Oxide growth stresses in an austenitic stainless steel determined by creep extension

The development of growth stresses during isothermal high-temperature oxidation can affect the mechanical integrity of protective oxide layers. The ability to measure the magnitude of these stresses is then important not only from the point of view of developing more robust degradation models, but also from the need to understand better the nature of these stresses and the role of alloy mechanical properties in their relaxation. Growth stresses within the chromia layer grown on 20Cr25Ni steels have been estimated in this paper from the creep extension observed during oxidation in air at 900degreesC. Two variants of the steel have been used, one containing 0% Si and the other 0.76% Si, sufficient to form a silica interlayer between chromia and steel. In each case, growth stresses were compressive but, at any given exposure time, those in the Si-free alloy were up to a third smaller than in the Si-bearing alloy. However, oxidation rates in the absence of silicon were higher than in the alloy containing silicon and when growth stresses were compared at a given oxide thickness there was much less difference between the alloys. This indicates that the silica interlayer has no large direct effect on the chromia growth stress. Both alloys show that the Growth stress decreases with oxide thickness from approximately -1.6 GPa at 1.2 mum to approximately -0.3 GPa at 4-mum thick.