surface treatments

TRIVALENT C H R O M E C O N V E R S I O N C O A T I N G F O R ZINC A N D ZINC ALLOYS BY NABIL ZAKI SURTEC INTERNATIONAL, ZWINGENBERG, GERMANY; www.surtec.com

In the late 1990s the End-of-Life-Vehicle (ELV) directives were introduced in Europe, mandating the recycling of 85% of vehicles by weight by the year 2006. Recycled components should be free of compounds or elements known to be haz- ardous to man or the environment. Among the restricted items are carcino- genic hexavalent chromium compounds. Consequently, the directive imposed a limit of 2 g of Cr(6) per vehicle to be met by July 2003. Leachable hexavalent chrome can be found in the conversion coating layer used to passivate zinc and zinc alloy plated surfaces. The amount of Cr(6) varies from 5 to 400 mg/m2 depending on the type ofpassivation used, and is typically low for light blue coat- ings and heavier for the thicker yellow and olive green types.

To insure compliance with the new regulation and eliminate the need for con- stant monitoring, the European car industry opted for the total removal of hexa- valent chrome from their plated finishes. The U.S. car industry soon adopted the same restrictions and standardized finishes across their worldwide operations.

The search for a viable Cr(6)-free conversion coating with similar functional prop- erties led researchers ultimately to the selection of trivalent chrome passivation as the most adequate alternative to date. Availability of raw materials, safety, economical considerations, and ease of adaptability to existing finishing plants were important factors in the selection of this technology as a viable replacement process.

TRIVALENT CHROME PASSIVATION TECHNOLOGY Trivalent chromium compounds are readily available, noncarcinogenic, and safe to handle. Some compounds are used extensively in many applications such as dyeing and waterproofing of fabrics, printing, wood preservation, chrome plating, and other industrial processes. Trivalent chrome conversion coating technology was introduced commercially in the late 1980s as an earlier attempt at replacing carcinogenic hexavalent chrome from as many processes in metal fin- ishing as possible. Although there were no regulations or specifications requir- ing this change at the time, platers realized the environmental and safety advan- tages from such a substi tution, along with improved and more reliable performance. The first generation of Cr(3) conversion coatings was limited to pro- ducing blue-bright passivation designed for light service conditions, meeting 12 to 24 hr of neutral salt spray (NSS) to white zinc corrosion. Further development was needed to meet the full range of automotive and other industry requirements for extended corrosion resistance.

The mechanism ofCr 6. conversion coating and its corrosion resistance was used in developing a comparable substitute process.

Upon immersion of zinc plated parts in hexavalent chromating solutions, zinc is oxidized at the interface by the Cr 6., which is reduced to Cr 3~, dissolves at a con- trolled rate in the acidic solution, and reacts with Cr 3~ to form zinc 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

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Table I. Average Film Composition of Hexavalent Yellow Chromate Conversion Coating Composition % by ~c

Cr 6* 8.7

Cr 3+ 28.2

S (as sulfate) 3.27

Zn 2* 2.12

Na* 0.32

Water 19.3

Oxygen 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 Cr 6+ oxidation step. They rely on the direct reaction of Cr 3+ 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 60°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 Cr(3) Thin Layer Cr(3) Thick Layer

Parameters Passivation Passivation Passivation

Make up, % volume 0.5-1.5% 2-8% i0-13%

pH 1-2 1.8-2.4 1.6-2.2

Temperature, °C 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

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chromitAL TCP Trivalent Chrome Passivation for Aluminium

Sur-Tec 650 chromitAL provides exce[|ent bare corrosion resistance on unpainted surfaces and enhances adhesion of organic coatings.

App|ications include plain and anodized aluminum.

LL i ~ L! ̧ ~¸ i

i !i~ii!~iiiii~ii!!ii~ii~iii~i~i~iiii!i~!i~ililili~i ~

i̧ ¸¸ !̧̧ ¸¸ i̧̧ ¸¸2¸ !̧̧ ̧

}

Fig. 1. Passivation film thickness.

