A chromium tube is a type of cylinder made from a steel metal with a significant percentage of chromium, such as 1% or more chromium. The chromium may be added in the iron or manganese ingots or as a by-product of the refining of other metals. Chromium is a strong, hard, brittle, oxidation-resisting alloy which is commonly used for making steel and other high-carbon alloys such as ferro-chromium.
The tubes of the invention are not only easier and cheaper to fabricate than the chromium steel tubes heretofore known, but they are also more ductile, strong and uniform. They may be straightened or bent, hot or cold, to a considerable extent without danger of breakage, and they are more readily annealed and air-cooled than the tubes heretofore known.
They are readily softened by heating them to a temperature in excess of about 750 C, but not so hot that they become brittle, even though they contain less than 5% chromium. Rapid cooling, that is, from elevated temperatures used in hot-forming or annealing, does not produce marked hardening or brittleness either, although slow cooling may have some effect.
The chromium is reduced to a chromium +3 state by using a suitable reducing agent which has an oxidation reaction product that will not contaminate the resulting chromium +3 reaction product, such as methanol, ethanol, n-propanol, hydrochloric acid, hydrogen gas, formaldehyde, formic acid, carbon monoxide, peroxide and sodium borohydride.
In addition, a suitable acid species is employed in the reduction to adjust the pH of the mixture to an optimum level for the reaction. The acid species can be any of the common oxidation-reducing acids, such as sulfuric acid, phosphoric acid, hydrochloric acid and most caustic alkalies and alkali carbonates. The amount of the acid species required to provide a stiochiometric amount of both protons and counter-ions for the chromium +3 reaction product and for the inert sodium cation will depend on the stiochiometry of the reduction reaction.