無氰鍍鋅三價鉻化成膜老化之研究

Influence of an Aged Cr(III) Bath on the Corrosion Resistance of Cr(III) Conversion Coatings

本研究探討三價鉻化成液老化對化成膜耐蝕性的影響。首先模擬鍍液消耗之情形，分別對三價鉻化成液進行稀釋，濃度分別稀釋為2/3和1/2。此外，增加鋅離子濃度以觀察鋅離子對化成膜耐蝕性能的影響。極化曲線結果顯示，以三價鉻化成液濃度降低後化成膜之腐蝕電流會增加且造成結構變化；於鋅離子效應的研究中發現，當化成液中鋅離子的含量低於2000 ppm時其腐蝕電流變小，但是濃度增加至5000 ppm時，化成膜之耐蝕性能開始往下降。奈奎斯圖的結果顯示，化成膜僅在三價鉻化成液濃度未稀釋前呈現收斂現象；於鋅離子的研究中發現，未添加鋅離子的化成膜阻抗值僅1.75 kΩ，添加500 ppm與1000 ppm鋅離子時，化成膜阻抗值提升至10 kΩ與12 kΩ。

This study was aimed to investigate the effect of an aged Cr(III) bath on the corrosion resistance of Cr(III) conversion coatings. In order to simulate the Cr(III) conversion bath under an aged condition, we diluted the Cr(III) conversion bath to 2/3 and 1/2 times of its original concentration. Furthermore, we added zinc ions into the Cr(III) bath to examine their influence on the corrosion resistance of Cr(III) conversion coatings. The results of linear polarization (LP) analyses demonstrated that the conversion coating formed in the original bath showed a better corrosion resistance than those formed in the dilute baths. Moreover, when the concentration of zinc ion in the bath was less than 2000 ppm, the corrosion resistance of the prepared coatings was enhanced. However, the corrosion resistance of the prepared coating was decreased when the content of zinc ions in the bath went up to 5000 ppm. According to the AC impedance analysis, the coating prepared in the original bath was thinner than those prepared in the dilute baths. The impedance of the prepared coating in the absence of zinc ions in the bath was 1.75 kΩ. As the zinc ion content in bath went up to 500 ppm and 1000 ppm, the corrosion resistance of the prepared coatings increased to 10 kΩ and 12 kΩ, respectively.