pH值和温度对镁合金应力腐蚀行为的影响.docx

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Introduction
Magnesium (Mg) alloys are extensively used in various engineering applications due to their unique combination of favorable properties such as high strength-to-weight ratio, good machinability, and excellent damping characteristics. However, their corrosion behavior has been a subject of great concern in the past. When exposed to aggressive environment, Mg alloys undergo various degradation processes including corrosion, which highly affects their mechanical integrity and performance. Stress corrosion cracking (SCC) is one of the most critical degradation phenomena that occur in Mg alloys. Thus, understanding the factors that influence the SCC behavior of Mg alloys is crucial for their successful applications. This paper aims to investigate the effect of pH value and temperature on the SCC behavior of Mg alloys.
Background
SCC is a unique type of corrosion that occurs under sustained tensile stress, in the presence of a corrosive environment. SCC occurs when the applied stress in a material exceeds the yield strength of the material at its operating temperature in a corrosive environment. The resulting cracked component can lead to catastrophic failure. The occurrence of SCC leads to the failure at a lower stress level that the material otherwise would sustain.
Magnesium alloys are subject to SCC in a range of environments including, for example, saltwater, moist air, and acidic media. Many researches have been conducted to investigate the SCC behavior and mechanism of Mg alloys, but the correlation between the environmental conditions and SCC susceptibility is still not fully understood.
Experimental Procedure
In this study, the SCC behavior of Mg alloys was investigated under different pH values and temperatures. The Mg alloys used in this experiment contained 1% Yttrium (Y) and % Zirconium (Zr). The oxide layer on Mg alloy specimens was formed by immersing them in boiling distilled water for 60 minutes to obtain a uniform corrosion product layer. The SCC tests were conducted by subjecting the specimens to 80% of their ultimate tensile strength at different pH values and temperatures, while being immersed in a % NaCl solution. The pH values chosen were 2, 4, 6, and 8. The temperature range tested was 25°C, 40°C, 60°C, and 80°C.
Results
The results obtained from the SCC tests showed that the SCC susceptibility of the Mg alloys varied significantly with pH and temperature. The SCC behavior of the Mg alloys increased with increasing temperature and decreasing pH value. Specifically, the SCC susceptibility was highest at a pH value of 2 and increased from 25°C to 80°C. The results revealed that the Mg alloys are more susceptible to SCC at lower pH values and higher temperatures. Furthermore, SEM analysis revealed that the SCC cracks were mainly transgranular, and the extent of SCC cracks increased with decreasing pH value and increasing temperature.
Discussion
The results obtained in this study suggest that the SCC behavior of Mg alloys is highly influenced by environmental factors such as pH and temperature. The observed dependence on pH suggests that an acidic environment promotes the SCC behavior of Mg alloys. This could be due to the fact that at lower pH values, the corrosion rate of Mg alloys is higher, which leads to a significant accumulation of hydrogen and promotes the SCC behavior. With an increase in temperature, the corrosion kinetics are enhanced, which makes the environment more aggressive and contributes to the more rapid propagation of cracks.
The observed SCC behavior of Mg alloys can also be explained in terms of the electrochemical reaction that takes place between the alloy and the environment. In an acidic environment, the rate of the hydrogen evolution reaction is high, which leads to the accumulation of hydrogen. The accumulation of hydrogen at the crack tip leads to embrittlement and promotes the cracking process.
In conclusion, this study revealed that the SCC susceptibility of Mg alloys is highly dependent on pH value and temperature. The findings suggest that an acidic environment and high temperatures lead to a significant increase in the SCC behavior of Mg alloys. The study results can be useful in selecting suitable operating conditions and designing advanced corrosion control coatings for Mg alloys in various engineering applications. However, more research is needed to determine the precise mechanism of SCC behavior of Mg alloys under different environmental conditions.