Aluminising of Monel 400 alloy by pack cementation medhod; observation of growth and oxidation kinetics

In this study, aluminum coating was applied to Monel 400 nickel-copper alloy through the pack cementation method to investigate the oxidation behavior of the coating layer. Aluminum coating was performed at low temperature using the pack cementation method at temperatures of 600, 650, 700, and 750°C for durations of 2, 4, and 6 hours. The pack mixture was prepared using metallic Al powder, Al2O3 as an inert filler, and Ammonium Chloride (NH4Cl) as an activator. The microstructures of the resulting coatings were characterized using SEM and EDS analyses, and phase analyses were conducted using XRD analysis. SEM analysis revealed that the coating layers were homogeneous, compact, and pore-free, and there was a strong bond between the coating and the matrix. The thickness of the coating layer was measured from the surface to the matrix, and values ranging from 6 µm to 20 µm were observed. It was determined that the lowest temperature, 600°C, was sufficient for the accumulation of an alumina layer and a successful coating layer was obtained. Additionally, the pack cementation process carried out at 650°C and a processing time of 6 hours provided the thickest coating layer. The EDS analysis of a sample subjected to a 6-hour treatment at 600°C, resulting in the formation of a layer. The observations reveal varying stoichiometric ratios within the formed layer, indicating changes in the composition. Additionally, an increased presence of an oxygen-rich surface layer suggests a certain degree of oxidation. Specifically, upon closer examination of point 2, the presence of the nickel aluminide phase is evident. Furthermore, the analysis indicates the migration of copper towards the surface. As one moves towards the matrix of the layer, a decrease in the aluminum content is observed. These findings provide valuable insights into the elemental distribution and composition of the formed layer during the 6-hour treatment. X-ray diffraction (XRD) analysis was performed on the samples that underwent the alumisation process for 2 and 4 hours at a temperature of 600, 650, 700 and 750 °C in order to investigate the phase composition. The XRD analysis results indicate the presence of Ni2Al3 and Ni3Al phases in the resulting coating layer formed through the alumination process. The XRD diffraction patterns corresponding to the materials treated with the alumination process for 2,4 and 6 hours at 600, 650, 700 and 750 °C. The growth kinetics of the coating were analyzed by correlating the depth of the alumina layer with time and process temperature. The hardness values of the alumina layer formed on the surface were measured, and an increase in hardness values was observed with increasing process time and temperature. The thickness of the aluminide layers resulting from the aluminum coating process on Monel 400 alloy was assessed, and the hardness measurements were conducted on multiple points within the metallographically prepared samples. The measurements were performed at incremental distances from the surface to the matrix, allowing for the evaluation of hardness variations across the aluminide layer. The hardness measurements were continued until reaching the core hardness value of the material. The hardness values exhibited changes depending on the temperature and duration of the aluminum coating process. The initial hardness of the Monel 400 alloy was measured at 230 HV. Subsequently, the obtained aluminide layers displayed hardness values ranging from 450 to 700 HV, exhibiting an increasing trend with prolonged treatment durations. The thickness of the coating layer was measured from the surface to the matrix and it was observed that the values varied between 3 µm and 40 µm. It was determined that the lowest temperature of 600°C was sufficient for the deposition of the aluminite layer and a successful coating layer was obtained. In addition, the box cementation process with a process temperature of 650°C and a process time of 6 hours provided the thickest coating layer. A microhardness scan test was performed from the outermost surface of the nickel aluminide intermetallic coating to the base matrix to determine the general mechanical properties. The hardness values of the aluminide layer formed on the surface were measured and an increase in hardness values was observed with increasing processing time and temperature. It was observed that the hardness of Monel 400 alloy, which was 230 HV, increased up to 1280 HV depending on the time and temperature of the coating. A homogeneous and continuous aluminium-rich coating layer on the surface was achieved at a temperature of 650°C and a retention time of 6 hours. Considering the coating thicknesses, temperatures and times obtained after coating, the formation kinetics of the aluminium coating were calculated. The kinetics of the coating layer was calculated as 117,960 kJ/mol. DTA-TG thermal analyses were performed to determine the oxidation and thermal resistance properties of the coating. The growth kinetics of the coating was analysed in relation to time and process temperature over the depth of the aluminide layer. The aluminium coated samples were subjected to oxidation tests at 750, 800 and 850°C for 72 hours. The coated samples were weighed before the oxidation analysis and at the determined temperatures and the weight gains were compared. It was determined that the highest weight gain was in the sample subjected to oxidation at 850°C. In the oxidation analysis, it was observed that the coating layer formed on the samples fell off with the increase in time at 800 and 850°C. The activation energy calculated after oxidation was calculated as 176,121041 kJ/mol.