The surface of the CrMnFeCoNi alloy after sulfiding at 600 °C shows three regions
Source: BAM, Division Materialography, Fractography and Ageing of Engineered Materials
The use of more and more varied fuels implies an increased list of criteria that need to be addressed when choosing a material for a combustion chamber and its supply pipes. This is especially important in case of impurities containing fuels such as biogas – the materials must be very resistant against corrosion, and even more so because the process takes place at temperatures above 500 °C. The high temperatures increase the diffusion rate of all elements and thus speed up the corrosion compared to processes at room temperature. One of the corroding element is sulphur and its gaseous components, whose contents in fuels have luckily been submitted to strict regulations in the last decade.
In a first step it is necessary to understand the early stages of the corrosion. How does the sulfur attack the material around it, which is usually of metallic nature? Here, this question is investigated on a rather new class of materials called high entropy alloys or multicomponent alloys. These materials differ from classic alloys by the lack of base elements. Classic alloys are based on one predominant element (steel – iron; titanium alloys – titanium; superalloys – nickel and cobalt, sometimes iron) while the high entropy alloys do not have a base but consist of five or more "base" elements of very similar concentration. The alloy that was chosen for this investigation consists of chromium, manganese, iron, cobalt and nickel and is of equimolar composition, i.e., CrMnFeCoNi.
The material is submitted to an atmosphere that consists of noble gas argon and 0.5 % sulfur dioxide. The process takes place at 600 °C for 6 h. The corrosion products are observed by different methods such as x-ray diffraction, scanning and transmission electron microscopy and the results are compared with thermodynamic calculations. It was found that the sulfur does not diffuse into the bulk material but interacts only at the surface – preferentially at grain boundary triple junctions. These regions are well visible in the figure (left): they are characterized by thick manganese sulfates depicted in white. Manganese is known for diffusing fast from the inside of the material towards the surface, where it interacts with sulfur and oxygen. The rest of the surface shows different oxides and, in some rare places, chromium sulfates. The only beneficial product, i.e., one that can protect the material from further corrosion, is the chromium oxide. Its formation shall be enhanced and promoted in the subsequent studies.
Illustration: The schematic perpendicular cut shows the corrosive products
Source: BAM, Division Materialography, Fractography and Ageing of Engineered Materials
Early Material Damage in Equimolar CrMnFeCoNi in Mixed Oxidizing/Sulfiding Hot Gas Atmosphere
Anna Maria Manzoni, Wencke Mohring, René Hesse, Leonardo Agudo Jácome, C. Stephan-Scherb
published in Advanced Engineering Materials, page 2101573, 2022
BAM Department Materials Engineering, Division Materialography, Fractography and Ageing of Engineered Materials
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