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Molybdate-Based Alternatives to Chromating As a Passivation Treatment for Zinc

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Molybdate-Based Alternatives to Chromating As a Passivation Treatment for Zinc Molybdate-Based Alternatives To Chromating As a Passivation Treatment for Zinc By Peter T. Tang, Gregers Beth-Nielsen and Per Mailer Zinc-plated parts are typically passi- place chromates. In the early stages of vated with chromate-based solutions development, an extensive optimization to reduce corrosion. Chromates, of the treatment parameters (tempera- however, are a cause of environ- ture, duration, concentrations, etc.) re- mental concern, for their toxic ef- sulted in the formulation of two baths- fects on plants and wildlife, and al- one with a molar ratio of Me/P equal to lergic effects on workers who come 0.33 and a pH value of 2.1, and the other in contact with them. with Me/P equal to 0.66 and a pH value of A molybdate-based alternative has 4.6. These two baths were both tested, Fig. I—Typical potential versus time chart, mea- sured during the passivation of a cyanide-zinc been developed that can be used to and will be referred to as 0.33b solution sample. Potentials are measured versus SCE. replace chromates in a wide range of and 0.66’ solution. applications. The process has been tested in a project set up by the Pre-treatment ences in the temperature will not cause Danish Government in 1989, at the There is hardly any gas evolution dur- changes in the coating properties. When Centre of Advanced Electroplating ing a treatment with these alternative passivation is completed, the samples (CAG) in Denmark. Procedures used processes. The amount of zinc dissolved are washed in clean (preferably de-ion- for the alternative process are simi- during surface treatment is modest. It is ized) water and dried at room tempera- lar to those used in chromating. The important that the zinc surface be clean ture (or in hot air up to 60 “C). corrosion protection provided by the and without oxides, organic surfactants The potential versus a saturated process on zinc-plated parts is com- or other films that may hinder the forma- calomel electrode (SCE) was measured parable to chromates. Depending on tion of the passivation layer. for several of the samples during treat- test conditions, especially pH value, Parts should be passivated immedi- ment. Although there were some differ- the molybdate/phosphate process ately after the zinc coating has been ences in the results, the basic pattern was found to be better than chro- applied (they should not be allowed to was the same. mate at low pH values, equal to dry). If the zinc plating solution is cya- As illustrated in Fig. 1, a process time chromates in outdoor exposure tests nide-based (cyanide-zinc), and if the zinc of more than two min is unnecessary. and prohesion tests, but not as good solution contains additives, it is neces- There is indication that dipping for too in neutral tests, such as salt spray. sary to rinse the parts in a very dilute nitric ong can be a disadvantage. This edited version of a presenta- acid (about 0.15 mole/L HNO3. This pre- tion from the joint AESF/EAST Ses- treatment (called a bright dip) will clean Post-treatment sion at SUR/FIN® ‘94—lndianapolis, the zinc surface and remove traces of experiments have shown that molyb- discusses the effectiveness, cost, organic additives (especially important date-based solution passivated surfaces stability,0 chemistry, layer composi- for cyanide-zinc). accept post-treatments, such as paints, tion, and performance of this alter- sealers and lacquers. Tests have indi- native process. Passivation Conditions cated that some organic sealers (also The passivation process itself is simply a involving a short dip in a warm solution) hromated zinc parts are used in dip for approximately two min in the mo- will improve corrosion resistance of the the automotive, aerospace, elec- Iybdate-based bath at 60 ‘C. To ensure a combined system. tronics, and many other indus- uniform concentration of the active com- c tries. The passivation treatment ponents throughout the bath, and to avoid Performance Evaluation used to create the protective film on fluctuations in temperature, a good stir- The molybdate/phosphate-based treat- zinc contains significant amounts of ring is recommended, using air bubbling, ment performed well in a number of Cr6+-compounds, which are consid- sample movement, or something similar. tests. Figures 2–7 illustrate tests used ered undesirable because of environ- The process time is not critical, al- and results. mental and health concerns. though baths with high pH values usually A molybdate/phosphate treatmenta has require longer dipping intervals for a good Coating Properties been developed as an alternative to re- layer to develop. Process times of be- After the first promising results with the tween 80-130 sec for the 0.33 solution, molybdate-based treatment, numerous and 100-180 sec for the 0,66 solution, tests became very important. Through- aMolyPhos, patent WO 93/10278 pending b MolyPhos 33 are acceptable. A temperature range of out the development and optimization of o o cMolyPhos 66 60 C ± 5 C is adequate. Small differ- he methods,’ corrosion resistance was Fig. Z-Corrosion measurement by titration (CMT). Fig. 3-Typical CMT charts for cyanide-zinc passi- A simplified drawing of the corrosion cell and the vated with solutions 0.33 ( ) and 0.66 (0). for selected passivated and unpassivated zinc basic flow of information. coatings. 