A thermal cracking process using tubes, pipes, and coils made of.An outermost surface covering not less than 55% of stainless steel, said surface having a thickness from 0.1 to 15 microns and being a spinel of the formula Mn.sub.xCr.sub.3-xO.sub.4 wherein x is from 0.5 to 2 is not prone to coking and is suitable for hydrocarbyl reactions such as furnace tubes for cracking.