Current applied

The amount of externally applied current needed to change the corrosion potential of a freely corroding specimen by a few millivolts (usually 10 mV) is measured. This current is related to the corrosion current, and therefore the corrosion rate, of the sample. If the metal is corroding rapidly, a large external current is needed to change its potential, and vice versa. 

Cathodic protection using sacrificial anodes or applied current can retard or eliminate tuberculation. However, costs can be high and technical installation can be very difficult. Costs are markedly reduced if surfaces are coated (see Material substitution below). 

Applied current devices as well as sacrificial anodes have frequently been used to decrease corrosion associated with deposition. The effec-... 

Cathodic protection (involving the use of applied current or sacrificial anodes)... 

Linear polarization instruments provide an instantaneous corrosion-rate data, by utilizing polarization phenomena. These instruments are commercially available as two-electrode Corrater and three electrode Pairmeter (Figure 4-472). The instruments are portable, with probes that can be utilized at several locations in the drilling fluid circulatory systems. In both Corrater and Pairmeter, the technique involves monitoring electrical potential of one of the electrodes with respect to one of the other electrodes as a small electrical current is applied. The amount of applied current necessary to change potential (no more than 10 to 20 mV) is proportional to corrosion intensity. The electronic meter converts the amount of current to read out a number that represents the corrosion rate in mpy. Before recording the data, sufficient time should be allowed for the electrodes to reach equilibrium with the environment. The corrosion-rate reading obtained by these instruments is due to corrosion of the probe element at that instant [184]. 

Fig, 1,26 E Vi, log (curves for the corrosion of a metal in a reducing acid in which there are two exchange processes (c,f. Fig, L24) involving oxidation of M—are reduction of —vH2. Note that (o) the reverse reactions for exchange process are negligible at potentials removed from E, (b) the potential actually measured is the corrosion potential E , which is mixed potential, and (c) the E vs. (,pp curves (where ijppi is the applied current density) when extrapolated intersect at corr. 

Fig. 1.62 Potential/current curves for a metal polarised (a) cathodically and (b) anodically. The horizontal intercepts xy, x y, x"y" with AB and CB represent the local cell currents respectively, and yz, y z, y z" the externally applied currents (cathodic and anodic). In bimetallic corrosion yz, y z, etc. will be the galvanic current /gjjv, flowing from to (see...

If the applied current density is reduced when a tin anode has been made passive in alkaline solution with the formation of a brown him and evolution of oxygen, the surface him changes to one of yellow colour and dissolution of tin as stannite ions proceeds freely . This effect is exploited in the electrodeposition of tin from sodium or potassium stannate solutions. 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

In the past 100 years many approaches tested intentionally or unintentionally have failed. Nevertheless, the underlying concepts may be tested again today with promise, applying current knowledge in biology and chemistry. 

Figure 1.3. Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt deposited on YSZ, an O2 conductor. T = 370°C, p02=4.6 kPa, Pc2H4=0.36 kPa. The catalytic rate increase, Ar, is 25 times larger than the rate before current application, r0, and 74000 times larger than the rate I/2F,16 of 02 supply to the catalyst. N0 is the Pt catalyst surface area, in mol Pt, and TOF is the catalytic turnover frequency (mol O reacting per surface Pt mol per s). Reprinted with permission from Academic Press.

As shown schematically in Figure 1.4, ions arriving under the influence of the applied current or potential at the three-phase boundaries catalyst/solid electrolyte/gas form there adsorbed species (0(a), Na(a)) which have only three possibilities ... 

Figure 4.49. Transient effect of constant applied current (I=+10 pA) on the rate (r) of C2H4 oxidation on Ir02/YSZ, on catalyst work function (AO) and on catalyst potential (Uwr)-Conditions T=380°C, pc>2 = f 5 kPa and PC2H4 =0.05 kPa.88 Reprinted with permission of The Electrochemical Society.

Figure 4.50. Transient effect of constant applied current (I=+300 pA) on the rate of C2H4 oxidation on Ir02 and on 75mol% Ir02 - 25%Ti02 and 25% Ir02 - 75%Ti02 composite catalysts deposited on YSZ. Note the decrease in p upon increasing the Ti02 content and the appearance of permanent NEMCA in all cases.124...

Figure 4.51. Transient effect of a constant applied current on the rates of C02, N2 and N20 production, on NO conversion (XN0) and on catalyst potential (Uwr) during NO reduction by propene in presence of gaseous 02 on Rh/YSZ.70 Reprinted with permission from Elsevier Science.

Figure 5.11. Effect of applied current on induced work function change on Pt/p"-Al203 26 dashed line catalyst labeled26 Cl, T = 291°C, pQ2 = 5 kPa, pc2H4 2.1xl0 2 kPa solid lines catalyst labeled26 C2, T = 240°C, p02 = 21 kPa, Inset Effect of applied current on computed initial dipole moment of Na on Pt ( ) I>0, (A) I<0.26 Reprinted with permission from Elsevier Science.

Figure 8.6. Effect of applied current (left) and corresponding catalyst potential Uw (right) on the rate of C2H4 oxidation on a Rh surface which is reduced under open-circuit conditions.13 Pq2 1.3 kPa, pc2H4=7.4 kPa. O, T=320°C, r0= 1.74x1 O 7 mol/s , T=350°C, ro=6.5x]0 7 mol/s A, T=370°C, r0=8.4xlO 7 mol/s Filled symbols open-circuit conditions. Reprinted with permission from Academic Press.

Figure 8.17. Effect of applied current on the increase in the rate of C2H6 oxidation 27 Pc h6=1-65 kPa, pO2=107 kPa O, T = 500°C , T = 460°C A, T = 420°C. Reprinted with permission from Academic Press.

A striking feature of the effect of current on the CO oxidation oscillations is shown in Fig. 8.33. It can be seen that the frequency of oscillations is a linear function of the applied current. This holds not only for intrinsically oscillatory states but also for those which do not exhibit oscillations under open-circuit conditions, such as the ones shown on Fig. 8.31. This behaviour is consistent with earlier models developed to describe the oscillatory behaviour of Pt-catalyzed oxidations under atmospheric pressure conditions which are due to surface Pt02 formation35 as analyzed in detail elsewhere.33... 

Similar studies utilizing Au electrodes on YSZ showed again that the selectivity and yield of C2 hydrocarbons can be significantly affected by applying currents or potentials to the cell.40,41,53 The behaviour with Au appears to be qualitatively similar with that obtained with Ag electrodes although electrophilic behaviour is also reported.40,41... 

Figure 8.50. Rate and catalyst potential response to a step change in applied current during CH3OH dehydrogenation and decomposition on Ag. The experimental time constants x are compared with 2FNG/I T=660°C, Pch30h=5.2 kPa.56 Reprinted with permission from Academic Press.

Figure 9.10. Ethylene epoxidation on Ag/p"-Al203 Transient effect of a negative applied current (Na supply to the catalyst) on the rates of ethylene oxide and C02 formation and on catalyst potential (work function) and Na coverage22 T=260°C, P=5 atm, p02=17,5 kPa, Pc2H4=49 kPa, 0.6 ppm C2H4CI2. Reprinted with permission from Academic Press.

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