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URN etd-0802104-094837 Statistics This thesis had been viewed 2814 times. Download 1612 times. Author Wei-Ting Yang Author's Email Address email@example.com Department Materials Engineering Year 2003 Semester 2 Degree Master Type of Document Master's Thesis Language English Page Count 86 Title The Corrosion Improvement of Magnesium Alloy by Pulsed DC Plasma Enhanced Chemical Vapor Deposition of Hexamethyldisilazane Coatings Keyword AZ61 Mg alloy corrosion property Pulsed DC PECVD system Pulsed DC PECVD system corrosion property AZ61 Mg alloy Abstract Mg alloys have been extensively used commercially in the mechanical structure of electronic industry, aerospace and vehicle industry. Nevertheless, poor corrosion resistance of magnesium alloys has posed a bottleneck for further utilization of Mg alloys in industry. Although there are many methods which have been successfully employed to improve the corrosion resistance, the improvement is limited and deserved a further investigation to enhance the corrosion resistance of Mg alloy for value-added applications. The pulsed DC PECVD, a economic thin film process, was carried out to deposit the passive thin film of plasma-polymerized PHMDSZ (hexamethyldislazane) to improve the corrosion resistance of AZ61 Mg alloy. The corrosion resistance was accessed with potentiodynamic tests, electrochemical impedance spectra (ECS) and salt spray test in this study. The surface morphologies were observed with SEM and AFM. The chemical structure and hydrophobility of the film was investigated with FTIR and measurement of water contact angle, respectively. The results indicate that the improvement can mitigate the corrosion of AZ61 Mg alloy by 4 orders and the reproductivity is very good in contrast to the process of DC PECVD. Since less sporadic arcs, which will damage the deposited film with crater-shaped defects whereon, occur for the process of the pulsed DC PECVD. From the total resistance of ECS analysis, the films still exhibit protection against corrosion even after 384hrs immersion in 3.5wt% Na2SO4 solution. Since the plasma-polymerized film is pinhole-free, the three Randle circuits in series were used to simulate the electrochemical cells. During the immersion, the resistance of the coating decreases gradually and the capacitance of the coating decreases monotonically. The results indicate that the deterioration of the deposited after long-term immersion can be attributed to the water intake since the high dielectric constant of water was incorporated into the film to enhance the dielectric constant. The water intake will facilitate the ion movement in the film and make the insulating performance deteriorates. Advisor Committee Mu-Rong Yang - advisor
Chao-Sung Lin - co-chair
Ko-Shao Chen - co-chair
Files Date of Defense 2004-05-03 Date of Submission 2004-08-02