Abstract
Currently, surface treatments lead to inducing a superficial layer of several nanometers up to micrometer, which in some cases can be protective. In this experimental work, an oxide layer was generated under different atmospheres (CO2 and steam atmospheres) during the thermal aging treatment of two different maraging grades, 300 and 350. Afterwards, this layer was microstructural and mechanically characterized by advanced characterization techniques at the micro- and submicron length scale to highlight some information related to the generated oxide layer. The results showed that the oxide layer (in both grades) was made up of several compounds like: TiO2, MoO3, hematite (α-Fe2O3), and CoFe2O4, this being the majority compound distributed homogeneously throughout the layer. Furthermore, a nickel-rich austenitic phase at the interphase was mainly made up cobalt ions (Co2+), instead of iron ions (Fe2+), within the spinel lattice.
Highlights
The oxidation process of maraging steels was addressed by several authors until the1990s [1,2,3,4,5]
On the basis of the results found in this work, several points can be highlighted; one of them is that TiO2 is in the innermost layers of the oxide, because it was not detected by X-ray photoelectron spectroscopy (XPS)
This observation is in fair agreement with References [55,56,57] and Klein et al [1,2,3], based on thermodynamic information from experiments conducted under a steam atmosphere, which highlighted that, in the first stage of oxidation process, all elements were oxidized, and mainly titanium, which exhibited high reactivity with oxygen, but it ended up being covered by iron oxides because of the faster kinetics of the oxide formation reactions of this element
Summary
The oxidation process of maraging steels was addressed by several authors until the. 1990s [1,2,3,4,5] In each of these publications, different conditions such as time, atmosphere, and maraging steel grades were used to obtain an oxide layer with a spinel-type microstructure that could act as protective layer which is able to be employed in different sectors. Klein et al [1,2] observed that, during the oxidation process of steels under different atmospheres (CO2 , steam and air) at 485 ◦ C for 3 h, it is possible to growth a spinel iron oxide Fe3 O4 , known as magnetite, as follows: Steam atmosphere: 3Fe + 8H2 O = Fe3 O4 + 4H2 O + 4H2 , (1).
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