Study On Corrosion Behavior Of Three Kinds Of Austenitic Steels In Simulated Flue-gas/Coal-ash Environments

High temperature corrosion behavior and corrosion mechanism of three kinds of austenitic steels（S30432,HR3 C,NF709R） at 700 ℃ was studied in the synthetic flue-gas and flue-gas/Coal-ash environments. The microstructure and composition of the scales were investigated by scanning electron microscope/energy dispersive spectroscope and X-ray diffraction. The influence of material composition and corrosion medium on the corrosion behavior was investigated. Based on the above results, the corrosion resistence property of these steels was also be compared. In addition, the impat of chloride on corrosion behaviors of HR3 C in flue-gas/Coal-ash environments at 650℃ was studied by the same method as above.The corrosion results of these steels in flue-gas environments show that, a lot of iron and chromium oxides are formed on the surface of austenitic steels, which are closely related to composition and elements of steels.The higher Cr content in the alloy, the more Cr2O3 in the scale. The corrosion products of HR3C（Cr 25wt%） is mainly Cr2O3, the main products of NF709R（Cr 22wt%） is Cr2O3 and Fe2O3. In S30432（Cr 18wt%） the corrosion products is mainly iron oxides（Fe2O3 and Fe3O4） as the Cr content is low.The Cr content of NF709 R was between them（Cr 22wt%）, the corrosion products of NF709 R contained a number of Cr2O3, while a highe content of Fe2O3. The three kinds of steels all generate little sulfides in affected zone. The corrosion of the steels is mainly oxidation corrosion with light sulfidation in metal/scale interface and internal sulfidation in Substrate under the scale. The sulfidation accelerates destroy the oxide film and enhance the corrosion.The corrosion results of these steels in flue-gas/coal-ash environments indicate that, the main corrosion products are iron oxides. The iron oxides tend to exist in Fe3O4 phase when serious corrosion happened, and exist as Fe2O3 after slight corrosion. High content S element was detected in metal/scale interface and internal sulfidation region from all test materials. The increasing of SO₂ content promotes the formation of low melting point eutectic sulfate on the surface, and accelerate the sulfidation in initiation stage. The liquid sulfates can reacted with the oxides, or even directly react with the metal substrate. Materials mainly suffered low temperature hot corrosion by liquid sulfate.The corrosion results of HR3 C in flue-gas/coal-ash environments with Chloride indicate that, Cr2O3 scale in vulnerable in chlorine-containing environment, it’s easy to suffer corrosion by chlorine and lose protect function to matrix. As a Fe base material with high Cr content, HR3 C is very sensitive to chloride. the main corrosion products of HR3 C is iron oxides. while contain some Cr2O3,NiCr2O4 and NiFe2O4. Ni element enrichment in the corrosion process, while the Cr and Fe elements are a large number of consumption. A large number of holes was detected in the metal matrix under the scale. The Chloride can significantly promote corrosion of HR3 C in flue-gas/coal-ash environments, and make the corrosion production spall easier. The corrosion is the interaction oxide, sulfide and chloride in the presence of molten salt.The comparison of three kinds of austenitic steels corrosion properties indicate that all the materials have a strong resistance to high-temperature corrosion in fuel-gas atmosphere except S30432, the fireside corrosion sustained by superheater materials is mainly of the fuel-gas/coal ash corrosion. the continuous and compact Cr2O3 oxide scale is the key role of materials to resist hot corrosion sulphate salt. The higher Cr content in alloy, the ability to resist sulphate salt hot corrosionin is better. The increasing of SO₂ content can promotes the corrosion both in fuel-gas and fuel-gas/coal ash environment. In fuel gas/coal ash environment, when the SO₂ concentration increase to 1vol%, even the HR3 C whose Cr content up to 25wt%, its corrosion resistance also declined sharply. In fuel-gas/coal-ash environment with 1%SO₂, all test steels suffer serious hot corrosion. The Fe element in austenitic steels dissolved in molten salt make the corrosion fast at propagation stage. Overall,The fireside corrosion resistance performance of materials is HR3C>NF709R>S30432 under the fuel-gas/coal-ash environment.