Abstract
This paper presents the results of microstructural examinations on slurry aluminide coatings using scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. Aluminide coatings were produced in air atmosphere on austenitic high-temperature creep resisting cast steel. The function of aluminide coatings is the protection of the equipment components against the high-temperature corrosion in a carburising atmosphere under thermal shock conditions. The obtained coatings had a multilayered structure composed of intermetallic compounds. The composition of newly developed slurry was powders of aluminium and silicon; NaCl, KCl, and NaF halide salts; and a water solution of a soluble glass as an inorganic binder. The application of the inorganic binder in the slurry allowed to produce the coatings in one single step without additional annealing at an intermediate temperature as it is when applied organic binder. The coatings were formed on both: the ground surface and on the raw cast surface. The main technological parameters were temperature (732–1068°C) and time of annealing (3.3–11.7 h) and the Al/Si ratio (4–14) in the slurry. The rotatable design was used to evaluate the effect of the production parameters on the coatings thickness. The correlation between the technological parameters and the coating structure was determined.
Highlights
High-temperature corrosion is a common problem in various industrial processes, such as heat treatment, chemical processing, refining, etc. in, for example, petrochemical, aerospace, and gas turbine industries
In this work, a mixture of three selected salts was used. ese salts applied in the slurry are a flux and have good properties from an ecological and economic point of view. e existence of oxides on both aluminium particles in the slurry and the surface of samples is a barrier for diffusion processes. e oxide shell is dissolved by this flux during annealing. e coating is formed in a liquid state. e liquid state guarantees high rates of the diffusion reaction, and the interruption of the oxide shell accelerates processes in the liquid state
Results and Discussion e coatings were successfully produced for each set of assumed technological parameters. e microscopic observations proved that the microstructures of the coatings depended on the production parameters
Summary
High-temperature corrosion is a common problem in various industrial processes, such as heat treatment, chemical processing, refining, etc. in, for example, petrochemical, aerospace, and gas turbine industries. On the surface of Cr-rich materials, a protective chromium oxide layer (Cr2O3) is formed and provides the carburisation resistance at temperatures below 1050°C [1]. The addition of aluminium and silicon to the high-temperature creep resisting alloys to develop full protection involves metallurgical compromise in strength and ductility. That protect the base material in industrial processes, need to inhibit the diffusion of elements, and must offer resistance to thermal shocks that occur during quenching following the carburising process [8]. In addition to aluminium in the coating, very important is the presence of silicon, which decreases the carbon solubility in austenite and is one of the main elements improving the resistance to carbon absorption at high temperatures. E innovative composition of the slurry has forced the determination of optimum production parameters of the protective coatings on austenitic high-temperature creep resisting cast steel. In this work, a mixture of three selected salts was used. ese salts applied in the slurry are a flux and have good properties from an ecological and economic point of view. e existence of oxides on both aluminium particles in the slurry and the surface of samples is a barrier for diffusion processes. e oxide shell is dissolved by this flux during annealing. e coating is formed in a liquid state (the temperature of annealing is above the temperature of aluminium melting). e liquid state guarantees high rates of the diffusion reaction, and the interruption of the oxide shell accelerates processes in the liquid state
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