Acids silicon-iron alloys

Fig. 3.63 Limits of use of silicon irons for handling hydrochloric acid solutions (former Ministry of Supply data) for Fe-18Si and Fe-14-5Si alloys...

Quinoline Salicylic acid Silicon Dinitrogen tetroxide, linseed oil, maleic anhydride, thionyl chloride Iodine, iron salts, lead acetate Alkali carbonates, calcium, chlorine, cobalt(II) fluoride, manganese trifluoride, oxidants, silver fluoride, sodium-potassium alloy... 

An acid-resisting alloy known as illium contains Ni 60-65, Cu 6-42, Cr 21-07, Mo 4-67, W 2-13, with traces of aluminium, silicon, iron, and manganese. It melts at 1300° C., and has a tensile strength of 50,000 lb. per square inch. It is remarkably resistant to corrosion, a 25 per cent, solution of nitric acid having no apparent influence upon it in twenty-four hours.2... 

Reduction of a mixture of silicon and iron oxides with coke produces an alloy of iron and silicon known zs ferrosilicon. It is used in the production of acid-resistant steel alloys, such as duriron, and in the deoxidation of steel. Aluminum alloys for aircraft are strengthened with silicon. 

Separation of Sip4 by the Conway microdiffusion method is a slow process [2,3], The sample to be analysed (e.g., copper, aluminium, iron alloys, dolomite, titanium dioxide) is decomposed with acids in a polystyrene Petri dish to which hydrofluoric acid is subsequently added. The Sip4 evolved is trapped in NaOH solution, e.g., in another Petri dish placed beside. Both vessels are placed inside another, tightly closed, polystyrene vessel. To achieve quantitative separation of silicon, the reaction is carried out at 70°C for 18 h. 

At the turn of the century the cost of elemental silicon was 1600/lb. Improvement of reduction conditions allowed this price to fall to 0.10/lb by 1931 and made the use of the element more practical. Elemental silicon is employed as an additive to iron to give silicon-steels, which are more acid resistant, to alloy copper for production of silicon bronzes and as an additive to aluminum or magnesium to improve the strength of these structural materials and provide resistance to corrosion. The use of silicon as a semiconductor had begun by 1940 and after discovery of the direct process reaction became the necessary starting point for the production of silicone polymers103. 

A wide variety of inorganic materials have been used to precipitate or collect trace metals from solution. The most direct approach is a cementation process, which is one that removes the trace pollutants from solution by reduction with a metal and plating onto that metal surface. Although this process may be slow, the filtration is usually quick, since decantation is often sufficient. Finely divided cadmium extracts copper, selenium, and mercury from nitric and sulfuric acid solutions (66). When copper was used to preconcentrate mercury from water or biological fluids prior to atomic absorption analysis, the detection limit was 1-2 X 10 g (67, 68). Iron (69), zinc (70), and tungsten (71), as metals, have also been used to obtain a deposit of several trace metals from aqueous systems as dilute as 10 ppb for subsequent analysis. Elemental tellurium can be produced in solution by reduction using tin(II) chloride or sulfur dioxide, and coprecipitates silver (72) and selenium (73). Granulated silicon-metal alloys were used to remove metal ions from water and brine by reduction as well (74, 75). 

When 4.5% chromium is added to the alloy, it becomes resistant to severe chloride-containing solutions and other strongly oxidizing environments. The chromium-bearing grade (Durichlor) will handle hydrochloric acid up to 80°F (27°C). Hydrofluoric add causes rapid attack. Table 4.5 lists the compatibility of high-silicon iron with selected corrodents. 

Nickel/silicon alloy (10% silicon, 3% copper, and 87% nickel) is fabricated only as castings and is rather brittle, although it is superior to the iron/silicon alloy with respect to strength and resistance to thermal and mechanical shock. It is comparable to the iron/silicon alloy in corrosion resistance to boiling sulfuric acid solutions at concentrations above 60%. Therefore, it is chosen for this and other arduous duties where its resistance to thermal shock justifies its much higher price compared with iron/silicon alloys. 

Silicon used for diffusion treatment of carbon steels enhances corrosion resistance to sulfuric acid. Such a treatment has the surface durability of iron/silicon alloys without their marked brittleness. 

Chromium, silicon and other alloying elements are used to create cast irons for corrosion resistance in specific environments. Silicon-containing cast irons are used for sulfuric acid duty. 

Metallic magnesium is produced by either chemical or electrolytic reduction of its compounds. In chemical reduction, first magnesium oxide is obtained from the decomposition of dolomite. Then ferrosilicon, an alloy of iron and silicon, is used to reduce the MgO at about 1200°C. At this temperature, the magnesium produced is immediately vaporized and carried away. The electrolytic method uses seawater as its principal raw material magnesium hydroxide is precipitated by adding slaked lime (Ca(OH)2, see Section 14.10), the precipitate is filtered off and treated with hydrochloric acid to produce magnesium chloride, and the dried molten salt is electrolyzed. 

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. 

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

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