Factors governing breakaway oxidation of FeCrAl-based alloys

At temperatures above around 1100 C the life time of FeCrAl based alloy components can be limited by oxidation. Growth and spalling of the protective alumina scale leads after long exposure times to a depletion of aluminium in the alloys, eventually resulting in breakaway oxidation. This life time limit can be predicted using a recently developed model, taking into account scale growth rate (characterized by the parameters k and n), initial alloy Al content (Co), critical Al content for protective alumina formation (CB), oxide adherence and component geometry. Based on the evaluation of long term oxidation data for a number of commercial and model FeCrAl alloys it is shown that the life time can substantially be increased by decreasing the oxide growth rate and/or increasing Co, whereby application of the latter factor is in most practical cases limited due to restrictions imparted by the alloys' mechanical properties. For typical commercial ODS materials CB is around 1 wt-%, however, this value is strongly affected by the exact alloy composition, especially Cr-content. CB seems to be higher for dispersion strengthened alloys than for conventional wrought materials. The adherence of the oxide scale not only depends on type and exact amount of reactive element (oxide) addition but also on other common minor alloying additions, such as Ti. Indications were found, that oxide adherence is also affected by the mechanical strength of a material and/or component. (orig.)