Inhibition by Passivation

Schematic polarization curve of a metal with active dissolution and passive range and a passivating redox system creating passivity (solid line) and maintaining 

---

To provide active corrosion inhibition by passivating the metallic substrate or by reducing the rate of the oxygen-reduction reaction... 

The reduction of chromate films is an important method of corrosion inhibition by passivation (section 10.2.2) ... 

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. 

Corrosion-inhibiting primers based on this technology have been in continuous service since they were first utilized with nitrile epoxies in the late 1960s. These inhibitors function by passivating the aluminum. In this process, water permeating the adhesive bondline carries a certain amount of inhibitor to the oxide surface. 

The ways in which inhibitive anions affect the corrosion of zinc are mainly similar to those described above for iron. In inhibition by chromate, localised uptake of chromium has been shown to occur at low chromate concentrations and in the presence of chloride ions Thus under conditions unfavourable for inhibition, pore plugging occurs on zinc. Inhibitive anions also promote the passivation of zinc, e.g. passivation is much easier in solutions of the inhibitive anion, borate , than in solutions of the non-inhibitive anions, carbonate and bicarbonate , A critical... 

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

Although Li+ appears to have no nutritional function, plants have the ability to absorb the cation, the uptake being inhibited by Ca2+ [248], The transport of Li+ in plant cells has received little attention its movement can be passive or mediated by Na+ channels [249] and in some reports Li+ actually blocks ion channels for instance it inhibits voltage-dependent K+ channels in Chara cells [250]. Li+ is unevenly distributed throughout the plant, being most concentrated in the cell walls [251]. 

Addition of sodium dodecyl benzene sulfonate to dilute alkaline electrolyte depresses the passivation of zinc surface [275]. Owing to the dodecyl benzene sulfonate adsorption, the passive layer on zinc has a loose and porous structure. Zinc electrodissolution was inhibited by the presence of sodium metasdicate [276] and some acridines [277]. The protection effect was described by a two-parameter equation. 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

In principle, interfacial recombination processes can be inhibited by modifying the interface. The use of t-butylpyridine in the DSSC electrolyte solution to increase its photovoltage is one example [2,97]. We wished to explore general methods for passivating interfacial recombination sites in DSSCs that might allow the use of a variety of redox couples and therefore facilitate making a viable solid-state DSSC. 

Figure 9 An illustration depicting the two methods of interface passivation used here, (a) Reaction (5) alone is inhibited by electropolymerizing a film of insulating PPO, polytp-henyleneoxide-co-2-allylphenyleneoxide), on the Sn02 substrate, (b) Both reactions (4) and (5) are inhibited by coating exposed oxide surfaces with poly(methylsiloxane).

In an in vitro study in an adipocyte cell line (3T3L1 cells), >75% of the 2,2, 4,4, 5,5 -hexabromobiphenyl taken up by the cells was associated with subcellular fractions that contained 85% of the cellular triglyceride, with only 20% of the compound found in the microsomal plasma-membrane fraction (Kraus and Bernstein 1986). This study also found that inhibition of respiration by cyanide at a concentration that completely inhibited oxygen consumption did not affect uptake of 2,2, 4,4, 5,5 -hexabromobiphenyl, supporting the assumption that because of their lipophilic nature, PBBs penetrate membranes by passive diffusion. 

---