Corrosion Properties of Rapidly Solidified Mg-Al-Zn Alloy Thin Plate in Aqueous NaCl and Eco-coating Process for the Mg Alloy to Protect from Corrosion

• Main Authors: Bing-Lung Yu, 余秉隆
• Other Authors: 汪俊延
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/38336067866161161075

博士 === 中興大學 === 材料工程學系所 === 95 === ABSTRACT Metallurgical factors that may affect the corrosion characteristics of die-cast Mg alloys include the microstructure and the chemical composition. Corrosion performance of a specified alloy with a specific composition (such as AZ91D) is determined by its microstructure. Corrosion properties of hot-chamber die casting thin plate (e.g., 1.4 mm thick in present study) was addressed briefly in previous studies. Hence in this study, the corrosion of hot-chamber die cast AZ91D thin plate (1.4 mm in thickness) was investigated in terms of its microstructure, to elucidate the role of die chill skin in corrosion. The die chill skin composed of a thin layer of chill zone and a thick layer of interdendritic Al-rich-α Mg/Al12Mg17 β particles/α-Mg grains composite microstructures. The chill zone had fine columnar and equiaxed grains and contained a distribution of sub-micro Mg-Al-Zn intermetallic particles. Beneath the chill zone, Al12Mg17 β particle was irregularly shaped but did not have an interdendritic network morphology. Furthermore, Al-rich-α phase (also known as eutectic α) was in the interdendritic network, which occupied a higher volume fraction than the β phase in the die skin layer. Corrosion characteristics were studied via constant immersion and electrochemical tests. The sample without the die skin on surface corroded more slowly. The inferior corrosion performance of die skin was considered to be related to particle-like β phase independ¬ently distributing in die skin and the high volume fraction of the interdendritic network of Al-rich-α phase contained in the die skin, owing to the high cooling rate during solidification. The Al-rich-α phase does not increase the corrosion resistance of the AZ91D alloy. The above results showed that the sample without the die skin had superior corrosion resistance. It must spend time on the removing of the die skin layer. Hence, the purpose of the following study is to develop the eco-coating process for the Mg alloy to protect from corrosion. The coatings on magnesium alloy usually act as a corrosion barrier to the environment. However, the coating must be crack free for applications otherwise the substrate material under the crack becomes local anode, leading to severe local corrosion. In the first topic of the eco-coating process for the Mg alloy, magnesium film was deposited on AZ91D specimen, acting as a sacrificial anode. The corrosion properties of the Mg-film coated specimen were estimated by electrochemical polarization experiments and constant immersion tests, both in 3.5% NaCl solution. Resistively heated tungsten coil heating system was used for vaporizing source. The Ecorr values of the coated specimens were -1.66 ~ -1.7 V/Ag/AgCl, which was evidently lower than that of the AZ91D substrate (-1.45 V/Ag/AgCl). According to the electrochemical analyses, the magnesium coating could be used as a distributed sacrificial anode, cathodically protecting the AZ91D substrate. Immersion tests showed that the uncoated specimen was severely corroded while the Mg film-coated specimen was well protected by the sacrificial anode of the magnesium film. Mg alloy is prone to galvanic corrosion because most other metals have a nobler electrochemical potential than Mg alloy. Hence, the second topic of the eco-coating process for the Mg alloy was that the biomimetically synthesized corrosion-resistant coating on Mg alloy. Abalone shell (aragonitic CaCO3) formed in seawater, which naturally has substantial corrosion endurance in chloride solution. Mg-Al-Zn (AZ91D) sample was treated in aqueous Ca2+/HCO3- at 50ºCfor aragonitic CaCO3/Mg,Al-hydrotalcite coating. The CaCO3/Mg,Al-hydrotalcite coating greatly improved Mg alloy’s galvanic corrosion resistance. Electrochemical tests showed that the Icorr values of the coated Mg sample was 7~10 µA/cm2, which was evidently lower than that of the AZ91D substrate. Self-healing of the CaCO3/Mg,Al-hydrotalcite coating deteriorated by scribing occurred. Not only the coating but also the corrosion performance of the coating could be re-healed. A two-stage process of the aragonitic CaCO3 growth on Mg alloy was reported, i.e., lateral growth stage and thickening stage. The former leaded to a continuous CaCO3 thin film covering on Mg sample. That is, Mg2+ ions, which came form Mg alloy surface due to self corrosion in aqueous Ca2+/HCO3-, induced CaCO3 to preferentially grow along sample surface until the sample was totally covered. Subsequently, the later stage caused the thickening of the coating.