Chromium hydride
Chromium hydrides are compounds of
The hydrogen in typical chromium hydride alloys may contribute only a few hundred parts per million in weight at ambient temperatures. Varying the amount of hydrogen and other alloying elements, and their form in the chromium hydride either as solute elements, or as precipitated phases, expedites the movement of dislocations in chromium, and thus controls qualities such as the
Material properties
Even in the narrow range of concentrations that make up chromium hydride, mixtures of hydrogen and chromium can form a number of different structures, with very different properties. Understanding such properties is essential to making quality chromium hydride. At room temperature, the most stable form of pure chromium is the body-centered cubic (BCC) structure α-chromium. It is a fairly hard metal that can dissolve only a small concentration of hydrogen.
It can occur as a dull brown or dark grey solid in two different crystalline forms:
An apparent unusual allotrope of chromium in a hexagonal crystal form was investigated by Ollard and Bradley by X-ray crystallography; however they failed to notice that it contained hydrogen.[1] The
A
Face-centered cubic CrH had the composition CrH1.7.[3] But in theory it would be CrH2 if the substance was pure and all the tetrahedral sites were occupied by hydrogen atoms. The solid substance CrH2 appears as a dull grey or brown colour. Its surface is easily scratched, but that is due to the brittleness of the hydride.[3]
Face-centered cubic chromium hydride also forms temporarily when chromium metal is etched with hydrochloric acid.[7]
The hexagonal form spontaneously changes to normal chromium in 40 days, whereas the other form (face-centered cubic) changes to the body-centered cubic form of chromium in 230 days at room temperature. Ollard already noticed that hydrogen is evolved during this transformation, but was not sure that the hydrogen was an essential component of the substance, as electrodeposited chromium usually contained hydrogen. Colin G Fink observed that if the hexagonal form was heated in a flame that the hydrogen would quickly burn off.[6]
Electroplating chromium metal from a chromate solution involves the formation of chromium hydride. If the temperature is high enough the chromium hydride rapidly decomposes as it forms, yielding microcrystalline body-centered cubic chromium. Therefore, to ensure that the hydride decomposes sufficiently rapidly and smoothly, chromium must be plated at a suitably high temperature (roughly 60C to 75C, depending on conditions). As the hydride decomposes, the plated surface cracks. The cracking can be controlled and there may be up to 40 cracks per millimeter. Substances on the plating surface, mostly
Superhexagonal[when defined as?] chromium hydride has also been produced by exposing chromium films to hydrogen under high pressure and temperature.[8]
In 1926 T. Weichselfelder and B. Thiede claimed to have prepared solid chromium trihydride by reacting hydrogen with chromium chloride and phenylmagnesium bromide in ether, forming a black precipitate.[9][10]
Solid hexagonal CrH can burn in air with a bluish flame. It is ignitable with a burning match.[11]
Related alloys
The hydrogen content of chromium hydride is between zero and a few hundred parts per million in weight for plain chromium-hydrogen alloys. These values vary depending on
and so on.Alloys with significantly higher than a few hundred parts per million hydrogen content can be formed, but require extraordinarily high pressures to be stable. Under such conditions, the hydrogen content may contribute up to 0.96% of its weight, at which point it reaches what is called a line compound phase boundary. As the hydrogen content moves beyond the line compound boundary, the chromium-hydrogen system ceases to behave as an alloy, and instead forms a series of non-metallic stoichiometric compounds, each succeeding one requiring still higher pressure for stability. The first such compound found is dichromium hydride (Cr
2H), where the chromium-to-hydrogen ratio is 1/0.5, corresponding to a hydrogen content of 0.96%. Two of these compounds are metastable at ambient pressures, meaning that they decompose over extended lengths of time, rather than instantaneously so. The other such compound is Chromium(I) hydride which is several times more stable. Both these compounds are stable at cryogenic temperatures, persisting indefinitely. Although precise details are not known.[13]-
Other materials are often added to the chromium/hydrogen mixture to produce chromium hydride alloy with desired properties. Titanium in chromium hydride make the β-chromium form of the chromium-hydrogen solution more stable.[citation needed]
References
- S2CID 4131298.
- ISSN 0925-8388.
- ^ ISSN 0925-8388.)
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: CS1 maint: multiple names: authors list (link - ^ . full text available
- ISSN 0013-4651.
- ISSN 0360-3199.
- ^ mellor. "The Chemical Properties of Chromium" (PDF). A Comprehensive Treatise on Inorganic and Theoretical Chemistry. p. 160.
- .
- ^ Raub, Christoph J. (September 1993). "Hydrogen in Electrodeposits: of Decisive Importance, But Much Neglected" (PDF). Plating and Surface Finishing: 35.
- .
Further reading
- Wolf, G. (1971). "Specific heat of chromium hydride CrHx from 11 to 300 K". Physica Status Solidi A. 5 (3): 627–632. ISSN 0031-8965.
- Khan, H.R; Ch.J Raub (1976). "Properties of chromium hydride". Journal of the Less Common Metals. 49: 399–406. ISSN 0022-5088.
- Kerle, Bettina; Mathias Opper; Sigrid Volk (6 September 2000). "Hexavalent Chromium Processes" (PDF). SurTec GmbH. Archived from the original (PDF) on 8 January 2009. Retrieved 1 August 2012.
- Stock, Allen D.; Kenneth I. Hardcastle (1970). "Phase and composition analysis of chromium hydride". Journal of Inorganic and Nuclear Chemistry. 32 (4): 1183–1186. ISSN 0022-1902.
- Khan, H.R.; A. Knödler, Ch.J. Raub, A.C. Lawson (1974). "Electrical and magnetic properties of chromium hydride". Materials Research Bulletin. 9 (9): 1191–1197. ISSN 0025-5408.)
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: CS1 maint: multiple names: authors list (link - Venkatraman, M; J.P Neumann (1991). "The cr-h (chromium-hydrogen) system". Journal of Phase Equilibria. 12 (6): 672–677. S2CID 97887064.