Phosphate and Sol-Gel Passivation Treatments of Zinc-Coated Sheet Steels
Date Issued
2014
Date
2014
Author(s)
Su, Hsiang-Yu
Abstract
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.
Subjects
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
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-103-F97527052-1.pdf
Size
23.54 KB
Format
Adobe PDF
Checksum
(MD5):cf2d6e93070c184b679b04e48ed32000