First-generation trivalent chrome conversion processes were based on fairly sta- ble Cr(3) complexes, which slowed their reactivity rates even at high temperatures. They produced rtlm thicknesses of 20 to 30 nm with limited corrosion resistance. To produce thicker passivation layers, a second-generation trivalent passiva- tion process was developed. It incorporates accelerators, modified complexors, and is operated at higher concentration and temperature to drive the reaction kinetics at a faster rate. When applied as recommended, film thicknesses of 300 to 380 nm, equivalent to those produced from yellow Cr(6) passivating solutions, were obtained. The film in this case consists of an insoluble barrier lay- er free of hexavalent chrome.

Table II and Fig. i compare operating conditions and resulting conversion coat- ing films produced by various passivating solutions.

500 450 400 350 300 250 200 1SO 100

50 0

Or(3) Blue Or(O) Yellow Cr{:l) Thick Layer

r'110% W C I EEl% W C nFI ra t w C

Fig. 2. Comparison of neutral salt spray results for Cr(6) versus Cr(3) passivation on zinc (WC [minus) white corrosion).

4 2 8

Zn Z~e 7.r~i8 Zn~i 12 7_.~C0

Fig. 3. Neutral salt spray results for Cr(3) thick layer passivation of zinc and zinc alloys.

TRIVALENT CHROMATE PASSIVATION PROPERTIES Trivalent chrome passivation films are similar to hexavalent types as far as the bar- rier portion of the coating is concerned in many respects; however, the absence of soluble Cr(6) compounds contributes to a range of new properties.

Corros/on Res/stance NSS corrosion testing data per ASTM B 117 of zinc and zinc alloy plated sur-

faces with Cr(6) and Cr(3) passivation are illustrated in Figs. 2 and 3. Results sup- port the concept of thick layer trivalent chrome passivation being a viable alter- native to conventional hexavalent chromate. In all cases, except for barrel plated work, its performance matches or exceeds that ofhexavalent types. This is due to mechanical damage to the passivation film during bulk processing and the absence of self-healing properties. This shortcoming is overcome by the use of seal- ers or topcoats to protect the film. Postpassivation treatment increases corrosion resistance further, and provides the flexibility to add a range of properties su'ch as lubricity, torque-tension modification for fasteners, and developing various col- ored finishes.

Fig, 4. Scanning electron micrograph of Cr(6) at left and Cr(3) at right. Passivation films dried at room temperature prior to heat treatment at 1,000[times].

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Fig. 5. Scanning electron micrographs of Cr(6) at left and Cr(3) at right. Passivation films after heat treatment at 200°C for I hr at 1,000[times].

Heat Resistance An advantage oftrivalent chrome passivation is its superior resistance to high tem- peratures. Unlike hexavalent chrome passivation films, they can be heated to 200°C or more for extended periods of time and still maintain up to 70% of their original resistance. Hexavalent chromate films dehydrate and fail entirely when heat- ed above 55°C for more than a few minutes. Figs. 4 and 5 show the surface analy- sis of both types of conversion coatings before and after heat treatment. After the coatings have formed and dried, hexavalent chromate films show a pattern of cracks or fissures as a result of partial dehydration of the adsorbed Cr(6) content. Upon heat treating, total dehydration takes place and Cr 6÷ is reduced to C? +, widening and deepening the cracks, exposing zinc, and resulting in premature cor- rosion failure. By contrast, the trivalent chrome passivation film, consisting of the more stable oxidation state Cr(3) compounds, is more homogeneous and crack free. It remains unchanged after heat treatment. This property is used to great advan- tage when zinc plated parts must be heat treated for hydrogen embrittlement relief. This is done typically without passivation, which would otherwise be destroyed. The need to replate with a thin layer of zinc after baking to apply an adherent conversion coating is eliminated. Parts passivated with trivalent chrome can be heat treated with no change in appearance and minimum loss of corrosion protection. The choice of sealers and topcoats must be carefully considered if parts are to be heat treated as some types of sealers could reduce this advantage either by corrosive chemical attack or by dehydrating and inducing cracking in the underlying passivation film.

