High Temperature Oxidation of 22wt.% Cr Austenitic Alloys upon Exposure to Hot Corrosion Environment with Na2so4

High temperature corrosion studies in today’s austenitic steels are of great interest to develop novel corrosion- and creep-resistant austenitic steels for advanced zero-emission power generation plants. These efficient power plants can offer advantageous considerations for the global need for energy and mitigation of climate change. However the aggressive environmental conditions poised challenges to current austenitic steels for long-term applications. To date, long-term application of austenitic steels has been restricted to service temperatures lower than 635 °C while future boilers are targeting temperatures of 700 – 750 °C. Therefore, extensive microstructural evolution studies of Fe-Cr-Ni steels exposed to high temperature conditions will contribute useful information to help retard degradation or enhance corrosion resistance properties while in search of novel compositions in austenitic steels.

Protective Cr-rich oxide films formed on austenitic stainless steels play an effective role to lower high temperature corrosion. These oxide scales of low porosity, good adherence, low growth rate, and high mechanical and thermodynamic stability can provide good corrosion resistance properties. Hence, in the formation of a protective Cr-rich oxide scale as a barrier to further corrosion, diffusional transport of Cr within the alloy to the oxide scale is critical.

The conditions of the exposure environments of the austenitic steels highly affect the growth and stability of these protective Cr-rich oxide scales. For example, the formation of volatile Cr species can lead to a depletion of Cr in the oxide scales and form Fe-rich oxide films, which inadequately function as a barrier to corrosion. Since the rate of diffusional transport of Cr in the oxide scale is compensated, accelerated localized attacks within the grains occur. In addition, the presence of H₂O and CO₂ in the exposure environment results in the formation of Fe-rich oxide scales and accelerated oxidation rates of most technical steels.

Sulphurous corrosive components (SO₂ and sulphate-based salt components) in commercial high temperature applications, for e.g. combustion, accelerate the corrosion rate, resulting in adverse modification of degradation properties in Fe-Cr-Ni steels. This deposit-induced intensified oxidation process is known as hot corrosion. In addition, Na and S exist as fuel impurities in power plants and lead to deposits of Na₂SO₄. These molten deposits can cause sulphate-deposit induced accelerated oxidation processes, which destroys the reaction-product oxide scales. Propagation modes of hot corrosion determined by these mechanisms are also dependent on the exposure temperatures and durations, and compositions of exposure gas, oxides and deposits. Hence, the influence of different exposure temperatures and durations on the mechanisms of the oxidation behavior of Fe-Cr-Ni steels studied in this work will help predict the behavior of austenitic steels in hot corrosion environments.

In this work, the isothermal high temperature oxidation behaviors of a 22 wt.% Cr austenitic stainless steels, Sanicro25 (42Fe22Cr25NiWCuNbN), subjected to various exposure temperatures have been investigated. The exposure environment consists of O₂+H₂O+CO₂+SO₂+Na₂SO₄. The alloys were subjected to respective exposure temperatures of 600, 700 and 750 °C with different exposure durations of 300 h and 1000 h. They were also compared to results observed in Sanicro25 alloys which were subjected to similar environmental conditions at 700 °C but for a shorter exposure duration time of 168 h [1].

Detailed microstructural investigations within the individual grains of the formed oxide, the oxide-metal interface and in the vicinity of the oxide-steel interface using XRD, SEM, FIB/SEM, TEM and EDX have been performed. These results yield insights to the microscopic mechanisms governing the oxidation behavior of steels with deposits of Na₂SO₄when subjected to aggressive high temperature environments related to oxy-fuel combustion.

References

1.  L. Intiso, N. Mortazavi, L.-G. Johansson and M. Halvarsson, Submitted, 2014.