Thermodynamische und kinetische Untersuchungen zur Korrosion von Fe, Ni und Cr, sowie ausgewählten Legierungen bei Gegenwart eines geschmolzenen KCl - ZnCl 2-Eutektikums

• Thermodynamic and kinetic investigations on the corrosion of Fe, Ni and Cr as well as of selected alloys in the presence of a molten KCl-ZnCl2 eutectic

Ruh, Andreas; Spiegel, Michael (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2008)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2007

Abstract

In the presence of heavy metal containing chloride melts, metallic materials show a strongly accelerated corrosion. In this work, the influences of the elements Fe, Cr, Ni and Si and the partial pressures of O2 and HCl on the salt melt-induced high temperature corrosion have been studied, as well as the quantity of the chloride deposit. Primarily, thermogravimetric investigations have been conducted on the pure metals Fe, Cr and Ni but also on the alloys Alloy 625, Alloy 625Si and Fe-10%Cr, which have been covered by a chloride mixture containing 50 mol.% ZnCl2 and 50 mol.% KCl. The experiments have been carried out in Ar - O2- and Ar - O2 - HCl-containing atmospheres at 320°C. The studies were complemented by experiments on the solubility of metals in a 50 mol.% ZnCl2 - 50 mol.% KCl melt. Thermodynamic calculations have been done for the interpretation of the results. The exposure of Fe covered by a ZnCl2 - KCl deposit in O2-containing atmospheres results in an increased corrosion rate. 3 kinetic stages have been observed: a: an incubation phase, b: a linear stage and c: a stage that follows either a logarithmic or a parabolic rate law. It was shown that the kinetic constants depend on the amount of the salt deposit and on the oxygen concentration in the gas atmosphere. According to thermodynamic calculations the partial pressure of Cl2 is established by the oxidation of ZnCl2 to ZnO and Cl2. The reason for the intense corrosion is attributed to the high solubility of FeCl2 in the 50 mol.% ZnCl2 - 50 mol.% KCl melt, which was demonstrated by a newly developed phase diagram. The addition of HCl to the reaction gas generally yields an increased formation of iron chloride and a further acceleration of the corrosion rate. This traces back to the formation of Cl2 by the oxidation of HCl in the gas atmosphere. Furthermore, thermogravimetric experiments also show evaporation of iron chloride and zinc chloride. In contrast, in Ar - O2 - H2O-atmospheres a lower mass gain has been detected at longer exposure times. The reduction of the Cl2 partial pressure by the reaction of Cl2 with H2O forming HCl and O2 (reverse Deacon reaction) is the main reason for this behaviour. Chromium covered by ZnCl2 - KCl shows in Ar - O2 still a relatively good corrosion resistance, which however decreases significantly by adding HCl. In Ar - 8 vol.% O2 - 1000 vppm HCl a very complex kinetics is observed which has two stages showing a mass gain and one stage showing a mass loss. During corrosion, chromium chloride is formed that reacts partially by forming chromium oxide and chromate during a subsequent stage. Ni shows in both, Ar - O2- and Ar - O2 - HCl-containing atmospheres a quite good corrosion resistance beneath the chloride melt. The addition of HCl increases the mass gain only slightly in comparison to Fe and Cr. For the interpretation of the possible reason, thermodynamic calculations show on the one hand that the standard free energy of formation of NiO is slightly higher than that of NiCl2 and on the other hand that NiCl2 is scarcely soluble in the ZnCl2 - KCl melt. Alloy 625 and Alloy 625Si react in Ar - 8 vol.% O2 in the presence of a ZnCl2 - KCl-melt by showing a low mass change. During the reaction of both alloys in Ar - 8 vol.% O2 - 2000 vppm HCl and in the presence of the chloride melt, an accelerated corrosion was detected. Even in this case relatively large quantities of a chromium-rich chloride appear as corrosion product. Beyond that a negative effect on the corrosion behaviour arises also from the alloying element Si. Fe-10%Cr covered by a ZnCl2 - KCl-deposit shows a strongly accelerated corrosion in Ar - 8 vol.% O2 as well as in Ar - 8 vol.% O2 - 2000 vppm HCl. The thermodynamically favoured reaction of chromium chloride to oxide, the formation of iron chloride, supported by the release of chlorine, and the solubility of iron chloride in the ZnCl2 - KCl-melt are to be noted as possible reasons for that. A comparison of thermogravimetrically determined mass gains for the corrosion of Fe beneath 50 mol.% ZnCl2 - 50 mol.% KCl with mass gains, simulated by a modelling program shows in general a relatively good congruence. However, discrepancies exist in reference to the parabolic/logarithmic stage and to the chemical composition of the scale. Finally, this work showed that the element nickel might most likely reduce the susceptibility of metallic materials to corrosion under the investigated conditions. In contrast, chromium has a disadvantageous effect on the corrosion behaviour. Alloying silicon to nickel-base alloys also results in a decrease of the corrosion resistance.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Ceramics and Institute of Mineral Engineering [524110]