Steam oxidation of thermally deposited coatings from 304 L and recycled 316 L/Z100 steels: Influence of temperature, coatings microstructure and steel recycling

The application of thermally sprayed stainless steels coatings is a well-established approach to protecting low-alloy steels against high-temperature degradation. In this study, we investigated both: (1) the influence of microstructural features of the coatings and (2) the recycling of stainless steels on the course of high temperature steam oxidation. It was achieved by thermally spraying coatings on C45 steel from 304 L and a new type of 316 L/Z100 steel, obtained as a result of mixed scraps recycling. Steam oxidation tests carried out in the temperature range 600–800 °C for 500 h in pure steam show temperature depended mechanism of oxidation, which was defined and described. The formation of the scales with a multi-layered structure was observed, consisting of Fe₂O₃, Fe₃O_4, Cr₂O₃, Fe- or Cr-rich Cr_xFe_x-2O₃ and (Mn,Cr,Ni)_xFe_3-xO₄ spinels. The relative ratio of the phases and their presence in particular scale layers varied depending on the material studied and the exposure temperature. The key findings are that recycling of steel deposited as protective coating has led to an unintended change in Si and C concentrations, which may affects course of oxidation. Also, the microstructure of the initial coatings, consisting of porosity and a significant volume of oxides formed during steel deposition, inhibits the formation of a homogeneous layer of protective Cr₂O₃. As a result, the established values of oxidation kinetics constants (in range of 8.55E-12 to 1.50E-10 g³·cm⁻⁶·s⁻¹ for 316 L/Z100 and 2,71E-12 to 4.60E-10 g³·cm⁻⁶·s⁻¹ for 304 L coatings) and oxidation activation energies (156.4 kJ·mol⁻¹ for 316 L/Z100 and 211.5 kJ·mol⁻¹ for 304 L coatings) differ significantly from those of bulk steels under the same conditions reported in the literature.