Model About the Course of Corrosion Reactions of Austenitic Steels in H2S-, HCl- and CO2-Containing Atmospheres at 680 °C
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Material selection for thermal cracking processes of anthropogenic resources is challenging. H2S and HCl are formed, and corrosion occurs at high temperatures. Several steels were investigated in the past. In all cases, corrosion layers were characteristically structured. On top, large chromium sulfide crystals are located, followed by a Cr2O3 layer, followed by a layer enriched in chlorine and nickel. In this paper, we propose a model on the course of the corrosion reactions. By considering the water–gas shift reaction as well as the influence of H2S, the layer formation can be explained. As an example S31400, 240 h corrosion time at 680 °C was chosen.
KeywordsHigh-temperature corrosion Austenitic stainless steel Corrosion mechanism
In several industrial processes, construction materials suffer from chlorine and/or sulfide high-temperature corrosion, e.g., ethylendichloride production, gasification of biomass, coal or waste. Most processes work at higher partial pressures of oxygen, where metallic materials are able to form protective oxide layers. However, if chlorine is present, porous and non-protective corrosion layers are formed [1, 2, 3, 4, 5].
The mechanism of chlorine-induced high-temperature corrosion in oxidizing atmosphere is well established and was investigated by numerous authors [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. At temperatures above 400 °C, chlorine or low molecular chlorine-containing compounds, like HCl, are able to move through the initial, protective oxide layer. The mechanisms how chlorine or HCl is able to do so are still part of investigations and not clear yet. Below the oxide layer, metal chlorides are formed, which are volatile and evaporate. The gaseous metal chlorides diffuse out of the oxide layer until they reach areas with a higher oxygen partial pressure. There they react to metal oxides and HCl. The formed HCl is able to reenter the reaction cycle and an autocatalytic corrosion process is initiated, which leads to rapid material failures. Generally, chromium forms its oxide in most environments due to its high affinity to oxygen [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20].
Stability diagrams help to estimate the thermodynamic stability of a distinct phase when a metal is subjected to corrosion by two or more oxidants. These diagrams were often used by authors to explain the corrosion of metals in mixed oxidant atmospheres [7, 8]. In order to obtain more realistic results, in case of involvement of volatile compounds (e.g., FeCl2), the concept of quasi-stability diagrams was introduced by Schwalm et al. [1, 2, 3]. These quasi-stability diagrams are based on a limiting vapor pressure of the volatile species of 10−4 bar which, in this case, is the metal chloride. Quasi-stability diagrams reach their limits when a third oxidant is added to the gas mixture.
In coal gasification or thermal cracking of municipal solid waste processes, H2S is additionally present beside HCl and CO2 [21, 22]. The influence of H2S on the chlorine-induced high-temperature corrosion is not well investigated. Only Bakker et al. [21, 22] and Pan et al.  dealt with this topic and even found contradictory results in some cases.
During earlier experiments, corrosion resistance of several steels and nickel-based alloys was investigated in H2S- and HCl-containing atmosphere at high temperatures [4, 5, 6]. The potential for oxidation in the gas mixture was low because CO2 and no O2 were present. Because the interpretation of the observed corrosion layers was non-trivial and was only suggested in the previous publications; in this paper, a model about the course of the corrosion reaction is proposed based on additionally performed thermodynamic calculations. As an example alloy the austenitic stainless steel S31400 (1.4841), which was tested at 680 °C for 240 h in the corrosive gas atmosphere was chosen.
Materials and Methods
All corrosion experiments were performed in an inert silica tube, which was heated with a tube furnace. In order to reach a well-defined substrate surface, all specimens were polished with 1000 grit SiC paper, washed, degreased and gauged before they were placed on the specimen holder in the silica tube. The whole system was purged with nitrogen for 30 min before the experiment was started and during the heating process. Until the furnace reached the required temperature, the testing gas mixture was added with 120 ml/min. By purging the system with nitrogen for 30 min before the furnace was turned off, the experiment was stopped.
Chemical composition of S31400 (1.4841) in wt%
Thermodynamic calculations were performed with the software package FactSage 7.2 (© 1976–2018 Thermfact and GTT-Technologies). The Gibbs free energies of given chemical reactions were obtained with the FactSage module Reaction using the FactPS compound database (FACT pure substances database). The built-in calculation routine is based on Gibbs energy minimization and accesses only compound databases. Further, the calculation assumes all gas phase species to be ideal and neglects expansivity and compressibility of solid and liquid phases.
Metallographic Preparation of the Samples After the Corrosion Experiments
In order to prepare cross sections of the corroded samples, they were cold-mounted in epoxy resin (Struers® Epo Fix). A vacuum guaranteed the absence of gas inclusions. Polishing was performed water-free to avoid the dissolution of the water-soluble metal chlorides. With the exception of the 3-µm polishing, SiC paper was used. The bounded grains maintained corrosion products way better than loose grains. Isopropyl alcohol was used as lubricant.
