HOT CORROSION OF NICKEL-CHROMIUM AND NICKEL-CHROMIUM-ALUMINUM THERMAL-SPRAY COATINGS BY SODIUM SULFATE-SODIUM METAVANADATE SALT

The hot corrosion behavior of low-pressure plasma-sprayed (LPPS) 80 wt % Ni-20 wt% Cr and flame-sprayed. (FS) 75 wt% Ni-20 wt% Cr-4.0 wt% Al coatings on type 304 (UNS S30400) stainless steel (SS) by thin fused f ilms of sodium sulfate (Na2SO4) and 0.7 mol fraction Na2SO4-0.3 mol fr action sodium metavanadate (NaVO3) at 900 degrees C in a 1% sulfur dio xide (SO2)-oxygen (O-2) atmosphere was studied using an electrochemica l method. The 80% Ni-20% Cr coatings were tested in the as-sprayed con dition. 75% Ni-20% Cr-4% Al coatings were tested under as-sprayed and laser-glazed conditions. Evolution of the fused salt film chemistry, i n terms of the thermodynamic activities of O-2 and sodium oxide (Na2O) , was monitored using a ztrconia O-2 probe and a fused-silica Na senso r. Results permitted qualitative evaluation of the corrosion resistanc e of the coatings and could be interpreted in terms of previously prop osed hot corrosion mechanisms.