Anodic protection basis

Stern, eta obtained potentiostatic polarisation curves for titanium alloys in various solutions of sulphuric acid and showed that the mixed potentials of titanium-noble metal alloys are more positive than the critical potential for the passivity of titanium. This explains the basis for the beneficial effects of small amounts of noble metals on the corrosion resistance of titanium in reducing-type acids. Hoar s review of the work on the effect of noble metals on including anodic protection should also be consulted... 

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. 

Anodic protection finds its basis in the understanding of active-passive behavior. By increasing the potential of the component to be protected, it moves from an actively corroding situation to one where passivity can be induced. Such techniques can be quite cost-effective, but must be applied under well-controlled operating conditions because slight overprotection or... 

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... 

In anodic protection installations, potentiostats are used in principle, i.e., dc current sources with an electronic system ensuring the maintenance of a given potential value on the surface of polarized metal. Industrial potentiostats differ from laboratory ones by their significantly higher power output. Usually, a ten-fold excess of power of the potentiostat is assumed in relation to the power calculated on the basis of the current required for maintaining the passive state. Approximate parameters of industrial potentiostats are as follows ... 

As already mentioned, no general principles concerning the design of anodic protection systems have been prepared up to now. Such problems as the number and location of cathodes and reference electrodes are solved experimentally. However, on the basis of information obtained for many industrial objects with anodic protection, some principles may be established concerning the method of polarization of the structure (Foroulis, 1980). 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

By contrast, if additional electrons were introduced at the metal surface, the cathodic reaction would speed up (to consume the electrons) and the anodic reaction would be inhibited metal dissolution would be slowed down. This is the basis of cathodic protection. 

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Claims are sometimes made that the use of cathodic protection devices eliminates the need for any type of water treatment chemical, including oxygen scavengers (on the basis that oxygen in the FW increases the rate of zinc anode corrosion, producing both zinc ions and hydroxide ions and resulting in the removal of 02 from the BW electrolyte). Such claims that corrosion protection devices provide a complete program are spurious. 

On the other hand, anything that makes iron behave more like the cathode prevents corrosion. In cathodic protection, iron makes contact with a more active metal (stronger reducing agent), such as zinc. The iron becomes cathodic and remains intact, while the zinc acts as the anode and loses electrons (Figure 21.2IB). Coating steel with a sacrificial layer of zinc is the basis of the... 

To shed light on the reaction pathway for the sdective anodic oxidation, we examined the electrolysis of the 24-protected derivatives (51a, 51b), and the 3-epimer (71) of 68. In case of the 24-protected derivatives (51a, 51b), the corresponding 3-ketone derivatives were not obtained at all and the starting compounds were recovered. Here again, it was confirmed that the 24-hydroxyl group was indispensable to this selective anodic oxidation. The 3a,23-dihydroxyl derivative (71) was also found to give no product by anodic oxidation. On the basis of above mentioned evidence, although some other pathways could be considered, an electrochemical process would be proposed for the present anodic oxidation. 

The almost neutral environment of carbonated concrete hinders formation of a protective film on the steel and thus the basis for conditions of passivity. The anodic curve therefore shows a progressive increase as shown in Figure 7.6, curve a. On a semilogarithmic scale (E vs log i) over a wide range of current densities the anodic curve is a straight Hne with a slope of between 60 and 120 mV/decade. 

The Evans diagram is also very useful in estimating the current required in the external circuit to stop the process of corrosion. If an external current is appHed cathodicaUy (negative current), the potential on the cathodic polarization line crosses the equihbrium potential of the anode and the anodic reaction is not thermodynamically feasible. Thus, the corrosion process stops. This process is the basis of cathodic protection and is discussed in Chapter 15. 

With intelligent eombinations of these simple models it is possible to make approximate estimations for other geometries. Typieal of the three models dealt with above is that both the anode and the cathode potential are constant over the respective electrode surfaces. Analytical solutions exist also for some other geometries. On the basis of such solutions (and partly on an empirical basis) several formulae for the resistance of the electrolyte volume near the anode - the anode resistance Ra - have been developed. Such formulae for various anode geometries are used in design of cathodic protection systems [10.25-10.34]. 

By measuring the potential of the protected structure, the degree of protection, including overprotection, can be determined. The basis for this determination is the fundamental concept that cathodic protection is complete when the protected structure is polarized to the open-circuit anodic potential of the local action cells [10]. 

Observing the polarization diagram for the copper-zinc cell in Fig. 5.2, it is clear that, if polarization of the cathode is continued, using external current, beyond the corrosion potential to the thermodynamic potential of the anode, both electrodes attain the same potential and no corrosion of zinc can occur. This is the basis for cathodic protection of metals. Cathodic protection, discussed further in Chapter 13, is one of the most effective engineering means for reducing the corrosion rate to zero. Cathodic protection is accomplished by supplying an external current to the corroding metal that is to be protected, as shown in Fig. 5.14. Current leaves the auxiliary anode (composed of any metallic or nonmetaUic... 

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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