The Corrosion Experiment at 680 °C
An XRD analysis of the corrosion products revealed the phases Cr3S4, Cr2S3 and Cr2O3. No metal chlorides were found using XRD. This is most probably due to the very hygroscopic character of metal chlorides, which may react with atmospheric water and therefore are not crystalline. Furthermore, there was an enrichment of nickel in the border area of the base metal, next to the corrosion products. Chromium and iron were depleted in this area and were found in the corrosion products as Cr3S4, Cr2S3, Cr2O3. The volatile FeCl2 re-sublimated in the colder parts of the silica tube. The enrichment of nickel reveals that there is no reaction between this metal and the gas atmosphere. A possible course of corrosion reactions was developed to explain this setup of corrosion products.
Suggested Corrosion Mechanism
Early Stage of Corrosion
The vapor pressure of CrCl2 is about 10−2 mbar at 680 °C, which causes slow evaporation rates.
The explanation for the chromium sulfide crystals on the surface and Cr2O3 layer formation can be found in the water–gas shift reaction, which forms H2O from CO2 and H2.
The Water–Gas Shift Reaction
The presence of H2S influences the catalytic reaction at low as well as high temperatures. At low temperatures, the decrease in reaction rate is negative for H2 production. In CVD processes, H2S also reduces the reaction rate of H2O formation, which reduce Al2O3 powder formation in the gas phase [28, 29].
Steady-State Stage of Corrosion
After the first attack of HCl and the formation of a porous layer of corrosion products, diffusion processes determine the further development of the corrosion process. HCl will still diffuse to the metal to form metal chlorides and hydrogen. The amount of hydrogen will increase in the layer, but it will also diffuse outwards. As shown above, due to the presence of H2S and missing of a catalytic surface, the water–gas shift reaction is negligible in the main gas flow.
The absence of oxygen in deeper parts of the corrosion layer can be explained by the fact that CO2 is converted completely into H2O and subsequently to Cr2O3 near the surface.
The fact that H2S suppresses the water–gas shift reaction, and that there is no H2O in the main gas flow, explains the absence of oxygen in the chromium sulfide crystals.
Thermodynamically, the formation of iron sulfide from FeCl2 is possible as well. However, no sulfides of iron were detected at 680 °C, and only FeCl2 crystals were present in the cold parts of the testing equipment. Kinetic reasons seem plausible for the suppression of the conversion of FeCl2 to FeS with H2S.
The enrichment of nickel in relation to the steel matrix (Fig. 3) can be explained by its low reactivity with HCl (Fig. 5a) and the fact that iron and chromium are lost due to the evaporating FeCl2 and CrCl2. The chlorine content below the corrosion layers is caused by remaining metal chlorides, which could not be evaporated until the experiment was completed and the furnace was turned off.
For the corrosion of austenitic steels like S31400 in a H2S-, HCl- and CO2-containing environment at 680 °C, a characteristic setup of the corrosion products was observed. On top of the substrate, large chromium sulfide crystals were detected, followed by a porous layer containing Cr2O3, and in contact with the metallic substrate, a porous area was found containing chlorine and an enrichment of nickel with respect to the metal matrix.
The phenomenon of separated regions with Cr2O3 and chromium sulfides can be explained by the peculiarity of the water–gas shift reactions, which forms H2O from CO2. Inside the corrosion layers, CO2 can react with already present hydrogen to H2O, which immediately forms Cr2O3 via the reaction with CrCl2. Excessive CrCl2 diffuses out of the corrosion layers and reacts on the surface with H2S to Cr3S4 and Cr2S3. The reason that oxides and sulfides are not mixed can be explained by the influences of H2S on the water–gas shift reaction.
Open access funding provided by Montanuniversität Leoben. The material was provided by Salzgitter AG, which is gratefully acknowledged.
Alexander Schmid and Gregor Mori contributed to conceptualization; Alexander Schmid contributed to methodology; Edith Bucher contributed to software; Alexander Schmid, Gregor Mori and Roland Haubner contributed to validation; Alexander Schmid contributed to formal analysis; Alexander Schmid contributed to investigation; Alexander Schmid and Gregor Mori contributed to resources; Alexander Schmid and Edith Bucher contributed to data curation; Alexander Schmid contributed to writing—original draft preparation; Alexander Schmid, Gregor Mori and Roland Haubner contributed to writing—review and editing; Alexander Schmid contributed to visualization; Gregor Mori and Roland Haubner supervised the study; and Gregor Mori contributed to project administration.
Compliance with Ethical Standards
Conflict of interest
The authors declare that they have no conflict of interest.
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