替代六價鉻製程研究

Study on Hexavalent Chromium Process Alternatives

陳黼澤(Fu-Je Chen)

指導教授 ： 潘永寧 ； 林招松

國立臺灣大學/工學院/機械工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.01943

摘要

六價鉻在材料防護上常應用於電鍍鉻和鈍化處理。然而，六價鉻屬於劇毒物質，對人體及自然生態造成極大的危害，故亟需開發非六價鉻表面處理製程。本論文包含替代六價鉻電鍍製程與電鍍鋅鋼件的非六價鉻鈍化處理製程兩部份。其中，替代硬鉻高硬度與耐磨特性的相關電鍍製程是針對具有潛力的三價鉻電鍍與鎳鎢、鎳磷和鈷磷合金；替代鉻酸鹽鈍化處理製程則著重於三價鉻鈍化與鈦酸鹽鈍化處理製程。
三價鉻電鍍顯示非晶質鉻鍍層延展性低，含有高密度裂紋，鍍液系統添加有機或無機硫化物會有含硫分子吸附於陰極表面以抑制碳共鍍，形成結晶質三價鉻鍍層。鎳鎢合金顯示硬度與鍍層鎢含量以及微結構相關，適當熱處理晶界弛豫作用改變晶界結構，有助於提升鎳鎢合金硬度。鎳磷合金電鍍顯示脈衝電源形式能夠提高鍍層磷含量並改善電流效率和調控內應力，同時不添加應力調整劑鍍液系統即能夠製備與純鎳相匹配的低內應力。鎳鍍層會隨著熱處理溫度增加歷經回復、再結晶與晶粒成長行為而導致鍍層軟化，但鈷鍍層高溫穩定性較佳，相對於純鎳和純鈷單質金屬，鎳磷合金與鈷磷合金透過熱處理會有析出硬化效益，可使鍍層硬度達到最大值，值得注意的是純鈷和鈷磷合金高溫場合會有結構相變態，同時鈷基地含有高密度疊差和退火雙晶而有細化結晶區塊的效益，故鈷磷合金抗高溫軟化能力優於鎳磷合金。
電鍍鋅鋼板表面六價鉻鈍化膜緻密性高，而三價鉻鈍化膜則屬於多孔性結構，三價鉻皮膜是由三價鉻與二價鋅的氧化物/氫氧化物所組成，適量硼酸有助益於常溫三價鉻成膜，同時烘烤處理提升皮膜緻密性，極化阻抗因而大幅提昇。鈦酸鹽鈍化膜含有高密度孔洞結構，但鋅基材介面有較為緻密的阻障層，鈦酸鹽皮膜會隨自然脫水乾燥時效性即會呈現穩定狀態以提高抗蝕性能。

關鍵字

硬鉻 ； 三價鉻電鍍 ； 鎳鎢合金 ； 鎳磷合金 ； 鈷磷合金 ； 脈衝電鍍 ； 三價鉻鈍化 ； 鈦酸鹽鈍化

並列摘要

Hexavalent chromium has been used extensively in chrome plating and conversion coating treatment. However, hexavalent chromium is a highly toxic substance that threatens human health as well as the natural environment. Thus, the development of surface treatments free of hexavalent chromium is required. This study includes the replacements of hexavalent chromium surface treatments in electrodeposition system and passivation system on zinc-electrogalvanized steel plate. In which, the prospective replacements of hard, wear resistant electrodeposition focused on trivalent chromium electrodeposititon, nickel tungsten, nickel phosphorus, and cobalt phosphorus electrodeposition; the replacement of chrome passivation treatment was concentrated on trivalent chromium passivation and titanate-salt passivation systems.

Results showed that amorphous trivalent chromium electrodeposits had low ductility and contains high density surface cracks. Addition of organic or inorganic sulfur compounds in the bath suppressed carbon codeposition via adsorption of sulfur-containing molecules on the cathode, forming a crystalline trivalent chromium electrodeposit. Nickel tungsten alloys showed a relationship between hardness and microstructure at various tungsten contents. Proper heat treatment changed grain boundary microstructure and improved hardness of nickel tungsten alloy through grain boundary relaxation. Nickel-phosphorus electrodeposits with high phosphorus content and low internal stress have been plated at high current efficiency using pulse current in a nickel sulfamate bath free of stress reducers. Results showed that through controlling the duty cycle of a pulse current, Ni-P deposits with a broad range of internal stresses can be produced. Pure nickel electrodeposits softened at higher heat treatment temperatures due to recrystallization and grain growth behaviors. On the other hand, pure cobalt electrodeposits had better high temperature stability. Heat treatment of nickel phosphorus and cobalt phosphorus alloys resulted in precipitation hardening which optimized coating hardness. It was noticeable that pure cobalt and cobalt phosphorus alloys had structural transformation at higher temperatures. High density dislocation pile-up and annealing twins were found at the cobalt matrix. Both caused a grain refinement effect on the crystalline domains. Thus, cobalt phosphorus alloys are more superior in high temperature mechanical properties than nickel phosphorus alloys.

Hexavalent chromium passivation on electrogalvanized steel had a dense coating microstructure, whereas trivalent chromium passivation had a porous coating microstructure. Trivalent chromium coating consisted of oxides/hydroxides of trivalent chromium and divalent zinc. Added appropriate amount of boric acid aided formation of trivalent chromium coatings under room temperature conditions. Meanwhile, baking treatment increased coating density and much improved polarization resistance. Titanate conversion coatings contained high density porous structure. However, a dense barrier microstructure existed on the zinc-substrate interface. Naturally dehydrated titanate conversion coating rendered a state of stability and improved corrosion resistance.

並列關鍵字

hard chromium ； trivalent chromium electrodeposition ； nickel tungsten alloy ； nickel phosphorus alloy ； cobalt phosphorus alloy ； pulse electrodeposition ； trivalent chromium conversion coating ； titanate conversion coating

參考文獻

13. 陳黼澤，“鎳磷與鈷磷合金電鍍”，國立臺灣大學材料科學與工程學研究所碩士論文，2005年7月。

31. 黃大展，“鎳鎢合金電鍍”，國立臺灣大學材料科學與工程學研究所碩士論文，2008年7月。

1. L. Xia, R. McCreerya, “Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024-T3 Probed by Vibrational Spectroscopy,” Journal of the Electrochemical Society, Vol. 145, No.9, 1998, pp. 3083-3089.

2. N. V. Mandich, “Chemistry & Theory of Chromium Deposition: Part II. Theory of Deposition,” Plating and Surface Finishing, Vol. 84, No. 6, 1997, pp. 97-101.

4. A. Baral and R. Engelken, “Modeling, Optimization, and Comparative Analysis of Trivalent Chromium Electrodeposition from Aqueous Glycine and Formic Acid Baths,” Journal of the Electrochemical Society, Vol. 152, No. 7, 2005, pp. C504-C512.