Diffusion stable aluminide coatings: Behaviour under steam and oxy-fuel fire-side conditions at 650 °C and accelerated diffusion tests at 900 °C

Two diffusion aluminide coatings obtained at 1050 °C were produced to protect ferritic steels both from steam oxidation and fire-side corrosion at 650 °C. One of them was a simple aluminide coating, whereas the other one was enriched with Cr prior to the aluminizing step. As opposed to aluminide coatings obtained at 700 °C, the new “as deposited” coatings do not exhibit through-thickness cracks and the Al content at the surface is lower. The Cr enriched coating has twice as much Cr at the surface than the aluminide coating produced under the same conditions but without Cr enrichment. Cr at the surface may increase the lifetime of this coating as it is known that it decreases the critical amount of Al required to maintain a protective alumina scale. The two aluminide coatings are very stable under atmospheric steam, as well as under an oxy-combustion atmosphere at 650 °C. They show no significant degradation by diffusion, nor evidence of substrate attack despite the development of some cracks, likely formed during the tests cooling cycles. None of these cracks reached the substrate. This behaviour contrasts that of the coating obtained at 700 °C, which exhibits fast diffusion during exposure at 650 °C. Accelerated diffusion experiments were also carried out at 900 °C under steam, comparing the Cr enriched and unenriched aluminide coatings in order to establish if the higher Cr content present in the enriched coating allows maintenance of the protective alumina scale for longer exposures. However, Cr from the substrate diffused outwards equalizing the Cr surface content of both coatings.