Microscopy of high temperature oxidation of iron and some stainless steels

Doctoral thesis, 2007

This thesis concerns the high temperature oxidation of iron and some stainless steels. The oxide scales, as well as the subjacent metal, were investigated using a range of electron and ion microscopy techniques, including SEM, TEM, EDX and FIB. The aim was to link the microstructural observations to the oxidation mechanisms of the materials. It was shown that iron exposed to O2 with 100 ppm SO2 forms a thinner more dense oxide scale than iron exposed in pure O2. The oxide scales consisted of two magnetite layers below a hematite layer. The inhibitive effect is attributed to the presence of iron sulphide that blocks active sites on the hematite surface, slowing down the formation of oxygen ions. The impact of water vapour was studied on the oxidation of iron, the ferritic/martensitic stainless steel X20 and on the austenitic stainless steel 353MA. Iron oxidation was studied through an ESEM in-situ investigation at 500 ºC. Several factors are suggested to influence the local oxide growth rate of iron: (i) the surface of the metal grain, (ii) the thickness of the hematite layer, (iii) the oxide grain size, and (iv) the exposure environment. The two alloys sometimes experience breakaway oxidation in O2/H2O mixtures, because their oxide scales are depleted in Cr due to the formation of CrO2(OH)2(g). The transition from a thin protective Cr-rich oxide scale to non-protective Fe-rich oxide scale is faster, i.e. the entire surface is covered with thick oxide within a short time after the initiation of breakaway oxidation, for the ferritic/martensitic stainless steel than for a typical austenitic stainless steel. This can be attributed to the higher bulk diffusion rate and the higher density of faster diffusion paths (steel grain boundaries). Alloy 353MA, exposed at 700 ºC, forms a Cr-rich healing layer beneath the Fe-rich oxide some time after breakaway oxidation. It is suggested to be a result of the high Cr/Fe ratio of the alloy. The behavior at 900 ºC was different. In spite of the loss of Cr from the oxide scale, breakaway oxidation did not occur. This is suggested be a combined effect of more rapidly chromium diffusion, spinels formed at the oxide/gas interface and the high Ni content in the alloy. The influence of KCl on the initial breakdown of the protective scale on the austenitic stainless steel 304L was studied in dry and wet oxygen at 600 ºC. The rapid breakdown of the protective scale is suggested to be caused by the formation of K2CrO4, depleting the protective oxide in chromium. Cl is suggested to play a minor role in the initial breakdown of the protective scale.