Passivation partial

Cyclic voltammetry was chosen for polymer deposition as a one-step process. Figure 15.2a presents a typical voltammogram obtained for polypyrrole electrodeposition in the presence of p-TSA as the dopant. In the first cycle, the strong passivation of A1 is present and expressed by the peak at +1.5 V vs SCE. After the first cycle, the surface is passivated, partially by the polymer deposition and partially by the formation of the corresponding oxide which dramatically decreases the current intensity in the next cycles. Polypyrrole oxidation takes place at potentials close to 0 V vs SCE followed by a reduction at lower potentials. The peak appearing at -0.75 V vs SCE only within the first two cycles is attributed to an anion exchange reaction. 

At high temperature, siUcon carbide exhibits either active or passive oxidation behavior depending on the ambient oxygen potential (65,66). When the partial pressure of oxygen is high, passive oxidation occurs and a protective layer of Si02 is formed on the surface. 

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. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

Uses. The use of titanium alloys for cast partial dentures offers light weight, low cost, good ductihty, adequate stiffness, chemical passivity toward foods and oral fluids, and biocompatibiHty with the oral tissues. 

These three passive systems are important in the technique of anodic protection (see Chapter 21). The kinetics of the cathodic partial reaction and therefore curves of type I, II or III depend on the material and the particular medium. Case III can be achieved by alloying additions of cathodically acting elements such as Pt, Pd, Ag, and Cu. In principle, this is a case of galvanic anodic protection by cathodic constituents of the microstructure [50]. 

Surface films are formed by corrosion on practically all commercial metals and consist of solid corrosion products (see area II in Fig. 2-2). It is essential for the protective action of these surface films that they be sufficiently thick and homogeneous to sustain the transport of the reaction products between metal and medium. With ferrous materials and many other metals, the surface films have a considerably higher conductivity for electrons than for ions. Thus the cathodic redox reaction according to Eq. (2-9) is considerably less restricted than it is by the transport of metal ions. The location of the cathodic partial reaction is not only the interface between the metal and the medium but also the interface between the film and medium, in which the reaction product OH is formed on the surface film and raises the pH. With most metals this reduces the solubility of the surface film (i.e., the passive state is stabilized). 

Due to both carbonization and penetration of chloride ions, steel will pass from a passive to an active condition and (consequently) may corrode. If the mortar is completely surrounded by water, oxygen diffusion in wet mortar is extremely low so that the situation is corrosion resistant because the cathodic partial reaction according to Eq. (2-17) scarcely occurs. For this reason the mortar lining of waste pipes remains protective against corrosion even if it is completely carbonated or if it is penetrated by chloride ions. 

Besides the use of anodic polarization with impressed current to achieve passivation, raising the cathodic partial current density by special alloying elements and the use of oxidizing inhibitors (and/or passivators) to assist the formation of passive films can be included in the anodic protection method [1-3]. 

Passivating inhibitors act in two ways. First they can reduce the passivating current density by encouraging passive film formation, and second they raise the cathodic partial current density by their reduction. Inhibitors can have either both or only one of these properties. Passivating inhibitors belong to the group of so-called dangerous inhibitors because with incomplete inhibition, severe local active corrosion occurs. In this case, passivated cathodic surfaces are close to noninhibited anodic surfaces. 

Fig. 21-6 The dependence of the passivation process on the shape of the cathodic partial current potential curve (a) Anodic partial current potential curve, (b) cathodic partial current-potential curve without local cathode rest potential (c) cathodic partial current potential curve with local cathode rest potential I7j p.

This area will be passivated by the increase in pH due to the cathodically produced OH ions, and partially cathodically protected by the electrons liberated by the anodic processes within the pit. The tubercle thus results in an occluded cell with the consequent acidification of the anodic sites. Wranglen considers that in view of the fact that crystals of FeClj -4H20 are sometimes observed at the bottom of a pit the solution within the pit is a saturated solution of that salt, and that this will correspond with an equilibrium pH of about 3-5. 

At sufficiently high rates of flow in natural waters enough oxygen may reach the surface to cause partial passivity, in which case the corrosion rate may decrease. In sea-water, owing to the high concentration of chloride ions, the corrosion rate increases with velocity. In one series of tests, corrosion under static conditions was 0-125mm/y, 0-50mm/y at 5 ft/s and 0-83 mm/y at 15 ft/s. 

It should be noted that the data refer mostly to the behaviour of the alloys in H2SO4. Passivity is, however, influenced by the composition of the solution as well as that of the metal and for this reason the influence of alloying additions may be different in solutions containing other ions. In particular, CI and other similarly aggressive ions have a large influence and may prevent passivation, either completely or partially. If passivity cannot be maintained over the entire surface of the metal, pitting develops, and this is considered later. 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Stainless steel has been tried as an inert anode, mainly under laboratory conditions and with only partial success. Even at low current densities in fresh water the majority of alloys pit rapidly, although others show the ability to remain passive at a low current density . However, at practical current densities, the presence of chloride ions, deposits on the anode or crevice corrosion at the anode support lead to rapid failure , but it may be possible that stainless steel could give useful service under certain conditions and with particular alloys . ... 

Etch primers partially fulfil the roles of both pretreatment and primer. They contain phosphoric acid for surface passivation and are based on polyvinyl butyral ... 

Papin s digester, 177 Partial pressures, lawr of, 171, 265, 274 Pascal s law, 40 Passive resistances, 91 Path, 44... 

Surface recombination processes of charge carriers are mechanisms that cannot easily be separated from real semiconductor interfaces. Only a few semiconductor surfaces can be passivated to such an extent as to permit suppression of surface recombination (e.g., Si with optimized oxide or nitride layers). A pronounced dip is typically seen between the potential-dependent PMC curve in the accumulation region and the photocurrent potential curve (e.g., Fig. 29). This dip may be partially caused by a surface... 

Titanium, Ti, a light, strong metal, is used where these properties are critical— in widely diverse applications such as jet engines and dental fixtures such as partial plates. Although titanium is relatively reactive, unlike scandium it is resistant to corrosion because it is passivated by a protective skin of oxide on its surface. The principal sources of the metal are the ores ilmenite, FeTiO , and rutile, Ti02. 

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