A~e~r~nce Trivalent chrome passivation produces a range of colored films. Thin layers are typically iridescent blue, while thicker coatings are pale green to yellow blue depending on whether the zinc is alloyed and the specific alloying element. Since hexavalent chrome is the source of yellow color in conventional conversion coatings, this color is not usually available in Cr(6)-free coatings unless induced by dyes or other metals and their oxides. The use of transparent sealers and topcoats can modify the appearance of the coating producing silver-white or pale- colored films free of iridescence. Black coatings may be obtained with specially modified trivalent chrome passivating solutions containing metals, such as cobalt or iron, but are difficult to control. Deep uniform black finishes are best produced on zinc-iron alloys. Other alternatives for black finishes over trivalent

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chrome coatings include organic topcoats applied by the dip-spin process, suit- able for bulk-processed parts such as fasteners and other barrel-plated work. After passivation and rinsing in such a process, parts are transferred into baskets, immersed in the blackening solution, spin-dried in a centrifuge under con- trolled rotational speed to remove excess liquid, then dried at a specified curing temperature.

POSTPASSIVATION TREATMENT The use of postpassivation may be necessary in order to accomplish one or more of the following functions:

1. Modify the color of the conversion coating film 2. Provide specific lubrication or coefficient of friction for threaded components 3. Extend the corrosion resistance particularly for barrel and bulk processed parts.

There are several commercially available types of postpassivation treatments. Thorough testing of the selected system under simulated end use conditions is recommended in order to qualify the process for the intended application. This may include heat treatment prior to accelerated corrosion or torque-tension testing on components for near engine exposure.

Seale~ These are types of products that will react with the conversion coating film and result in more durable resistant finishes. Examples of sealers are silicate-based products applied at either room or elevated temperatures. The latter will leach out a small portion of the passivation film and leave a heavier protective layer of sil- icate reaction products on the surface. This type of sealers may not be adequate for high-temperature exposure depending on the type and degree of alkalinity of the residual silicate film and their resistance to cracking. Aged residual film alkalinity may represent hazardous handling problems and should be ade- quately investigated. Other types of sealers may contain phosphates, silanes, and transition metals.

Topcoats These may be organic lacquers, polymers, lubricants, waxes, oils, and oil emulsions with suspended particles and coloring dyes. They may be applied by immer- sion, spray, or dip-spinning. A topcoat used commercially for zinc plate consists of coatings containing zinc or aluminum flakes to provide protection against gal- vanic corrosion between the fasteners and magnesium or aluminum surfaces. The dip-spin process has also been used to apply black finishes with torque modifi- cation properties for fasteners and washers. Generally, drying prior to the appli- cation of topcoats produces more uniform, heavier films with better corrosion protection.

TYPICAL PROCESS SEQUENCE

1. Zinc or zinc alloy plate 2. Rinse 3. Activate (dilute acid) 4. Trivalent chrome passivate 5. Drag out rinse (optional) 6. Rinse (counter-current flow)

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Table IlL Automotive Standards 8-9/an of Zinc Plate with Hexavalent Chrome-Free Passivation and Sealer (NSS Test Requirements per ASTH B 117)

Hrs-White Hrs-Red Heat Treatment Auto Maker Corrosion Corrosion Required Conditions

GM May be specified 3044 Clear/Blue~Black 120 240 150°C/1 hr Yellow-Iridescent 120 360 150°C/1 hr

Ford WSS-M 12Pl 7B l/B3 Clear/Silver white 96 384 No Clear/Silver white 72 360 Yes 120°C/4 hrs Daimler Chrysler May be specified PS 1207 - R Iridescent Rack 200 Iridescent-Barrel 160 irridescent Rack 120 No sealer iridescent Barrel 100 No sealer As of Aug. 200i (Subject to change).

7. Dry (optional or as recommended) 8. Seal and/or topcoat 9. Dry

PROCESS C O N T R O L Trivalent chromate conversion coatings are applied over acid or alkaline elec- troplated zinc and zinc alloys in conventional plating lines replacing existing hexa- valent chrome tanks, with little or no modification to the line. Provisions for heat- ing may be required along with proper ventilation. Postpassivation may be used inqine if it is compatible with the operation and space for extra stations is avail- able. Organic topcoats, especially for barrel plating processes, are best applied off- line.

