FeCrAl (Kanthal ®) alloys are high-temperature resistant alloys of iron, chromium, and aluminum, with excellent oxidation resistance at elevated temperatures. In most cases are used for electrical heating and structural elements resistant to elevated temperatures (up to 1400 °C). The oxidation resistance of these alloys is related to the formation of thermodynamically stable and oxygen-impermeable alumina (Al2O3) on the surface, which inhibits further oxidation of the material. Despite their excellent oxidation resistance, the life span of the elements made of these alloys is limited if they are subjected to thermal cycling at temperatures above 1000 °C. The reason for this is cracking of the alumina scale, due to different temperature expansion compared to the alloy matrix, and the consumption of the aluminium in the subsurface area of alloy matrix, due to the constant re-formation of the alumina scale. Once the aluminium content in the subsurface area is reduced beneath critical concentration (<3 wt.%), the alloy can no longer reform the continuous alumina scale, resulting in oxidation of iron and alloying elements and thus gradual destruction of the heating or structural element.
The purpose of the master's thesis was to increase the concentration of aluminum in the surface layer of FeCrAl alloy (® Kanthal AF), which would extend the ability to recover the aluminium oxide layer and consequently the life span of the elements made of this alloy. In order to increase the concentration of aluminum in the subsurface layer, the alloy was treated by the pack aluminization process.
The pack aluminization process was carried out in the tube furnace in which samples were annealed, buried in a powder mixture of 1 wt.% Al, 1 wt.% NH4Cl and 98 wt.% Al2O3 at various times and temperatures. The aluminization was followed by high-temperature oxidation of the aluminized and for comparison also non-aluminized samples. The oxidation was carried out at 1200 °C in an air atmosphere. The aluminized and oxidized samples were analyzed by a scanning electron microscope (SEM). The surface condition, the diffusion depth of the aluminium and the structure of the oxide coating were investigated. We find out that annealing temperature, annealing time and the composition of the powder mixture are crucial to the depth at which aluminum will diffuse into the base material during the pack aluminization process and which phases will form on the surface.
The experiments were part of a project to extend the life span of diesel engine glow plugs.