Potential anodic current, effect

Figure 2. Effect of different surfactants on the anodic current potential (vs. SCE) curves obtained in 2 N NaOH. Key A, no surfactant B, anionic surfactant C, neutral surfactant and D, cationic suifactant.

In an impressed-current cathodic protection system the power source has a substantial capacity to deliver current and it is possible to change the state of polarisation of the structure by altering that current. Thus effective control of the system depends on credible potential measurements. Since the current output from any given anode is substantial, the possibility of an IR error which may reach many hundreds of millivolts in any potential measurements made is high. Thus the instant-off technique (or some other means of avoiding IR error) is essential to effective system management. 

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. 

Because of the different potential distributions for different sets of conditions the apparent value of Tafel slope, about 60 mV, may have contributions from the various processes. The exact value may vary due to several factors which have different effects on the current-potential relationship 1) relative potential drops in the space charge layer and the Helmholtz layer 2) increase in surface area during the course of anodization due to formation of PS 3) change of the dissolution valence with potential 4) electron injection into the conduction band and 5) potential drops in the bulk semiconductor and electrolyte. 

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

The C.P.D. is obtained directly by the magnetron and the capacitor (or Kelvin) methods. Other methods which have proved successful rely on the variation of anode potential in a diode with constant cathode conditions. In this case, since adsorption changes the effective anode potential, the applied potential necessary to restore the anode current to its original value is equal to the C.P.D. between the two surfaces. As considered in Sec. II, a true average work function is measured in the C.P.D. method when the two conductors are separated by a distance which is much greater than the size of the patches on the surface. These conditions are invariably fulfilled in the capacitor and the space-charge-limited diode methods. 

An interesting example of the kinetic effect in semiconductor photocorrosion is photopassivation and photoactivation of silicon (Izidinov et al., 1962). Silicon is an electronegative element, so it should be dissolved spontaneously and intensively in water with hydrogen evolution. But in most of aqueous solutions the surface of silicon is covered with a nonporous passivating oxide film, which protects it from corrosion. The anodic polarization curve of silicon (dashed line in Fig. 20) is of the form characteristic of electrodes liable to passivation as the potential increases, the anodic current first grows (the... 

On the basis of oxidation potentials, current-potential relationships, and isotope effects, an electron-transfer mechanism is suggested for the anodic oxidation of methyl N,N-dialkyl substituted carbamates, which can reasonably explain the formation of all three types of products. Also, N-acylazacycloalkanes are converted anodically at a platinum electrode in R0H-Et4NBF4 into a-monoalkoxy or a,a -dialkoxy derivatives depending on the electrolysis conditions employed.198... 

The Effect of Illumination. In an alkaline solution, an n-GaP electrode, (111) surface, under illumination shows an anodic photocurrent, accompanied by quantitative dissolution of the electrode. The current-potential curve shows considerable hysterisis as seen in Fig. 2 the anodic current, scanned backward, (toward less positive potential) begins to decrease at a potential much more positive than the onset potential of the anodic current for the forward scanning, the latter being slightly more positive than the Ug value in the dark, Us(dark). 

This is the steady-state current which is theoretically predicted if stage 1 is the rate-determining step in the sub-stages sequence represented in Equations 4.8 1.12. An important parameter to compare both in theory and experimentally is the Tafel slope or the transfer coefficient which results from it. Therefore, Equation 4.30 has to be written in a form that contains only one exponential term. Since the considered I-E curve is an oxidation wave, the effect of the reduction (second term in the right-hand part of Equation 4.30) will be negligible with potentials that are situated sufficiently far away from the equilibrium potential, and for the anodic current the following applies ... 

Results. The presence of Pt reduces the corrosion rate of Ti by shifting the free corrosion potential to more noble values (Fig. 6) where the Ti dissolution rate is slower. This shift is produced by the catalytic effect of Pt on hydrogen recombination which alters the cathodic reactions at the alloy surface. At the corrosion potential, the cathodic and anodic currents are equal. Although the shift in corrosion potential reduces the anodic current, anodic dissolution of Ti still occurs. The long-term corrosion rate of a surface alloy depends upon what happens to the Pt as the Ti is being dissolved. If Pt is removed from the surface, the corrosion rate will increase as the implanted volume of the alloy is dissolved. If Pt builds up on the surface, the corrosion rate should remain low. 

Figure 8.6 illustrates the effect of basic substrates on the oxidation of H2 in DMF. As the basicity of the substrate increases, the anodic peak potential for H2 shifts from —0.3 V versus SCE (C10,j") to —1.0 V (HO-) and the peak current decreases. The potential shifts are consistent with Eq. (8.28) and the H-atom-facilitated oxidation of water [bases (B) facilitate formation of [H2+0-] via proton transfer (B + H2+0- — BH+ + HO). In the limit hydroxide ion in H20 at unit activity becomes the primary substrate with its oxidation product (HO-) coupling with H2 (1 atm) via Pt—H(s)... 

Professor S. Srinivasan and his team have studied the effect of pressure and characteristics of the current-potential relations in a hydrogen-oxygen fuel cell with a proton exchange membrane (Y. W. Rho, O. A. Velev, S. Srinivasan, and Y. T. Kho,./. Electrochem. Soc. 141 2084, 2089, 1994). In this problem, it is proposed to study the applicability of the theoretical dependence of the cell potential as a function of pressure. The temperature is 25 °C and it may be assumed that the pressure of the gas in each of the compartments, i.e., the anodic compartment (hydrogen) and the cathodic compartment (oxygen), are the same, Pn =Po P- For the formation of water in its standard state, the relevant thermodynamic quantities are ... 

A significant increase in anodic current from 1.8 pA to 21 pA was observed when CaCl2 was used as the supporting electrolyte in place of KC1. A similar effect was observed by Katz and co-workers for the oxidation of NADH at a cysteamine/PQQ modified electrode25. A slight shift of the oxidation potential from 0.25 to 0.21 V was also observed. LSV also showed similar results for DEAET. 

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