How rust dust and pipe scale lower the temperature at which acetylene decomposes

Acetylene does not need a flame to decompose. A hot surface will start it, and the temperature needed depends heavily on what is on that surface. In a clean steel pipe the figure is 425 °C. With a gram of rust or pipe scale present it falls to 280–300 °C, and with caustic potash to 170 °C. Those numbers explain several rules of acetylene plant design that otherwise look like housekeeping: how vessels are heated, how lines are cleaned, and why deposits in driers and purifiers matter.

The key work is by Miller and Penny, presented at the Institution of Chemical Engineers Symposium on Chemical Process Hazards, Manchester, 1960. Acetylene was admitted to an evacuated steel pipe 38 cm long and 1.3 cm in diameter, heated to a known temperature. The lowest wall temperature that initiated decomposition was then found with and without powders in the tube.

The Hölemann work (Forschungsber. Nordrhein-Westfalen No. 102, 1954) also found that the capillary figure did not depend on heating rate (100–300 °C/s), bore or length of the heated zone, and that pre-annealing in air raised it by 30–60 °C and a tin coating by 40–50 °C. The surface condition moves the number; the geometry hardly does.

The powder does not catalyse the explosion in the ordinary sense. As compiled by S. A. Miller (1965), the explanation is that condensation reactions of acetylene begin on the powder surface, those reactions are themselves strongly exothermic, and they raise the local surface temperature to the point where the gas decomposes.

That is consistent with what is known about acetylene polymerisation. The same compilation records that trimerisation of acetylene to benzene releases about 90 % of the energy of full decomposition to carbon and hydrogen, and that polymerisation starts from about 300 °C, lower on catalytic surfaces. A deposit that promotes condensation is therefore a small heater sitting on the pipe wall. Iron oxide, lime, silica, sodium hydroxide and potassium hydroxide are all listed among the promoters; finely divided platinum and pyrophoric iron, nickel or cobalt become incandescent in contact with cold acetylene.

Oil acts differently. A paraffin-oil film raised the capillary initiation temperature rather than lowering it, but oil is unwelcome in acetylene service for other reasons, including its effect on flashback arrestor elements.

Cullis and Nettleton (Trans. Faraday Soc., 1963, 59, 301) measured the minimum temperature at which acetylene reacts on various solid surfaces. The calcium silicate they tested was made from sand and lime for DA cylinder porous mass.

The porous mass in a modern cylinder is therefore inert at any normal service temperature, but it is not inert in a fire, and charcoal masses of the older type become active about 70 °C lower than calcium silicate. This is one more reason that cylinders exposed to fire or external heat are handled as described on the cylinder fire page. The compilation by S. A. Miller (1965) adds that neither acetone nor nitrogen affected the rate of this surface reaction.

The figures above concern pure acetylene decomposing with no oxygen present. IS 14814:2023 gives the autoignition temperature of acetylene as 305 °C (cl 4.5.2) and the ignition temperature as 325 °C (cl 4.5.1), the reverse of the usual relationship, so both are quoted with their clauses; they refer to acetylene burning in air. Both sets of numbers matter, but they apply in different places: the air figures outside the plant and wherever air can enter, and the decomposition figures inside pipes, vessels, driers and purifiers. The flammability and ignition temperature page sets out the air-side data.