| Summary: | 碩士 === 元智大學 === 化學工程學系 === 93 === Recently, chromating conversion treatment on magnesium alloys has provided good corrosion protection worldwide. However, the liquid wastes generated from surface coating may cause serious environmental pollution on the earth and the contamination of Cr species are strictly prohibited in Taiwan. Chromate-free conversion technique on magnesium alloys is prevailing and may be an appealing environmental friendship alternative. Therefore, the main objective of this study was to investigate a phosphating conversion treatment on magnesium alloys by chromate-free solution routes. Due to the phosphating treatment on AZ91D alloy surfaces required a longer reaction time of 10 min, adding proper surfactant for accelerating agent may decease reaction time approximately of 3 min that can reach commercial thickness of coated films determined by metallographic optical macroscope (MOM).
Experimentally, 10~30 mL/L surfactant of triethanolamine (TEA) having a low molecular weight was added to the treating solution in order to decrease the required treating time. By MOM tenique, a thickness of 1.4 μm for treated AZ91D alloy was also obtained by non-accelerator bath within 10 min. Thickness of 4.1 and 18.5 μm were separately formed on AZ91D alloys in the treating solution bath containing 30 mL/L accelerator within 4 and 20 min, respectively. In other words, by adding the treating solution of 20 mL/L optimially of 0.8 μm/min treating rate calculated bascally with non-accelerator solution within 2 min, that it is enough to reach the required thickness of 1.4 μm.
Generally, the thickness of 1 mm or less formed by AZ91D alloy on notebook casing. Due to the reqirement of thinner thickness of AZ91D alloy, the grinding and polishing procedures of AZ91D alloy were needed before surface modification. The innovatory fixed tools used for AZ91D alloy polishing were therefore designed successfully. This fixed tool with 100-600 rpm (rate per minute) could meet the commercial standard of grinding and polishing procedures of surface-modified AZ91D alloy.
Understanding of the equations used for the formation of chromate-free conversion film, may provide the reasons for investigating the phosphating procedure in the surface treatment of AZ91D alloys. Investigating the properties of using a pH value of 3-4 in phosphating bath and formation of the unstable intermediates of Mn(H2PO4)2 or MnHPO4, may enhance the clarification of the original additives of different commercial formula used for the surface treatment of AZ91D alloys.
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