An insert made from Jethete M152 type stainless steel (Fe–12Cr) was used for moulding a polymer product, but failed in-service due to the formation of oxidation/corrosion products. The investigation of the failure mechanism has been carried out using scanning and transmission electron microscopy (SEM and TEM). A high density of fine precipitates, 50–200nm in size, were observed in the etched samples by SEM. TEM analysis combined with convergent beam electron diffraction (CBED) showed that the precipitates are (Cr, Fe, Mo) 23 C 6 carbides which posses Fm3¯m space group with a lattice parameter of 1.07±0.03nm. The formation mechanisms of the precipitates were studied by investigating a range of heat treatments.