Passivity causes

Two basic mechanisms cause biological corrosion. Biologically produced substances may actively or passively cause attack. Each mechanism either accelerates preexisting corrosion or establishes a new form of metal loss. Often the distinction between active and passive attack is vague. 

Passivity is also readily detected electrically by the difference in potential between passive and active iron,1 the former having a lower potential. This is a particularly convenient method of detecting passivity caused by anodic polarisation. 

The silicon surface in nonfluoride and nonalkaline solutions is spontaneously passivated due to the formation of a thin native oxide film at a rate depending on many factors as discussed in Chapter 2. For n-Si samples in aqueous solutions under illumination the occurrence of passivation causes a decrease of the photocurrent as shown in Fig. 5.11. " In the absence of HF, photocurrent rapidly reduces to near zero due to the formation of an oxide film. The stationary photocurrent increases with increasing HF concentration. For a given light intensity, there is a HF concentration above which the photocurrent does not decrease from the initial value. The surface is free of oxide film at this HF concentration. 

First-order reactions may be favored under these conditions. In addition, overpotentials for the reaction of the substrate at the electrode may be avoided and reactions may be accelerated. Furthermore, if electrode passivation causes problems for direct electrolyses, an indirect pathway could be an advantage. In indirect processes often there are obtained better selectivities with redox catalysts. 

Above 400 mV, reactions at the magnetite electrode are dominated by the oxidation to maghemite with the concurrent release of electrons (anodic current) and Fe(II) to solution (equation 5). Increasing positive potentials accelerate this reaction. Above 400 mV, the current density remains relatively constant up to the potential at which H2O begins to dissociate (Figure 2). Constant currents as functions of increasing positive potentials are commonly attributed in metal and metal oxide electrodes to passivation caused by the formation of unreactive oxidized surfaces. In magnetite, such passivation... 

Nevertheless, corrosion of steel reinforcements does occur in structures such as concrete bridge decks and parking garages. These problems have been studied for many years and result from the breakdown of passivity caused, for example, by salt in the environment or in the concrete [59]. Passivity may also be lost after several years if air diffuses through the concrete to the reinforcements, converting alkaline Ca(OH)2 to less alkaline CaCOs. Corrosion processes for steel in concrete are illustrated in Fig. 7.15 [60]. 

As discussed above, the earthquakes were not caused by active fault movements. The thrust movements of the fault solid rock walls were passively caused by the uplifting of powerfully expanding and migrating natural gases in the faults. The passive thrust movements could be only a few tens meters. They could not push, vibrate or force the seawater of thickness from three to six thousand meters to form the extremely large tsunami waves. 

Ferreira, M., Varela, H., Torres , R.M., and Tremiliosi-Filho, G. (2006) Electrode passivation caused by polymerization of different phenolic compounds. Electrochim. Acta, 52 (2), 434-442. 

Details of the above reactions are given in Chapter 2. The films do not remain protective any more if the pH falls due to carbonation. They do not remain adherent to the reinforcement any longer and loose their passivity causing the reinforcement surface to be exposed to corrosive species. The ingress of CO2 is a major factor which breaks down the passivity and dissolves the protective oxide layer on the reinforcement surface. 

In this case, the integrated redimdancy protocol does not guarantee the absence of such discrepancies, but rather controls their safety with respect to the rest of the system. This is done by processing the emitted outputs, or eventually, if the discrepancy is sustained, by sending a passivation causing a return into the isolated mode. These points are detailed below in sections 11.4.8 and 11.4.9. 

The anodes can be made of graphite which tolerates high current densities without passivation, but are subject to gradual corrosive attack causing a... 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

Tantalum is not resistant to substances that can react with the protective oxide layer. The most aggressive chemicals are hydrofluoric acid and acidic solutions containing fluoride. Fuming sulfuric acid, concentrated sulfuric acid above 175°C, and hot concentrated aLkaU solutions destroy the oxide layer and, therefore, cause the metal to corrode. In these cases, the corrosion process occurs because the passivating oxide layer is destroyed and the underlying tantalum reacts with even mild oxidising agents present in the system. 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

For example, chloride and duoride ions, even in trace amounts (ppm), could cause the dissolution of aluminum metallization of complimentary metal oxide semiconductor (CMOS) devices. CMOS is likely to be the trend of VLSI technology and sodium chloride is a common contaminant. The protection of these devices from the effects of these mobile ions is an absolute requirement. The use of an ultrahigh purity encapsulant to encapsulate the passivated IC is the answer to some mobile ion contaminant problems. 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Generally, pitting corrosion only occurs on passivated metals when the passive film is destroyed locally. In most cases chloride ions cause this local attack at potentials U > U q. Bromide ions also act in the same way [51], The critical potential for pitting corrosion UpQ is called the pitting potential. It has the same significance as in Eqs. (2-39) and (2-48). 

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