Cathodic reactant

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

The equilibrium potentials and E, can be calculated from the standard electrode potentials of the H /Hj and M/M " " equilibria taking into account the pH and although the pH may be determined an arbitrary value must be used for the activity of metal ions, and 0 1 = 1 is not unreasonable when the metal is corroding actively, since it is the activity in the diffusion layer rather than that in the bulk solution that is significant. From these data it is possible to construct an Evans diagram for the corrosion of a single metal in an acid solution, and a similar approach may be adopted when dissolved O2 or another oxidant is the cathode reactant. 

It follows from this that the limiting current density / l is the most significant parameter in a corrosion reaction in which oxygen is the cathodic reactant, and that any factor that increases / l will increase the corrosion rate, since at E ... 

Figure 1.32 shows the E-i curves for a metal corroding in an acid in which both dissolved oxygen and ions act as cathode reactants (note that... 

Crevices or deposits on a metal surface or any other geometrical configuration that results in differences in the concentration of the cathodic reactant... 

Differences in aeration or in the concentration of other cathode reactants Differences in temperature... 

The area of the metal in contact with the lower concentration of the cathode reactant, although there are exceptions to this rule... 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

Different types of crevices are shown in Fig. 1.49, and reference should also be made to Section 19.2 for other examples. The phenomenon is referred to as crevice corrosion, and is characterised by a geometrical configuration in which the cathode reactant (usually dissolved oxygen) can... 

Most cases of crevice corrosion take place in near-neutral solutions in which dissolved oxygen is the cathode reactant, but in the case of copper and copper alloys crevice corrosion can occur owing to differences in the concentration of Cu ions however, in the latter the mechanism appears to be different, since attack takes place at the exposed surface close to the crevice and not within the crevice in fact, the inside of the crevice may actually be cathodic and copper deposition is sometimes observed, particularly in the Cu-Ni alloys. Similar considerations apply in acid solutions in which the hydrogen ion is the cathode reactant, and again attack occurs at the exposed surface close to the crevice. 

Increase in velocity may increase the rate by bringing the cathode reactant more rapidly to the surface of the metal thus decreasing cathodic polarisation, and by removing metal ions thus decreasing anodic polarisation. 

Increase in velocity may decrease the rate by bringing the cathode reactant to the surface at a rate that exceeds i i,, thus causing passivation. 

In addition to the mechanical damage of the protective film, velocity or movement will also bring the cathode reactant more rapidly to the metal surface thus decreasing cathode polarization. 

For situations controlled by anodic dissolution of a film P = 1/density of metal, but if the corrosion is controlled by the cathodic reaction P = 1/density of metal x nc Ma/na Me where n and M are the number of electrons and the molecular masses of anodic and cathodic reactants. 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

This represents a special case of high-level turbulence at a surface by the formation of steam and the possibility of the concentration of ions as water evaporates into the steam bubbles . For those metals and alloys in a particular environment that allow diffusion-controlled corrosion processes, rates will be very high except in the case where dissolved gases such as oxygen are the main cathodic reactant. Under these circumstances gases will be expelled into the steam and are not available for reaction. However, under conditions of sub-cooled forced circulation, when cool solution is continually approaching the hot metal surface, the dissolved oxygen... 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

Rosenfel d" considers that SO2 can act as a depolariser of the cathodic process. However, this effect has only been demonstrated with much higher levels of SO2 (0-5%) than are found in the atmosphere (Table 2.4) and the importance of this action of SO2 has yet to be proved for practical environments. However, SO2 is 1 300 times more soluble than O2 in water" and therefore its concentration in solution may be considerably greater than would be expected from partial pressure considerations. This high solubility would make it a more effective cathode reactant than dissolved oxygen even though its concentration in the atmosphere is comparatively small. 

When corrosion occurs, if the cathodic reactant is in plentiful supply, it can be shown both theoretically and practically that the cathodic kinetics are semi-logarithmic, as shown in Fig. 10.4. The rate of the cathodic reaction is governed by the rate at which electrical charge can be transferred at the metal surface. Such a process responds to changes in electrode potential giving rise to the semi-logarithmic behaviour. 

In principle it is possible for water to act as a cathodic reactant with the formation of molecular hydrogen ... 

When a cathodic process occurs at a finite rate the concentration of the electron acceptor (cathode reactant) at the metal/solution interface (x = 0) will become less than that in the bulk solution c, and as the rate increases it will continue to decrease until it becomes zero, i.e. as soon as the electron acceptor arrives at the interface electron transfer occurs. 

Since in most cases of corrosion in which transport of the cathodic reactant (HjO , dissolved O2, Fe , HNO3 acid, etc.) is rate determined, the anodic curve intersects the cathodic curve at /V, then... 

Cathodic Reactant species which is reduced at a cathode. 

The stability of membranes against thermomechanical and chemical stresses is an important factor in determining both their short- and long-term performance. Transport and mechanical properties of membranes affect the fuel cell performance, while the lifetime of a fuel cell is mostly dependent on the thermomechanical and chemical stability of the membrane. Thermomechanical and chemical degradation of a membrane will result in a loss of conductivity, as well as mixing of anode and cathode reactant gases. 

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