Metal Finishing Guide Book

2013

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Table I. Average Film Composition of Hexavalent Yellow Chromate Conversion Coating Composition % by wt Cr6+ 8.7 Cr3+ 28.2 S (as sulfate) 3.27 Zn2+ 2.12 Na+ 0.32 Water 19.3 Oxygen Bal. chromium oxide compounds. An increase in the pH at the interface causes the trivalent chrome compounds to precipitate on the surface, forming a gelatinous film consisting of hydrated chromic-chromate, chromium hydroxide, and zinc and other 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+ oxidation step. They rely on the direct reaction of Cr3+ with the dissolved zinc and produce insoluble barrier layers of zinc chromium oxide precipitated on the surface in a similar fashion as with Cr(6) passivation, although under different reaction kinetics. Table I shows an average composition of a hexavalent chrome yellow conversion 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 mechanically 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) content is not dehydrated by exposure to temperatures above 50 to 60OC for an extended period of time. The balance of the film composition consists of insoluble trivalent chrome compounds of zinc, oxides, sulfates, and water. This portion of the film, defined as a barrier coating, provides the bulk of the corrosion protection and accounts for about 90% by weight of the film. It is worthwhile 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 corroTable II. Operating Conditions and Coating Films Parameters Cr(6) Yellow Passivation Cr(3) Thin Layer Passivation Cr(3) Thick Layer Passivation Make up, % volume 0.5-1.5% 2-8% 10-13% pH 1-2 1.8-2.4 1.6-2.2 Temperature, OC 20-25 20-25 55-56 Time, Sec 30-45 30-60 30-60 Agitation Mechanical or air Mechanical or air Mechanical or air Activation Dilute acid Dilute nitric or sulfuric Dilute nitric or sulfuric Film color Yellow iridescent Blue bright iridescent Light green hue Film thickness, nm 300-350 60-80 300-350 438

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