Metal Finishing Handbook 2012- PDF

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Get Black to Work. The Industry’s Blackest Black Easy to Make Up Easy to Maintain Up to 250 hours Salt-Spray to White 800.456.1134 www.metalfinishing.com/advertisers Jet Black B2 Trivalent r Black Passivate for Alkaline and Acid Chloride Zinc. Haviland’s Surface Finishing Chemistry is formulated, b...

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Get Black to Work. The Industry’s Blackest Black Easy to Make Up Easy to Maintain Up to 250 hours Salt-Spray to White

800.456.1134

www.metalfinishing.com/advertisers

Jet Black B2 Trivalent r Black Passivate for Alkaline and Acid Chloride Zinc. Haviland’s Surface Finishing Chemistry is formulated, blended and packaged in Grand Rapids, Michigan.

“GLOBAL PRODUCTS, LOCAL SERVICE AND ADVICE YOU CAN RELY ON”

1(:86(5)5,(1'/72 h to wc

Zinc–iron/Tridur ZnFe H1

200 ml/l

45°C (40–50°C)

pH 5.5 (5–6.5)

>240 h to wc

Zinc–nickel/Tridur ZnNi H1

200 ml/l

45°C (40–50°C)

pH 5.5 (5–6.5)

>300 h to wc

Table 1: Application Parameters and Corrosion Results of the Post-dip (Tridur Finish 300)

the passivate or the passivate with post-dip system. The conversion coating–like composition is confirmed by an XPS depth profile, recorded on a sample of Tridur ZnNi H1 with Tridur Finish 300 (20% v/v) applied (Fig. 8). Significant carbon concentrations are only found on the surface, likely due to adsorption of CO2 from the air or surface contaminaFigure 16: Log plot of polarization experiments of an tions. Within about 10 nm, the carexperimental black zinc passivate formulation. bon concentration falls to a very low level, not changing significantEcorr /mV icorr /μA/cm2 ly with increasing sputter depth. Sample The composition of the post-dip Experimental black zinc layer and the passivate’s conversion –1,059 89 passivate without final finish coating appear almost identical. A change in nickel concentration indiExperimental black zinc –1,055 40 passivate + Tridur Finish 300 cates a diffuse transition between the more post-dip-like and the more Experimental black passivate –1,050 16 passivate-like layers. Therefore, the + Corrosil Plus 501 post-dip contributes to an increase Table 2: Corrosion Potential (E ) and Corrosion Current in thickness of about 0.2 µm in this Densities (i ) from Tafel Analysis: Potentials vs. Ag/AgCl application. (3M KCl) However, the lack of sharp transitions is also due to the fact that the post-dip penetrates deeply into the passivate layer, effectively filling up micro cracks. Both the passivate layer and the post-dip layer Figure 17: Tridur Finish 300 applied to Tridur ZnNi H1 on bear reactive sites with regard to Zn/Ni (14% Ni, 8 μm) after 1,008 h in neutral salt spray coordination chemistry. During testing according to DIN EN ISO 9227. No voluminous the deposition at elevated temperwhite corrosion products were produced. ature, and especially in the subsequent hot-air drying process, the chromium(III) present in the passivate layer reacts with the post-dip solution’s components, finally building up the enhanced corr

corr

391

Final Finish Applied

Hours to White Corrosion

None/passivate only

24–48 h

Tridur Finish 300 (20% v/v)

312 h

Corrosil Plus 501 BG*

432 h

*Organic polymer/silicate-based sealer. Table 3: Minimum Corrosion Resistance of Different Finishes Applied to Black Passivated Zinc-Nickel (Tridur ZnNi H1): Neutral Salt Spray Testing (ISO 9227).

Conversion Coating

conversion coating. Figure 9 shows a structural proposal for this layer’s composition. Polynuclear chromium(III) complexes bearing µ-phosphato bridges are described in literature10–12 and they most likely contribute to the post-dip layer’s composition. Due to the very similar composition of Tridur Finish 300 layers and passivate

