Oxygen, determination cathodic reduction

Fig. 10.6 Polarisation diagram showing the limited role hydrogen evolution plays at the corrosion potential of steel in aerated neutral solution, the larger role in determining cathodic protection currents and the dominant role in contributing to current requirements at very negative potenitals. The dotted line shows the total cathodic current due to oxygen reduction and...

The formation of a rare earth metal oxide on the metal surface, impedes the cathodic reduction of oxygen and thus cathodic inhibition is achieved by the addition of a rare earth metal salt to a system. The surface atom concentration ratio, [Ce/Ce + M], where M is Fe, Al or Zn, is a function of cerium oxide film thickness determined by AES depth profiles as shown in Fig. 12.2. 

Some classes of vicinal oxygen-functionalized compounds are reduced to give stereoiso-meric products [174-180]. Cathodic reduction of benzil in acidic and neutral solutions led to the formation of cis and traw -stilbenediols. The cis—trans ratio could be determined by trapping the chemically unstable stilbenediols as the 0,0 -dimethyl derivatives during electrolysis in the presence of dimethyl sulfate [175]. The cis isomer, as expected, was always predominant. 

Anions of the halides, as well as sulfides, selenides, and tellurides, can be determined by means of anodic waves due to mercury-salt formation. Among the oxygen-containing anions—in addition to those of the metals mentioned above—cathodic reduction waves can be used for determination of bromates, iodates, periodates, sulfites, polythionates, etc. 

Acid solutions (low pH) are more corrosive than neutral or alkaline solutions. In ordinary iron or steel, the dividing line between rapid corrosion in acid solution and moderate or low corrosion is nearly neutral or alkaline solution at pH 7.5. In case of corrosion of iron or steel in aerated water, anodic reaction takes place at all pH values as per Eq. (1.1), but the corrosion rate varies due to changes in the cathodic reduction reaction as per Eq. (1.2). In the intermediate pH 4-10 ranges, loose, porous, ferrous oxide deposit shelters the surface and maintains the pH at about 9.5 beneath the deposit. The corrosion rate is nearly constant and is determined by uniform diffusion of dissolved oxygen through deposit in this range of pH. In more acidic solutions ([Pg.11]

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

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). 

It has been shown in the previous chapters that the product of the electrochemical reactions in the sulphide flotation system is determined by the mixed potential of the flotation pulp. The value of the potential is dependent on the equilibrium of anodic and cathodic process existing in the pulp. In general, the most important cathodic reaction existing in the pulp is the oxygen reduction. To rewrite Eq. (1-1) as the following ... 

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