Chromium vapor pressure, high temperature

Many elements evaporate, but many such as chromium (Cr), cadmium (Cd), magnesium (Mg), arsenic (As), and carbon (C) sublime, and many others such as antimony (Sb), selenium (Se), and titanium (Ti), are on the borderline between evaporation and sublimation. For example, chromium, which has a vapor pressure of 10 Torr 600°C below its melting point, is generally vaporized by sublimation. Carbon cannot be melted except under high hydrostatic pressure. Materials such as aluminum, tin, gallium, and lead have very low vapor pressures at temperatures above the points at which they are just-molten. For example, tin has a vapor pressure of lO Torr 1000°C above its melting point. Aluminum and lead have vapor pressures of 10 Torr at about 500°C above their melting points. 

If you require metal components on your vacuum system, select metals with low gas permeation, such as 300 series stainless steel. It is nonmagnetic and, like glass, is a poor conductor of heat and electricity. Stainless steel, also like glass, is relatively nonreactive, and therefore is less likely to rust or be affected by chemicals. If welding the stainless steel is required, select 304L stainless steel, which is low in carbon. Otherwise, at welding temperatures the carbon will combine with the chromium (within the stainless steel) to form chromium carbide and the corrosion protection of the chromium will be lost. Type 303 stainless steel should not be used for vacuum work because it contains selenium, which has a high vapor pressure. 

Chromium hexacarbonyl forms large, highly refractive crystals that can be sublimed in vacuum without change but decompose on heating above about 100°. The vapor pressure of chromium hexacarbonyl has been measured up to 125°. On exposure to light and air at room temperature... 

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. 

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