F/Nm

T/kN

KM10

μthread

μhead

μtot

Tridur Zn H1

120.3 ± 11.3

36.1 ± 0.01

0.33 ± 0.03

0.32 ± 0.04

0.22 ± 0.04

0.27 ± 0.03

Tridur ZnNi H1

150.9 ± 13.7

36.1 ± 0.02

0.42 ± 0.04

0.33 ± 0.07

0.35 ± 0.03

0.34 ± 0.03

Table 4: Friction Properties Determined on M10×50 Bolts (measurements ± standard deviation)

layers, it is very difficult to find some contrast between both layers by means of SEM imaging. However, Figure 10 shows an SEM image of a FIB cross section through a sample with Tridur Finish 300 applied to black passivated zinc–nickel (14% nickel). The image reveals a thickness of 100–200 nm for the passivate and the post-dip layer. Layer morphology. The morphology of the post-dip layer was investigated using different concentrations of Tridur Finish 300 applied to a black passivated (Tridur ZnNi H1) zinc–nickel alloy surface. The morphology of the deposit in dependence of the concentration of the post-dip bath was studied by means of SEM micrographs on samples of black passivated zinc–nickel (Figs. 11–14). The post-dip caulks the micro cracks of the black passivated zinc–nickel surface. The post-dip layer’s appearance itself resembles that observed with a hexavalent black chromate on zinc–nickel with regard to the mud-crack-like surface observed. Above 200 ml/l the post-dip layer’s cracks become larger in size (Fig. 15). This means that with excessive concentrations a lesser extent of the surface may be covered by the post-dip layer. No significant advantage concerning neither the aspect nor the corrosion protection could be determined with higher concentrations. Corrosion-protection properties. Corrosion-protection properties were investigated with different concentrations of the post-dip solutions (Tridur Finish 300) applied to black passivated zinc–nickel. It was found that a high level of corrosion protection was already established with 50 ml/l of Tridur Finish 300, not increasing significantly with higher concentrations (100–300 ml/l). However, the aspect of the parts finished was found to be best at 200 ml/l (20% v/v). Evaluation on black passivated zinc–iron (Tridur ZnFe H1) produced similar results. On black passivated zinc (Tridur Zn H1), 100 ml/l was found to be a suitable concentration. With regard to the decorative aspect of the finished surfaces as well as their corrosion-protection properties by means of neutral salt spray testing, the applica392

tion parameters shown in Table 1 have been proven in practice for application on some black passivates. Tridur Finish 300 can be applied in both rack and barrel applications. The bath parameters are the same for both methods and depend only on the composition of the underlying conversion coating. Tridur Finish 300 is no substitute for sealers in general. Usually the corrosion protection that can be expected from a chromium-based post-dip can be classified as slightly below that of a film-building sealer based on polymer dispersions or solutions (e.g., Corrosil Plus 501). The corrosion behavior was analyzed by recording polarization curves ±50 mV around the open circuit potential in a three-electrode set-up, including a platinum counter electrode, a Ag/AgCl (3M KCl) reference electrode, and the sample as the working electrode. The samples were immersed in aerated solutions of 50 g/l sodium chloride adjusted to pH 7. After 3 min of equilibrium time the open circuit potentials (ocp) were measured and the sample was then polarized from –50 to 50 mV vs. ocp at a sweep rate of 5 mV/s. The data were then plotted on a graph (Fig. 12). The results of the Tafel analysis of the data are summarized in Table 2. The registered corrosion currents correlate with corrosion rates. The surface with only the passivate and no post-treatment applied showed the highest corrosion rates. Reduced corrosion rates were observed on the surface with the post-dip applied to the black passivate, and even lower corrosion rates were found with the surface having the polymer-based sealer applied. Also, the sealed surface behaves in an electrochemical manner that is slightly nobler than the postdipped surface, which itself appears nobler than the passivate surface. This principal sequence in corrosion protection is confirmed by neutral salt spray testing on samples with Tridur ZnNi H1 with Tridur Finish 300 according to DIN EN ISO 9227 (Table 3). Fig. 17 shows three steel panels plated with a Zn/Ni-alloy (14% Ni, 8 µm), black passivated with Tridur ZnNi H1 and with Tridur Finish 300 applied as the final finish after 1,008 h in neutral salt spray testing (DIN EN ISO 9227). Only a small amount of non-voluminous zinc corrosion product formed on the rinsed and dried panels. Torque and tension properties. The friction properties of the new surface were evaluated on M10×50 (thread pitch 1.50) hex head bolts of property class 10.9. The bolts were plated with 8–10 µm of zinc (Protolux 3000) as well as with zinc–nickel (14% Ni, Reflectalloy ZNA) and respectively passivated with a black zinc (Tridur Zn H1) or black zinc–nickel passivate (Tridur ZnNi H1). Tridur Finish 300 (10% v/v for zinc and 20% v/v for Zn/Ni) was applied as the final finish after the passivate treatments. Twelve samples (Zn), respectively 20 bolts (Zn/Ni), were tested on a Schatz Analyse 5413-4504 testing machine at a tightening speed of 30 min–1 according to DIN EN ISO 16047. The results are summarized in Table 4. Higher friction figures have been determined for the zinc–nickel surface compared with the zinc surface. With both surfaces the friction behavior is essentially the same as that found with hexavalent chromium–based conversion coatings (e.g., black or yellow chromates) without any sealer or lubricant applied.

