Investigation of the HCl (g) attack on pre-oxidized pure Fe, Cr, Ni and commercial 304 steel at 400 °C

The paper presents the results from an investigation studying the ability of pre-oxidized metals and alloys to withstand chlorine attack in the form of gaseous HCl. The materials under investigation were pure Fe (s), Cr (s), Ni (s), and a commercial 18Cr–10Ni–Fe (304) alloy. The samples were pre-oxidized in different well defined environments, dry 10 vol.% O2 (g) + N2 (bal.), 10 vol.% O2 (g) + 5 vol.% H2O (g) + N2 (bal.) and 10 vol.% O2 (g) + 250 vppm SO2 (g) + N2 (bal.) for 24 h at 400 °C using a horizontal tube furnace. Afterwards the oxide films were characterized by GI-XRD, FEG-SEM, XPS and ToF-SIMS. The samples were then exposed further in 10 vol.% O2 (g) + 500 vppm HCl (g) + x (x = 5 vol.% H2O (g), 250 vppm SO2 (g)) + N2 (bal.). The exposure time was 100 h and after the exposures during the cool down process the reaction chamber was flushed with dry 10 vol.% O2 (g) + N2 (bal.). The corroded samples were then examined by the same techniques mentioned before. HCl (g) showed mainly to be aggressive toward the Fe (s) samples that form a relatively thick and porous oxide scale consisting of layered Fe2O3 (s)/Fe3O4 (s) during pre-oxidation, and the aggressiveness did not depend on the pre-oxidation conditions. All the other materials formed thin and dense oxides (20–100 nm) during pre-oxidation, and they did not suffer accelerated oxidation caused by HCl (g) during the subsequent exposure. The only exception was Ni (s) that had been pre-oxidized in an atmosphere containing SO2 (g), in this case Ni sulphides and sulphates were formed during pre-oxidation which in turn caused accelerated oxidation to Ni when subsequently exposed to HCl (g). HCl (g) readily reacts with NiSO4 (s) and Ni3S2 (s) and forms NiCl2 (s) and SO3 (g).