The characteristics and adhesive strength of a high Cr Fe alloy plasma spray coating applied to the inner surface of cylindrical aluminum containers of 17 NAS battery cell used to leveling electrical power for 0.2 to 10 years were studied. The operating temperature of the batteries was 593 K±15 K, and the number of charge/discharge cycles per year was 300 on average. A Sulfide layers consisting mainly of Cr 2 S 3 and Cr 3 S 4 was formed on the surface of the high Cr Fe alloy plasma spray coating, and the growth rate of sulfide layer was approximately 2 mm per year. Chromium sulfides also formed within the spray coating. Molten sulfur and Na 2 S x which are positive electrode active materials, penetrated the coating through the interfaces of the sprayed particles. The deepest sulfides almost reached the wall surface of the aluminum container. The adhesive strength of high Cr Fe alloy plasma spray coatings operating history of 3, 5, and 10 years, was in the range of 42 44 MPa, showing that the adhesive strength from the time of coating was maintained. After tensile testing, peeling only occurred at the middle region of the sprayed layer, indicating that the reason for the peeling of the coating was formation of chromium sulfide formed at the interfaces of the sprayed particles.
Fig. 1 Construction of sodium sulfur cell. In order to improve the adhesion of 75 mass Cr Fe alloy plasma spray coatings, which provide sulfide corrosion resistance for the inner surface of NAS battery high temperature type Al cylinders, the Al substrate temperature and the flattening behavior of the plasma spray particles were studied. Also, experiments were conducted to study how changes in the preheating temperature of the Al cylinder affected the spray coating's adhesion strength. The splat morphology of the spray particles changed significantly in relation to the preheating temperature of the Al substrate, from splash shaped to disk shaped as the preheating temperature increased. It was especially evident that the splat morphology of the particles changed sharply at over 433K, forming disk shaped splats with excellent adhesion to the substrate. A good correlation was found between the flatness of the spray particles and the adhesion strength of the coating formed by changing the preheating temperature of the Al cylinder. When the preheating temperature was set at over 433K, coating's with good adhesion strength were formed. It was therefore shown that by studying the flattening behavior of the plasma spray particles, an effective method could be found for improving the adhesion strength of the plasma spray coating. (Received April 24, 2007; Accepted June 13, 2007) Keywords: sodium sulfur battery, aluminum cylindrical container, preheating temperature, high chromium iron alloy coating, spray particle, particle flattening behavior, coating adhesion strength
In order to improve the adhesion of 75 mass Cr Fe alloy plasma spray coatings, which provide sulfide corrosion resistance for the inner surface of NAS battery high temperature type Al cylinders, the Al substrate temperature and the flattering behavior of the plasma spray particles were studied. Also, experiments were conducted to study how changes in the preheating temperature of the Al cylinder affected the spray coating's adhesion. The splat morphology of the spray particles changed significantly in relation to the preheating temperature of the Al substrate, from splash shaped to disk shaped as the preheating temperature increased. It was especially evident that the splat morphology of the particles changed sharply at over 433 K, forming disk shaped with excellent adhesion splats to the substrate. In addition, as the preheating temperature was increased, fewer coating defects such as blisters and peeling were seen on the spray coating. A good correlation was found between the flatness of the spray particles and the adhesion of the coating formed by changing the preheating temperature of the Al cylinder. When the preheating temperature was set at over 433 K, coating with good adhesion were formed. It was therefore shown that by studying the flattening behavior of the plasma spray particles, could be found an effective method for improving the adhesion of plasma spray coating.
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