鍍鋅鋼板之磷酸鹽鈍化處理及溶膠凝膠法鈍化處理

Phosphate and Sol-Gel Passivation Treatments of Zinc-Coated Sheet Steels

蘇香宇(Hsiang-Yu Su)

指導教授 ： 林招松

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.02919

摘要

披覆在鐵基材料上的鋅層可以具有障蔽保護以及犧牲陽極保護的效果，然而鋅層在潮濕的環境下容易腐蝕，為了提升鍍鋅鋼件的抗蝕性，通常會在鍍鋅處理後進行表面鈍化處理。經六價鉻鈍化處理後的鍍鋅鋼件展現出優異的抗腐蝕性質，但六價鉻物質具有致癌性，同時也會嚴重污染生態環境，為此，近年來六價鉻物質被限制使用或漸漸禁用，而研究非六價鉻鈍化處理的鈍化技術變的勢在必行。
磷酸鹽鈍化處理被廣泛應用於現今工業，磷酸鋅皮膜晶粒(Zn3(PO4)2•4H2O，Hopeite)呈不規則狀，晶粒間沒有辦法完全接合，故晶粒間易有殘存孔隙或是皮膜較薄處，而這些缺陷處常常是腐蝕反應發生的起源，本研究嘗試在磷酸鹽鈍化處理液中加入添加劑Ni(NO3)2或Mn(NO3)2，藉以增加磷酸鹽皮膜成核點，進一步減少磷酸鹽皮膜孔隙率。實驗結果顯示，無論添加何種添加劑，磷酸鹽皮膜晶粒大小均隨著磷酸鹽鈍化處理液中添加劑的濃度增加而減小，各個式樣在動電位極化曲線、電化學交流阻抗頻譜、以及鹽霧試驗下的抗蝕性皆隨著磷酸鹽皮膜晶粒尺寸減小而增加。利用XRD繞射分析以及XPS對添加不同添加劑之磷酸鹽鈍化皮膜進行結構以及成分分析後發現，經由二價鎳離子修飾後的磷酸鹽皮膜主要由磷酸鋅水合物構成，而經由二價錳離子修飾的磷酸鹽皮膜則是由磷酸鋅水合物以及磷酸鋅錳水合物構成。添加二價鎳離子以及二價錳離子均可以細化磷酸鹽皮膜、降低皮膜孔隙率，但由其成分組成可以得知兩者細化之機制不同，提升抗蝕性的機制也有所不同。二價鎳離子主要藉由促進鋅離子溶出，增加磷酸鹽皮膜的成核點，進而使得磷酸鹽皮膜晶粒尺寸縮小、降低磷酸鹽皮膜孔隙率；而二價錳離子是藉由增加磷酸鹽鈍化處理時離子碰撞的機率，進而使磷酸鹽皮膜孔隙率降低。
隨著工業蓬勃的發展，磷所造成的環境汙染也日益受到重視，為了防治磷對環境造成的傷害，有許多替代方案也日益被提出。本研究亦嘗試將矽烷化合物以溶膠-凝膠法對熱浸鍍鋅鋼板進行輥塗型鈍化處理，結果顯示鈍化後的熱浸鍍鋅鋼板之抗蝕性與試片前處理方式、前軀體水解時的pH值、水解時間、以及溶膠-凝膠溶液成分組成息息相關。此部分的研究略分為三個階段，第一階段研究TEOS無機型溶膠-凝膠塗膜的配置方式對於熱浸鍍鋅鋼板抗蝕性的影響，並以此為基石，研究溶膠-凝膠塗膜的特性。相較於未鈍化前的熱浸鍍鋅鋼板，經過TEOS無機型溶膠-凝膠鈍化處理後的熱浸鍍鋅鋼板在鹽霧試驗以及電化學交流阻抗下的抗蝕性質明顯改善，然而，經由表面形貌觀察卻還是可以觀察到塗膜具有裂紋，而這些缺陷在鹽霧試驗期間就是腐蝕反應發生的起點。第二階段則是研究TEOS/GPTMS無機/有機複合型溶膠-凝膠塗膜對於熱浸鍍鋅鋼板抗蝕性的影響，根據實驗結果顯示適當調配前驅物成分之後可以改變塗膜的特性，使得塗膜不具有裂紋缺陷，因此熱浸鍍鋅鋼板的抗蝕性得到進一步的提升，另一方面，塗膜的漆膜附著性也隨著有機前驅物的添加而得到了大幅的改善。為了賦予塗膜腐蝕抑制能力，本研究於第三階段嘗試在無機/有機複合型溶膠-凝膠溶液中加入具有腐蝕抑制效果的無機鹽類(硝酸亞鈰Ce(NO3)3•6H2O)，根據動電位極化曲線以及電化學交流阻抗頻譜分析的結果均指出，加入硝酸亞鈰的複合型溶膠-凝膠塗膜的腐蝕電流密度以及皮膜電容值均較未添加硝酸亞鈰前的無機/有機複合型溶膠-凝膠塗膜大，然而鹽霧試驗結果卻顯示含有硝酸亞鈰的複合型溶膠-凝膠塗膜具有較佳的抗蝕性，在Cross-cut鹽霧試驗中含有硝酸亞鈰的複合型溶膠-凝膠塗膜之塗膜缺陷區也保有較佳的耐蝕性，因此，混摻於複合型溶膠-凝膠溶液中的硝酸亞鈰在塗膜的腐蝕防護機制上扮演極重要的角色。

