Anodic polarization curves sulfuric acid

Fig. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

FIGURE 22.24 Anodic polarization curves for passivation and transpassivation of metallic iron and nickel in 0.5 kmol m-3 sulfuric acid solution with inserted sketches for electronic energy diagrams of passive films [32] /ip = passivation potential, TP = transpassivation potential, fb = flat band potential, /Fe = anodic dissolution current of metallic iron, Nl = anodic dissolution current of metallic nickel, and io2 — anodic oxygen evolution current. 

Fig. 4.14 Anodic polarization curves for Fe-10.5%C and SS (304 L) in 1 N sulfuric acid [47]. Reproduced...

As shown in Fig. 4.15, the anodic polarization curves obtained for steels with chromium content between 3.54% and 19.20% indicates that increasing the chromium content enhances the Fe-Cr-Ni alloy passivation ability by decreasing the critical current density of the alloys from 10 to 10 pA/cm [49]. The passive current also decreases from 50 pA/cm for 3.54% Cr alloy in sulfuric acid to 10 pA/cm for the alloy with 19.2% Cr. 

The corrosion resistance of TiN can be presented by anodic polarization curves. The polarization result of TiNo.gr sintered body determined at room temperature is shown in Figure 11.3.1 [11]. Since its electrode potential directly after immersion in dilute sulfuric acid is positive (4-0.016 V), it is not soluble for the dilute sulfuric acid. The current density increases rapidly with increasing the voltage, and then decreases reversibly from 0.1 to 0.5 V, showing passivation of TiN. This behavior resembles to anodic polarization curve of titanium. This would be due to substoichiometric composition TiN with excess amount of titanium. Dissolution of TiN is possible in hot fluoric nitric acid solution alone. 

FIGURE 11.3.1 Anodic polarization curve of TiN in dilute sulfuric acid. 

In acidic media, the metals iron, nickel and chromium have passivation current densities that increase in the order Cr < Ni < Fe. In Figure 6.11, the anodic polarization curves for the three metals in 0.5 M sulfuric acid (25 °C) are compared. Chromium has lower values of both ip and Ep than the other two metals. By alloying increasing amounts of chromium to steel one therefore improves the corrosion resistance. Experience shows that above a chromium concentration of 12 to 13%, a steel passivates spontaneously in contact with aerated water. It becomes "stainless", meaning it does not rust easily. Figure 6.12 gives the corrosion potential of different... 

Figure 10.11 Variation with applied potential of the friction coefficient between an AISI430 stainless steel and an aluminium oxide pin in 0.5 M sulfuric acid. Also shown is the anodic polarization curve of the steel [5].

Fig. 2-27 shows the rates of corrosion of Zr-702, Zr-705, and B-3 alloys in boiling solutions of hydrochloric acid. In contrast with the behavior of zirconium alloys in sulfuric and phosphoric acids (Figs 2-24 and 2-26), in hydrochloric acid the rate of corrosion of Zr-705 is lower than that of Zr-702 alloy. Anodic polarization curves of... 

Besides alloy composition (Table 2-19), the corrosion behavior of titanium in reducing acids is very dependent on acid concentration, temperature, and impurities in the acid (Schutz and Thomas, 1987). Anodic polarization curves of titanium in sulfuric acid solutions showed that the critical current for passivation at fixed temperature increased with the acid concentration (Levy, 1967 Peters and Myers, 1967) and with temperature at a given acid concentration (Levy, 1967). Fig. 2-29 shows the rates of corrosion of Ti Gr 2 and other alloys as a function of the concentration of pure hydrochloric acid at the boiling temperatures. As the acid concentration increases the rate of corrosion of Ti Gr 2 increases rapidly. 

Figure 12.3 Forward and backward potentiostatic anodic polarization curves for mild steel in 10% sulfuric acid at 22 and 60°C.

Fig.l. Typical anodic polarization curve of stainless steels in sulfuric acid solutions. Eoc natural corrosion potential, Ep passivity potential, Er rupture potential. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 9—4 shows the polarization curves observed for the transfer reaction of cadmium ions (Cd Cd ) at a metallic cadmium electrode in a sulfuric acid solution. It has been proposed in the literature that the transfer of cadmium ions is a single elemental step involving divalent cadmium ions [Conway-Bockris, 1968]. The Tafel constant, a, obtained from the observed polarization curves in Fig. 9-4 agrees well with that derived for a single transfer step of divalent ions the Tafel constant is = (1- P) 1 in the anodic transfer and is a = z p = 1 in the cathodic transfer. 

FIGURE 22.7 Polarization curves forthe anodic dissolution and the passivation of metallic iron in 0.5 kmolm 3 sulfuric acid solution at 25°C [9,10] fpe = anodic iron dissolution current, io7 — oxygen evolution current, p = passivation potential, and Etp — trans-passivation potential. 

As suggested earlier, the n-type GaAs does not corrode in the dark but does corrode under photoexcitation. Figure 22.21 shows semi-schematically the polarization curves for corrosion of an n-type GaAs electrode in sulfuric acid solution under the dark and photoexcited conditions [5-6]. It is seen that in acid solution the rc-type GaAs electrode does not corrode in the dark but does corrode under photoexcitation. The anodic dissolution occurring at the photoexcited rc-type electrode is essentially the same as that which will occur at the p-type electrode, except that the potential region... 

FIGURE 22.21 Polarization curves for the corrosion of a photoexcited n-type GaAs electrode in 0.5 kmol m-3 sulfuric acid solution [15] curves are exaggerated around Ecolr solid curve = photoexcited, dashed curve = dark, and H2 = cathodic hydrogen reaction coupled with anodic GaAs dissolution. 

Because of the presence of an oxide film, the dissolution rate of a passive metal at a given potential is much lower than that of an active metal. It depends mostly on the properties of the passive film and its solubility in the electrolyte. During passivation, which is a term used to describe the transition from the active to the passive state, the rate of dissolution therefore decreases abruptly. The polarization curve of a stainless steel in sulfuric acid, given in Figure 6.2, illustrates this phenomenon. In this electrolyte, the corrosion potential of the alloy is close to -0.3 V. Anodic polarization leads to active dissolution up to about -0.15 V, where the current density reaches a maximum. Beyond this point, the current density, and hence the dissolution rate, drops sharply. It then shows little further variation with potential up to about 1.1 V. Above that value the current density increases again because transpassive dissolution and oxidation of water to oxygen becomes possible. 

Fig. 1. Anodic charging curves for Pt/Pt electrode in 1 N sulfuric acid solution with chemisorbed organic substances before (a) and after (b) cathodic polarization of the electrode. The methanol was adsorbed at 480 mV, the other compounds were adsorbed at an open circuit (starting potential 500 mV). The concentrations were 0.5 mole/liter. The dotted lines represent the curves for the indifferent electrolyte (from data in [59]).

---