Cathodic-free corrosion

The current I is called the total current. In free corrosion, i.e., without the contribution of external currents (see Fig. 2-1), it is always zero, as given by Eq. (2-8). and are known as the anodic and cathodic partial currents. According to Eq. (2-10), generally in electrolytic corrosion anodic total currents and/or cathodic redox reactions are responsible. 

This reaction occurs with overall cathodic currents, i.e., with cathodic polarization. It can be practically ignored in the case of free corrosion of steel in a neutral solution. Other oxidizing media are of interest only in special cases. 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

Heterogeneous surface areas consist of anodic regions at corrosion cells (see Section 2.2.4.2) and objects to be protected which have damaged coating. Local concentrations of the current density develop in the area of a defect and can be determined by measurements of field strength. These occur at the anode in a corrosion cell in the case of free corrosion or at a holiday in a coated object in the case of impressed current polarization (e.g., cathodic protection). Such methods are of general interest in ascertaining the corrosion behavior of metallic construction units... 

Fig. 4-3 Schematic representation of the partial current densities in corrosion in free corrosion (a-c) and with cell formation with foreign cathodic structures (d).

Even in good alloys and under favorable conditions, the a value does not lie above about 0.6. In enamelled storage tanks where the current requirement is low, the a value can fall to as low as about 0.1. The cause of the high proportion of selfcorrosion is hydrogen evolution, which occurs as a parallel cathodic reaction according to Eq. (6-5b) or by free corrosion of material separated from the anode on the severely craggy surface [2-4, 19-21]. 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

Fig. 12-2 Local cathodic protection in a power station. deep anodes O horizontal anodes Potential readings Ccu-cuso4 volts (A) free corrosion potential before commissioning the cathodic protection (B) 4 months after switching on...

Cathodic protection of an uncoated ship is practically not possible or is uneconomic due to the protection current requirement and current distribution. In addition, there must be an electrically insulating layer between the steel wall and the antifouling coating in order to stifle the electrochemical reduction of toxic metal compounds. Products of cathodic electrolysis cannot prevent marine growths. On the contrary, in free corrosion, growths on inert copper can occur if cathodic protection is applied [23]. 

Further cell currents flow between the wells as a result of electrical connections established between them by the flow lines and of the different free corrosion potentials, thereby allowing them to behave as anode or cathode. The currents can amount to a few amps so that considerable corrosion damage can arise. The action of these cells can be prevented by building in insulators between the drilling and the field cable. 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

Results. The presence of Pt reduces the corrosion rate of Ti by shifting the free corrosion potential to more noble values (Fig. 6) where the Ti dissolution rate is slower. This shift is produced by the catalytic effect of Pt on hydrogen recombination which alters the cathodic reactions at the alloy surface. At the corrosion potential, the cathodic and anodic currents are equal. Although the shift in corrosion potential reduces the anodic current, anodic dissolution of Ti still occurs. The long-term corrosion rate of a surface alloy depends upon what happens to the Pt as the Ti is being dissolved. If Pt is removed from the surface, the corrosion rate will increase as the implanted volume of the alloy is dissolved. If Pt builds up on the surface, the corrosion rate should remain low. 

From this physical model, an electrical model of the interface can be given. Free corrosion is the association of an anodic process (iron dissolution) and a cathodic process (electrolyte reduction). Ther ore, as discussed in Section 9.2.1, the total impedance of the system near the corrosion potential is equivalent to an anodic impedance Za in parallel with a cathodic impedance Zc with a solution resistance Re added in series as shoxvn in Figure 13.13(a). The anodic impedance Za is simply depicted by a double-layer capacitance in parallel with a charge-transfer resistance (Figure 13.13(b)). The cathodic branch is described, following the method of de Levie, by a distributed impedance in space as a transmission line in the conducting macropore (Figure 13.12). The interfacial impedance of the microporous... 

Figure 7.123 shows the corrosion-fatigue behavior of the line pipe steel in the 3.5% NaCl solution at the corrosion potential, -440 30 mV (SHE) compared with -800 10 mV (SHE) when cathodically coupled (Ref 169). At the higher free-corrosion potential, the slope of re-... 

First, when the current intensity variations cover a very wide intervad, it is practically impossible to examine the kinetics of the electrochemical system accurately. In fact, the current range must be selected in view of the maximiun value of the current intensity that will be obtained during the performing of the polarization curve. Thus, in principle and in the course of each measurement, the study of the behaviour of a system near its free corrosion potential is practically incompatible with the examination of the current-voltage characteristic in the anodic and cathodic irreversibility zones. 

Property of cathode or cathodic reaction Property at free corrosion Property of electron Property of environment Property of reactant species... 

Anodic and cathodic processes may take place preferentially on separate areas of the surface of the reinforcement, leading to a macrocell. This can be established, for instance, between active and passive areas of the reinforcement. Current circulating between the former, which are less noble and thus function as anodes, and the latter, which are more noble and thus function as cathodes, accelerates the corrosion attack on active surfaces while further stabilising the protective state of passive ones. The magnitude of this current, known as the macrocell current, increases as the difference in the free corrosion potential between passive and active rebars increases, and decreases as the dissipation produced by the current itself at the anodic and cathodic sites and within the concrete increases. 

Non-carbonated and chloride-free concrete. In concrete that is not carbonated and does not contain chlorides, and in the absence of external cathodic polarization, hydrogen evolution, and thus consequent embrittlement, cannot take place. In this type of concrete, characterized by a pH above 12, hydrogen evolution can only occur at potentials below about —900 mV SCE. Passive steel under free corrosion conditions has much less negative potentials (Chapter 7) in the case of atmospherically exposed structures, the potential is between 0 and —200 mV (zone A of Figure 10.9). 

Cathodic protection (CP) is applied to structures already affected by corrosion, mainly induced by chlorides the steel is subjected to cathodic polarization, i.e. its potential is brought to values more negative than the free corrosion value, so that the corrosion rate is reduced. Corrosion can actually be stopped if a potential more negative than the value of repassivation (E, Figure 7.9) is reached. 

To this concern. Figure 20.5 shows the results of application of cathodic prevention to slabs subjected to ponding with a NaCl solution [43]. After about 700 days, initiation of rebar corrosion was detected in the control slab (in the free corrosion condition) at spots where the chloride content at the steel surface had reached more than about 1 % by mass of cement. From about that point in time, the slab receiving a very low current density of 0.4 mA/m showed a similar (instant-off) potential as the control slab and its 4-hour decay values became lower than 100 mV. The slab receiving 0.8 mA/m showed lower decays from about... 

This monitoring will provide information with respect to the material corrosion regime, i.e., active, passive, transpassive, pitting behaviour etc., and diagnostic possibilities exist only if the corrosion system is welt understood particularly as a result of laboratory studies - for example, the significance of the free corrosion potential or the potentials under impressed anodic or cathodic currents. 

Figure 7.67 S-N curves for steel in air, and in seawater under free corrosion and cathodic protection (CP).

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