| Summary: | 博士 === 國立臺灣大學 === 機械工程學研究所 === 98 === Hexavalent chromium has been used extensively in chrome plating and conversion coating treatment. However, hexavalent chromium is a highly toxic substance that threatens human health as well as the natural environment. Thus, the development of surface treatments free of hexavalent chromium is required. This study includes the replacements of hexavalent chromium surface treatments in electrodeposition system and passivation system on zinc-electrogalvanized steel plate. In which, the prospective replacements of hard, wear resistant electrodeposition focused on trivalent chromium electrodeposititon, nickel tungsten, nickel phosphorus, and cobalt phosphorus electrodeposition; the replacement of chrome passivation treatment was concentrated on trivalent chromium passivation and titanate-salt passivation systems.
Results showed that amorphous trivalent chromium electrodeposits had low ductility and contains high density surface cracks. Addition of organic or inorganic sulfur compounds in the bath suppressed carbon codeposition via adsorption of sulfur-containing molecules on the cathode, forming a crystalline trivalent chromium electrodeposit. Nickel tungsten alloys showed a relationship between hardness and microstructure at various tungsten contents. Proper heat treatment changed grain boundary microstructure and improved hardness of nickel tungsten alloy through grain boundary relaxation. Nickel-phosphorus electrodeposits with high phosphorus content and low internal stress have been plated at high current efficiency using pulse current in a nickel sulfamate bath free of stress reducers. Results showed that through controlling the duty cycle of a pulse current, Ni-P deposits with a broad range of internal stresses can be produced. Pure nickel electrodeposits softened at higher heat treatment temperatures due to recrystallization and grain growth behaviors. On the other hand, pure cobalt electrodeposits had better high temperature stability. Heat treatment of nickel phosphorus and cobalt phosphorus alloys resulted in precipitation hardening which optimized coating hardness. It was noticeable that pure cobalt and cobalt phosphorus alloys had structural transformation at higher temperatures. High density dislocation pile-up and annealing twins were found at the cobalt matrix. Both caused a grain refinement effect on the crystalline domains. Thus, cobalt phosphorus alloys are more superior in high temperature mechanical properties than nickel phosphorus alloys.
Hexavalent chromium passivation on electrogalvanized steel had a dense coating microstructure, whereas trivalent chromium passivation had a porous coating microstructure. Trivalent chromium coating consisted of oxides/hydroxides of trivalent chromium and divalent zinc. Added appropriate amount of boric acid aided formation of trivalent chromium coatings under room temperature conditions. Meanwhile, baking treatment increased coating density and much improved polarization resistance. Titanate conversion coatings contained high density porous structure. However, a dense barrier microstructure existed on the zinc-substrate interface. Naturally dehydrated titanate conversion coating rendered a state of stability and improved corrosion resistance.
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