Chromium in steels

Chromium in steel Discussion. The chromium in the steel is oxidised by perchloric acid to the dichromate ion, the colour of which is intensified by iron (III) perchlorate which is itself colourless. The coloured solution is compared with a blank in which the dichromate is reduced with ammonium iron(II) sulphate. The method is not subject to interference by iron or by moderate amounts of alloying elements usually present in steel. 

The above considerations will be illustrated by the simultaneous determination of manganese and chromium in steel and other ferro-alloys. The absorption spectra of 0.001 M permanganate and dichromate ions in 1M sulphuric acid, determined with a spectrophotometer and against 1M sulphuric acid in the reference cell, are shown in Fig. 17.20. For permanganate, the absorption maximum is at 545 nm, and a small correction must be applied for dichromate absorption. Similarly the peak dichromate absorption is at 440 nm, at which permanganate only absorbs weakly. Absorbances for these two ions, individually and in mixtures, obey Beer s Law provided the concentration of sulphuric acid is at least 0.5M. Iron(III), nickel, cobalt, and vanadium absorb at 425 nm and 545 nm, and should be absent or corrections must be made. 

The second term indicates that by increasing the melt temperature, e.g., by oxygen lancing, to 1850 °C, AG° for the reaction becomes more positive and the reverse reaction leading to retention of chromium in steel becomes favorable. Addition of a deoxidant, e.g., silicon as ferrosilicon, which forms an oxide stabler than Cr203, also leads to further recovery of chromium in steel ... 

Fredriksson H. Heilner L., The influence of carbon on the segregation of chromium in steel. Scand. J. Metallurgy 3 974),6 -68... 

The first patent for the use of chromium in steel was granted in 1865 - but the large-scale use of chromium had to wait until chromium metal could be produced by the alumino-thermic route, developed in the early 1900s and when the electric arc furnace could smelt chromite into the master alloy, ferrochromium. 

E.A.G. Zagatto, O. Bahia-Filho, M.F. Gine, H. Bergamin-Filho, A simple procedure for hydrodynamic injection in flow injection analysis applied to the atomic absorption spectrometry of chromium in steels, Anal. Chim. Acta 181 (1986) 265. 

J. F. Tyson and A. B. Idris, Determination of Chromium in Steel by Flame Atomic Absorption Spectrometry Using a Flow Injection Standard Additions Method. Analyst, 109 (1984) 23. 

E. A. G. Zagatto, O. Bahia F , M. F. Gin6, and H. Bergamin F, A Simple Procedure for Hydrodynamic Injection in Flow Injection Analysis Applied to the Atomic Absorption Spectrometry of Chromium in Steels, Anal. Chim. Acta, 181 (1986) 265. 

Other metals also can be "passivated." For example the presence of 12 percent chromium in steel renders the steel passive in an oxygenated environment because it promotes the formation of a thin but tightly-bound oxide layer. The presence of chromates in the electrolyte will encourage the formation of y-FeaOa on iron surfaces, isolating the surfaces from corrosion. 

Chromium in steel forms a carbide that hardens the metal. The chromium atoms may also occupy locations in the crystal lattice, which will have the effect of increasing hardness without affecting ductility. The addition of nickel intensifies the effects of chromium, producing a steel with increased hardness and ductility. 

Another major problem frequently encountered, specifically in the trace and ultratrace domains of ecological analysis, is the risk of contamination and/ or volatization of individual components. The risk of contamination is always present if the grinding elements themselves contain a high percentage of the substance to be determined (for example, iron and chromium in steel mills), or when certain readily volatilized combinations escape from the sample as a result of overheating. 

Quantitative gas chromatographic schemes now exist for the determination of beryllium in blood, urine, and tissue,chromium in serum," aluminum in uranium, aluminum, gallium, and indium, in aqueous solu-tions," iron in ore, chromium in steel, titanium in bauxite, aluminum, iron, and copper in alloys,uranium, tungsten and molybdenum in alloys and ores, " and the list continues to grow rapidly. In the ultratrace analysis of beryllium the lower limit of detectability is ca. 10 g. The gas... 

A very rapid oxidative disintegration of chromium-bearing minerals, rocks and alloys is obtained by fusing or sintering the finely pulverized material with potassium bifluoride (platinum spoon). Potassium chromate results and may be detected by means of the diphenylcarbazide reaction. The fluoride disintegration is particularly recommended for the detection of chromium in steels or special alloys, which are likely to contain molybdenum. The latter, in the form of molybdate ions, reacts with diphenylcarbazide to yield a red-violet color and thus impairs the test for chromium. However, the fluoride method yields no M0O4" ions, but instead complex [M0O3F2] ions, which do not react with diphenylcarbazide. 

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation at elevated temperatures, and the chromium—molybdenum steels are an important class of alloys for use in steam (qv) power plants, petroleum (qv) refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to 10%. As a group, the low carbon chromium—molybdenum steels have similar creep—mpture strengths, regardless of the chromium content, but corrosion and oxidation resistance increase progressively with chromium content. 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Hydrogen at elevated temperatures can also attack the carbon in steel, forming methane bubbles that can link to form cracks. Alloying materials such as molybdenum and chromium combine with the carbon in steel to prevent decarburization by hydrogen (132). 

Carbon disulfide is normally stored and handled in mild steel equipment. Tanks and pipes are usually made from steel. Valves are typically cast-steel bodies with chrome steel trim. Lead is sometimes used, particularly for pressure reUef disks. Copper and copper alloys are attacked by carbon disulfide and must be avoided. Carbon disulfide Hquid and vapor become very corrosive to iron and steel at temperatures above about 250°C. High chromium stainless steels, glass, and ceramics maybe suitable at elevated temperatures. 

In the stainless group, nickel greatly improves corrosion resistance over straight chromium stainless. Even so, the chromium-nickel steels, particularly the 18-8 alloys, perform best under oxidizing conditions, since resistance depends on an oxide film on the surface of the alloy. Reducing conditions and chloride ions destroy this film and bring on rapid attack. Chloride ions tend to cause pitting and crevice... 

Another useful element in imparting oxidation resistance to steel is silicon (complementing the effects of chromium). In the lower-chromium ranges, silicon in the amounts of 0.75 to 2 percent is more effective than chromium on a weight-percentage basis. The influence of 1 percent silicon in improving the oxidation rate of steels with varying chromium contents is shown in Fig. 28-26. 

C) 370/656X brittleness after exposure to temperatures between about 700 to 1. OSO-F. stainless steels. chromium stainless steels, over 13% Cr and any 400 Series martensitic chromium stainless steels low in carbon content (high Cr/C ratio). complex chromium compound, possibly a chromium-phosphorus compound. chromium steels at temperatures above about 700 F (370 C) keep carbon up in martensitic chromium steels and limit Cr to 13% max. 

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