鎂離子對於熱浸純鋅鋼板磷酸鹽化成皮膜的影響

熱浸鍍鋅鋼板具有優異的機械性質，廣泛用於各種工程及日常用品上。然而，熱浸純鋅鋼板在運送及存放過程中易腐蝕。故六價鉻鈍化處理在過去數十年常用來提升熱浸純鋅鋼板的抗蝕性，但六價鉻具有致癌性，同時嚴重污染生態環境，已嚴格限用。因此開發取代無鉻鈍化處理之製程更形迫切。本研究嘗試在磷酸鹽鈍化液中添加鎂離子，對熱浸純鋅板進行化成處理，以調整化成皮膜表面之結晶形態，藉以提昇化成皮膜的抗蝕性。試片經化成處理後，以掃瞄式電子顯微鏡、橫截面穿透式電子顯微鏡以及低掠角X-ray繞射分析皮膜的表面型態和微結構。以極化曲線與鹽霧試驗量測皮膜的抗蝕性質，以 450℃耐高溫試驗檢測皮膜的耐熱性。最後以百格試驗量測皮膜的附著性。結果顯示，熱浸純鋅鋼板磷酸鹽化成處理液中含有適量之鎂離子及鋅離子時，鎂離子可取代原本磷酸鋅(hopeite)化成皮膜中鋅離子之結晶位置，形成結構相似之磷酸鹽(hopeite(Mg-exchanged,syn. )皮膜，並增加化成液中陽離子與磷酸根之碰撞機會，在短時間化成處理製程(90秒)即可形成較多磷酸鹽皮膜沉積於試片表面，提升化成皮膜之覆蓋率和抗蝕性；隨著化成處理時間拉長，鎂離子對於皮膜覆蓋率和抗蝕性的影響不明顯。經由SEM表面形貌觀察，含鎂離子之化成液可形成晶粒尺寸較小且較為整齊之皮膜結構。有關化成液中鋅離子的效應發現，當化成液中鋅離子與鎂離子濃度達一定比例時，才有利hopeite(Mg-exchanged)之晶粒生成。而硝酸根為加速鋅底材溶解的氧化劑，對於磷酸鹽化成皮膜的結晶形態，並無顯著之影響。

關鍵字

熱浸鍍鋅鋼板；無鉻化成處理；磷酸鹽化成處理；鎂離子；磷酸鋅化成皮膜

並列摘要

In light of their excellent mechanical properties, hot-dip galvanized (GI) sheet steels have been widely used in various engineering applications and daily supplies. However, the pure zinc coating tends to suffer corrosion during transportation and storage. As a result, hexavalent chromium conversion coating has been employed for decades to improve the corrosion resistance of GI steel. However, hexavalent chromium is a highly toxic and carcinogenic substance. Its use has been limited by several international regulations. Therefore, the development of chromium-free conversion processes for GI steel is an urgent necessity. This study aims to study the effect of Mg2+ in phosphate conversion solution on the nucleation and growth of the phosphate conversion coating on GI steel. The surface morphology and microstructure of the conversion coating was characterized using scanning electron microscopy, cross-sectional transmission electron microscopy and glancing angle x-ray diffraction. The corrosion resistance of the coating was evaluated using electrochemical polarization measurement and salt spray test. The heat resistance of the coating was studied by heat treatment at 450℃ for 5 min. Finally, the adhesion of the coating was measured using a crosscut tape method The results show that in the phosphate conversion solution containing proper amounts of magnesium ions and zinc ions to, Mg2+ can replace the ionic sites of Zn2+ in hopeite (Zn2(PO4)3•4H2O), giving rise to a similar crystalline phase, i.e. hoepeite (Mg-exchanged) ,(Mg0.62Zn2.38(PO4)2). Moreover, Mg2+ promoted the collisions between cations and PO43-, effectively enhancing the nucleation rate of Zn phosphate crystals on GI steel. As the result, adding proper Mg2+ to phosphate conversion solution increased the coverage of phosphate crystals and improved the corrosion resistance of GI steel after a short time conversion coating treatment. The influence of Mg2+ became insignificant after prolonged conversion treatment. Nevertheless, Mg2+ in the conversion solution refined the size of phosphate crystals and led to the formation of an uniform conversion coating. As for the effect of Zn2+ in the solution, hopeite(Mg-changed) formed in the solution with Zn2+/ Mg2+ ratio exceeding a specific value. Finally, NO3- in the solution hardly affected the morphology and size of phosphate crystals although the presence of NO3- facilitated the dissolution of the GI coating.

並列關鍵字

hot-dip galvanized steel ； chromium-free conversion coating treatment ； phosphate conversion coating treatment ； Mg2+ ； Zn phosphate conversion coating

參考文獻

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鎂離子對於熱浸純鋅鋼板磷酸鹽化成皮膜的影響

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