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The Passivation of Zinc, Aluminum, and Their Alloys by Immersion In

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The Passivation of Zinc, Aluminum, and Their Alloys by Immersion In Corrosion resistant films from trivalent chrome based solutions applied to electrodeposited zinc and zinc alloys. By: Craig V. Bishop, Roland Vogel*, John R. Kochilla, Charlotta Wennefors Atotech USA, Rock Hill, SC, *Atotech GmbH, Berlin Germany INTRODUCTION Reviews11,12,13 have been published that As early as 19511 inventors compare C3P passivation with developed trivalent chrome based chromating and indicated that a shared passivation treatments for zinc (C3P). model for film formation, based upon These C3P solutions were an alternative early CrVI research14 had achieved a to existing hexavalent chromium (CrVI) consensus. This model describes film chromate treatments that were classified formation on the basis of: 1. Localized based upon characteristic film colors: rise in pH near the surface as protons are clear or ‘blue’, yellow, olive drab, and reduced to hydrogen gas and zinc black2. Although early C3P eliminated dissolves. 2. Precipitation of CrIII and hexavalent chromium they did not gain other metal ions, if present, that would commercial acceptance until the late bridge or olate through oxygen and 1970’s as a niche process. The process oxyanions in the higher pH region. produced a thin, ~40nm based upon Additional understanding of passivation XPS, clear film with modest corrosion based upon structural variation of the resistance expectation3,4 formed from solutions of inorganic CrIII compounds. Figure 1 XPS depth profile montage of C3P region illustrating the Zn Auger interference Pragmatic as well as environmental with Al X-ray excitation. concerns led to acceptance. C3P minimized blemishes due to reaction of CrVI and small amounts of codeposited iron common in, the then emerging, acid and non-cyanide alkaline zinc processes (CFZP). With the acceptance of CrIII derived films as clear passivates a large variety of processes were patented, marketed, or otherwise described to improve corrosion protection. Heated and/or high [CrIII] solutions were described in 19825 that claimed thick (~200nm) films from solely inorganic solutions. The use of di and tricarboxylic Zinc or zinc alloy has been suggested for acids as well as cobalt and rare earths ZnNi15 as well as zinc16.. was described beginning in 19836,7,8. Iridescent C3P for zinc nickel9 and black Post treatment for passivates and C3P on alloys of zinc iron with iron chromates are often recommended levels as low as 20 ppm, and zinc nickel dependent upon final application. (5-25%) range were developed in the Described as seals, if the thickness is early 1990’s10. less than one or two microns, as a CrVI advantages could be met with topcoat if greater than 4 microns. CrIII, but not all16.In the recent past, this reluctance to replace CrVI has changed due to the European Union adopted End of Life Vehicle (ELV). This initiative severely restricts the amount of CrVI, as Table 1 Summary of production scale C3P well as eliminating other toxic materials, testing. Trials completed/failed (in progress). in motor vehicles. With enthusiasm *due to insufficient thickness (3-5 microns). came realization that a very long list of C3P as well as non-chrome methods needed to be evaluated. It has taken an Organic Inorganic Top Totals immense ongoing international effort by Seal Seal Coated Organic Zn ZnFe 5/0 Zn 2/0 29/0(5) OEM’s, applicators and suppliers to sort CrIII 11/0(4) ZnFe through the offerings. ZnFe 2/0 4/0(1) ZnNi ZnNi 3/0 2/0 What C3P’s have survived? At Inorganic ZnFe ZnFe 3/0 ZnFe 21/1*(4) CrIII wo 3/0 ZnNi ZnNi 4/1* this time a universal CrIII or chrome free Co 6/0(2) ZnNi passivation chemistry that can be applied 5/0(2) to all zinc and zinc alloys, regardless of Inorganic Zn (3) (3) CrIII w Co performance requirements or anticipated Phosphate Zn ZnFe 4(6) post treatment, has not definitively CrIII (trace 1/0(2) Fe) (2) emerged. Organic acid based C3P with ZnFe cobalt and subsequent organic seal has 3/0 ZnNi(2) met or exceeded nearly all requirements Total 54/1(18) but has modest issues with operating costs, waste treatment, longevity, compatibility with dip spin and For fasteners, layers may have electrophoretic topcoats. Engineering specialized tribology characteristics to solutions are evolving to eliminate these ensure uniform torque and clamp load concerns but other C3P have achieved for joints assembled with automatic success in a variety of applications. machinery. The nomenclature’s of these Inorganic C3P, with and without cobalt, coatings are dry film lubricants (DFL), achieves many of the lower corrosion usually without additional corrosion requirements on zinc and performs as protection, or integral lubricants (IL), if well or better than organic C3P on some combined with a seal or topcoat. Seals, zinc alloys. Phosphate based C3P is as well as topcoats, can greatly increase 15 effective for blackening ZnFe and ZnNi the reliability of CrIII passivates . as well as zinc that is inadvertently alloyed with trace