Open-circuit corrosion

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

C, the corrosion current density, /, at the open-circuit corrosion potential, E. See also discussion in text. 

Fig. 1.40 Schematic anodic polarisation curve for a passivatable metal (solid line), shown together with three alternative cathodic reactions (broken line). Open-circuit corrosion potentials are determined by the intersection between the anodic and cathodic reaction rates. Cathode a intersects the anodic curve in the active region and the metal corrodes. Cathode b intersects at three possible points for which the metal may actively corrode or passivate, but passivity could be unstable. Only cathode c provides stable passivity. The lines a, b and c respectively could represent different cathodic reactions of increasing oxidizing power, or they could represent the same oxidizing agent at increasing concentration.

The results of the calculations reveal that the Ru losses from erosion, dissolution and open circuit corrosion (assuming shut-downs at a frequency of 12 per year), occurring... 

One must be wary of the use of anodic protection, in that any area that is not polarized completely into the passive region will dissolve at a high rate. The optimum protection range is shown in Fig. 16. Therefore anodic protection is more susceptible to the presence of crevices, deposits, or poor placement of polarizing electrodes than is cathodic protection. If a component is cathodically under protected, the maximum rate at which the unprotected area corrodes is the normal open circuit corrosion rate in anodic protection, underprotection results in high rate dissolution of the unprotected area and can therefore can lead to unexpected career changes. Understanding the manner in which current from an anodic protection system is distributed across a surface is important in such installations. The issues involved in current distribution are discussed in detail in Chapter 4. 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

Impressed-current cathodic protection entails the use of an external power source in combination with a stable anode. The potential of the specimen being protected is forced to negative values with respect to its open-circuit corrosion potential, and its rate of anodic dissolution is consequently reduced. The result of impressing a cathodic current on the structure is shown in Fig. 21M for the parameters used to draw this figure we obtain i = 48.8 pA/cm and E = - 0.554 V, NHE. Applying a cathodic current density of 72... 

The understanding gained by considering the Evans diagrams allows us to measure the corrosion current in a straightforward manner. First we must realize that the corrosion potential is in fact the open-circuit potential of a system undergoing corrosion. It represents steady state, but not equilibrium. It resembles the reversible potential in that it can be very stable. Following a small perturbation, the system will return to the open-circuit corrosion potential just as it returns to the reversible potential. It differs from the equilibrium potential in that it does not follow the Nemst equation for any redox couple and there is both a net oxidation of one species and a net reduction of another. 

The net current density observed at a potential , close to the open-circuit corrosion potential, is hence... 

These equations allow us to determine the corrosion current by making current-potential measurements in the range of about 20 mV around the open-circuit corrosion potential. 

In case that 8i in Eqn. (14) is small, the approximation becomes better in terms of Taylor expansion. Also, the constant term of polarization curve in Eqn. (1) representing open circuit corrosion potential can be eliminated and it only depends on the surface resistance = R. Eqn. (1) becomes Eqn. (15)... 

The constant term depends on the environmental conditions such as temperature, pH, concentration of oxygen and the reference electrode offset. But the differential method without the term has advantages on them, in case that the same reference electrodes are used in the short-time measurement. This formulation easily eliminates the effect of open circuit corrosion potential and reference electrode offset. If the potential or current density are constant in two boundary conditions, the differential boundary conditions are zero according to Eqn. (12) or Eqn. (13). 

The both numerical and experimental potential distributions are shown in Figure 7 and they show good agreement. It is noted that the good agreement can be achieved even if the open circuit corrosion potential is unknown, polarization curve is non-linear and unknown and there are the offsets of reference electrodes. 

These limiting conditions (i.e., when E = Ecorr, E Ecorr and E Ecorr) are analyzed as follows. Since Ecorr is the free or open-circuit corrosion potential, Iex must equal zero at this potential and, therefore, the curves of log Iex ox and log Iex red must approach very low values when plotted on logarithmic coordinates as observed in Fig. 4.13. At large positive deviations from Ecorr, reference to Fig. 4.13 shows that Ired M and Ired x become negligible, which allows Eq 4.48 to be written as ... 

The potentiostat can be set to polarize the WE either anodically, in which case the net reaction at the WE surface is oxidation (electrons removed from the WE), or cathodically, in which case the net reaction at the WE surface is reduction (electrons consumed at the WE). With reference to the potentiostatic circuit in Fig. 6.1, determination of a polarization curve is usually initiated by measuring the open-circuit corrosion potential, Ecorr, until a steady-state value is achieved (e g., less than 1.0 mV change over a five-minute period). Next, the potentiostat is set to control at Ecorr and connected to the polarization cell. Then, the set-point potential is reset continuously or stepwise to control the potential-time history of the WE while Iex is measured. If the set-point potential is continuously increased (above Ecorr), an anodic polarization curve is generated conversely, if the potential is continuously decreased (below Ecorr), a cathodic polarization curve is produced. 

Fig. 7.78 Stress corrosion potential ranges of pipeline steel in hydroxide, carbonate-bicarbonate, and nitrate solutions in slow strain-rate test. Strain rate 2.5 x 10 6 s 1. Arrows indicate open circuit corrosion potentials for each environment. Redrawn from Ref 68...

Another example of a galvanic cell reaction is provided by open circuit corrosion of the metal deposit. Freshly deposited (and particularly finely-divided) metals are more active than their bulk, compact counter parts. Corrosion of the mixed electrode deposit may ensue if the cathode surface is left under open circuit conditions metal dissolution is balanced via reduction of species such as dissolved oxygen, protons or higher oxidation states of transition metal ions. Illustrative (simplified) examples of such oxidising agents include the following ... 

The electrochemical behaviour of metals in anhydrous HF has been reviewed by Vijh, with particular attention to anodization, open-circuit corrosion, film formation, anodic dissolution, and evolution of F2. The dependence of the F2 overpotential at Ni in anhydrous HF on the current density has been investigated. At low current densities the overvoltage was mainly due to the potential difference across the anodic barrier film, whereas at high current density the electronic conduction of the film increased appreciably, resulting in a decrease in the potential drop. Other workers have shown that the process of H2 discharge in HF is affected by the addition of NaF, presumably by reducing the overvoltage on nickel. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

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