1 4 7 24 48 96 168 I Hours Fig. 5-Neutral salt spray test of Zn/Co (0.8% Co) Fig. 6-Prohesion test results. Solid bars represent Fig. 7-Outdoor exposure test showing the differ- plafed steel panels, protected by solution 0.66 solution O. 66; pattened bars are yellow chromate on ence between four popular zinc coatings and the (solid bars) and yellow chromate (patterned bars). acidic zinc substrate. Each cycle equals two hr. passivation processes molybdate-based (solid bars) Evaluated according to ISO 4540 at Danfoss A/S; and chromate (patterned bars). test confines with AESF B117. the only parameter investigated. New because this combination also had the ● Nitrogen (less than six percent) origi- tests, involving properties such as adhe- lowest corrosion rates, using CMT, be- nates from the additives used in the sion, paintability, electric conductivity, tween 4.0-7.0 µA/cm2 (twice as good as zinc baths. The presence of nitrogen heat resistance and more, had to be yellow chromate). The following compo- was not intended. planned and carried out. nents are found in the coating layers: ● Carbon is only found in the first few Chemical Composition ● Large amounts of zinc (16-22 atomic monolayer of the surface, and is im- The passivation coatings, or layers formed percent) were found in all coatings, possible to avoid. The carbon signal by the process on different surfaces and indicating that some zinc dissolved (C) has been removed in Fig. 5. alloys, have been studied in detail. For during the passivation process, but the studies of the surface and the chemi- was precipitated as part of the layer on Adhesion and Paintability cal composition of the coatings, Auger the surface. The Auger signal for so- It is important for the passivated surface analysis2 and several other methods, dium (Na) is at the same position as to accept paint and different printing tech- such as EDAX,12 XPS,13 and SEM 1 zinc (Zn), but by using XPS, it was niques. It is also important that the coat- have been used. This investigation indi- possible to separate the combined sig- ing be firmly attached to the surface, cates a significant difference between nal into some 13-15 percent Zn and 5- and that it be reasonably resistant to the 0.33 solution and the 0.66 solution. 7 percent Na. scratches and damage that may occur The 0.66 solution exhibits a homoge- during handling. neous surface with a constant composi- Oxygen (0) is found in large amounts Solution 0.33 does not work with acidic tion within the entire passivation layer. (45-50 percent), but is not surprising zinc baths and there also have been The 0.33 solution coatings are usually because molybdate and phosphate some problems with cyanide zinc. So, it is much rougher and thinner. hold four oxygen atoms. The amount not recommended for items exposed to of oxygen may be higher than the 50 rough handling or rubbing. The adhesion percent indicated in the diagram (Fig. problems do not occur with alloys such as 8), because it is the lightest element in zinc/nickel or zinc/cobalt. the film. No adhesion problems have been ob- served with solution 0.66, and it has been The amount of molybdenum (Me) is found to be better than chromate treat- quite constant in all tests (20 percent). ments in general. The oxidation state of molybdenum is The coating accepts ethanol-based 2+ believed to be M0 , except for the first paint, powder coatings and different “silk” 5+ 6+ Fig. 8-Auger profile of solution 0.66 on acidic zinc. few atomic layers where M0 and Mo or “screen” printings. Resistance to or- The sputtering rate is about 180 monolayers/min. have been identified. ganic thinners is good. Auger profiles show that the thickness Phosphorus (approximately seven per- Other Properties of the molybdate-based passivation layer cent) was expected to be higher. The The electrical contact resistance of pas- is within a range from 0.2-0.4 µm, and low amounts of active components in sivation layers formed with molybdate/ that the layer formed on acid zinc using the passivation film explain why the phosphate-based treatments is less than the 0.66 solution is significantly thicker bath is used up at a relatively slow rate.
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