Increasing Bath Life and Quality of Trivalent Passivates on Zinc Plate
Asterion’s Roger Sowinski says the use of proprietary organic pre-dips has also been effectively utilized to both extend the bath life and improve the overall quality of the finished part.
Q: What is the best way to increase the bath life and quality of trivalent passivates on zinc plate?
A: One of the more significant developments of the last 20 years in the metal finishing industry is the commercial development and application of trivalent passivates on zinc electroplate. These developments have allowed applicators to eliminate the use of toxic hexavalent-chromate conversion coatings, thereby safeguarding the health of all involved in the handling and processing of zinc-plated parts. Further, these trivalent passivates have a substantially positive impact on the environment. The following information addresses issues involved with producing a consistent blue-bright, thin-film passivate on electrodeposited zinc.
Trivalent passivates are typically acidic in nature and operate at a pH between 1.6 and 2.2, under most operating situations. Therefore, during the passivation step, both zinc and iron will be dissolved in the passivate solution, and their concentration in the passivating solution will increase. The zinc comes from the surface of the plated part and the iron comes from unplated areas (such as blind holes, tubular parts or parts lost from racks and improperly fitted barrel doors). These lost parts will, with time, have their zinc coating dissolved by the acidic passivate, followed by iron dissolution from the part itself. Increasing both iron and zinc can cause the problem of “yellowing” the parts. The mechanism of yellowing differs for zinc and iron. A chrome 3-zinc complex is formed in a good passivate film. This results in a blue-bright finish. As the iron increases in the passivate solution, a yellow-brown, zinc-iron-chrome 3-complex forms. These are competitive reactions, and the intensity of the yellow color will depend upon the iron concentration. The typical iron limitation is 300 to 1000 mg/l. Since this is a competitive reaction, the iron concentration can be moderated by increasing the chrome concentration in the passivate solution, thereby preventing the conversion film from turning yellow.
We also need to consider the increasing zinc content. Much higher zinc levels can be tolerated before a yellow color is observed as a result of high zinc. Trivalent passivates can tolerate zinc concentrations in excess of 10 grams/liter. As mentioned above, a chrome 3-zinc complex comprises a good passivate film. Zinc acts as a catalyst during the deposition. Therefore, if the concentration of the chrome 3 is high in the bath, the potential for too heavy a chrome film is possible, which leads to a yellow coloration. The real issue for extending the bath life of these passivates is developing a sound strategy to control the amount of dissolved metals in the bath.
Because zinc can be tolerated at a much higher level, a sound strategy would be to control the amount of dissolved iron in the passivate solution. One of the simplest methods of bath life extension is to keep the passivating solution free from parts. A periodic sweep of the solution (magnetic) to remove parts that have fallen from racks or out of barrels should be done on a daily basis. Further, the use of iron inhibitors in the passivate has proven to be an effective method of preventing the dissolution of iron from exposed steel surfaces by preventing the acid from attacking the surface. A low equilibrium concentration of iron can be achieved through the use of these inhibitors in which the amount of iron dissolved will be removed by amount of dragout.
For passivate solutions where the iron concentration is already at elevated levels and the parts have taken on a yellow hue, the addition of an organic acid should be considered. These act as weak complexing agents, whereby they inhibit the codeposition of iron in the passivating layer. Care should be taken to avoid adding too much acid, which would result in an acceleration of the iron dissolution. A specific ration between the amount of iron and acid can be calculated to effectively negate the yellowing effect of iron, depending upon the acid used. In the case of the inhibitor and organic acid usage, the finisher should consult with the supplier of their proprietary passivate(s) for their recommendations.
With proper bath maintenance and a sound strategy for iron control, the amount of zinc in the passivate should not present a serious discoloration problem. Common sense should be the guide in knowing when to dump the passivate solution.
Aside from the metallic ion contamination associated with trivalent passivation, organic contamination from organic occlusion products present in brighteners used for both acid and alkaline zinc plating process should also be considered. Since most zinc baths utilize organic brightening agents, these materials will codeposit in the zinc plate and will be dissolved in the acidic passivate solution. The simplest solution is to use an acid predip and rinse prior to passivation. Inorganic acids (typically Nitric acid made up at low concentration) should be used after plate and prior to the passivate solution. These acidic predips should be dumped on a regular basis. The use of proprietary organic predips has also been effectively utilized to both extend the bath life and improve the overall quality of the finished part.
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