熱浸鍍鋅鋼板之磷酸鹽、鉬酸鹽與釩酸鹽 複合鈍化處理

Abstract

熱浸鍍鋅鋼板(GI steel)廣泛應用於汽車、家電及建築等領域，為了提升鍍鋅層於貯存及運送時之抗蝕性，通常會先針對鍍鋅層施以鉻酸鹽(六價鉻)鈍化處理。六價鉻鈍化膜除了提供優良之障蔽保護(Barrier protection)外，同時亦具有自我癒合能力(self- healing ability)。然而，六價鉻具有致癌性，同時嚴重污染生態環境，目前已嚴格限用。本研究以輥塗型商用六價鉻皮膜(中鋼G5304鈍化液)為對照組，開發在防蝕及自我癒合保護機制與六價鉻皮膜相近的非鉻型皮膜。首先以浸泡製程(immersion process)對GI鋼板進行磷酸鹽處理，接著使用皮膜抗蝕性較佳之磷酸鹽系統，搭配鉬酸鹽及釩酸鹽系統以兩道次及單道次輥塗製程(roll coating process)對GI鋼板進行鈍化處理。研究結果顯示，當磷酸鹽溶液中含有鎂離子時，浸泡型磷酸鹽皮膜是由結晶型之磷酸鋅/磷酸鋅鎂混合物所構成。而提升溶液中鎂離子的濃度，可有效減小浸泡型磷酸鹽皮膜晶粒大小及降低整體皮膜孔隙率，故進而提升皮膜之抗蝕性。輥塗型單道次磷酸鹽皮膜是由非晶型之磷酸鋅/磷酸鋅鎂、氫氧化鋅所構成，其經過鉬酸鹽或釩酸鹽後處理之後，新生成許多球狀皮膜，故後處理液可藉由生成球狀皮膜來修補磷酸鹽皮膜之缺陷，並將磷酸二氫根、磷酸氫根及(四價氧化鉬及六價鉬酸根)或(四價氧化釩與五價釩酸根)併入於皮膜中，故皮膜抗蝕性及自我癒合能力獲得提升。相較於輥塗型兩道次磷酸鹽/鉬酸鹽皮膜，輥塗型兩道次磷酸鹽/釩酸鹽皮膜由於缺陷量較少(球狀皮膜密度較高)且併入較高含量之具較低溶解度氧化物(V2O4)及具較強氧化力的釩酸根，故其抗蝕性及自我癒合能力皆較佳。輥塗型單道次磷酸鹽(鉬酸鹽)皮膜及輥塗型單道次磷酸鹽(釩酸鹽)皮膜是由非晶型之由磷酸鋅/磷酸鋅鎂、氫氧化鋅、磷酸二氫根、磷酸氫根及(四價氧化鉬及六價鉬酸根)或(四價氧化釩與五價釩酸根)所構成，而其抗蝕性及自我癒合能力皆較輥塗型單道次磷酸鹽皮膜優異。相較於輥塗型單道次磷酸鹽(鉬酸鹽)皮膜，輥塗型單道次磷酸鹽(釩酸鹽)皮膜厚度較厚且併入較高含量之具較低溶解度氧化物(V2O4)及具較強氧化力的釩酸根，故其抗蝕性及自我癒合能力皆較佳。相較於鉬酸鹽後處理或添加劑，釩酸鹽後處理或添加劑皆賦予輥塗型磷酸鹽皮膜較佳之抗蝕能力及自我癒合能力，而其中又以釩酸鹽後處理之效果最佳。而相較於其他非鉻型皮膜[輥塗型單道次磷酸鹽(釩酸鹽)皮膜、輥塗型兩道次磷酸鹽/鉬酸鹽皮膜、輥塗型單道次磷酸鹽(鉬酸鹽)皮膜]，輥塗型兩道次磷酸鹽/釩酸鹽皮膜具有較低之缺陷量(較高密度之球狀皮膜)，故其具有較佳之障蔽保護功能(抗蝕性)，此外，併入較高含量之具較強氧化力的高價數金屬離子(五價釩酸根)亦賦予皮膜較佳之自我癒合能力，而提高釩酸鹽後處理液之釩酸根濃度(原來之兩倍)可製備抗蝕能力及自我癒合能力與鉻酸鹽皮膜相近之兩道次非鉻型皮膜。

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.