Long-term performance of aluminide coatings on Fe-base alloys

Aluminide coatings made by chemical vapor deposition on ferritic (Fe-9Cr-1Mo) and austenitic (Type 304L) substrates are being evaluated in humid air at 650°–800 °C. A humid air environment is used to identify coating failure during exposure, as uncoated substrates experience rapid oxidation at these temperatures. One goal of this work was to demonstrate the potential benefits and problems with alumina-forming coatings of two different thicknesses. The higher exposure temperatures were selected to accelerate the degradation of the coating by interdiffusion with the substrate. Another goal is to develop a lifetime model based on the results. The critical Al content of the coating at which failure occurs is a key parameter needed to complete the model, and a coating failure after ∼10 kh at 700 °C provides some information.