An Evolution of Trivalent Chromate Technologies
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Alternatives to the Hexavalent Chromates: An Evolution of Trivalent Chromate Technologies. By William Eckles and Rob Frischauf, Taskem, Inc. Fourth generation trivalent chromate hexavalent chrome in the work place, conversion coatings exhibit excellent except for hard chrome plating.1,2 The corrosion resistance and self-healing best alternative to hex-chrome is properties, not unlike hexavalent trivalent chromium for both chromate chromate conversion coatings. These conversion coatings and for decorative self-healing trivalent chromate chrome plating. Consequently, trivalent conversion coatings incorporate chromate conversion coatings are chemically inert nanoparticles in the replacing conversion coatings containing conversion coating. The nanoparticles hex-chrome. This process of replacing add thickness and corrosion resistance hex-chrome with trivalent chrome has a to the chromate film. The migration of history going back to the 1970s. the nanoparticles into scratches in the conversion coating film produces the First generation trivalent chromates were self-healing effect. mixtures that were similar in composition to hexavalent chromates, What surface coating technologies are except that the oxidizing power was available to fight against corrosion, now supplied by either hydrogen peroxide or that hexavalent chromates are no longer by nitrates, in place of chromic acid or available? Surface coatings increase chromate salts. These early trivalent service life with increased corrosion chromates were used to produce a blue protection, aesthetic appeal, and coating on alkaline non-cyanide zinc. increased wear resistance. Zinc The coatings were thin (~60 nm), electroplate with chromate coatings are powdery, and provide limited corrosion the low-cost, bulk-applied coatings that resistance. What corrosion resistance are widely specified. Electroplated zinc they did provide was often limited to the is given post-plating treatments prevention of “finger staining,’ but not consisting of chromate conversion much more. Typical neutral salt spray coatings, topcoats, and sealers. Until test (NSST) resistance was 3 or 4 hours recently, these chromate conversion to 5% white rust. coatings and sealers contained the hexavalent chromium ion from either Improvements that lead to what are chromic acid or chromates. referred to as second generation trivalent chromates, consisted of “more of the The End of Life Vehicles (ELVs) same.” Higher concentrations, higher Directive (2000/53/EC) of the European temperature, and the use of fluoride Union mandated the replacement of extended the NSST corrosion resistance hexavalent chrome by July 1, 2007 and to perhaps as high as 24 hours to 5% has effectively ended the use of white rust. 2006 SUR/FIN Proceedings 818 ©2006 SFIC The next step was the introduction of (sputtering), where the surface is “thick film” trivalent passivates that bombarded with ionized argon to relied upon the introduction of weak remove layers of the film. The revealed organic acids into the formulations.3 surface is then exposed to an electron This innovation provided corrosion beam that causes x-ray emission from resistance of about 48 hours to 5% white Auger electron transitions in the excited rust in salt spray corrosion testing for atoms within the film. films that were about 100 nm thick. True third generation trivalent chromates 100 90 were improvements based on the 80 70 Nano introduction of transition metals and 60 O 50 Cr higher operating temperatures, which 40 N 30 produced thick (~400nm), green Zn 4 20 coatings with red/green iridescence. 10 Elemental Percent 0 The corrosion resistance provided by 0 100 200 300 400 500 600 700 800 900 1000 these “green” coatings was a quantum Depth (Angstroms) leap beyond what could heretofore be th Figure 1. Ion milling of 4 generation achieved. NSST results were in excess trichromate film, using Auger detection. of 300 hours to 5% white rust. Another property of third generation trivalent As the sputtering proceeds, more and chromates is that they have little water more layers of the film are peeled away, of hydration in the film and are less as illustrated in Figure 1. At the surface affected by exposure to heat. Therefore, can be seen the presence of a baking will not significantly degrade spontaneously formed nano-particle them. topcoat, followed by the actual chromate conversion coating, that also contains Fourth generation trivalent chromate nano-particles. Finally, the milling films are also thick (~400nm), but with process reaches the zinc surface. The a blue, slightly iridescent color. These total film thickness milled is about films are also able to