Septum permeation of hydrogen from flames

Abstract

Flames consist of maintained chemical reactions, and the concentrations of labile chemical species such as hydrogen atoms, may greatly exceed the concentrations for chemical equilibrium at the flame temperature. This should influence the attack on metals by flames. To test this, apparatus has been constructed for measuring the rate of permeation of hydrogen and other gases from flames through metal septa. Relative rates of permeation have been compared for a hydrogen/oxygen/nitrogen flame: (1) with metal septa consisting of decarburized chrome steel, chrome steel, decarburized chromium/nickel alloy steel, or nickel. (2) with metal septa consisting of decarburized steel or nickel, when small quantities of sulphur dioxide or hydrogen sulphide are mixed with the hydrogen/oxygen/nitrogen flame. For the different metals and the same reference flame (free from sulphur) relative rates of permeation at around 820° C are in the sequence: nickel > chrome steel ≃ decarburized chromium/nickel alloy steel > decarburized chrome steel. Approximate relative rates are nickel, 1000; chrome steel, 300; decarburized chromium/ nickel alloy steel 300; decarburized chrome steel 10. The addition of small quantities of sulphur dioxide to the flame at once reduces the concentration of hydrogen atoms, as observed with a molybdenum trioxide test. Comparable amounts of hydrogen sulphide added to the flame have little or no effect. Both types of sulphur compound rapidly change the rate of permeation of hydrogen through the metal septa from the flame. An increased permeation rate is observed for decarburized chrome steel, and a decreased permeation rate for a decarburized chromium/nickel/iron alloy. An initial drop in permeation rate through chrome steel with sulphur dioxide in the flame is probably due to the decreased concentration of hydrogen atoms. The subsequent effects for both sulphur dioxide and hydrogen sulphide are attributed to the formation of sulphur compounds either on the surface or within the crystal lattice of the metals.