This study assesses the corrosion performance of aluminum-manganese (Al-Mn) coatings deposited on 42CrMo4 steel by atmospheric plasma spraying and (APS). Al-Mn alloy powder containing 25 at% Mn was gas atomized under nitrogen atmosphere using pure Al and Mn as starting materials. The powder was characterized by laser granulometry and SEM-EDX analysis. A fraction with particle sizes ranging from 10 to 60 μm was used as the feedstock powder. The APS coatings were characterized by optical image analysis, Vickers hardness measurements, and salt-spray testing. The Al-Mn coatings exhibited significantly higher hardness in comparison to pure aluminum. Red rust appeared after 240 h of salt-spray testing, leading to local coating detachment after 1000 h. The corrosion attack starts at coating defects such as microcracks, which can be attributed to the brittleness and nonuniform melting behavior of the powder and possibly inhomogeneities in the spraying process.

The atomization of molten materials using hot gases has advantages in comparison to the conventional gas atomization techniques. Some of these advantages favour the hot gas technology for the powder production for thermal spraying. Firstly, a considerably higher output of fine powder, in particular within the important particle size range between 5 and 30 µm, is a result of hot (inert) gas atomization. The reasons are the strongly increased gas exit velocity and the higher overall temperatures inside the interaction zone of the gas and the melt droplets. The enhanced shear forces acting on the molten liquid and the prolonged liquid state of the atomized particles lead to a more efficient atomization. Secondly, the extended time regime for liquid droplets facilitates the atomization of highly viscous melts, such as oxide melts, and results in more spherical particle shapes with good flowability. Thirdly, oxide or nitride powders can be generated directly from the molten metal by the usage of hot reactive gases or gas components for atomization. This paper describes the special features of a 25 bar hot gas atomization technique with pre-heated gases up to 1200°C and discusses its potentials for the generation of powders in view of their suitability in thermal spray applications.