鍍鋅鋼板矽氧烷鈍化處理研究與耐蝕機制探討

Abstract

熱浸鍍鋅鋼板為廣泛使用的鋼鐵材料之一，與未經熱浸鍍的鋼板相比，擁有優異的抗腐蝕能力，故大量運用在車輛、船舶、建築、通訊等領域。鍍鋅層具有障蔽保護以及犧牲陽極保護性能以抑制紅鏽發生。然而鋅在腐蝕環境中容易氧化，形成氧化鋅等白色鏽蝕物，影響鋼板美觀性及鋅層抗蝕能力。因此，藉由表面鈍化處理來延緩鍍鋅鋼板的腐蝕速率是相當重要的。本論文利用有機-無機矽烷輥塗鈍化處理來提升鍍鋅鋼板抗蝕性，分為： (1)探討不同矽烷前驅物、pH值、水解時間與腐蝕抑制劑對鈍化液穩定性及提升塗膜抗蝕能力影響；(2)探討矽烷化合物及添加腐蝕抑制劑後鈍化膜的耐蝕機制。 實驗結果顯示鈍化後的鍍鋅鋼板抗蝕性與矽烷前驅物種類、酸種類、pH值、加熱條件以及添加物息息相關。研究主要分為三個部分，第一部分為建立GA系統，以GPTMS(3-縮水甘油醚氧基丙基三甲氧基矽烷)與AEAPS(N-胺乙基-3-胺丙基甲基二甲氧基矽烷)配製方式探討對熱浸鍍鋅鋼板抗蝕性影響，並以此為基石研究含矽塗膜特性。此外，將GA系統與其他矽烷前驅物系統如GT(GPTMS與TEOS四乙基矽氧烷)、GO(GPTMS與OTES辛基三乙氧基矽烷)及GM(GPTMS與MTES甲基三乙氧基矽烷)等比較。從實驗結果發現，GA系統具有較佳之塗膜耐蝕性與溶液安定性，除了試片經48小時鹽霧測試腐蝕面積小於5%，溶液經過120天之黏度變化仍未超過10 cp。而透過添加苯甲酸與加熱等方式可再提升GA系統鈍化膜的抗蝕能力。然而，經熱鹼洗後再經24小時鹽霧測試試片全面鏽蝕，表示矽烷鈍化膜無法抵抗鹼基攻擊。第二部分為研究GA系統的溶液安定性、微結構與耐蝕機制。實驗結果發現GPTMS與AEPAS的長鏈可以使溶液具有較佳的安定性，且矽烷帶有的環氧基與胺基經開環反應增加塗膜交聯程度。同時，多元磷酸除了作為酸催化劑水解用途外，還兼具作為腐蝕抑制劑的功能，提高矽烷鈍化膜抗蝕性。此外，利用SEM觀察塗膜發現表面未產生裂縫，膜厚約為0.1~1.2 μm，表示塗膜薄且緻密。第三部分則是透過添加金屬離子以探討塗膜耐蝕性、耐鹼影響。從鹽霧試驗結果發現添加鈦、鋯、釩離子後塗膜耐蝕性大幅提升，經168小時試驗後腐蝕面積仍<5%，但抗鹼洗性未獲改善。此外，添加鈦、鋯離子會使溶液安定性下降，配製後1天隨即膠化無法使用，而添加釩離子則對溶液安定性影響較小。

Hot-dip galvanized (HDG) steel sheet is one of the widely used steel products. Compared to bare steel sheet, HDG steel sheet has superior corrosion resistance and is extensively used for automobile, ship, building, and communication equipment industries. Although Zn coatings provide both barrier and sacrificial protections to the underlying steel substrate, they tend to suffer corrosion in the atmosphere. The resulting white ZnO corrosion products adversely affect the cosmetic appearance and corrosion protection of Zn coatings. Passivation treatment is thus indispensable for HDG steel sheet. In this thesis, an organic-inorganic silane coating was developed to enhance the corrosion resistance of HDG steel sheet, including (1) the effect of silane precursors, pH, corrosion inhibitors, and hydrolysis time on the stability of the sol-gel solution and the corrosion resistance of the resulting coating; (2) the investigation of the corrosion mechanism of the silane coating with the addition of corrosion inhibitors. It was found that the corrosion resistance of silane coatings strongly depended on the types of silane precursors and acidic additives, pH, and pre-heating treatments. The first part of this thesis is to study the effect of the 3-glycidyloxypropyltrimethoxysilane (GPTMS) / 3-2-aminoethylaminopropyldimethoxymethylsilane (AEAPS) (GA) silane system and its preparation on the corrosion resistance of the silane coating on HDG steel sheet. The comparison of the GA system to GPTMS / tetraethoxysilane (TEOS), GPTMS / triethoxyoctylsilane (OTES), and GPTMS / methyltriethoxysilane (MTES) systems was made. Among these distinct systems, the GA system was the most stable and the resulting silane coating exhibited the best corrosion resistance. The GA sol underwent a viscosity change ≤ 10 cp after 120 days of aging at room temperature. Meanwhile, the GA-coated HDG steel sheet suffered less than 5 % corrosion after 48 h of the salt spray test (SST). The silane coating formed with the addition of benzoic acid to GA system was also markedly enhanced. But, after alkaline pretreatment, this coating suffered 100 % corrosion after 24 h of SST, indicating the coating lacks of resistance to alkaline attack. The second part of this thesis is to further elevate the stability of the GA sol and detail the microstructure and anticorrosion mechanism of the silane coating. The long side chains of GPTMS and AEAPS were found to confer sol stability. This is because the opening reaction between an epoxy ring and an amine contributes to the condensation of the gel coating. Moreover, polyphosphoric acid acts a role not only for acidic catalysis toward precursors hydrolysis but also as corrosion inhibitor to improve the corrosion resistance of silane coating. The silane coating was free of cracks and 0.1 ~ 1.2 μm in thickness, indicating it is compact and uniformly covers on top of the HDG steel sheet. The third part of this thesis is to explore the effect of the addition of inorganic salts such as titanate, zirconate, and vanadate. These inorganic salts were shown to enhance to corrosion resistance of the silane coating to 168 h of SST, but had insignificant improvement against alkaline attack. Finally, the addition of vanadate hardly influenced the sol stability; in contrast, the sol in the presence of titanate and zirconate completely jellified after 24 h.