A study of electropolishing behavior of commercial pure nickel in phosphoric-sulfuric mixed acids

• Main Authors: Tsung Heng Yang, 楊宗衡
• Other Authors: C. A. Huang
• Format: Others
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/44577398328054744720

碩士 === 長庚大學 === 機械工程學系 === 100 === The commercial pure nickel alloy (Nickel 200) has been widely used in chemical industries owing to its excellent corrosion resistance and anti-oxidation properties in high temperature environment. Generally, the surface of Nickel 200 is often polished in industrial applications. Except mechanical polishing, electropolishing is commonly used because of convenient operation and suitable for fine parts. In this study, the anodic polarization and electropolishing behavior of Nickel 200 in phosphoric-sulfuric mixed solutions with different volume ratios was studied by using rotating disc electrode. After electropolishing, the surface morphologies of Nickel 200 were examined with optical, scanning electron and atomic force microscopes. The results of polished surfaces were discussed with electropolishing parameters, such as rotation speed and solution composition. The results of anodic polarization test showed that an obvious transition from active to transpassive dissolution was found with an increasing the phosphoric acid content in the mixed acids. The limiting current density increased with an increasing the rotation speed. On the other hand, the potential range of limiting current plateau reduced apparently when the rotation speed and the phosphoric acid content. According to the results of electropolishing, three specific surface morphologies can be found namely. Furthermore, the possible formation mechanisms of these three specific surface morphologies were proposed based on experimental results. According to the AFM quantitative analyzing, the Nickel 200 could be well electropolishied at rotating speed of 200 rpm in the phosphoric-sulfuric mixed acid in a volume ratio of 2:1. Although adding water in phosphoric-sulfuric mixed acids could increase the limiting current density significantly, an excess addition of water narrowed the potential range of limiting current plateau and suppressed the brightening effect.