodic corrosion rate in terms of the current density ip as shown in Figure 9.1 is apparently due to the limited ionic mobility in the passive film [1]. 

Impressed current anodic protection is of Little importance in comparison to cathodic protection. It has been applied successfully to the protection of stainless steel and titanium alloys in the presence of acidic electrolytes. As in the case of impressed current cathodic protection, it is essential to control the electrode potential within suitable limits. Figure 10.35 indicates the application of anodic protection to a simple tank and to a more complex tube-and-shell heat exchanger. When anodic protection is commissioned, the current must be large enough to exceed icurr order to passivate the surface. The current then falls to fpAss current is increasingly deflected to more remote parts of the structure which passivate in turn. Once established, the current drain is minimal and is approximately equal to ipASs ... 

The basic disadvantage of sacrificial protection is the irreversible loss of anode material and the resulting need of its replacement in addition, corrosion products of the anode can pollute the environment. Also, the range of application of galvanic anodes is limited by the resistivity (the specific resistance) of the environment and relatively small values of the protective current. A schematic diagram of sacrificial anode protection is presented in Fig. 8-23. 

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