Open circuit corrosion potential

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.

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...

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. 

Since no charge can accumulate on any metal surface, the electrons generated by the anodic reaction must aD be used up by the cathodic reaction. This can only occur in Fig. 1 where the anodic and cathodic reaction Unes cross. At this point, the rate of the anodic reaction, the corrosion rate, equtds the rate of the cathodic reaction—in this case, hydrogen generation. This rate is called the open-circuit corrosion rate, abbreviated The potential at which the lines cross is called the open-circuit corrosion potential, abbreviated This type of plot is called an Evans... 

Due to its simplicity, open circuit corrosion potential measurements (see Chapter 20 of this manual) have been used in MIC studies for many years. Corrosion potential measurements as a function of time have been used to obtain information on MIC of steel, aluminum alloys, stainless steels, and other passive alloys. By itself, the corrosion potential of plain carbon and low alloy steels indicates very little because these steels can corrode at a wide range of potentials. Rapid changes in the corrosion potential, however, can be used to indicate cathodic depolarization, or an enhancement of the anodic reaction, or to the formation of a semi-protective film. 

In electrochemical noise (EN) measurements fluctuations in potential or current are measured as a function of time. The measurements can be done (see Chapter 7) either without or with an externally applied signal. In the first case one monitors the open circuit corrosion potential of the test metal versus a suitable reference electrode or versus a second electrode of the seune material exposed under identical conditions. The advantage of this technique for use in MIC research is that there is no external signal to disturb the biological community on the metal surface. Alternatively, one can measure fluctuations in potential (E) at an applied current (I), or the reverse, fluctuations in I at an applied E. It has also been suggested that one could couple the metal of interest to a platinum electrode and measure the noise... 

The main concept that most of the corrosion data interpretation is based on was first introduced by Wagner and Traud (1938), according to which galvanic corrosion is an electrochemical process with anodic and cathodic reactions taking place as statistically distributed events at the corroding surface. The corresponding partial anodic and cathodic currents are balanced so that the overall current density is zero. This concept has proven to be very useful, since it allowed all aspects of corrosion to be included into the framework of electrochemical kinetics. Directly deduced from this were the methods of corrosion rate measurement by Tafel line extrapolation, or the determination of the polarization resistance Rp from the slope of the polarization curve at the open circuit corrosion potential... 

An important variant of potentiodynamic polarization is the cyclic polarization test. This test is often used to evaluate pitting susceptibility. The potential is swept in a single cycle (or slightly less than one cycle), and the size of the hysteresis is examined along with the differences between the values of the starting open-circuit corrosion potential and the return passivation potential. The existence of the hysteresis is usually indicative of pitting, while the size of the loop is often related to the amount of pitting. 

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