前處理對於AZ31B錳酸鹽化成皮膜抗蝕性及接觸阻抗之影響

The Influence of Pretreatment on the Corrosion Resistance and Contact Resistance of Permanganate Conversion Coating of AZ31B Magnesium Alloy

林翰(Han Lin)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202102380

摘要

鎂合金具備低密度，與高比剛性比強度、而且在抗震能力、電磁遮蔽效應均有良好的表現，是輕量化的首選金屬之一，因此鎂合金近年來其應用領域逐漸被擴大，其中在電子產品中鎂合金的應用需要較低的接觸電阻來確保電磁屏蔽功能並且又需要具有一定的抗蝕能力，因此本研究旨以使用不同前處理方法，來達到降低鎂合金化成皮膜之電阻以及同時提升其抗蝕能力。
本篇研究使用SEM微結構分析、EDS成分分析、歐傑電子表面分析、接觸阻抗測試以及電化學分析，來探討AZ31B鎂合金在錳酸鹽化成處後之抗蝕能力以及接觸阻抗。並且在後續引入化學拋光和鹼洗前處理，試圖提升鎂合金之抗蝕能力也同時降低其接觸阻抗。SEM表面微結構顯示，初研磨完之鎂合金表面有明顯的刮痕，而在經過化學拋光處理後形貌變得較為平坦，但拋光後的表面會發現到有一些突起的二次相，判斷為二次相與底材間的選擇性溶解。 在後續引入錳酸鹽化成處理的部分，SEM結果顯示僅研磨完之試片經過化成後短時間即可快速成膜，而拋光過後之式樣在短時間化成處理後的成膜較不完整，不過在後續延長化成時間可以得到較平整的化成膜，但在突起的二次相附近發現有較多缺陷的存在，推測二次相會影響附近成膜的完整性，造成其抗蝕能力無法明顯的提升，不過在接觸阻抗的測試中有經拋光處理之試片接觸值有明顯下降，推測為皮膜的平整有助於降低其阻抗值。
為了要更好的提升錳酸鹽膜層的抗蝕性，減少二次相附近的缺陷，因此在後續的研究中使用氫氧化鈉溶液做為鹼洗液，以減少底材與二次相之間的活性差異。由歐傑電子的影像分析可以觀察到，底材經過拋光後於表層二次相周圍氧訊號分布較不均勻，顯示其表層之氧化層厚度較不均勻，推測其為造成成膜不均勻的主因；而在化學拋光後進行鹼洗處理的試樣其結果則顯示表層具有相對均勻的氧化層。而在後續的化成實驗中，可以從SEM下觀察到有經過氫氧化鈉前處裡的式樣，較不容易發現二次相附近有明顯缺陷，推斷鹼洗處理可以讓後續的錳酸鹽化成膜能夠更均勻完整的披覆上底材。電化學結果顯示有經過鹼洗處理的式樣在後續的化成能夠擁有最優秀的抗蝕能力，同時接觸阻抗的結果顯示，拋光鹼洗後再進行化成擁有最低的阻抗值。

關鍵字

AZ31B鎂合金 ； 錳酸鹽化成處理 ； 微結構 ； 電化學交流阻抗 ； 接觸阻抗

並列摘要

The properties of magnesium including low density, high specific rigidity ratio, good performance in shock resistance and electromagnetic shielding effect and so on. It is one of the preferred metals for light weight, so the application field of magnesium alloy has been gradually expanded recently. The application of magnesium alloys in electronic products requires low Interfacial contact resistance (ICR) to ensure electromagnetic shielding function and good corrosion resistance. Therefore, this research aims to use different pretreatment methods to reduce the Interfacial contact resistance (ICR) of magnesium alloy coatings and at the same time Improve its corrosion resistance.
In this study, surface morphology and composition were investigated by scanning electron microscopy SEM, EDX and AES. And the electrochemical performance was tested through EIS, PDP and hydrogen evolution, futher more the contact resistance were also been tested.
We used AZ31B as substrate. The first part of this study, chemical polishing treatment was applied before the permanganate conversion coating treatment. Attempted to improve the corrosion resistance of magnesium alloys and also reduced its contact resistance. The SEM surface microstructure showed that after been chemical polished, the surface of magnesium alloy becames smoother, but we found some protrusions caused by selective dissolution between the precipitates and the substrate. In the subsequent part we applied permanganate treatment to improve corrosion resistance, SEM results showed that in a short time chemical conversion treatment, the polished sample had incomplete film compare with the unpolished sample. However, a smoother film could be obtained by prolonging the conversion coating time, but many defects were found near the precipitates. It was speculated that the precipitates would affect the integrity of the nearby film, causing the corrosion resistance not be significantly improved. In the interfacial contact resistance test, the resistance value of the polished sample had significantly decreased. we speculate that the flatness of the film helps to reduced its resistance value.
To better improve the corrosion resistance of the manganate coating and reduce the defects near the precipitates, sodium hydroxide solution was used as the alkaline wash solution in the following research, hoping to reduce the difference in activity between substrate and the precipitates. We found out that the oxygen signal near the precipitates was uneven after the substrate was polished by using AES analysis, showing that the thickness of the oxide film precipitates was inconsistent. We assumed that leads to subsequent formation defects. Compared with the previous specimen, the specimen with alkaline wash treatment after been chemical polished had a relatively uniform oxide film. In the following chemical conversion experiment, it can been observed from the SEM that the defects near the precipitate were significantly reduced when the alkaline washing process was added. It is inferred that the alkaline washing treatment allow the subsequent permanganate treatment to form a uniform film, which can cover the substrate more completely. The electrochemical results showed that the alkaline washed samples provided better corrosion resistance in the subsequent conversion coating. At the same time, the contact resistance was further reduced.

並列關鍵字

AZ31B ； permanganate conversion coating ； microstructure ； electrochemical impedance spectroscopy ； interfacial contact resistance

參考文獻

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