Long-term (> 10,000h) oxidation behavior of aluminide coatings made by chemical vapor deposition (CVD) or pack cementation on representative ferritic (Fe-9Cr-1Mo) and austenitic (type 304L stainless steel) are being studied in air + 10% water vapor in the temperature range of 650-800µC. Unlike the uncoated alloys, which are severely attacked in this environment, the CVD aluminide coatings have not failed after 16,000h at 700µC or 10,000h at 800°C. The pack aluminide coatings also show good oxidation protection after testing for ~5,000h at 700°C. In addition, initial efforts have been made to fabricate coatings at lower aluminizing temperatures and the current results suggest that FeAl coatings can be synthesized at temperatures as low as 700°
Aluminum-containing coatings are of interest for protection of ferritic and austenitic alloys in advanced power-generation applications because of their formation of an external alumina scale that remains stable to very low oxygen pressures.[1-5] Alumina-forming alloys are not as severely affected by the presence of water vapor in steam or exhaust gas streams as chromia formers.[5-7] Without a coating, Fe-base alloys can undergo accelerated attack in steam/exhaust environments forming voluminousFeOx.[6-10] Prior work has shown that aluminide coating failure can occur due to cracking in the coating as a result of the difference in coefficient of thermal expansion (CTE) between the substrate andaluminide coating.[6,11] The other anticipated failure mechanism is related to loss of Al from the coating, primarily due to interdiffusion with the substrate.[12,13] One approach to reduce the CTE mismatchinduced strain in the coated alloy is via reducing the coating thickness. In addition, a recent study [14] on creep behavior of ferritic/martensitic steels coated with aluminide coatings indicated that thick aluminide coatings (~250 µm) adversely affect the creep resistance of the substrate because of their low creep strength. As illustrated in Figure 1, for a high creep load (140 MPa), compared to the uncoated alloy, the creep rates were significantly higher for the 2mm thick specimens with a pack aluminide coating of ~230 µm. With a lower load of 110-120 MPa, ~50% decrease in creep rate was noted with reduction of coating thickness from 230 µm to 40 µm. Therefore, thinner coatings in the range of 50- 100 µm are preferred for concerns of both CTE mismatch and creep resistance. However, thinner coatings have a lower Al reservoir that limits their capability to reform the Al2O3 scale in the event of scale spallation. Also, when exposed to an H2S-containing environment, thicker coatings with a higher surface Al content showed better sulfidation resistance.[6]
In this study, model aluminide coatings made by a laboratory chemical vapor deposition (CVD) process with different thicknesses are being tested in humid air. The CVD coating process was selected for the initial testing because of its superior control over coating composition, purity and microstructure, which is critical for evaluation of the long-term coating performance. For subsequent laboratory testing, similar aluminide coatings were made by a more commercially viable pack cementation process to compare their performance to the CVD coatings.
