熱浸鍍鋅鋼板之輥塗型磷酸鹽／鉬酸鹽皮膜微結構與自我癒合能力之研究

Microstructure and Self-healing Ability of Phosphate/Molybdate Passive Films on Hot-dip Galvanized Steels Prepared by a Roll Coating Process

本研究利用輥塗製程於熱浸鍍鋅鋼板（GI steel）上備製磷酸鹽／鉬酸鹽皮膜，並評估皮膜自我癒合的能力。鹽霧試驗結果顯示，相較於單道次磷酸鹽或鉬酸鹽皮膜，兩道次磷酸鹽／鉬酸鹽之刮痕區白銹覆蓋率較低。刮痕區之WDS分析結果顯示，刮痕區經過鹽霧試驗之後，鉬、磷的訊號強度小幅上升，表示皮膜於腐蝕環境中可釋放出鉬、磷物種並遷移至刮痕區鈍化裸露之鋅底材。交流阻抗分析使用一含有5%氯化鈉水溶液、鈍化後GI鋼板及未鈍化GI鋼板之模擬刮痕槽進行分析。分析結果顯示，隨著浸泡時間的增加，未鈍化GI鋼板阻抗亦隨之增加，顯示槽體中未鈍化GI鋼板受到鈍化後GI鋼板所釋放出之鉬、磷物種保護，故進而提升GI鋼板之抗蝕性。而當鈍化後GI鋼板為兩道次磷酸鹽／鉬酸鹽皮膜時，未鈍化GI鋼板具有最佳之總阻抗，與鹽霧試驗之結果一致，兩道次磷酸鹽／鉬酸鹽皮膜具有最佳之自我癒合能力。

Phosphate/molybdate passive films were prepared by a roll coating process on hot-dip galvanized (GI) steels, and the self-healing ability of the film was studied. The results from salt spray tests showed that the fraction of white rust area within a scratch on the phosphate/molybdate film prepared by a two-step roll coating process was less than that on the phosphate or molybdate passive film prepared by a one-step roll coating process. According to WDS (Wavelength-dispersive spectroscopy) analyses, the intensities of Mo and P within the scratch were enhanced after the salt spray test as compared with those before the test, indicating that Mo and P species were released from the passive film, migrated to the scratch, and passivated the bare zinc surface. AC impedance measurements were conducted in a 5 wt% NaCl solution with a simulated scratch cell consisting of one as-passivated and one as-received GI samples. Electrochemical impedance spectroscopy results demonstrated that the total impedance of the as-received GI sample increased with prolonged immersion times, which suggested this sample was protected from corrosion in the cell through the release of Mo and P species from the passive film. When the coating of the as-passivated GI sample in the scratch cell was the phosphate/molybdate passive film, the as-received GI sample displayed the greatest impedance. This was consistent with the salt spray test results that the phosphate/molybdate passive film had the best self-healing ability.