Long Term Diffusion Studies in Fe Aluminide Coatings Deposited by Slurry Application on Ferritic Steel

Abstract

Diffusion iron aluminide coatings have shown excellent resistance to high temperature oxidation in air, corrosive atmospheres and steam. A study of the diffusion behaviour of slurry applied diffusion aluminide coatings deposited on ferritic steel have been carried out under a 100% flowing steam atmosphere for up to 50,000 h at 650 °C. The results have shown that initially, the coating forms by outward growth possibly including the dissolution of the steel in molten aluminium. At later stages, during exposure to steam at 650 °C, aluminium diffuses inward and moreover, Fe also diffuses outward resulting in the progressive development of Kirkendall porosity. Results have also indicated that in order to form a pure protective Al2O3 scale the Al wt.% has to be > 4. Below this content Al-Fe mixed oxides develop exhibiting a less protective behaviour.

Comparison between field and laboratory steam oxidation testing on aluminide coatings on P92

“…The coating exhibits through thickness cracks likely formed by mismatch between the thermal expansion coefficients of the different phases. On exposure to steam at 650 8C coatingsubstrate interdiffusion continues and the Al rich Al 3 Fe and Al 2 Fe 5 phase transforms into FeAl whereas the AlN precipitation zone grows in thickness [18] (Fig. 8b).…”

“…The cracks, present in the coating prior to testing, neither propagate beyond the coating-substrate interface, nor become sites of substrate attack. It has already been shown that the surface of the cracks also develop protective Al 2 O 3 which eventually fills them [18]. The KOMET 650 exposed aluminide samples were prepared on P91 instead of P92.…”

“…Coating-substrate interdiffusion is important at 650 8C [18] and results in the transformation of the initially present Al richer phases (FeAl 3 and Fe 2 Al 5 ) into FeAl. Moreover, as Al continues diffusing inwards, the AlN precipitation zone increases in thickness reaching approximately 40 mm on both laboratory and field tested specimens (Fig.…”

Interdiffusion behavior of Al‐rich oxidation resistant coatings on ferritic–martensitic alloys

“…For exposures at 700 °C, the C b of 3.5% was used, which was established from the thin coating tested in air + 10 vol% H 2 O 14. Additional C b values (5–13.5%), recently reported for Fe 2 Al 5 ‐based slurry aluminide coatings tested in steam at 650 °C 9, were also included in the prediction. The predicted coating lifetime and maximum diffusion depth are summarized in Table 4.…”

“…There are several concerns associated with the use of Al‐rich coatings on ferritic–martensitic and Fe‐base austenitic alloys, including: (1) the difference in coefficient of thermal expansion (CTE) between the aluminide intermetallic phases (e.g., Fe 2 Al 5 , FeAl, or Fe 3 Al) and the substrates 6, 7, (2) diffusion of Al into the substrate, which not only depletes the Al reservoir, but also may destabilize the phases intended to strengthen the alloys 8, 9, and (3) the mechanical integrity in thermal cycling of more brittle, high‐Al aluminide phases such as Fe 2 Al 5 10–13. Long‐term laboratory exposures 14, 15 were conducted at 650–800 °C in air + 10 vol% water vapor on Al‐rich coatings made in laboratory‐scale chemical vapor deposition (CVD) 16 and pack cementation processes 17, 18.…”