酸性鍍鋅三價鉻化成膜製程之研究

A study on Cr(III) Conversion Coatings Formed during Acid Zinc Plating

本研究藉由比較酸性鍍鋅及鹼性鍍鋅製程的差異，了解酸性鍍鋅製程對三價鉻化成膜之影響。研究中使用開環電位法觀察成膜時間對電位的影響，場發射電子顯微鏡（FE-SEM）來觀察皮膜表面形貌，電化學交流阻抗法（EIS）分析化成膜的腐蝕阻抗，以鹽霧（SST）和極化曲線分析皮膜耐蝕性能，而化成膜的組成成分由二次離子質譜儀分析（SIMS）得到。開環電位法發現鹼性鍍鋅在進行三價鉻化成反應時，其表面電位變化有20 mV，相較於酸性鍍鋅則為10 mV。而且當成膜時間達100 s時，鹼性鍍鋅的電位變化已趨近於零，反觀酸性鍍鋅的電位變化，仍隨著成膜時間的增加而持續地減小。表面形貌的觀察發現，酸性鍍鋅化成膜表面呈現裂紋的結構，而鹼性鍍鋅化成膜則為一緻密的表層結構。電化學交流阻抗及線性極化測試結果顯示，酸性鍍鋅和鹼性鍍鋅三價鉻化成膜之阻抗值分別為27 kΩ和53 kΩ，而腐蝕電流則分別為3.85×10-5 A/cm2及1.527×10-6 A/cm2。此外，鹽霧測試的結果顯示，鹼性鍍鋅之三價鉻化成膜達鹽霧時間144小時，而酸性則僅達72小時的鹽霧時間。電化學及鹽霧測試結果均顯示鹼性鍍鋅三價鉻化成膜之耐蝕性能優於酸性鍍鋅的系統。

Acid zinc plating has the advantages of high efficiency, good conductivity, and low energy consumption. Therefore, this technique may find itself in a lot of applications. In this study, we explored the difference between the alkaline zinc plating and the acid zinc plating. Furthermore, the influence of acid zinc plating on the corrosion resistance of Cr(III) conversion coatings was investigated. In addition, the morphologies and the corrosion resistance of the Cr(III) conversion coatings were examined by means of scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), linear polarization (LP) analysis, and salt spray test (SST). Results of open circuit potential (OCP) measurements showed that the variation in OCP during the alkaline zinc plating was 20 mV, and it declined to 10 mV during the acid zinc plating. Furthermore, the OCP during the alkaline zinc plating held steady after 100 s, while the OCP during the acid zinc plating continued to decrease as the plating time increased. SEM images indicate that the coating formed during the acid zinc plating exhibited a surface morphology with a number of cracks, but the coating formed during the alkaline zinc plating showed a compact, even structure without cracks. The results from EIS, LP, and SSTs demonstrated that coatings formed by alkaline zinc plating showed better corrosion resistances than those formed by acid zinc plating.