achieve over 300 400 nm. Fourth generation trivalent hours in NSST to 5% white rust. A chromates contain the nano-particles in unique property of this fourth generation the chromating solution and are not a trivalent chromate is that it exhibits self- post-chromate topcoat. healing properties, somewhat like the self-healing of hexavalent chromate The chrome-containing layer in this film films. The unique ability to self-heal is is relatively thin for a chromate that the result of incorporating nano-particles achieves more than 300 hours in NSST in the formulations. These chemically testing, and consequently contains much inert particles spontaneously form a less trivalent chromium than third topcoat on the chromate, while the parts generation films. Corrosion resistance are being processed in the trivalent of 600 hours to 5% white rust has been chromating solution. reported; twice what is achievable with yellow hexavalent chromates! These layers can be seen in Figure 1, which illustrates the results of an “Self-healing” is the property of analytical process called ion milling chromate films which allows small 2006 SUR/FIN Proceedings 819 ©2006 SFIC imperfections or cracks in the film to be Figure 3 shows the relationship between filled with more “chromate” from the chrome concentration and NSST for a chemicals already present in the fourth generation iridescent, trivalent chromate film and water from the chromate. Figure 4 shows the very environment; i.e., the salt spray cabinet. important relationship between In the case of hexavalent chromates, temperature and time to white rust. It these “healing” chemicals include can be noticed that the optimum chromate ion that diffuse into scratches temperature is about 30 degrees in the film and further react with zinc to centigrade, a significantly lower repair the scratch with more chromate temperature than third generation conversion coating. trivalent chromates. Therefore, fourth generation trivalent chromates are considered “low temperature”. 500 400 300 Figure 2. Self-healing illustration using 200 nanoparticles to fill voids. 100 Hours to WR 0 Self-healing with nano-particles is 15 25 35 45 illustrated in Figure 2. Here the nano- Temperature particle coating can be seen filling a cut in the chromate film. Q-Panels with Figure 4. Corrosian resistance vs. operating crosscuts have gone more than 300 hours temperature. in NSST without corrosion. In Figure 5 the effect of immersion time Fourth generation chromates do not use on corrosion in salts spray can be seen. organic acids, so waste treatment It should be noted that most of the problems caused by complexed metals corrosion protection can be attributed to are avoided. These fourth generation the first 20 seconds of immersion at 30 films are totally inorganic. Baking will degrees. not significantly degrade them. 500 500 400 400 300 300 200 200 100 Hours to WR Hours to WR 0 100 1.534.56 0 Chrome concentration(g/L) 10 20 30 40 50 60 70 Immersion Time (sec.) Figure 3. Corrosion resistance vs. chrome concentration. Figure 5. Corrosion resistance vs. immersion time 2006 SUR/FIN Proceedings 820 ©2006 SFIC Figure 6 shows that for the best results in generation trivalent chromates, requiring salt spray, the optimum pH is 2.0, with a operators and vendors to work together fairly narrow range of operating pH from to achieve optimum performance. 1.8 to 2.2. At pH<1.8, the film is dissolving as well as forming, and at pH>2.2, the nano-particles begin to fall References: out of solution before they can form a 1. The Environment Agency, 2006, film. www.environmentagency.gov.uk 2. Directive 2002/95/EC of the European Parliament and Council. 500 3. Klos, US Patent 5,368,655; 1994. 400 4. Preikschat, et al, US Patent 6,946,201. 300 200 100 Hours to WR 0 1.6 1.8 2 2.2 2.4 2.6 pH Figure 6. Corrosion resistance vs. operating pH. Iron and zinc are the most significant contaminants in the chromating solution. Levels of iron can reach 300 ppm without any decrease in corrosion protection, whereas zinc can reach 20,000 ppm in these chromates without any detrimental effects. Iron will discolor (blacken) the film long before it reduces corrosion resistance. Self-healing and low temperature operation are unique characteristics of fourth generation trivalent chromates. As with all thick film trivalents, careful control of operating parameters is necessary to achieve the promised performance. This cannot be emphasized enough. Operating outside the recommended limits in the operating instructions will result in poor performance, in terms of corrosion resistance, but can also cause the chromating solution to decompose. Therefore, there is a new learning curve for successful operation of fourth 2006 SUR/FIN Proceedings 821 ©2006 SFIC.