|MadSci Network: Chemistry|
Hi Tomasz, Several elements can be alloyed with iron to make stainless steel, however, chromium is the most significant component in stainless steel. Iron and chrome both react with oxygen to form oxides at the metal surface. The chrome oxide forms a protective layer on the metal surface which slows the rate of oxidation. Iron oxide is more permeable and the base metal continues to react and oxidize. Aluminum is a very reactive metal. The thin layer of oxide on the surface protects the base metal from reacting. If you place liquid mercury on aluminum metal, the mercury forms an amalgam that is more permeable and destroys the protective layer. On a humid day the aluminum will burn in air! Your second question is more difficult to answer. Low thermal expansion alloys generally have high nickel content (40 to 50%). The curie point of these alloys is much lower than for carbon steels. The curie point is the temperature at which a ferromagnetic material changes to become parramagnetic. Magnetostriction is a change in dimension which occurs as the magnetization of the crystal structure is changed. In a low thermal expansion alloy, the composition is carefully controlled so that the effects of thermal expansion are offset by magnetostriction. Low thermal expansion steels with high nickel content are much more expensive than plain carbon steels used in applications such as rails. I would be interested in the composition of a low thermal expansion steel used for rail applications. Bob Novak Specialist, Process R&D Carpenter Technology
Try the links in the MadSci Library for more information on Chemistry.