Passivity transpassive dissolution

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

Passivation looks different when observed under galvanostatic conditions (Fig. 16.2b). The passive state will be attained after a certain time t when an anodic current which is higher than is applied to an active electrode. As the current is fixed by external conditions, the electrode potential at this point undergoes a discontinuous change from E to Ey, where transpassive dissolution of the metal or oxygen evolution starts. The passivation time t will be shorter the higher the value of i. Often, these parameters are interrelated as... 

For metallic iron and nickel electrodes, the transpassive dissolution causes no change in the valence of metal ions during anodic transfer of metal ions across the film/solution interface (non-oxidative dissolution). However, there are some metals in which transpassive dissolution proceeds by an oxidative mode of film dissolution (Sefer to Sec. 9.2.). For example, in the case of chromium electrodes, on whidi the passive film is trivalent chromium oxide (CrgOj), the transpassive dissolution proceeds via soluble hexavalent chromate ions. This process can be... 

Figure 23 Schematic polarization curve for metal that spontaneously passivates but pits upon anodic polarization. A hysteresis loop, which can appear during a reverse scan, is shown ending at Erp. One dotted line shows behavior for anodizing conditions, while the other shows transpassive dissolution.

Passivation potential — Figure 1. Polarization curves of three metals in 0.5 M H2SO4 with active dissolution, a passive potential range, and transpassive dissolution and/or oxygen evolution at positive potentials Ep(Cr) = -0.2 V, -Ep(Fe) FP(Ni) = 0.6 V [i]... 

Fig. 2D Schematic HE plots for a system undergoing corrosion and passivation. Active dissolution region A-B. Passive region C-D. Transpassive region where pitting occurs D-E.

In electrochemical experiments of nickel in sulfuric acid, passive behavior can be seen, but the passive range is relatively small and transpassive dissolution is found at high potentials (Fig. 21). The passive behavior is the result of a NiO or Ni(OH)2 film. At higher temperatures, active dissolution of nickel appears [29]. 

Fig. 4 Eiectrochemicai frameworks for the intergranuiar corrosion of aiioys that exhibit uniform passivity prior to sensitization in the environment given. Case (a) different primary passive potentiais and active dissoiution regions for the grain boundary and grain matrix such as is observed for Fe-Cr and Fe-Ni-Cr aiioys. ICC occurs over the potential range at which the matrix is passive while the grain boundary is active. Case (b) different critical potential for grain boundary and matrix. The critical potentials have been shown to be associated with pitting, repassivation, and/or transpassive dissolution.

The last electrochemical framework for IGC also involves passive materials. IGC can occur when grain boundaries develop lower critical potentials associated with pitting or transpassivity. Pitting or transpassive dissolution then occurs preferentially along grain boundaries in solutions in which oxidizing conditions exist. This situation is shown in Fig. 4(b). 

Chromium is an example with an extremely thin passive layer and a much smaller corrosion current. At 1.0 V the passive film is oxidized to chromate. This is the region of transpassive dissolution (Figure 10.17). 

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. 

The oxidation of trivalent chromium oxide to hexavalent oxide is also observed in passive films on stainless steel. An interesting application of this effect is electropolishing. The object made of stainless steel to be polished is connected as the anode in an electrochemical cell that contains a suitable electrolyte, normally a mixture of concentrated sulfuric and phosphoric acids. Transpassive dissolution then leads to electropolishing, provided the dissolution reaction is under mass transport control. Protrusions on the surface are favored by mass transport and therefore they dissolve more rapidly, leading to leveling of the surface [34]. 

Figure 3-2. b) Schematic pocential-pH diagram showing the regions of immunity, corrosion (active and transpassive dissolution), and passivation. 

A typical example of the application of EIS is the investigation of passive films on Zn, Zn-Co, and Zn-Ni (Fig. 7-18), which were carried out to explain the difference in the corrosion behavior of pure and low-alloyed zinc by the possible formation of electron traps through the incorporation of cobalt or nickel into the oxide film (Vilche et al., 1989). Passive films of zinc in alkaline solutions are known to be n-type semiconductors with a band gap Eg = 3.2 eV (Vilche et al., 1989). The n-type character arises from an excess of zinc atoms in the nonstoichiometric oxide. The impedance measurements in 1 N NaOH solution were carried out at potentials at which Faraday reactions like transpassive dissolution and oxygen evolution do not interfere. The passive layer was formed for 2 h at positive potential before the potential was swept in the negative direction for the impedance meas-... 

Passive and Transpassive Dissolution of Nickel in Acidic Solutions The kinetics of nickel dissolution in the passive and transpassive ranges M remained totally unclear until the application of a very low frequency impedance technique. A general model was proposed on the basis of an extensive study of anion effects [143]. In the passive state, the fiequency domain had to be extended far below ImHz and long-term stability was obtained only by using single-crystal electrodes [144]. 

In contrast to selective dissolution, evenly dissolving alloys can be dealt with, up to a sophisticated level including non-steady-state responses, by macroscopic, i.e., kinetic descriptions. As shown in Figure 23, Olivier [1851] pointed out that the steady polarization curves of Fe-Cr alloys in a 0.5 M sulfuric solution display the decay of active and passive currents with increasing Cr content and the emergence of the transpassive dissolution of Cr to the hexavalent state. 

M. Bojinov, I. Betova, R. Raicheff, G. Fabricitts, T. laitinerr, and T. Saario, Mechanism of transpassive dissolution and secondary passivation of chromirrm in sulphuric acid solutions, Mater. Sci. Forum, 289-292 1019 (1998). 

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