Phosphate, Molybdate and Vanadate Composite Passivation Treatment for Hot-dip Galvanized Steel
Date Issued
2011
Date
2011
Author(s)
Tsai, Cheng-Yang
Abstract
Hot-dip galvanized steel (GI steel) is widely applied to automobile, electrical appliance and construction fields. To protect Zn coating against corrosion during transportation and storage, hexavalent Cr treatment is generally adopted. Hexavalent Cr coating provides superior barrier layer protection and self-healing protection. However, a strict restriction was imposed on the use of chromate passivation because of the toxicity of hexavalent Cr. Therefore, non-chromate treatments have received an ever-increasing attention. The purpose of this study is to develop a roll coating non-chromate passivation treatment for GI steel (commercial roll chromate passive coating as comparison). This study prior investigated the phosphate treatment for GI steel by immersion coating process. Then, the composite phosphate, molybdate and vanadate treatment for GI steel conducted by two-step and one-step roll coating process were explored. Results showed that the presence of Mg2+ in the phosphate solution, the phosphate coating prepared by immersion coating process was composed of mixed Zn phosphate hydrate and (Zn, Mg) phosphate hydrate with high degree of crystallinity. Elevating Mg2+ in the solution markedly reduced the phosphate grain size and increased the population density of the phosphate grains. As a result, the porosity of the phosphate coating was reduced with increasing solution Mg2+, which, in turn, improved the corrosion resistance of the coating. The one-step roll phosphate coating was porous, amorphous and was composed of mixed Zn phosphate/(Zn, Mg) phosphate and Zn(OH)2. The one-step roll phosphate coating underwent the molybdate or vanadate post treatment, the defects of the phosphate coating were mainly repaired and the incorporation of (H2PO4-, HPO42-) and (MoO2, MoO42-) or [V2O4 , V10O27(OH)5−, V10O286− …] was also detected. As a result, the corrosion resistance and self-healing ability of the coating were both improved. Compared to the two-step roll phosphate/molybdate coating, the two-step roll phosphate/vanadate coating had a better corrosion resistance and self-healing ability was owing to a less amount of the defects (more spherical particles) present in the coating, higher content of low-valent oxide with lower solubility (V2O4) and high-valent anion with greater oxidizing power [V10O27(OH)5−, V10O286− …] incorporated into the coating. The one-step roll phosphate(molybdate) coating and phosphate(vanadate) coating are both porous, amorphous and were composed of mixed Zn phosphate/(Zn, Mg) phosphate, Zn(OH)2, (H2PO4-, HPO42-) and (MoO2, MoO42-,) or [V2O4 , V10O26(OH)24-, V10O27(OH)5-...] and they showed a better corrosion resistance and self-healing ability than those of the one-step roll phosphate coating. Compared to the one-step roll phosphate(molybdate) coating, the one-step roll phosphate(vanadate) coating displayed a better corrosion resistance and self-healing ability was owing to a higher content of low-valent oxide with lower solubility (V2O4) and high-valent anion with greater oxidizing power [V10O26(OH)24-, V10O27(OH)5-...] incorporated in the relatively-thick coating. Compared with the molybdate post treatment or additives, vanadate post treatment or additives rendered a better corrosion resistance and self-healing ability of the phosphate coating, vanadate post treatment especially. Among the roll coating non-chromate passivation treatment (the two-step roll phosphate/vanadate, the one-step roll phosphate(vanadate), the two-step roll phosphate/molybdate, the one-step roll phosphate(molybdate), the two-step roll phosphate/vanadate demonstrated the best corrosion resistance was due to a less amount of the defects (more spherical particles) present in the coating. Besides, a higher content of high-valent anion with greater oxidizing power [V10O27(OH)5−, V10O286− …] incorporated into the coating contributed the best self-healing ability of the two-step roll phosphate/vanadate coating. Morover, elevating the concentration of [V10O27(OH)5−, V10O286− …] in the post solution rendered a similar corrosion protection and self-healing ability of the two-step roll phosphate/2vanadate coating as compared to those of commercial roll chromate coating.
Subjects
Hot-dip galvanized steel
phosphate
molybdate
vanadate
passivation
corrosion resistance
self-healing ability
SDGs
Type
thesis
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