Depassivation

The cleaning or depassivation eflect is of great importance in sonoelectrochemistry, as it can be employed to wash off surface-adsorbed species and reduce blocking of the electrode by adsorption of reaction products. This eflect has been reported, for example, for the depassivation of iron electrodes and for the removal of deposits and in the presence of polymer films on the electrode surface. However, damage of the electrode surface, especially for materials of low hardness such as lead or copper, can also occur under harsh experimental conditions and applied intensities [70, Tf, 80]. 

Corrosion resistance of stainless steel is reduced in deaerated solutions. This behavior is opposite to the behavior of iron, low-alloy steel, and most nonferrous metals in oxygenated waters. Stainless steels exhibit very low corrosion rates in oxidizing media until the solution oxidizing power becomes great enough to breach the protective oxide locally. The solution pH alone does not control attack (see Chap. 4, Underdeposit Corrosion ). The presence of chloride and other strong depassivating chemicals deteriorates corrosion resistance. 

Pits form grooves or furrows on wall surfaces on stainless steels (Figs. 7.5 and 7.6). Contents leak out of pit interiors and depassivate areas immediately below the active corrosion sites. Grooves form, fol-... 

The triggering mechanism for the corrosion process was localized depassivation of the weld-metal surface. Depassivation (loss of the thin film of chromium oxides that protect stainless steels) can be caused by deposits or by microbial masses that cover the surface (see Chap. 4, Underdeposit Corrosion and Chap. 6, Biologically Influenced Corrosion ). Once depassivation occurred, the critical features in this case were the continuity, size, and orientation of the noble phase. The massive, uninterrupted network of the second phase (Figs. 15.2 and 15.21), coupled... 

However, if the interpretation of the potentials measured for regions with a covering as uniform as possible and aeration or moisture is extended to estimate the potential gradients corresponding to the explanation for Fig. 3-24, there follows the possibility of classifying the state of corrosion [52-54]. Furthermore, the sensitivity of the estimate can be raised by anodic polarization according to the explanation given for Fig. 2-7, because the depassivated steel is less polarizable than the passive steel in concrete [43]. 

Corrosion likelihood (depassivation) Steel/concrete potential Uccso, (V)... 

When active, as in a pit or a crevice or when depassivated by mechanical damage of oxide film or chemical removal in nonoxidizing acid. 

Fig. 2.35 Mechanical stresses and corrosive effects to which a joint endoprosthesis is exposed in the human body W-wcar (mechanical and/or depassivation), B-bending (corrosion fatigue), T-torsion, G-general and localised attack.

Although aluminium is a base metal, it spontaneously forms a highly protective oxide film in most aqueous environments, i.e. it passivates. In consequence, it has a relatively noble corrosion potential and is then unable to act as an anode to steel. Low level mercury, indium or tin additions have been shown to be effective in lowering (i.e. making more negative) the potential of the aluminium they act as activators (depassivators). Each element has been shown to be more effective with the simultaneous addition of zinc . Zinc additions of up to 5% lower the anode operating potential, but above this level no benefit is gained . Below 0 9 7o zinc there is little influence on the performance of aluminium anodes . Table 10.10 lists a number of the more common commercial alloys. 

Sodium The FW sodium (Na) content is clearly a factor in the formation of sodium hydroxide in BW and an excess may promote various forms of caustic-induced corrosion. Also, high sodium levels may lead to the depassivation of steel surfaces caused by high pH generation, which reduces the corrosion resistance of boiler steel. 

Sulfate Sulfate ions have reactions similar to chloride. They are also depassivation agents (especially if allowed to concentrate sufficiently) and can greatly accelerate the risk of stress corrosion... 

Chloride leakage with BX softening NOTE Caused by inadequate rinsing. BW carryover Increased BD Temporary loss of control Depassivation... 

Cl- and S042+ leakage (DI). Anodic depolarization, leading to depassivation and corrosion... 

Condenser leaks. Fouling, deposition Chloride depassivation and pitting corrosion of FW lines and boiler... 

Condenser leaks permit dissolved solids, oxygen, organics, and suspended solids to enter the CR system and from there to enter the FW system. This is a very serious problem and leads to feed system fouling and deposition and the depassivation of metal surfaces, which in turn induces pitting corrosion to occur. 

Chloride is both a cathodic depolarizing agent (depolarizer) and a depassivating agent, and the presence of high chlorides in steam-water circuits significantly increases the risk of stress corrosion cracking of austenitic steels (type 300 stainless). 

Sulfate ions have reactions similar to those of chloride. They are corrosion-causative agents (similar to oxygen and hydrogen) of the various types of concentration cell corrosion. In addition, they also are depassivation agents and may greatly accelerate the risk of stress corrosion mechanisms. Saline corrosion pits resulting from high concentrations of chloride and sulfate salts also may be associated with low pH corrosion because hydrochloric acid and sulfuric acid can form within the pit, under deposits. 

Other forms of concentration-cell corrosion include Caustic gouging Saline corrosion Combination of free caustic and concentrating effect causes severe metal wastage. High chlorides and sulfates, result in corrosion from depolarization and depassivation effects... 

NOTE Where cross-contamination takes place, rapid pitting corrosion of depassivated surfaces may develop. This in turn requires a cleaning and repassivation program to be initiated. 

Depassivation of metal surfaces Depolarization, due to chloride leakage Deposit... 

The passive layer is subsequently formed from Ti02+ and was described as Ti02 x H20. During corrosion CO or C02 is formed from the carbon of the carbide. The electrochemical behaviour of TiC in acid electrolytes was reinvestigated with respect to the depassivation of titanium substrates for anodic PbOzdeposition by M. Cappa-donia et al. [122] using XPS. 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Chlorine derivative of cyanamide possesses a high reactivity and can show depassivating properties, but it is difficult to unambiguously identify the compound that is formed because the presence of all possible compounds in the melt will be reflected by the same signal at 630 cm"1 [11],... 

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