Corrosion current density rate quantities

Generally, the difference in form between ideal and real responses, which is exclusively due to the experimental procedures adopted for performing the polarization curve, affects the determination of the values of the corrosion current density and the Tafel slopes. The practical impossibility of a well-defined determination of these quantities, which, with reference to the law (2), characterize the behaviour of the system under examination, may result in an unsatisfactory formulation of the reaction mechanisms and an incorrect evaluation of the corrosion rate. 

Table 3 shows that a high corrosion rate of the specimen can noticeably influence the determination of the quantities Be and lea- The latter quantity designates the apparent corrosion current density derived from the experimental curve without eliminating the contribution of the solution resistance to overvoltage. Thus, with reference to the interval examined, an important result is achieved the difference between the two corrosion current density determinations is found to be a monotonic increasing function. 

In summary, the study by Zhou et al. " ° confirmed the findings of the prior work by Liu et al., ° that electrochemical noise analysis is an effective method for monitoring the corrosion rate of metals and alloys in high subcritical and supercritical aqueous solutions. The method is readily calibrated and, when used to estimate the noise resistance, and yields a quantity (the polarization resistance) that is directly related to the corrosion current density and hence the corrosion rate through the Stem-Geaiy relationship. This... 

Corrosion (spontaneous dissolution) of the catalyticaUy active material, and hence a decrease in the quantity present. Experience shows that contrary to widespread belief, marked corrosion occurs even with the platinum metals. For smooth platinum in sulfuric acid solutions at potentials of 0.9 to 1.0 V (RHE), the steady rate of self-dissolution corresponds to a current density of about 10 A/cm. Also, because of enhanced dissolution of ruthenium from the surface layer of platinum-ruthenium catalysts, their exceptional properties are gradually lost, and they are converted to ordinary, less active platinum catalysts. 

Corrosion Rate. The current densities give us an estimate of the rate of the electrochemical reactions involved in corrosion, but they do not give us a direct measurement of the corrosion rate. A simple, empirical expression can be used to quantify the rate of corrosion in terms of a quantity called the corrosion penetration rate (CPR) ... 

In nitric acid, the cathodic depolarizer (passivator) is nitrous acid, HNO2. This must form first in sufficient quantity by an initial rapid reaction of iron with HNO3. As nitrous acid accumulates, anodic current densities increase, eventually reaching 4riacai- Passivity is then achieved, and the corrosion rate falls to the comparatively low value of about 2gmd [14]. 

Measurement of current or current density is the most common output of electrochemical corrosion tests. This quantity is usually related either to the corrosion rate or to some features of the corrosion process, such as surface redox reactions that can change the corrosion characteristics. The relationship between the measured current and the corrosion rate or mechanism depends to some degree on... 

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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