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Development of a process for cyanide-free alkaline bright zinc plating in conventional galvanic plants and the substitution of chromium (VI) in zinc passivation
- Bundesministerium für Bildung und Forschung (BMBF)
Fertigstellung des Projekts:
Fertigung / produzierende Unternehmen
The process developed, which uses a centrifuge installation and is the subject of a patent application, provides improved corrosion protection in relation to conventional cyanide-free processes. Heating costs are systematically reduced, and energy losses are lower at any bath temperature required, from 0 to 100ºC.
Zinc is among the most common metals to be recovered by galvanic separation. Zinc galvanised components are mainly used in the automobile industry, the construction industry and in electrical engineering.
The problem, in environmental terms, is that virtually all zinc and zinc alloy surfaces are treated with chromium (VI) compounds in order to improve corrosion protection and bonding properties (e.g. for enamelling). Chromium (VI) is a sensitising agent and is acutely toxic in large quantities. Zinc chromate in particular is carcinogenic. Since limit values in waste water are extremely low, industrial waste water treatment must involve a chemical reduction process.
These are the background conditions to the project for the development of a new cyanide-free zinc galvanising process and the formulation of the addition agents required for this purpose.
The new process must satisfy the following requirements:
- Reliability will need to be superior to that of the conventional cyanide-free process, if the technically superior but toxicologically problematic cyanide zinc process is to be completely superseded.
- For ecological reasons, the complexing agent cyanide should not be replaced by other harsh complexing agents.
- It should be possible for the process to operate without the production of waste water.
By the use of appropriate formulations, it has been possible to develop chromium (III)-based passivation processes which provide a comparable performance to that of chromium (VI)-based processes. This procedure is described as “Chromiting”.
Practical tests on a technical scale have been undertaken in a dipping bath and in a centrifuge installation.
Centrifuge chromiting has proved to be a particular success:
- The comparatively high chromium (III) concentration accounts for the relatively high startup costs of Chromiting. The use of a centrifuge installation allows a significant cost reduction to be achieved. Lower heating costs are also associated with the smaller initial volumes involved.
- In a closed centrifuge system, with its comparatively low energy losses, the bath solution can be maintained at any temperature required, from 0 to 100ºC.
- Carry-over in centrifuge installations is only some 10-25% of that associated with conventional installations of comparable capacity, since the solution is centrifuged and returned to the system after processing.
- Using a boiling Chromiting solution, coatings with corrosion protection values of 800 – 1000 hours to initial corrosion have been achieved – this surpasses all values achieved hitherto.
- Subject to appropriate operation, components are less susceptible to scratching, thereby improving protection times to initial corrosion.
- As a minimum requirement for the improvement of durability, the reservoir of Chromiting installations should be coated with a synthetic material.
The Chromite coating, the Chromiting process and the composition of the Chromiting solution were the subject of a patent application of 19th April 1996. An application for the registration of the name “Chromiting” as a trademark has been filed.
Autor/Projektleiter: Dr. Rolf Jansen
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