CONCLUSIONS The development of a trivalent chromium–based post-dip solution has been 393

demonstrated and its properties investigated. The post-dip solution does not act like a sealer but reinforces the trivalent chromium–based conversion coating. A µ-phosphate-bridged chromium(III) complex structure, bearing a similar constitution as that of the passivate layer, has been proposed. In the course of the development of this additional step of substituting hexavalent with trivalent chromium, several efforts were necessary to adjust the formulation to achieve both the requirements for decorative appearance as well as those of corrosion protection. The objective was to develop a post-treatment process that acts as a second conversion coating and, therefore, can also easily be applied in normal plating equipment. This was successfully achieved with an elaborate new additive system. This system governs the deposition process in the background without significantly contributing to the layer’s composition. The corrosion-protection properties of surfaces with Tridur Finish 300 applied are found to be excellent but slightly lower than those of surfaces treated with film-building, polymer-based sealers. The tribological properties of the Tridur Finish 300-treated surfaces were essentially the same as those from hexavalent chromates. Although developed with black passivates in mind, the new Tridur Finish 300 final finish process can be applied to any trivalent chromium–based conversion coating in both rack and barrel applications. While satisfying the high decorative demands issued when switching to trivalent conversion coatings, the new process achieves the corrosion-protection demands of the automotive industry, even with respect to non-sealed black passivated surfaces.

NOTES 1. Wilhelm, E.J., US Patent 2,035,380. 2. Johnson, D.M., US Patent 2,559,878. 3. Directive 2000/53/EC of the European Parliament and of the Council of 18th of Sept. 2000, on end-of-live-vehicles. 4. Directive 2002/95/EG of the European Parliament and of the Council of 27th of Jan. 2003. 5. Directive 2002/96/EC of the European Parliament and of the Council of the 27th of Jan. 2003, on waste electrical and electronic equipment. 6. Lukaszewski, G.M., Redfern, J.P. Nature 1961;190:805–6. 7. Bard, A.J., Frankel, M, Stratmann, M. Encyclopedia of Electrochemistry. Vol. 4. Weinheim: Wiley-VCH, 2003. 8. Jelinek, T.W. Galvanisches Verzinken. Saulgau: Eugen G. Leuze Verlag, 1982. 9. Sonntag, B, Vogel, R. Galvanotechnik 2003;10:2408–13. 10. Redfern, J.P., Salmon, J.E. J Chem Soc 1961;291. 11. Springborg, J. Acta Chem Scand1992;46:906–8. 12. Haromy, T.P., Linck, C.F., Cleland, W.W., Sundaralingam, M. Acta Cryst 1990;C46:951–7.

394

plating processes, procedures & solutions TRIVALENT CHROMIUM FOR ENHANCED CORROSION PROTECTION ON ALUMINUM SURFACES BY HARISH BHATT, ALP MANAVBASI, AND DANIELLE ROSENQUIST, METALAST INTERNATIONAL, INC., MINDEN, NEV. Chromate conversion coatings have been routinely applied on aluminum-based surfaces in order to improve corrosion characteristics and adhesive properties. The conventional chromate conversion coating process uses highly oxidizing toxic hexavalent chromium (Cr+6) compounds and ferricyanide. The metal finishing industry has been developing less toxic alternative coatings in order to comply with environmental regulations and substance restriction legislation, such as the European Union’s Restriction of Hazardous Substances (RoHS) directive. One promising alternative is the trivalent chromium–based environmentally friendly conversion coating. This article will describe a new trivalent chromium process for chromate conversion on aluminum with high corrosion protection, good paint adhesion, low cost, quick and simple processing, and all while meeting the stringent requirements of military specifications. It is QPL (Qualified Product List) approved by the United States Navy–Defense Standardization Program under Governing Spec MIL-DTL-81706-B. In addition, this article will outline various chromate conversion techniques for aluminum. It will address a new, environmentally friendly, cost-efficient, and performance-oriented chromate conversion coating with a unique and patented trivalent chromium pre- and post-treatment chemistry for aluminum.

