The Influence of Corrosion Conditions on Corrosion Characteristics of Cobalt-based Alloy

• Main Authors: Chien-hsun Wang, 王建勛
• Other Authors: J.M. Chou
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/57547305179344844804

碩士 === 義守大學 === 材料科學與工程學系碩士班 === 97 === The cobalt-based alloy of cobalt-based bimetallic cylinder was chosen to investigate its corrosion behavior by immersing in three acid solutions (6% FeCl3, 20% HCl, 60% H2SO4) with different immersion time (5 minutes, 30 minutes, 3 hours, 0.5 days, 1 day, 2 days, 3 days). In addition, the electrochemical experiments, such as open circuit potential and dynamic polarization tests, were also employed to understand the changes of the corrosion current and voltage in the same acid solutions. After the immersion test and electrochemical experiments, optical microscopy (OM) was employed to observe the morphology of corroded surface. The phase was identified by X-ray diffractometer (XRD). scanning electron microscope (SEM) was used to examine the microstructure., and inductively coupled plasma atomic emission spectroscopy (ICP-OES) were conducted to determine the metal ion species. The results show that the cobalt-based alloy consists of Co, W3CoB3, σ-CrCo, Co0.72Fe0.28 and σ-FeCr can be distinguished to Co-rich, Cr-rich and W-rich phases. According to the results of immersion test, the corrosion rate cobalt-based alloy decreases with increasing immersion time. Both the weight loss and the corrosion depth increases while immersion time increases. The relative corrosion resistance of cobalt-based alloy to acid solution is 6% FeCl3>20% HCl>60% H2SO4. The results of dynamic polarization test shows that no significant passivation occurs in the solutions of 6% FeCl3 and 20% HCl when the current increased, but slight passivation occurred in the solution of 60% H2SO4 (-119mV to-55mV). Comparing the results of open circuit potential tests with dynamic polarization tests, the cobalt-based alloy immersed in the solution of 6% FeCl3 has been attacked at 5 minutes, while the Co-based alloy starts to be attacked at 0.5 day at last in the solutions of 20% HCl and 60% H2SO4. All the results of the Co-based alloy in the three solutions exist the silicon oxide, iron chromium oxide and cobalt iron oxide. Morphological examination observation revealed that the Co-rich phase has worse corrosion resistance than the other two phases.