Resumo:
X22 steel is a metallic alloy classified as a martensitic stainless steel, commonly utilized in the construction of machinery and equipment operating at high temperatures. However, its resistance to oxidation is significantly limited at temperatures near its operational limit, compromising its practical applications. In this study, the objective was to enhance the oxidation resistance of X22 steel using the aluminization coating technique via HAPC. The process parameters for HAPC were based on pure aluminum (Al) and aluminum-chromium (Al-Cr) donor masteralloys. Temperature and time variations ranged between 650-800 °C and 4-36 h, respectively. NH4Cl, NaCl, and KCl salts were employed to activate the processes. Additionally, thermodynamic conditions for aluminum deposition in HAPC processes were evaluated using HSC Chemistry software. The results obtained demonstrate the viability of forming coatings with practical significance on X22 steel using both masteralloys. When utilizing pure aluminum (Al) as the masteralloy, coatings were achieved at a temperature of 800 °C, with process times of 16 and 36 h, activated with NaCl and KCl salts. The aluminized thickness varied between 50-85 μm, forming a compositional gradient of aluminum in the Fe2Al5, FeAl, and Fe3Al phases in most cases. As for the Al-Cr masteralloy, a coating was obtained under specific conditions: a temperature of 750 °C for 36 h, activated by NH4Cl salt. The aluminized thickness was 50 μm, with the formation of ductile phases FeAl and Fe3Al. This condition allowed for a reduction in process temperature while preserving the properties of X22 steel. In oxidation tests conducted at 800 °C for 288 h under ambient atmosphere (in air), the coatings on X22 steel provided satisfactory protection at high temperatures. Mass gains varied between 0.71-2.20 mg/cm² for the coatings formed using pure aluminum (Al) as the masteralloy and 5.10 mg/cm² for the coating formed using aluminum-chromium (Al-Cr) as the masteralloy. Uncoated X22 steel exhibited a mass gain of 47.27 mg/cm². Regarding the effect of the temperature used in HAPC processes on the microstructure and hardness of X22 steel, minor changes were observed at the maximum process temperature of 800 °C. However, implementing a proposed mode of external furnace cooling (REF) after HAPC processes allowed for adequate preservation of the hardness of X22 steel.
