Effect of specimen thickness on chromia scaling of Ni25Cr in N₂–O₂–H₂O test gases at 1000°C

Abstract

The isothermal oxidation behaviour of a Ni25Cr model alloy was studied using specimens of different thicknesses at 1000uC in dry and wet N 2 -1%O 2 . The oxidation mechanisms were evaluated using thermogravimetry and SEM/electron backscatter diffraction analyses of oxide scale cross-sections. The oxidation rates decreased with increasing specimen thickness and increasing water vapour additions in the gas. The findings can be explained by considering the effect of H-defects and in-scale stress state on point defect concentrations in the chromia lattice.

Scaling Kinetics and Scale Microstructure of Chromia Scales Formed on Ni-25%Cr Model Alloy during Oxidation in H2o-Containing High and Low pO2 Test Gas at 1000°C

“…Haensel et al [14,20,21] followed up and investigated the oxide scaling kinetics of different thick specimens for Ni-25%Cr model alloy in dry and wet oxygen at 1000°C. Their results led to the conclusion of a variability of the concentration of each defect type contained in the Cr sub-lattice in the chromia scales formed during exposure at fixed experimental conditions [14].…”

“…In this way the defect structure of the protective chromia scale formed during exposure can be made independent of the test gas parameter, i.e. pO 2 [14], which are otherwise closely linked together [9,15]. The great advantage of this procedure is one more degree of freedom.…”

Impact of Grain Boundary Density on Oxide Scaling Revisited

“…However, the true power of the thermogravimetric (TG) analysis is in that it actually monitors the oxidation process, and while microscopy tells of what has happened, the relevance of the images for the oxidation process is by necessity only implicit. The comparative study of chromia scaling of Ni25Cr at 1000 °C in dry and wet N 2 -1%O 2 [29] is particularly instructive. TG reports dramatic suppression of oxidation rate under humid conditions, while microscopy reports no change in grain size distribution.…”

“…Fundamentally, transport paths may be sealed either by explicit grains coarsening or by the 'clogging' of grain boundaries. Indeed, hydrogen residing in interstitial sites in the form of hydroxide ions was suggested in [29] to restrict transport in the crystalline grain boundaries. And yet both grains coarsening models developed here are able to describe the early subparabolic signatures of the TG data provided in [29] (see Fig.…”

“…Experiment from[29], wet N 2 -1%O 2 . Full lines and dotted lines refer to the superparaboliccubic and the parabolic-logarithmic models, respectively.Inverted colours: red for 1 mm and black for 0.25-mm specimens for contrast.…”