Passivators silicate

Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

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. 

Silt, sand, concrete chips, shells, and so on, foul many cooling water systems. These siliceous materials produce indirect attack by establishing oxygen concentration cells. Attack is usually general on steel, cast iron, and most copper alloys. Localized attack is almost always confined to strongly passivating metals such as stainless steels and aluminum alloys. 

Another class of inhibitors in near-neutral solutions act by stabilising oxide films on metals to form thin protective passivating films. Such inhibitors are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate and borate. Passivating... 

One of the reasons for the low selectivity of the mesoporous Ti silicates is their surface hydrophilicity, which is caused by the presence of a large number of surface Si-OH and Ti-OH groups. Because these mesoporous materials are better suited than TS-1 to the oxidation of large, bulky molecules, the passivation of these OH groups (e.g., by silylation) may improve catalyst activity and selectivity. Attempts have been made to reduce the concentrations of such OH groups by silylating them with various alkyl silanes (Table LI) (273). 

For moderate anodic potentials the dissolution rate becomes enhanced and the ratio of H2/Si is reduced due to the contribution of the electrochemical reaction path [Pa6]. At the electrode surface Si-H as well as Si-OH groups are present. For higher concentrations of the silicate monomer produced by reaction (4.3) silicate polymerization takes place [Ni6]. Passivation takes place for more anodic potentials, due to formation of Si-O-Si bonds according to reaction (4.2). 

Sepiolite passivates most of the nickel via formation of non interactive silicate-like materials. Heating at high temperatures induces migration of nickel to the interior and of vanadium to the exterior of the catalyst surface. Metal-surface interactions are observed also in Ni-loaded kaolin microspheres however, V on kaolin behaves like bulk V20g with respect to reduction, thus explaining this xlay s inability to passivate V-contaminants. 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

Notes Program needs good silicate dispersant polymer, to avoid risk of silicate scales. Also, provide good passivation and clean system. Allow for existing silicates in water. ... 

It is important not to leave the system empty of water for any long period, as rapid surface rusting will take place. As soon as the closed-loop system is declared free of contamination, sufficient corrosion inhibitor is added to provide long-term corrosion protection. The corrosion inhibitor is usually an anodic, passivating formulation, typically based on nitrite or tannin (and often in combination with phosphate, silicate, borate, or molybdate, etc.). Finally, after confirmation that the entire system is adequately treated (which usually requires the inhibited water in the system to be recirculated for a further 16 to 24 hours), the system is signed off and handed over. 

Most usually, a preoperational cleaning (POC) process/passivation program uses a chemical cleaner formulation based on a polyphosphate such as SHMP or STTP, together with various dispersants and surfactants. Where polyphosphate is not permitted to be discharged to sewer, silicates can often be used. Formulations may also include NaEDTA and sometimes specific corrosion inhibitors such as tolyltriazole (TTA). 

Many metals form conductive silicides, which, like SiC, are resistant to oxidation through the formation of stable passivating layers of silicates or silica on their surfaces at high temperatures. Molybdenum disilicide (MoSi2) has been developed as a heating element for use in air at temperatures above 1500 °C. Its resistivity behaves as is expected for a metal, increasing from about 2.5 x 10-7 fim at room temperature to about 4 x 10-6 Qm at 1800 °C. 

A reagent that promotes the appearance of a precipitate on the metal surface, possibly catalysing the formation of a passive layer, for example hydroxyl ion, phosphate, carbonate, and silicate. 

The mitigation of corrosion can be achieved economically by the use of corrosion inhibitors. Chromate has been extensively used in an aqueous environment for the protection of aluminium, zinc and steel. Although chromates are cheap and effective, they are not acceptable because of their toxicity. Alternate inhibitors such as molybdates, organic inhibitors such as phosphonates, mixtures of phosphates, borates and silicates and surfactants like sulfonates have been used in place of chromates. Chromates are anodic inhibitors and help to form passive oxide on the metal surface. 

The determination of metal purity and the elemental composition of alloys is of utmost importance to the metallurgical industry. Microwave-assisted digestion is often well-suited to metals and metallurgical samples that pose no difficulty and dissolve readily and safely with the aid of microwaves [148,186-196]. For example, hydrofluoric acid can be used in closed vessels to digest silicate matrices and stop the hydrolysis of refractory elements without loss of volatile fluorides or passivation. After cooling, boric acid can be added to complex unreacted hydrofluoric acid [14]. The solid sample itself may absorb microwave radiation, thus creating a heated surface on which the acid or acids can react. Microwave muffle furnaces are commercially available [197] based on oven linings made... 

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