The critical operating parameters are the chrome content, pH, and temperature. Analytical methods for trivalent chrome using spectrophotometric or simple

titration techniques are commonly used and readily available. Chrome content

Cr(3)+Sealer

Cr(3)+Sealer +HT 120C/4hr

Cr(3)+Sealer

Cr(3)+Sealer + HT 120C14hr

hPi

0 100 200 300 400 500 600

I WMte Cnrm.qinn

O Ford-WSS,.M21P 17- B1/B [] Cr(3) Pass vation

Fig. 6. Cr(3) passivation surpasses requirements for Ford.

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0 Cr(3) Clear -=

Cr(3) Clear, BIBr + Sealer X

Cr(3) Black + Sealer X !'

Cr(3) Yellow + Sealer J !' I

Cr(3) Clear

Cr(3) Clear, BIBr + Sealer X I

Cr(3) Black + Sealer X I

Cr(3) Yellow + Sealer J

hrs

100 200 300 400 500 600

,,

White Corrosion ]

,R~I Corrosion l [] GMW 3044 [] Cr(3) Passivation

Fig. 7. Cr(3) passivation meets or surpasses requirements for GM.

affects the ultimate film thickness. Temperature and pH control the reaction kinetics, film strength, and adhesion.

EFFECT OF C O N T A M I N A N T S The most common contaminants are zinc and iron resulting from processed parts. Excessive amounts of these metals result in the formation of uneven thin coat- ings and yellowing of the deposit with possible reduction in corrosion resistance. Removal methods of these metals by precipitation and selective ion exchange are available.

Drag-in of alkalinity affects the bath pH and can be corrected with mineral acids as recommended for the specific process.

BATH LIFE Trivalent chrome passivating solutions have much longer bath life than their hexa- valent counterparts. There is no composition imbalance resulting from gradual depletion of Cr(6) and buildup of reduced Cr(3). The corrosive effect is lower, and dissolution and buildup of zinc and iron are reduced. As a result, the solution composition is more stable over time. Unless there is gross contamination, these baths will perform satisfactorily for long periods of time. Under proper steady state conditions, the solution can last indefinitely.

The use of a drag-out tank and multiple counter-current flow rinsing after the passivation step is recommended. Solution from the drag-out can be returned to the process tank, reducing chemical consumption drastically. Evaporators can be used with the drag-out tank to improve the reuse rate of the chrome solution. When passivation is operated at an elevated temperature, the returned solu- tion volume is balanced by the evaporation in the process tank. Evaporators can also be installed on the chromating tank with no adverse effect on its components.

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Cr(3)-CIr/BIBr - Rack ~ - -

Cr(3)-ClrlBIBr - Barrel m p

Cr(3) light/grn - Rack

Cr(3) light/gm - Barrel

Cr(3) + Sealer- Rack

Cr(3) + Sealer- Barrel

hrs to white corrosion

100 200 300 400

[] Daimler Chrysler PS 1207-R

lCr (3 ) Passivation

Fig. 8. Cr(3) passivations meets or surpasses requirements for DaimlerChrysier.

W A S T E T R E A T M E N T Since the process is hexavalent chrome-free, the classical sulfite reduction step is eliminated. In principle, simple neutralization will precipitate chromium and oth- er metal hydroxides. An efficient method to improve total separation of chrome from its complexed form consists of lowering the solution pH with sulfuric acid, followed by lime neutralization and settling. Accurate methods of treatment, however, must be developed for the specific passivating process used to meet local and federal waste disposal requirements. The combination of reduced solution replacement and lower use of treatment chemicals improves the economics of the process.

SPECIF ICATIONS Several U.S. and European automotive specifications are now available calling for hexavalent chrome-free conversion coatings for zinc and zinc alloy plating. These specifications take into account the need for sealers and topcoats in order to achieve desired performance criteria. Additionally, some specifications require heat-treating components prior to NSS as part of the qualifying testing criteria. Table III lists some of the U.S. automotive specifications published as of this writ- ing. Figs. 6, 7, and 8 illustrate sample performance data of commercially available trivalent chromate passivation in relation to these specifications. The performance shown reflects results obtained from actual production lines and can be exceed- ed through process modification and optimization.

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