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.
The relationship between roughness caused by blasting and adhesion of spray coatings on aluminum container substrates was studied through various experiments as part of measures for improving the adhesion of the 75mass%Cr-Fe alloy plasma spray coating for sulfi dation corrosion resistance, which is applied on the inner surface of cylindrical Al containers of high-temperature type NAS batteries. Surface roughness of mRa2.8 -7.3 was acquired by using Al 2 O 3 particle size #100 (212 -75 m) to #46 (600 -250 m) grit. In order to achieve uniform roughness and a clean surface, a combination of blasting when the nozzle was being inserted from the top of the container, and air blowing when the nozzle was being removed was done. It was determined that when Al 2 O 3 particles of size #100 grit was used, a good anchoring shape was formed throughout with a roughened surface of mRa 2.8.When the internal surface of 3000 Al cylindrical containers were continually blasted using particle size #100 grit, the initial surface roughness of mRa3.7 -3.9 only deteriorated to about mRa2.6. A 75mass%Cr-Fe alloy spray coating was applied to the Al cylindrical containers that were roughened using particle size #100 grit. This coating showed cracks by a bending test, but no peeling occurred. This coating was examined by a tensile strength test and showed good adhesion at 64 -66 MPa.Through experiments, it was proven that spray coatings formed on the Al cylindrical containers after receiving optimal blasting with particle size #100 grit had good adhesion and corrosion resistance after being used for NAS batteries that stored electrical power for about nine years.
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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