Find the Right Coating for Every Zinc Die Casting Component
Zinc die cast components are used for many different purposes. Therefore, possible surface treatments are just as numerous. Which coating is most suitable for a given application is explained in this article.
When comparing zinc die casting to other casting materials, it is characterized above all by its high dimensional accuracy, high strength even with thin-walled molded parts and comparatively good corrosion resistance. Even the most demanding geometries can be realized with this material.
Choice of Several Coating Processes for Zinc Die Cast Parts
Structural parts made of die cast zinc are used in a wide variety of applications and in many areas of everyday life, e.g. in the automotive, mechanical and apparatus engineering, electrical engineering, electronics and construction industries (Fig. 1). This variety of possible applications also requires a wide range of processing methods of the material - depending on the area of application. Different kinds of surface treatment alter the corrosion protection, optical appearance or tribological properties of the parts.
Depending on your preferred techniques and requirements, the most important methods to choose from are:
- Decorative galvanic coating, e. g. copper, nickel, chrome;
- Functional galvanic coating, e. g. zinc + passivation;
- Direct passivation;
- Pretreatment and subsequent painting.
Decorative plating allows for stable and very uniform metallic surfaces (Fig. 2). For this purpose, zinc die castings are treated in a sequence of different process baths, which ultimately produce a high-quality decorative final layer. A typical layer sequence is shown schematically in Figure 3.
The first three layers of the sketched sequence are used for adhesive strength, levelling of roughness and corrosion resistance. Only the fourth layer has a direct effect on the appearance of the surface. Depending on the process, the surface can appear glossy or matt (figure 4).
Alternative Methods for Chrome Plating
In order to increase the abrasion resistance, to avoid nickel layers (risk of nickel allergy) and to preserve the well-known metallic-blue color, the parts are chromium-plated in the last step. In the past, only electrolytes containing chromium (VI) were used. However, chromic acid is toxic and chromium (VI) compounds have been included in Annex XIV of the Reach Regulation.
Therefore, alternative methods have long been worked on. In the meantime, it is now possible to deposit chromium layers from electrolytes containing chromium (III). Depending on the application, different optic characteristics can be achieved, as shown in Fig. 5: white type, dark type and anti-salt type.
Functional Galvanizing Improves the Optical Appearance
Functional electrogalvanizing not only increases corrosion protection, but also improves the appearance of the surface (Fig. 6). The electrolytically deposited zinc is homogeneous and, if necessary, creates an even surface, which makes the component appear shinier and more uniform. The desired appearance and the required corrosion protection of the component can be achieved by selecting the subsequent chromium (III)-containing passivation:
- Thin film passivation: bluish iridescent with moderate corrosion protection;
- Thick film passivation: red-green iridescent with excellent corrosion protection (Fig. 7);
- Black passivation: black with good corrosion protection.
- An additional coating applied can further improve corrosion protection, heat resistance or tribological properties [1,2].
For the direct passivation of zinc die casting, processes containing chromium (III) are also applied. Their main advantage is the simple and cost-effective process control. Compared to galvanic plating with subsequent passivation, the bath sequence and treatment times are shorter, which allows fora higher material throughput. However, this process only provides a technical coating with good corrosion protection, but without any decorative properties at all (Fig. 8).
Chromium (III) Passivation Provides a Good Basis for Painting
Direct passivation is also an excellent basis for subsequent painting. The good corrosion protection of the chromium (III) passivation becomes obvious when parts of the component are not painted or only partially painted, either intentionally for reasons of dimensional accuracy or unintentionally due to the complicated geometry of the components.
Compared to conventional iron phosphating as a pre-treatment for painting, chromium (III) passivation is much more suitable. After 120 hours exposed to salt spray mist, the iron phosphating process already reveals a distinct infiltration at the test scoring. Unpainted areas show voluminous zinc corrosion and even at the inner edges, which are not completely covered by the applied powder coating, the corrosion breaks through. In contrast, chromium (III) passivation does not show any signs of infiltration at the test scoring. Furthermore, uncoated or incompletely coated areas show almost no signs of corrosion.
Multi-Metal Pre-Treatment Enables a Process Bath for Different Materials
In the area of pretreatment, it is becoming more and more common to treat different base materials in the same process baths, on the one hand to save space, but on the other hand also because the components are becoming increasingly complex and often consist of a combination of different materials.
The Surtec 609 Zetacoat multi-metal pre-treatment is especially suitable for this purpose. This process is used to treat steel, zinc (hot-dip galvanized, electrolytically galvanized and zinc die casting) and aluminium. Compared to traditional zinc phosphating, Zetacoat offers significant advantages: almost no sludge accrues in the process bath, short overall process, environmentally friendly and free of phosphates, zinc, nickel, manganese, nitrite and VOC [3,4,5,6].
Several Processes for Surface Finishing
It can be summarized that a wide variety of processes can be used for the surface finishing of zinc die casting. Which method is ultimately used depends on:
- the desired and required optics
- the required corrosion protection
- technical requirements such as electrical conductivity, wear protection, combination with other materials and avoidance of contact corrosion
The table (left) provides an overview of the various surface finishes and compares the essential properties of the processes.
Jansen, Rolf and Preikschat, Patricia (2001):"Chromium (VI)-replacement on zinc — post-treatment processes in practice" in: Report volume on the 23rd Ulm Dialogue 2001, Bad Saulgau: Eugen G. Leuze Verlag, p. 33-41
Jansen, Rolf and Preikschat, Patricia (2001):"Chromating and passivation on zinc and zinc alloys" in: Jahrbuch Oberflächentechnik, volume 57,2001, Hanover: Giesel Verlag, p. 71-83
Volk, Peter (2006):"Alternatives to chromium (VI) pretreatment" in: besserlackieren, 5/2006, No. 9, S 7
Rausch, Werner (2005): The phosphating of metals. Bad Saulgau: Eugen Leuze Verlag, 3rd edition
5] Pohl, Jürgen and Möller, Ralf (2008):"New Cr (III)-based pretreatment" in: JOT Journal für Oberflächentechnik 2008, No. 11, p. 58-61
Volk, Peter (2010):"Chromium (III)-containing passivation of zinc die-casting" in: Giesserei 2010, Issue 7, pp. 22-27
Peter Volk is Head of Research & Development Metal Pre-Treatment/Industrial Parts Cleaning at Surtec International GmbH in 64625 Bensheim, Germany
this article was first published by blechnet.
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