The Effect of Grain-Boundary Diffusion on the Oxidation of Low-Chromium Steels

Abstract

Even though the oxidation behavior of steels is generally considered as to be widely understood, a closer look reveals some open questions, e.g. regarding the influence of the substrate grain size on the overall oxidation kinetics. At temperatures below 570°C the main constituent of the oxide scale formed on top of low alloy steels is magnetite. As shown by gold marker experiments it grows outward and inward at the same time, the latter exhibiting a gradual transition to the more stable spinel compound FeCr2O4. As indicated by intergranular-oxidation attack below the superficial scale, inward oxide growth seems to be driven by oxygen transport along the grain boundaries serving as fast diffusion paths. This is supported by thermogravimetric oxidation tests in air on low-Cr steels with varying grain size: The smaller the grains the higher the oxidation rate. Recently, a numerical model for the diffusive transport processes based on the finite-difference approach has been developed, which distinguishes between fast grain-boundary diffusion and bulk diffusion. Qualitatively, it is capable to predict the relationship between substrate grain size and inward oxide growth kinetics. Together with the thermodynamic tool ChemApp and in combination with a data set for the Fe-Cr-O system the mechanism-based simulation of the overall oxidation process of low-Cr steels is possible.

Effect of grain size on the oxidation of Fe–13Cr–5Ni alloy at 973K in Ar–21vol%O2

“…It is also important to note that due to the high Cr content in these alloys [2,7,8], the formation of the complete protective layer takes place before any preferential formation of chromia at the grain boundaries, and therefore the oxidation depends strongly on the outward diffusion of Cr. On the other hand, in the case of an alloy with very low Cr content, irrespective of its grain size, Cr-rich oxides with corundum structure could only be formed at grain boundaries, diffusion of Cr would never be sufficient to effect lateral growth of Cr-containing corundum structure oxides to form a continuous layer [2,6,12].…”

“…However, for transition Fe-Cr alloys with critical Cr content, enhanced diffusion of Cr by grain refinement may aid the selective oxidation of Cr and formation of the protective chromia scale [5,7,8], whereas high density of grain boundaries may also lead to more severe internal oxidation because they provide more nucleation sites for oxidation [6,11,12]. Therefore, the grain size effect on the oxidation of the transition alloys is still an open question and should be studied in detail.…”

“…A decrease in grain size brings a large concentration of grain boundaries which act as preferential and faster transport paths, and mainly results in (a) an enhanced selective oxidation of Cr and quicker formation of protective scales [5,7,8], (b) increase in the sites for oxide nucleation [6,11,12], or (c) increase in different morphology of the scales at grain boundaries and grain interiors [13,14]. Depending on the predominance of any of these effects, a change in grain size may have different influences on the oxidation of the steels.…”

“…Many previous works have examined the grain size effect on high temperature oxidation performance [2,[5][6][7][8][9][10][11][12][13][14]. A decrease in grain size brings a large concentration of grain boundaries which act as preferential and faster transport paths, and mainly results in (a) an enhanced selective oxidation of Cr and quicker formation of protective scales [5,7,8], (b) increase in the sites for oxide nucleation [6,11,12], or (c) increase in different morphology of the scales at grain boundaries and grain interiors [13,14].…”

Effect of shot peening on high temperature oxidation behaviour of boiler steel: experimental results and simulation

“…A computer based model was used to calculate the growth of the inner scale in steels. 4 The diffusion of the species such as O and Cr was treated by numerically solving Fick's second law by applying the two-dimensional finite difference method (Crank-Nicolson approach). 5 The model was named as InCorr and it can differentiate bulk and grain boundary diffusion.…”