Flexible coating

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The flexible zinc-nickel coating is a galvanic coating process in which the coating can be reshaped or bent after it has been applied . The coating serves to protect against corrosion , which is also guaranteed after forming.

Materials

Steel / ferrous metals come into consideration as coating material or base material or carrier materials ( substrates ) for zinc - nickel layers .

Basic principle of the zinc-nickel coating

Zinc-nickel coatings are deposited using electrolysis . A voltage is applied to a conductive solution containing metal ions, as a result of which a metallic layer is deposited on the electrodes . During the metallic deposition, the components whose coating is desired act as cathodes .

Development of the zinc-nickel alloy layer for coating

The development of the zinc-nickel coating resulted in the 1980s from the increasing demands that the automotive industry in particular places on corrosion protection against temperature, road salt and climatic loads. Zinc-nickel coatings are used when the classical electrolytic galvanizing abuts by high temperatures or aggressive environmental conditions to their limits. The nickel incorporation rates of the zinc-nickel alloy layers are around 10–15%.

Another reason for using zinc-nickel layers is their compatibility with aluminum. With the increasing use of add-on parts or even body parts made of aluminum, an alternative solution to zinc layers was necessary, because zinc leads to pitting ( contact corrosion with aluminum ) in contact with aluminum . Steel components provided with zinc-nickel layers, on the other hand, prevent contact corrosion with aluminum. The special requirements for the coatings are regulated by the automotive industry with standards that specify, for example, the necessary corrosion protection (example: 720 hours in the salt spray test without the formation of red rust).

Special features of the flexible zinc-nickel coating

Application of a flexible zinc-nickel layer

The flexible, deformable zinc-nickel alloy layer has an installation rate of 12-15% nickel and is deposited electrolytically. A high level of corrosion protection is achieved even with thin layers of 6–10 µm. By using a special electrolyte in conjunction with closely defined bath management parameters, the zinc-nickel process specifically changes the crystal structure , grain size and yield point of the deposit.

This creates a ductile, malleable, flexible zinc-nickel alloy surface.

Special features after coating

Pipes and sheet metal parts such as spindle tubes or magnet housings as well as other components can also be bent, reshaped or flanged after they have been coated with flexible zinc-nickel . No additional sealing is necessary. Corrosion protection is fully guaranteed even after the coating and forming process ( 720 hours in the salt spray test without red rust) and thus meets the current automotive standards.

In the case of parts coated with conventional, transparently passivated zinc-nickel processes, the coating can crack and flake off during deformation, which reduces the corrosion resistance. Therefore, preformed parts are often coated.

If flexible zinc-nickel is used, pipes and similar products can be coated straight or in their original condition. Since straight pipes are easier to store, handle and transport than curved pipes, it reduces the handling process and effort if the forming process takes place after coating. Bent pipes must be closed with plugs during the coating process in order to prevent the ingress of liquid, as this liquid would otherwise cause corrosion inside the pipeline. In addition, the rack systems (product carriers) of the coating systems can be loaded more efficiently with straight tubes, so that higher quantities can be coated in one work step.

Post-treatment of zinc-nickel surfaces

Post-treatment options can be used to temporarily protect the uncoated inner surfaces of, for example, pipes from corrosion. With a special solution that only reacts with the uncoated iron surface inside, for example, storage protection lasting four weeks can be achieved without the inside area corroding.

application

Flexible zinc-nickel layers are mainly used in the automotive industry (automotive & suppliers), e.g. B. for pipes and hydraulic lines as well as for punched or flanged sheet metal parts. There are also applications in the hydraulics industry (e.g. oil or fuel lines) and in mechanical and plant engineering.

literature

  • Jelinek, TW: Practical electroplating technology. EG Leuze Verlag, Saulgau, 2013.
  • Unruh, JNM and Hoffmann, R .: Lexicon for surface technology. EG Leuze Verlag, Saulgau, 2013.

Web links

Individual evidence

  • JOT Journal for surface technology, Springer Vieweg Springer Fachmedien Wiesbaden GmbH, Wiesbaden, edition 9/2008, page III-IV
  • WOMag, WOTech Technical Media, Waldshut, issue 2/2013; Pages 26–29