本研究將低碳鋼於 450 ℃ 進行第一階段熱浸鍍鋅 10 分鐘，再以 Zn-5Al 與 Zn-5Al-2Mg 之兩種鋅合金鍍浴進行第二階段鋅合金熱浸鍍 2.5 分鐘。透過各合金層之交流阻抗分析與等效電路擬合，進一步搭配合金層之元素分布，輔助探討鎂添加對於鍍層電性表現與耐蝕性關聯。
實驗結果顯示，一階段熱浸鍍鋅鋅鍍層為均勻之鐵鋅合金相組成，隨著鐵鋅合金層中鋅含量的提升，使鍍層活性增加，導致電荷轉移阻抗 (Rct) 降低；二階段熱浸鍍 Zn-5Al 與 Zn-5Al-2Mg 鍍層具有相似的微觀結構，由外向底材可區分為鋅鋁(鎂)層、枝狀鐵鋁層、網狀鐵鋁層、內鋅鋁(鎂)層。其中越靠近底材之合金層，陰極元素之佔比面積越大(鐵鋁相)，導致陽極 (富鋅相、鎂鋅相) 之陰極保護效果不佳，容易發生電荷轉移，出現電荷轉移阻抗 (Rct) 數值下降之趨勢。但經EIS 量測與等效電路擬合顯示，添加鎂元素仍有效提升二階段熱浸鍍鋅鋁各合金層之電荷轉移阻抗 (Rct)。透過鎂與鋅之腐蝕電位差異，鎂的優先腐蝕可以延緩鋅腐蝕電流釋放，延長陰極保護之時間，使耐蝕性上升。

In this study, a two-stage hot-dip galvanizing process was conducted on low-carbon steel at 450 ℃. In the first stage, the steel was immersed in a zinc bath for 10 minutes. Subsequently, the second stage involved hot-dip coating with two types of zinc alloy baths, Zn-5Al and Zn-5Al-2Mg, for 2.5 minutes. The electrical performance of the coatings and their corrosion resistance were investigated by analyzing the interfacial impedance of each alloy layer and fitting an equivalent circuit model. The distribution of elements in the alloy layers was also examined to assist in exploring the relationship between the addition of magnesium and the electrical properties and corrosion resistance of the coatings.
The experimental results demonstrate that the zinc coating in the first-stage hot-dip galvanizing process exhibits a uniform iron-zinc alloy phase composition. As the zinc content in the iron-zinc alloy layer increases, the reactivity of the coating increases, leading to a decrease in the charge transfer impedance (Rct). In the second-stage hot-dip coating with Zn-5Al and Zn-5Al-2Mg, similar microstructures are observed. From the outer layer to the substrate, distinct layers can be identified, including a zinc-aluminum (magnesium) layer, a dendritic iron-aluminum layer, a mesh-like iron-aluminum layer, and an inner zinc-aluminum (magnesium) layer. The alloy layers closer to the substrate exhibit a larger proportion of the cathodic element (iron-aluminum phase), which results in poor cathodic protection effectiveness for the anodic phases (zinc-rich and magnesium-zinc phases) and a tendency of decreased charge transfer impedance (Rct).

However, measurements obtained through electrochemical impedance spectroscopy (EIS) and fitting with an equivalent circuit model reveal that the addition of magnesium effectively enhances the charge transfer impedance (Rct) of the alloy layers in the second-stage hot-dip zinc-aluminum process. Due to the difference in corrosion potential between magnesium and zinc, the preferential corrosion of magnesium can slow down the release of corrosion currents from zinc, prolonging the duration of cathodic protection and thus improving the corrosion resistance.

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