CHROMATE CONVERSION OF ALUMINUM Chromate conversion coatings have been used for several decades in the aerospace industry to improve the corrosion resistance of aluminum alloys. Chromate conversion coatings have also been used to passivate zinc, cadmium, copper, silver, magnesium, tin, and their alloys. Chromate coatings, similar to phosphate coatings, are processes of chemical conversion because they contain both substrate metal and depositing species. However, chromate coatings are formed by the reaction of chromic acid or chromium salt water solutions. Chromate conversion coatings usually exhibit good atmospheric corrosion resistance. These conversion coatings form an ideal substrate for paints by providing a clean, essentially inert surface, which provides optimum conditions for adhesion. The application of chromated aluminum can cover a wide range of functions. Conversion coatings can provide mild wear resistance, better drawing or forming characteristics, and may be used to provide a decorative finish. In addition, they are also ideal for pretreatment prior to organic coating. Most organic coatings applied directly to aluminum surfaces will not adhere well, and if subjected to any deformation they will tend to flake off, exposing the bare aluminum. Scratching off the paint surface would also provide a nucleation site for aluminum corrosion and further undercutting of the coating. 395

DESIRABLE CHARACTERISTICS OF HEXAVALENT CHROMATE PASSIVATES • Prevents oxide formation • Provides color • Slow corrosion in prototypic tests (e.g., salt spray, rooftop, etc.) • Provides adhesion for organics (e.g. paint) • Prevents corrosion of painted surfaces • Conductive • Thin • Flexible

• • • • • • • • • •

Lubricious Easily applied Stable for weeks or months Durable Resilient (self healing) Coats in recesses Easy to strip Inexpensive equipment Single tank Inexpensive (charge-up cost)

The successful application of this conversion process requires the aluminum to be clean and free of organic soils, oxides, and corrosion products. Therefore, a pretreatment process is required that can be applied to aluminum and provides a suitable basis for subsequent coatings. Conversion coatings that can be used on aluminum alloys and are compatible with most paint systems have been developed. The name “conversion coating” describes a process of chemical reaction that results in a surface film. As a result of this reaction and conversion, the film becomes an integral part of the metal surface, which exhibits excellent adhesion properties. Chromate conversion coatings are a thin chemical film, usually less than 0.25 microns in thickness and are electrically conductive.

HEXAVALENT CHROMATES Historically, hexavalent chemistry has been used to process aluminum chromate conversion parts. Chromate passivation systems containing Cr+6 compounds are an extremely versatile group of aqueous chemistries that are extensively used in a diverse range of electroplating and metal treatment processes. They impart many beneficial and essential characteristics to metallic substrates and deposits obtained from a number of techniques, such as zinc electroplating. Chromate conversion coatings on alloys are formed by the reduction of chromate ions and the development of a hydrated Cr2O3 barrier layer, which provides corrosion resistance and further protection due to residual chromate ions. Hexavalent-based passivation (Cr+6) exhibits a number of desirable characteristics. The process will passivate the surface of zinc and zinc alloy electrodeposits with a thin film that provides end-user benefits such as color, abrasion resistance, and increased corrosion protection. When damaged, these hexavalent chromates possess a unique “self-healing” property. This means that soluble Cr+6 compounds contained within the passivation films will re-passivate any exposed areas. Hexavalent chromate has wet, gelatinous film drying at the surface. Subsurface moisture (dehydrating in approximately 48–72 hours) provides self-healing and lubricity characteristics. The deposits are harder than conventional trivalent chromate film, and they offer torque and tension to meet the finishing requirements of fasteners. Unfortunately, the Cr+6 used in generating cheap and very effective coatings poses serious health hazards as well as waste treatment prob396

Conversion Coating

Pretreatment

% Passed

% Failed

Enhanced

Etched

81

19

Standard

Etched

31

69

Enhanced

Non-etched

90

10

Standard

Non-etched

53

47

Table 1: Effect of an Additive on Corrosion Resistance

lems. Chrome sores, which are severe damage to mucous membranes and skin lesions, occur from exposure to the ever-present chrome-mists and aerosols in job shops. Environmental guidelines and regulations are in place that restrict and prohibit its usage. The finishing industry is developing less toxic alternatives in order to comply with substance restriction legislation and directives from the European Union. The most significant directive is RoHS, signed on Jan. 27, 2003, which went into effect July 1, 2006. The restriction covers six hazardous substances: lead, mercury, cadmium, Cr+6, polybrominated biphenyls (PBB), and polybrominated diphenyl ether (PBDE). Another European Union legislative action, the second edict that also contains Cr6+, is the End of Life Vehicle (ELV) directive, which went into effect on July 1, 2007. Four heavy metals included in ELV directive include: cadmium, lead, mercury, and Cr+6 (approximately 70% of total heavy metals is Cr+6). Industry has been actively following any new development to replace Cr+6. The most common alternative is trivalent chromium, which is environmentally friendly. However, there are still some weaknesses with trivalent chromate coatings. In order to achieve equal or better corrosion resistance compared with hexavalent chromate, in most cases a sealer or a topcoat is required. Some chemical manufacturers now offer better salt spray performance without any sealers o...