關鍵字

鍍鋅鋼件 ； 磷酸鹽鈍化處理 ； 添加劑 ； 溶膠-凝膠鈍化處理 ； 腐蝕抑制劑 ； 自我癒合能力 ； 電化學交流阻抗頻譜分析

並列摘要

Zinc coating provided two main functions on the steel substrate, the first one is the barrier protection which prevents the steel substrate form the corrosion factors attacking, and the other one is the sacrificial protection over the steel substrate. To further protect the Zn-coated steel against corrosion, hexavalent chromium (Cr6+) based passivation treatment is generally employed. However, the use of hexavalent chromium is restricted recently due to its high toxicity, signifying an urgent necessity to develop chrome-free treatments. Phosphate passivation treatment is generally adopted in order to enhance corrosion resistance of the steel substrate. Crystalline hopeite shows good adhesion on the zinc-coated substrate; however, the irregular crystals result in the existence of the opening pores on the coating, which is detrimental to corrosion resistance. The corrosion resistance of the coating is closely related to the coating porosity. Once the nucleation density is increased, the coating coverage is improved. The coating porosity is thus reduced, giving rise to better corrosion resistance. In this study, Ni2+ ions and Mn2+ ions were added separately to the treating solution in order to minimize the porosity of the phosphate conversion coating. Results showed the phosphate grain size and the coating porosity were reduced with the presence of Ni2+ or Mn2+ ions, and the corrosion resistance of the phosphate coating was thus enhanced. Although the nucleation sites of phosphate grains were increased with the presence of Mn2+ or Ni2+ in the phosphating solution, the mechanism of the nucleation site increment was different for the two cations. For the electrogalvanized steel phosphated in the presence of Ni2+, the enhanced dissolution of zinc played a major role in increasing the nucleation sites. On the other hand, the enhancement in nucleation rate in the presence of Mn2+ mainly resulted from the increased impingement of the reacting ions in the phosphate treating solution. In this study, the sol-gel passivation treatment on hot-dip galvanized sheet steels by using roll-coating was also concerned. The corrosion protection performance of the sol-gel coating on hot-dip galvanized sheet steel was closely related to the pre-treatment of the zinc-coated substrate, the pH value of the hydrolyzed solution, the hydrolysis time of the sol-gel solution, and the constituents of the so-gel solution. The pure inorganic coatings made from TEOS enhanced the corrosion resistance of the HDG steel under the salt spray test; however, its corrosion protection was limited by the presence of cracks. The incorporation of organic groups, GPTMS, reduced the volume shrinkage during the drying process. The formation of cracks was thus eliminated and the corrosion resistance of the HDG steel was further improved. Moreover, the paint adhesion was also enhanced with the addition of organic groups. The corrosion current density measured via potentiodynamic polarization and the coating capacitance evaluated via EIS increased with the addition of the cerium nitrate to the sol. This increase was likely due to the decrease in the compactness of the coating. However, the incorporated Ce species was leached out during corrosion. As a result, the addition of Ce nitrate to the sol studied improved the corrosion resistance of the coating, as evaluated via the salt spray test.

並列關鍵字

Zinc-Coated steel ； Phosphate Passivation Treatment ； Additive ； Sol-Gel Passivation Treatment ； Corrosion Inhibitor ； Self-Healing Ability ； Electrochemical Impedance Spectroscopy

參考文獻

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