Defining Failure Criteria for Extended Lifetime Metallic Coatings

Aluminide coatings have been studied for many years because of their excellent corrosion resistance in oxidizing, sulfidizing and carburizing environments. One of the factors that inhibits their application is a lack of sufficient data about their potential benefits. In order to address this issue, model coatings are being fabricated using a chemical vapor deposition (CVD process on ferritic (Fe-9Cr-1Mo) and austenitic (304L) substrates. The performance of both high and low Al activity CVD processes is being characterized. The thicker coating produced by the high Al activity process is more resistant to a sulfidizing environment due to its higher Al content. Both types of coatings provide excellent protection during 1h cycles at 700°C in air+10vol.%H2O, an environment that severely attacks the uncoated alloys. Diffusion and corrosion tests are being conducted to quantify the rate of Al loss from the coating. The remaining data needed to develop a coating lifetime model is the minimum Al content needed for the coating to be protective as a function of application temperature and environment. This failure criteria is being investigated using cast Fe-Al alloys with low Al contents (10-20at.%) at test temperatures of 700°1200°C. In the lower temperature range, Cr present in the coatings should play an important role in determining the critical Al content of the coatings because it had a significant effect on the performance of cast Fe-Al-Cr alloys in air+10vol.%H2O at 700°C.