Stainless steels alloying elements

A small, but important, sector of the steel market is that of the alloy steels the low-alloy steels, the high-alloy "stainless" steels and the tool steels. Alloying elements are added to steels with four main aims in mind ... 

US Patent 6,183,542 was issued in 2001 for a palladium membrane process. This process provides an apparatus that can handle high flow rates of gas, per unit area of membrane, while using a minimal amount of hydrogen-permeable material. This is accomplished by using stainless steel mesh elements to reinforce the thin-walled, palladium or palladium alloy membranes. This process also provides the ability to withstand large pressure gradients in opposite directions and thus will make it easier to clean membranes that have been clogged with contaminants. 

Elemental chlorine is obtained by the electrolysis of molten NaCl (Downs cell). Elemental fluorine is obtained by the electrolysis of KHE2 in a cell made of Monel metal (a stainless steel alloy). Both of these processes are dangerous. Why ... 

Stainless steel refers to steel alloys with a minimum chromium content of 10.5% that forms a thin chromium oxide film on the steel s surface to improve corrosion resistance, which makes the steel stainless. Other alloying elements typically include nickel and molybdenum to improve materials properties, e.g. fabrication. 

Note finally that, as mentioned in the Introduction, the corrosion of the substrate may also damage irreversibly a microstructured device under the severe conditions of fuel processing reactions. For example, under water vapor pressure, many detrimental effects can occur, such as surface migration of Ni in stainless-steel alloys, surface oxidation of metals (Fe to Fe203), surface enrichment with trace elements able to alloy/react with the coated catalyst (Sn, Pb, Cl ions) and poison it or surface substrate restructuring. 

Not much is known about astatine because it is very rare, is radioactive, and decays very quickly. Would you predict the chemical and physical properties of astatine to be more like those of a metal or nonmetal Defend your answer on the basis of astatine s location on the periodic table. Elemental chlorine is obtained by the electrolysis of molten NaCl (Downs cell). Elemental fluorine is obtained by the electrolysis of KHE2 in a cell made of Monel metal (a stainless steel alloy). Both of these processes are dangerous. Why ... 

Knife-line attack, immediately adjacent to the weld metal, is a special form of sensitization in stabilized austenitic stainless steels. Stabilizing elements (notably Ti and Nb) are added to stainless steels to prevent intergranular corrosion by restricting the formation of Cr-rich grain boundary precipitates. Basically, these elements form carbides in preference to Cr in the austenitic alloys. However, at the high temperatures experienced immediately adjacent to the weld fusion zone, the stabilizer carbides dissolve and remain in solution during the subsequent rapid... 

Common alloying elements include nickel to improve low temperature mechanical properties chromium, molybdenum, and vanadium to improve elevated-temperature properties and silicon to improve properties at ordinary temperatures. Low alloy steels ate not used where corrosion is a prime factor and are usually considered separately from stainless steels. 

Specify stabilized grades of stainless steel. An alternative method to prevent chromium carbide formation is to charge the alloy with substances whose affinity for carbon is greater than that of chromium. These substances will react preferentially with the carbon, preventing chromium carbide formation and thereby leaving the chromium uniformly distributed in the metal. The carbon content of the alloy does not have to be reduced if sufficient quantities of these stabilizing elements are present. Titanium is used to produce one stabilized alloy (321) and niobium is used to provide another (347). Note the cautions below. 

Cladding. The Magnox reactors get their name from the magnesium-aluminium alloy used to clad the fuel elements, and stainless steels are used in other gas-cooled reactors. In water reactors zirconium alloys are the favoured cladding materials. 

Localised corrosion can occur if the passivating element of an alloy is locally depleted from the matrix. This may occur in the sensitized regions of stainless steels for example, as outlined above. Other alloy components too. 

Tomashov, N. D., Chernova, G. P. and Marcova, O. N., Effect of Supplementary Alloying Elements on Pitting Corrosion Susceptibility of 18Cr-14Ni Stainless Steel , Corrosion, 20, 166 (1964)... 

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