Thermal spray coatings for molten carbonate fuel cells separator plates

Molten salt corrosion at the wet seal of separator plates is one of the pri ncipal life-limiting factors of molten carbonate fuel cells (MCFC. The wet seal must therefore be coated with an aluminide layer that is commonly prod uced by ion vapor deposition (IVD) of Al followed by heat treatment. Howeve r, this coating only lasts approximately 20 000 h and not the 40 000 h expe cted for a cell life. Moreover. the IVD Al coating is also very expensive s ince only a few plates can be coated simultaneously due to size limitations of the existing commercial vacuum chambers employed in IVD. The need of he at treatment further increases costs, particularly since it requires long h eating and cooling cycles in order to minimize distortion of the thin stain less steel plates. Thermally sprayed coatings constitute an alternative tha t requires neither containment nor heat treatment. and also provides the po ssibility of depositing materials more resistant to molten carbonates than plain aluminides. However, separator plates coated by thermal spray suffer distortion. due both to sand blasting (usually required prior to coating), and to the heat transfer process that occurs during the spraying process. I n this work, commercially available coatings have been applied by plasma sp ray and high velocity oxyfuel (HVOF), employing alternative surface prepara tion methods. Moreover, substrate pre-heating and/or cooling during deposit ion were examined in order to eliminate substrate distortion. FeCrAl. and N iAl as well as a quasi-crystalline approximant alloy AlCoFeCr were deposite d on AISI 310 foils, and after optimization the resulting coatings were cha racterized by means of scanning electron microscopy-energy dispersive spect roscopy (SEM-EDS). The optimized coatings were then tested by immersion in a 62 mol.% Li2CO3/38 mol.% K2CO3 molten carbonate eutectic mixture at 700 d egreesC and by electrochemical impedance spectroscopy. lVD Al coatings were also tested for comparison purposes. The results indicate that FeCrAl exhi bits a higher molten salt corrosion resistance than IVD aluminide coatings whereas NiAl was attacked shortly after the beginning of the test. The QC a pproximant AlCoFeCr resisted 1000 h of attack but its composition changed. Grinding of the substrate prior to coating resulted in good adhesion and su bstrate distortion was minimized by Ar cooling during deposition. (C) 2001 Elsevier Science B.V. All rights reserved.