Corrosion kinetics current

There are a number of mechanisms that pose potential problems to predicting dissolution rate kinetics as the system approaches saturation. Part of this conundrum originates from current models of glass corrosion kinetics that cannot yet incorporate these unanticipated phenomena into a mathematical equation that is consistent with the constraints of thermodynamics or kinetics. These phenomena include (1) alkali-hydrogen exchange (2) dissimilar reactivity of... 

In the simplest case where the oxidation reaction of a semiconductor material (42a) proceeds exclusively through the valence band and the reaction of reduction of the Ox component of the solution exclusively through the conduction band (see Fig. 13a), corrosion kinetics is limited by minority carriers for either type of conductivity. In fact, it can be seen from Fig. 12 that icorr(p) = i"m(p) and 1 ( ) = ipim(n), where ijj are the limiting currents of minority carriers (symbols in parentheses denote the type of conductivity of a sample under corrosion). Since the corrosion rate is limited by the supply of minority carriers to the interface, it appears to be rather low in darkness. The values of [Pg.283]

Figure 4. Polarization curves of carbon corrosion and oxygen evolution reactions based on measured carbon corrosion kinetics for Pt/Vulcan and Pt/Graphitized-Vulcan and oxygen evolution kinetics for Pt/C catalysts. The upper horizontal dotted line denotes a current density equivalent to oxygen crossover through membrane from cathode to anode.

Recent kinetic studies indicate that carbon corrosion can be significant under normal transient operation.56,57,60-62 The rate of voltage change, common in the automotive application, enhances cathode carbon-support corrosion.16 Hence, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling. Currently, the relationship between fuel cell performance decay and accumulated carbon-support loss is only empirical.22 More effort has to be made to incorporate mechanisms that can accurately quantify voltage decay with carbon-support loss.31,32... 

The description of corrosion kinetics in electrochemical terms is based on the use of potential-current diagrams and a consideration of polarization effects. The equilibrium or reversible potentials Involved in the construction of equilibrium diagrams assume that there is no net transfer of charge (the anodic and cathodic currents are approximately zero). When the current flow is not zero, the anodic and cathodic potentials of the corrosion cell differ from their equilibrium values the anodic potential becomes, more positive, and the cathodic potential becomes more negative. The voltage difference, or polarization, can be due to cell resistance (resistance polarization) to the depletion of a reactant or the build-up of a product at an electrode surface (concentration polarization) or to a slow step in an electrode reaction (activation polarization). 

A higher exchange current density impHes increased reaction rate, while lower exchange current density (higher polarization resistance) indicates sluggish corrosion kinetics. Equation (1.10) can be written in terms of charge transfer resistance as ... 

Here, the current is directly proportional to the thickness of the probe via some probe constants. Hence, the slope of the current/time curve is directly proportional to the corrosion rate, and changes in the slope represent the corrosion kinetics [36]. 

The reason for the success of Evans diagrams in corrosion is that they combine thermodynamic Victors ( values) with kinetics factors (i values). The usefulness of corrosion kinetics in the study of corrosion rates is, therefore, obvious. The exchange current densities have been included in the polarization diagram by Stem, and such diagrams are called Stem diagrams. Evans diagrams do not include exchange current densities. 

In this chapter, we attempt to evaluate state-of-the-art commercial conventional-carbon-support MEAs for their carbon corrosion kinetics, the relationship between cell voltage loss and carbon-support weight loss, and the fife projection of the catalyst support under automotive operating conditions. These operational conditions include steady-state operation, transient, start/stop, and unintended deviations from nominal run parameters. On the basis of these analyses, we elucidate (1) which operational conditions result in severe carbon corrosion, (2) whether current conventional-carbon-support MEAs are robust enough to meet automotive durability targets, and (3) if a state-of-the-art corrosion-resistant carbon-support MEA is absolutely required for improving automotive fuel cell durability. 

In civil engineering structures exposed to moisture, electrochemical corrosion can lead to extensive damage on metal members this damage is due to galvanic connections - so-called corrosion cells - that produce local electric currents in the metal. From equilibrium considerations, the risk of formation of electrochemical corrosion cells in a given civil engineering structure can be predicted and in many cases the risk can be limited. On the other hand, one must realize that equilibrium considerations cannot give information of corrosion rate and thus the extent of a foreseeable corrosion attack. The latter subject is addressed separately under Corrosion kinetics . 

EIS has been applied extensively to the analysis of the corrosion mechanism of iron and other metals in aqueous solutions. To characterize a given corrosion process, it is practically advisable to obtain a full AC frequency scan of the system, including sufficient low-frequency response and small amplitude voltage perturbation with cyclic voltammetry, before acquiring the response data such as current, voltage, and polarization resistance Most corrosion kinetics studies have been done on uniformly corroding surfaces where the dissolution of the metal is uniform all over the surface in contact with the electrolyte [43]. Localized corrosion and stress corrosion cracking can also be analyzed by impedance methods such as local EIS (Section 13-4). 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

In maldug electrochemical impedance measurements, one vec tor is examined, using the others as the frame of reference. The voltage vector is divided by the current vec tor, as in Ohm s law. Electrochemical impedance measures the impedance of an electrochemical system and then mathematically models the response using simple circuit elements such as resistors, capacitors, and inductors. In some cases, the circuit elements are used to yield information about the kinetics of the corrosion process. 

Although Table 2.16 shows which metal of a couple will be the anode and will thus corrode more rapidly, little information regarding the corrosion current, and hence the corrosion rate, can be obtained from the e.m.f. of the cell. The kinetics of the corrosion reaction will be determined by the rates of the electrode processes and the corrosion rates of the anode of the couple will depend on the rate of reduction of hydrogen ions or dissolved oxygen at the cathode metal (Section 1.4). 

Figure 10.5 demonstrates that, even when semi-logarithmic cathodic kinetics are not observed, partial or total cathodic protection is possible. Indeed, Fig. 10.5 shows that the corrosion rate approximates to the limiting current for oxygen reduction (/,ij and the current required for protection is substantially lower than when semi-logarithmic cathodic behaviour prevails. 

It was indicated earlier that the cathodic current was a poor indicator of adequate protection. Whilst, to a first approximation the protection potential is a function of the metal, the current required for protection is a function of the environment and, more particularly, of the cathodic kinetics it entails. From Fig. 10.4 it is apparent that any circumstance that causes the cathodic kinetics to increase will cause both the corrosion rate and the current required for full (/") or partial (1/ — /, ) protection to rise. For example, an increase in the limiting current in Fig. 10.5 produced by an increase in environmental oxygen concentration or in fluid flow rate will increase the corrosion rate and the cathodic protection current. Similarly, if the environment is made more acid the hydrogen evolution reaction is more likely to be involved in the corrosion reaction and it also becomes easier and faster this too produces an increased corrosion rate and cathodic current demand. 

Electrochemical noise This is a non-perturbation method and is defined as random low frequency low amplitude fluctuations either of the potential or current in a corroding system. Analysis of the corrosion potential noise can provide information relating to both the mechanism and kinetics of the cor-... 

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