iron. At present, Despite the strides of C3P, widespread sealing of these coatings is necessary for replacement of CrVI did not occur until reliable corrosion protection. Based recently. Factors including costs of upon application, desired final materials and qualification, final appearance, and performance different appearance, additional processing steps, substrates employ require different alloy, and energy requirements did not passivation methods to achieve cost compete well against chromate applied effectiveness. to non-deliberately alloyed zinc. Many In this paper we will summarize amps per liter of accumulated testing. our morphological, compositional, and Standard steel and brass (used to structural characterization studies of C3P determine iron concentration) Hull cell panels were used as base metal substrate Table 2 Summary of C3P characterization without modification, except for certain tests. XRPD measurements where an Morphology Thickness [CrVI] (dpc Application Other (nm) ug/m2) Organic Gen. Zn 100-400 <0.1 Std Zn yellow ZnO in film., no CrIII w Co smooth, occ. alternative, generally spinnel. Co cracks with seal, 'clear' alloy. reduces heat variation Inorganic ibid. Alloy 70-200 <0.02 or nd Alloy tie coat. With Ibid. Alloy CrIII wo Co function of ZnFe and seal. With sensitive ([Fe], alloy ZnNi as clear. [Ni]). Inorganic ibid. Alloy 70-200 Lower corrosion req'ts Low T app. CrIII w Co function of (blue). Improved [Co] and corrosion resistance dense? with seal. Phosphate Rough, Zn trace Fe <0.1 Black coatings. Low T app. CrIII dense crack 40-80 nm. pattern ZnFe 80-200 nm Organic Low rms, 800-1500 0 Improve corrosion Seal inorganic resistance. Can be nano integrated with particles lubricant Inorganic 0 Interiors, when low Seal cost needed, and with fluid compatibility issues. ‘amorphous’ electroless nickel coating18 coatings and seals that have met or was applied prior to zinc or zinc alloy exceeded the expectations of recent deposition due to concern about the production scale screening. We will not rolling texture present with sheet steel. characterize the C3P used for modest Plating was solely direct current and corrosion protection. They have been done in two liter beakers at uniform 20 widely accepted and previously ASF current density until thickness of 8 reviewed. microns was achieved. The panels were treated with CrIII based passivates, described as organic CrIII with Co, EXPERIMENTAL: Testing was done inorganic CrIII, inorganic CrIII with Co, at laboratory and production scale. At and phosphate based CrIII. In many the laboratory scale, commercially 17 cases CrIII derived film was available electrodeposits of zinc, zinc subsequently treated with either an nickel (12-15%), and zinc iron were organic or inorganic seal. These CrIII used. With few exceptions, noted in the processes and seals are also results section, these solutions were from commercially available processes19 pilot line process tanks that had over 10 . The resulting panels were used for analytical which interferes with CrVI 3s. The measurements combination of Cr3S and Cr3p appears a reliable indicator of CrVI For production scale testing, the to be zinc or zinc alloy, passivate and seal presence based upon additional were applied in a continuous sequence experiments with CrVI and CrIII by commercial applicators. For the dip prepared by drying aqueous mixtures of spin and electrophoretic paint tests the potassium dichromate and chrome e (figure 3). Complicating XPS resulting plated and passivated parts chlorid were then transported to either the quantification is the observation that vendor of the paint who performed or CrVI signal intensity dropped after arranged the topcoat application. All argon ion sputtering. Diphenylcarbizide (DPC) paints used in the study are available 13 from major suppliers and chrome free as testing was used to determine trace verified by EDS. presence or absence of CrVI for Hexavalent yellow chromate comparison to XPS. controls were produced from solution as Neutral salt spray (NSS) testing was described in reference 11. Other conducted in accordance with ASTM chemicals were obtained from Fisher B117 and all panels and parts were Scientific. heated as noted, then stored at room Instrumentation includes a temperature for 24 hours, prior to being in NSS. Hitachi S4500 cold field emission (CFE) placed ZnNi alloys produce an unusual SEM with Noran EDS, PHI 5600 XPS corrosion product in NSS testing (ESCA) System using monochromatic referred to as gray veil. This is not a Al X-ray source and Argon ion beam for voluminous corrosion and, unlike white depth profiling and charge neutralization corrosion products, it is not visible if a with an XPS beam diameter of ~1 mm, part is wetted with deionized water or Scintag X1 XRPD equipped for grazing removed if the surface is rubbed with a incidence X-ray analysis (GIXA), tool such as a q-tip. The appearance of Bruker D8 4 circle XRPD, NEC 5SDH gray veil in C3P on ZnNi does not Tandem Pelletron Ion Beam Accelerator correlate with corrosion failure such as for RBS and PIXE, and a Digital white corrosion or red rust.
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