Influence of gas phase composition on the kinetics of chloride melt induced corrosion of pure iron (EU-project OPTICORR)

Abstract

Heat exchanger tubes in waste-fired boilers are usually attacked by aggressive gases like oxygen, HCl and water vapour as well as by solid and molten salts. Especially if the salt deposit is molten, these compounds cause acceleration of the corrosion process. This study focuses on kinetic investigations on the influence of gas phase composition (O2, HCl and H2O) on the chloride melt induced corrosion of Fe. Thermogravimetric (TG) tests were conducted on Fe covered by a 50 mol.% KCl/50 mol.% ZnCl2-salt mixture at T = 320°C, first in an Ar - O2 - atmosphere. In these experiments the amounts of salt deposit (15 and 30 mg/cm2) and oxygen (4–16 vol.%) were varied to investigate their influences on corrosion kinetics. The TG curves show three kinetic stages, i.e. an incubation phase, a linear stage and a logarithmic/parabolic stage [1]. Further it could be shown that the duration of the incubation phase and the slope of the linear stage depend on the amount of the salt deposit as well as on the oxygen partial pressure (p(O2)). The incubation time increases with increasing amount of salt deposit and decreases with increasing p(O2). Experiments that were carried out in an Ar – O2 – HCl atmosphere (500–2000 vppm HCl, 4–16 vol.% O2) yield enhanced mass gain in comparison to TG tests without HCl. In atmospheres containing at least 1000 vppm HCl corrosion starts immediately without going through an incubation phase following a linear rate law. In the next stage the TG curve follows a rate law that is either parabolic or logarithmic. At higher amounts of HCl (2000 vppm, 8 vol.% O2) the TG curve shows a relative mass loss after about 55 h of corrosion indicating an evaporation of iron chloride. This phenomenon was also observed at 1000 vppm HCl and 16 vol.% O2 but not in the other experiments presented in this study. Experiments carried out in Ar – O2 – H2O are discussed also. In general the TG curves show lower mass gains after some hours of corrosion in comparison to TG tests without H2O but at the beginning a shortened incubation phase is evident.