Transpassive corrosion

Nitrate ions have a special influence by inhibiting pitting corrosion in neutral and acid waters atU> [Eq. (2-50)] [48,52], corresponds to a second pitting potential and is designated the inhibition potential. The system belongs to group IV, with pitting corrosion at U U... 

High-alloy steels with >16%Cr = Halide-free cold acids 0.2/1.1 -0.1/0.8 Protection against active and transpassive corrosion... 

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... 

Figure 11 shows idealized polarization curves for the cases where the temperature is above the CPT (pitting) and below the CPT (transpassive corrosion). These polarization curves show the pitting potential ( ),), transpassive potential (E,), and repassivation potential (E ). Ep and E, are defined as the potentials at which the current density unambiguously... 

The special construction of the spectrometer permits not only a safe specimen transfer without chemical changes, but also a well-defined specimen pre-treatment by sputtering previous to the electrochemical preparation. This is very important in the case of alloys because active dissolution or etching and transpassive corrosion or electropolishing may change the surface by preferential dissolution of one component. The altered surface composition may have an effect on the kinetics of passivation and on the composition of the passive layer, formed subsequently as has been... 

Note that jjh vviU be in the range of mV, while the total overpotential ri U can reach 100 V and more. Figure 9 gives a very good example of various ITR at the oxide surface of passive Fe. Here, the experimental parameter f/(hess) is chosen to eliminate the pH-dependence [24]. These experimental results were used to explain the difference between t]h and U [16, 28]. For example, the independence of the passive corrosion (15,16,18) = (1) of U means that )]h = 0 or A h = const. The transpassive corrosion, on the other hand, indicates an increase of Acpn and )]H > 0 [24]. Note that the anodic dissolution of Fe + decreases with increasing U near CpB, which is due to the enrichment of electrons or Fe + respectively at the oxide surface. The pH-dependence can be explained by Fq. (8) [16, 24]. 

If protection breaks down at any point, corrosion will be extremely rapid at that point because of the low resistance path formed. On the other hand, if the metal potential is made too positive, then the region of passivation may be passed and transpassive corrosion in the form of pitting will occur. To avoid such things happening, extensive monitoring and control facilities are required. 

The phenomena ofbreakdown ofpassive films at more noble values of potential leads to an accelerated rate of corrosion (transpassive corrosion). The potential at which the breakdown or rupture of protective film takes place and the current density rises sharply is called transpassive potential (Iitranspassive)- In Fig. 3.26, the effect of chromium... 

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