Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table I. Average Film Composition of Hexavalent Yellow Chromate Conversion Coating Composition Cr6+ Cr3+ S (as sulfate) Zn2+ Na+ Water Oxygen % by wt 8.7 28.2 3.27 2.12 0.32 19.3 Bal. consisting of hydrated chromic-chromate, chromium hydroxide, and zinc and oth- er metal oxides. The film traps some soluble hexavalent chrome as well from the solution. The reaction is fast and takes place at ambient temperatures. Trivalent chrome passivating solutions work on the same basic principle except for the omission of Cr6+ Cr3+ oxidation step. They rely on the direct reaction of with the dissolved zinc and produce insoluble barrier layers of zinc chromi- um oxide precipitated on the surface in a similar fashion as with Cr(6) passiva- tion, although under different reaction kinetics. Table I shows an average composition of a hexavalent chrome yellow con- version coating film over zinc plate. The film consists of 8 to 10% of leachable Cr(6). This is the portion of the film responsible for the self-healing effect associated with this type of passivation. If the film is scratched or mechanical- ly damaged, moisture dissolves the leachable Cr(6) restoring the conversion film and resealing the damaged area. This mechanism works as long as the Cr(6) con- tent is not dehydrated by exposure to temperatures above 50 to 60O C for an extended period of time. The balance of the film composition consists of insol- uble trivalent chrome compounds of zinc, oxides, sulfates, and water. This portion of the film, defined as a barrier coating, provides the bulk of the cor- rosion protection and accounts for about 90% by weight of the film. It is worth- while noting that trivalent compounds amount to a third of the composition as Cr(3). The thickness of a typical yellow hexavalent chrome passivation film on zinc plate was measured at 350 nm. It was postulated that excluding the hexavalent chrome portion from this film while maintaining its thickness would still provide essentially similar corro- sion resistance properties. Some differences are to be expected such as a change in the yellow appearance typically associated with Cr(6) compounds. Table II. Operating Conditions and Coating Films Cr(6) Yellow Parameters Make up, % volume pH Temperature, O Time, Sec Agitation Activation Film color 430 C Passivation 0.5-1.5% 1-2 20-25 30-45 Mechanical or air Dilute acid Film thickness, nm Yellow iridescent 300-350 2-8% 1.8-2.4 20-25 30-60 Mechanical or air Dilute nitric or sulfuric Blue bright iridescent 60-80 Cr(3) Thin Layer Passivation Cr(3) Thick Layer Passivation 10-13% 1.6-2.2 55-56 30-60 Mechanical or air Dilute nitric or sulfuric Light green hue 300-350

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