Replacing Hexavalent Chromium in Passivations on Zinc Plated Parts
The use and benefits of passivations based upon hexavalent compounds, particularly on zinc plated parts, are well established and understood in finishing. Through the changing attitudes within society, the continued long-term use of this technology is under question and review. We examine the background and comment on the alternative strategies available that are most likely to commercially succeed . . .

By Paul C. Wynn, Business Manager
and Craig V. Bishop, Special Projects Director
McGean-Rohco (UK) Ltd.

 

 

TABLE I—Desirable Characteristics of CrVI Passivates
• Prevents Oxide Formation
• Provides Color
• Slow corrosion in prototypic tests
(eg. salt, spray, roof top, etc.)
• Provides adhesion for organics
(eg. paint)
• Helps prevent corrosion of painted
surfaces (eg. creep)
• Conductive
• Thin
• Flexible 		• Lubricious
• Easily applied
• Stable for weeks or months
• Durable
• Resilient (repairs itself)
• re-hydrated after baking
• Coats in recesses
• Easy to strip
• Inexpensive equipment
• Single tank
• Inexpensive

TABLE II—The CrVI Chromate Film (Olation Polymer)
The CrVI Chromate Film

TABLE III—Biestek and Weber
  mg / dm2 Cr CrIII mg CrVI / dm2 dm2 / 2g ft2 / 2g
Cd Clear 1 30% 90% 0.27 7407 797
Zn Clear 1 30% 90% 0.27 7407 797
Al Clear 3 30% 80% 0.72 2778 299
Al Yellow 10 35% 70% 2.45 816 88
Zn Yellow 12 35% 70% 2.94 680 73
Cd Yellow 15 38% 70% 3.99 501 54
Mg Yellow 15 38% 70% 3.99 501 54
Zn Olive 23 42% 70% 6.762 296 32
Cd Olive 23 42% 70% 6.762 296 32

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