Aluminide coatings on stainless steels by pack cementation: High temperature reactivity

Alumina-forming or chromia-forming alloys are good candidates to resist to oxidation under high temperature exposures, because they form protective alumina or chromina layers on the alloy surface. Aluminide compounds are the most effective materials because they combine many beneficial properties (low density, high temperature mechanical properties...) which lead to the formation of a Al2O3 scale. The aim of this study was to form aluminides on the surface of a model alloy (Fe-30Cr) using a pack cementation process. The aluminised alloys were characterised using a SEM coupled with a EDX as well as using XRD. Superficial and cross-section observations revealed that the coatings consisted in iron, chromium and aluminium. Cross-section analyses showed a progressive diffusion of aluminium through the substrate. The high temperature oxidation resistance of the aluminised alloys was tested by oxidising the substrates at 1000degreesC for 100 hours in air under the atmospheric pressure in a thermogravimetric analyser (TGA). The oxidation rate was 100 times lower on aluminised samples compared to the bare steel. The high temperature oxidation resistance of the aluminised alloys was due the formation of a protective alumina layer on the stainless steel surfaces.