Sodium chloride metal surface

For example, with nickel-abased alloys, the formation of a nickel oxide film seems to be pre- requisite for obtaining a polished surface a finish of this quality, of 0.2 ym Ra, has been claimed for a Nimonic (nickel alloy) machined in saturated sodium chloride solution. Surface finishes as fine as 0.1 ym Ra have been reported when nick el- chromium steels have been machined in sodium chlorate solution. Again, the formation of an oxide film on the metal surface has been considered to be the key to these conditions of polishing. 

Metallic sodium. This metal is employed for the drying of ethers and of saturated and aromatic hydrocarbons. The bulk of the water should first be removed from the liquid or solution by a preliminary drying with anhydrous calcium chloride or magnesium sulphate. Sodium is most effective in the form of fine wire, which is forced directly into the liquid by means of a sodium press (see under Ether, Section II,47,i) a large surface is thus presented to the liquid. It cannot be used for any compound with which it reacts or which is affected by alkalis or is easily subject to reduction (due to the hydrogen evolved during the dehydration), viz., alcohols, acids, esters, organic halides, ketones, aldehydes, and some amines. 

If the ECM of titanium is attempted in sodium chloride electrolyte, very low (10—20%) current efficiency is usually obtained. When this solution is replaced by some mixture of fluoride-based electrolytes, to achieve greater efficiencies (> 60%), a higher voltage (ca 60 V) is used. These conditions ate needed to break down the tenacious oxide film that forms on the surface of titanium. It is this film which accounts for the corrosion resistance of titanium, and together with its toughness and lightness, make this metal so useful in the aircraft engine industry. 

Surface Finish. As well as influencing the rate of metal removal, electrolytes also affect the quality of surface finish obtained in ECM. Depending on the metal being machined, some electrolytes leave an etched finish. This finish results from the nonspecular reflection of light from crystal faces electrochemicaHy dissolved at different rates. Sodium chloride electrolyte tends to produce a kind of etched, matte finish when used for steels and nickel aHoys. A typical surface roughness average, Ra is about 1 ]lni. 

At 100—300°C sodium readily wets and spreads over many dry soHds, eg, sodium chloride or aluminum oxide. In this form the metal is highly reactive (7), but it does not easily wet stainless or carbon steels. Wetting of stmctural metals is influenced by the cleanliness of the surface, the purity of the sodium, temperature, and the time of exposure. Wetting occurs more readily at >300°C and, once attained, persists at lower temperatures (5). 

MicrobiologicaHy influenced corrosion, which results from the interaction of microorganisms and a metal, is receiving increased emphasis (1,3,9). The action of microorganisms is at least one of the reasons why natural seawater is more corrosive than either artificial seawater or sodium chloride solutions. Microorganisms attach to the surfaces of metals and can, for example, act as diffusion barriers produce metaboHtes that enhance or initiate... 

It is appropriate to consider first the crevice corrosion of mild steel in oxygenated neutral sodium chloride, and then to consider systems in which the metal is readily passivated. Initially, the whole surface will be in contact with a solution containing oxygen so that attack, with oxygen reduction providing the cathodic process, occurs on both the freely exposed surface and the surface within the crevice (Fig. 1.50). However, whereas the freely exposed surface will be accessible to dissolved oxygen by convection and diffusion, access of oxygen to the solution within the crevice can occur only... 

Salt solutions When a zinc sheet is immersed in a solution of a salt, such as potassium chloride or potassium sulphate, corrosion usually starts at a number of points on the surface of the metal, probably where there are defects or impurities present. From these it spreads downwards in streams, if the plate is vertical. Corrosion will start at a scratch or abrasion made on the surface but it is observed that it does not necessarily occur at all such places. In the case of potassium chloride (or sodium chloride) the corrosion spreads downwards and outwards to cover a parabolic area. Evans explains this in terms of the dissolution of the protective layer of zinc oxide by zinc chloride to form a basic zinc chloride which remains in solution. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Sodium a-olefinsulfonate, sodium silicate, sodium alkyl sulfate, sodium chloride, and sodium sulfate can be dissolved in water and cooled to give a gelled cleaning composition having high viscosity. Such mixtures are excellent cleaning compositions for ovens and other metal surfaces according to Watanabe and Schultz [95]. 

Chromate conversion coatings for aluminum are carried out in acidic solutions. These solutions usually contain one chromium salt, such as sodium chromate or chromic acid and a strong oxidizing agent such as hydrofluoric acid or nitric acid. The final film usually contains both products and reactants and water of hydration. Chromate films are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of accelerators such as cyanides, acetates, formates, sulfates, chlorides, fluorides, nitrates, phosphates, and sulfamates. 

In the present work, steel surfaces polished, phosphated and painted are studied using AC Impedance technique In order to evaluate the protection efficiency of a commercial phosphatlng solution. The AC Impedance behavior of painted metal has been correlated with the Immersion time In the phosphatlng solution and with the desirability of a phospho-chromic rinse (18-23). A comparison of the Impedance behavior of two different types of commercial paints Is made for various durations of Immersion In sodium chloride solution at room temperature, and also for various temperatures at a given duration of Immersion. 

Other factors indicated m the data of Tables 1 and 2 include Pour Point—defined as the lowest temperature at which the material will pour and a function of the composition of the oil in terms of waxiness and bitumen content Salt Content—which is not confined to sodium chloride, but usually is interpreted in terms of NaCl Salt is undesirable because of the tendency to obstruct fluid flow, to accumulate as an undesirable constituent of residual oils and asphalts, and a tendency of certain salt compounds to decompose when heated, causing corrosion of refining equipment Metals Content—heavy metals, such as vanadium, nickel, and iron, tend to accumulate in the heavier gas oil and residuum fractions where the metals may interfere with refining operations, particularly by poisoning catalysts. The heavy metals also contribute to the formation of deposits on heated surfaces in furnaces and boiler fireboxes, leading to permanent failure of equrpment, interference with heat-transfer efficiency, and increased maintenance. 

Catalyst bed plugging can arise in a variety of ways, but the overall effect of bed plugging is always the same expensive shutdowns and possibly complete renewal of the expensive catalyst. Thus, the deposition of rust, coke, or metal salts (e.g., sodium chloride) from heavier and dirtier feedstock may all contribute to the plugging of a catalyst bed. Vanadium and nickel may also be deposited onto the surface of the catalyst as well as into the pore system. Asphaltene deposition from residua and heavy oils is also a potential means of bed plugging— coagulation of the asphaltenes becomes appreciable at temperatures above 420°C (790°F) with the formation of hard, coke-like materials on the catalyst. 

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