Modelling and experiment of selective oxidation and nitridation of binary model alloys at 700 °C – The systems Fe, 1 wt.%{Al, Cr, Mn, Si}

Theoretical and experimental analysis of selective oxide and nitride formation in Fe–Al alloys

“…5, it can be clearly seen that severe internal corrosion starts after 60 min exposure, when the sample reaches temperatures above 800°C. This finding agrees nicely with observations from literature, which report corrosion depths of only a few micrometers after several hours isothermal oxidation in this temperature range [9,33].…”

“…Previous work [7][8][9] has examined the formation of internal aluminum nitride precipitates in iron-aluminum alloys under N 2 atmospheres with low partial pressures of oxygen. In these samples, the concurrent inward diffusion of dissolved oxygen and nitrogen caused both external scale formation and internal precipitation.…”

Influence of Gas Composition and Exposure Cycle on the Formation of Surface and Subsurface Oxides in Iron-Aluminum-Based Alloys at High Temperatures

“…The thermodynamically less favorable nitride phases can then be found at lower oxygen activities close to the corrosion front-since nitrogen also diffuses faster than oxygen. [9] This can also be seen from the EDX measurements in Figure 5, which were taken at the interface between aluminum nitrides and aluminum oxides. Here, the upper part of the precipitates (closer to the sample surface) consists of oxides, whereas the bottom part (closer to the internal corrosion front) does not show any oxide, which indicates nitride formation.…”

“…Hence nitrogen will be released into the metal lattice, where it diffuses toward the corrosion front prior to combining with the metallic alloy addition. Macroscopically, this effect looks like oxygen is ''pushing'' the nitrogen further inwards, [6,9] although it is in fact a transformation from the initially formed nitrides into the thermodynamically more stable oxide phases. This result is particularly interesting for further processing steps, as it suggests that the morphology of the initially formed aluminum precipitates is determined by nitrides, which may later be transformed into oxides.…”

“…However, it is often readily possible to remove an external iron oxide scale layer in silicon-free steels, formed during casting, reheating, and hot rolling. [7] If the internal oxidation underneath the oxide layer is too heavy, [8,9] this can cause problems during downstream processing (e.g., crevice corrosion in pickling solutions [10] or uncoated spots during galvanizing). By designing the gas composition in a reheat furnace in such a way that an outer scale layer may form, the internal oxide formation could be reduced.…”