GB462380A - Process for the manufacture of articles resistant to gaseous corrosion - Google Patents

Abstract

To impart to an alloy article the property of resistance to corrosion by a given gaseous substance, e.g. oxygen or sulphur, at a given temperature or partial pressure, the alloy containing an alloying element which gives with said gaseous substance a compound having a dissociation pressure lower than that of the corresponding compound of the basis metal or metals but which is not present in quantity sufficient to give satisfactory corrosion resistance when exposed to the gaseous substance at the given temperature and pressure, the article is heated in an atmosphere containing the said gaseous substance at a partial pressure above the dissociation pressure of the compound of the gaseous substance and the alloying element, but below the given partial pressure and so low that a coating is produced which consists to such a preponderating extent of the compound of the alloying element and is sufficiently thick, compact, and adherent that the article is resistant to corrosion under the given conditions. The partial pressure of the gaseous substance may be progressively increased during the treatment and at the beginning of the treatment may be between the dissociation pressure of the compound of the gaseous substance and the alloying element and that of the compound of the gaseous substance and the base metal. For example a ferrous alloy containing 12 per cent of chromium or 4 per cent of aluminium may be rendered resistant to corrosion by oxygen at about 1000 DEG C. by heating at about 1000 DEG C. in oxygen at a pressure of between 0,1 and 5 mm. of mercury. The partial pressure used varies according to the percentage of alloying element, the temperature of treatment, and the temperature and pressure at which the article is to be used. In the case of ferrous alloys and oxygen it in no case exceeds 10 mm. of mercury. Corrosion resistant decorative articles may be made by the process, different parts of an article being made of metal containing different alloying elements. For example one part may be made of copper containing chromium, another part of copper containing aluminium, and a third part of copper containing cobalt, the article being heated in oxygen at a low pressure, e.g. between 0,1 and 5 mm. of mercury, to form a green, gray, and blue article, the surface consisting mainly of chromium oxide, aluminium oxide, and cobalt oxide.