DEGRADATION OF SIMPLE AND Si-MODIFIED ALUMINIDE COATINGS IN TYPE II HOT CORROSION CONDITIONS

摘要

Si-modified aluminide coatings are widely applied to metal alloys employed in the hottest sectionsof aeronautical turbines to fight against hot corrosion. The Al/Si slurries to fabricate the coatingsoften contain chromates, which are carcinogenic. Therefore, different studies on Cr(VI)-free waterbasedslurries to deposit Al/Si coatings have been carried out in different research groups [1-3].Here, we focus on the mechanisms of formation of the Si-modified aluminide coatings on purenickel and report thereafter on the hot corrosion mechanisms at 700℃ for up to 300 h. Theinfluence of the testing method (salt embedment vs. salt spray) with Na_2SO_4 and of theatmosphere (air vs. 0.5% SO_2/SO_3) is discussed. The beneficial effect of Si will be highlighted bycomparing with a simple aluminide coating. It is shown that the oxide scale formed on the Al/Si isso thin in all conditions that it cannot be detected - neither by SEM/EDS and Raman spectroscopy,let alone by XRD. Barely any degradation is found on the entire surface that appears heat tintedwith no pit. Minor change of the coating thickness is solely related to interdiffusion. In contrast, thesimple aluminide coating develops some pits associated to an insufficient ability to form protectivealumina scales at 700℃ in such conditions. As a result, significant S ingress occurs within thecoating although the thickness remains constant in the un-attacked areas. Such observations willbe discussed to describe the underlying type Ⅱ hot corrosion mechanisms.