Ion passive

Figure 44 shows the coverage 0 against an NaCl concentration that converges to a value less than 1.0 as the concentration of CPion decreases even in the absence of CT ions, passive film has already been broken at least about 15%. This is the reason why the slope 1/6) in Fig. 42 seems to be constant at the region of low NaCl concentration. The pH of the test solution remains constant, that is, 5.7 0.1, so this phenomenon may be attributed to a change in the role of the passivity-destroying ions from CP ions to H+ ions.95,96... 

Fig. 5.41 Approximate anodic polarization curve for iron and cathodic polarization curves for oxygen under several conditions and for nitrite ions. The polarization curves are used to estimate the effects of these environments on corrosion rate. Estimated ECOrr ar d icorr for the several environments are C1, clean surface, aerated C2, surface with corrosion product, aerated C3, clean surface, deaerated C4, clean surface, deaerated plus nitrite ions, passivated C5, clean surface, aerated plus nitrite ions, passivated...

A few years later, the high ceiling diuretics were discovered. The most-used of these, furosemide (frusemide) 14.2), elicits a peak diuresis far greater than can be obtained with any thiazide. It is used to reduce oedema (dropsy) (Muschaweck and Hajdu, 1964). It acts by inhibiting resorption of chloride ions (and hence sodium ions, passively) in the ascending renal tubule at a site below where the thiazides act. There is no action on carbonic anhydrase. Ethacrynic acid (a phenoxyacetic acid) acts similarly, but is harder to control. 

In the presence of chloride ions passivation of steel needs higher pH value. Several hypotheses were proposed to explain the mechanism of passive film destmction in the presence of chlorides [342]. Possibility of chloride ions penetration to passive film, the effect of electric field generated around the adsorbed chloride ions, promoting of Fe " ions diffusion from the surface of metal are listed. Other factors will be discussed farther. 

In conclusion, intracellular [K" ] as measured indirectly by chemical methods generally yielded high values which resulted in the overestimation of the K" " equilibrium potential. To account for an Ej greater than E across the peritubular cell boundary, an active K" influx mechanism was postulated. However, the direct electrometric measurement of intracellular potassium yields the true value of E. This study has shown that E = E for both the peritubular and the luminal cell membranes. In other words, both the luminal and peritubular cell membranes act as potassium electrodes. Therefore, as in skeletal muscle (Khuri et aZ.j 1972), a Donnan-type equilibrium holds with respect to potassium ion passive distribution across both boundaries. 

It mnst be noted that it is impossible to use all these mechanisms in one coating. For example, pigments whose dissolved ions passivate the metal surface require the presence of water. This rules out their use in a true barrier coating, where water penetration is kept as low as possible. 

Beland suggests that corrosion protection comes both from the ability of the tripolyphosphate ion to chelate iron ions (passivating the metal) and from tripolyphosphate ions ability to depolymerize into orthophosphate ions, giving higher phosphate levels than zinc or molybdate phosphate pigments [23]. 

Chloride ions are required for the production of HCl in the stomach, where they accompany the H+ ions, which are secreted actively the Cl ions follow the H+ ions passively to maintain electroneutrality. Because of the high concentration of Cl ions in the stomach, persistent vomiting can lead to a state of chloride deficiency. 

When an experiment such as shown in Figure 18.9 is conducted in solutions of increasing concentration of NaCl, the behavior shown in Figure 18.11 is observed. Both the critical and the passivation current densities are increased and the breakdown potential becomes less positive until, at sufficiently high concentration of the cr ion, passivation can no longer be observed. 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

From polarization curves the protectiveness of a passive film in a certain environment can be estimated from the passive current density in figure C2.8.4 which reflects the layer s resistance to ion transport tlirough the film, and chemical dissolution of the film. It is clear that a variety of factors can influence ion transport tlirough the film, such as the film s chemical composition, stmcture, number of grain boundaries and the extent of flaws and pores. The protectiveness and stability of passive films has, for instance, been based on percolation arguments [67, 681, stmctural arguments [69], ion/defect mobility [56, 57] and charge distribution [70, 71]. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Active Transport. Maintenance of the appropriate concentrations of K" and Na" in the intra- and extracellular fluids involves active transport, ie, a process requiring energy (53). Sodium ion in the extracellular fluid (0.136—0.145 AfNa" ) diffuses passively and continuously into the intracellular fluid (<0.01 M Na" ) and must be removed. This sodium ion is pumped from the intracellular to the extracellular fluid, while K" is pumped from the extracellular (ca 0.004 M K" ) to the intracellular fluid (ca 0.14 M K" ) (53—55). The energy for these processes is provided by hydrolysis of adenosine triphosphate (ATP) and requires the enzyme Na" -K" ATPase, a membrane-bound enzyme which is widely distributed in the body. In some cells, eg, brain and kidney, 60—70 wt % of the ATP is used to maintain the required Na" -K" distribution. 

AletabolicFunctions. The chlorides are essential in the homeostatic processes maintaining fluid volume, osmotic pressure, and acid—base equihbria (11). Most chloride is present in body fluids a Htde is in bone salts. Chloride is the principal anion accompanying Na" in the extracellular fluid. Less than 15 wt % of the CF is associated with K" in the intracellular fluid. Chloride passively and freely diffuses between intra- and extracellular fluids through the cell membrane. If chloride diffuses freely, but most CF remains in the extracellular fluid, it follows that there is some restriction on the diffusion of phosphate. As of this writing (ca 1994), the nature of this restriction has not been conclusively estabUshed. There may be a transport device (60), or cell membranes may not be very permeable to phosphate ions minimising the loss of HPO from intracellular fluid (61). 

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

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. 

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