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

Abstract

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.% O 2 (g) + N 2 (bal.), 10 vol.% O 2 (g) + 5 vol.% H 2O (g) + N 2 (bal.) and 10 vol.% O 2 (g) + 250 vppm SO 2 (g) + N 2 (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.% O 2 (g) + 500 vppm HCl (g) + x ( x = 5 vol.% H 2O (g), 250 vppm SO 2 (g)) + N 2 (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.% O 2 (g) + N 2 (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 Fe 2O 3 (s)/Fe 3O 4 (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 SO 2 (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 NiSO 4 (s) and Ni 3S 2 (s) and forms NiCl 2 (s) and SO 3 (g).