Transpassive dissolution

C2.8.6(c). This increase occurs far below eitlier transpassive dissolution (oxide film dissolution due to tire fonnation of soluble higher oxidation states (e.g. Cr,0., ... 

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. 

Tin anodes dissolve by etching corrosion in acid baths based on stannous salts, but in the alkaline stannate bath they undergo transpassive dissolution via an oxide film. In the latter the OH" ion is responsible for both film dissolution and for complexing the tin. Anodes must not be left idle because the film dissolves and thereafter corrosion produces the detrimental divalent stannite oxyanion. Anodes are introduced live at the start of deposition, and transpassive corrosion is established by observing the colour of the film... 

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... 

Beck, F. Kaus, R. Oberst, M. (1985) Transpassive dissolution of iron to ferrate(VI) in concentrated alkali hydroxide solutions. Electro-chim. Acta 30 173-183... 

At E > 1.0 V transpassive dissolution as QxCY2 is obtained in 0.5 M H2SO4, with a strong increase of the current density (Fig. 5). At E > 1.7 V oxygen evolution contributes to the large anodic current. These characteristic electrochemical properties indicate the importance of Cr as an alloying additive to obtain corrosion-resistant alloys, especially at negative potentials and in acidic solutions. 

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]... 

One of the anodic oxidative dissolution of metal oxides frequently encountered in the corrosion of stainless steels is the transpassive dissolution of metallic chromium ... 

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]. 

The breakdown/adsorption mechanism At first, the oxide film breaks down owing to mechanical stress. Then, Cl is adsorbed at the bare metal and enhances active dissolution. The Sato model [11] focuses on pitting and transpassive dissolution. It assigns the breakdown to the electrostriction pressure resulting from the high field strength E. s produces... 

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). 

While the previous examples were limited in the anodic polarization potential either by transpassive dissolution or by oxygen evolution valve metals can be polarized to potentials of up to 100 V and above. Examples are aluminum, titanium, tantalum, hafnium, and zirconium. Formation characterization and properties of these oxides were treated in Chapter 9. 

Temperature can also be used as an acceleration factor in a fashion similar to potential. Many materials wiU not pit at a temperature below a critical value that is often extremely sharp and reproducible [56-62]. At low temperatures, extremely high breakdown potentials are observed, corresponding to transpassive dissolution, not localized corrosion. Just above the critical pitting temperature (CPT), pitting corrosion occurs at a potential that is far below the transpassive breakdown potential. This value of CPT is independent of environmental parameters and applied potential over a wide range, and is a measure of the resistance... 

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