Galvanizing Technique and Its Key Role

The galvanizing technique has developed over the past several decades from a serious experimental coating method into a scientifically-based technology. Coating methods have been systematically developed by taking into account the type, composition and properties of layers, with particular attention paid to specific process parameters (such as deposition rate, the degree of electrolyte action and distribution quantity). With the aid of this method, it is possible to separate pure metal and alloy layers in almost any desired thickness. The properties of such metallic layers are essentially different from metals and alloys produced metallurgically. The reasons for this should be sought in the microscopic or submicroscopic structure of such metal layers. This phenomenon is clearly evident in the case of Vickers micro-hardness. From the information in Table 1, it can be seen that metal layers produced by the galvanizing technique are harder than metals obtained metallurgically. The galvanizing technique enables the production of conversion layers, the separation of composition systems from metallic and non-metallic deposits (dispersion layers), the implementation of partial layers (selective metal separation), the repair of damaged coatings (part recycling) and the preparation of parts from complexly shaped surface profiles (galvanic shaping). Due to the multiplicity of these possibilities, a few examples are shown in table 2.

1.1 Metallic Layers

The following applications relate to the specific properties of metallic layers and coatings and their fields of application as specified in table 2.

1.1.1 Chromium Layers

Galvanically separated chromium layers are distinguished by high tarnish resistance, hardness and wear resistance and low friction coefficient and minimal adhesion tendency. With thicknesses between 0.2 and 0.6 μm and colour tones between bright silver and matt, they can be used for decorative purposes. Thus special design effects can be realised. In addition to bluish-white chromium, black chromium layers with thicknesses between 0.5 and 2 μm are also used for optical devices and cameras. Hard chromium refers to a chromium coating applied to structural parts with a thickness between 10 and 500 μm as required. By using hard-chromium-plated cylinders, valves and piston rods of diesel locomotives, it has been possible to extend general maintenance intervals from 10,000 kilometres to over 1 million kilometres. Hard-chromium-plated parts are used in mining, aircraft manufacturing, hydraulic and shaping technology and in the production of medical and surgical instruments.

1.1.2 Precious Metal Layers

Even very thin layers of precious metals show great effect. Galvanic separation of precious metals therefore plays an important role in electrical/electronic engineering, optics, aviation and space travel and in the production of chemical apparatus. In special cases it permits partial limited coating, for example on electronic contacts or cut strips. High-frequency conductors silver-coated by galvanization, which serve the same purpose as massive silver conductors, is another example. Precious metal separation enables the production of gold alloys of different values with special technological properties. Such gold coatings perform very well wherever there is risk of metal allergy. There is no need for further explanation that they provide excellent corrosion protection. Palladium layers are used not only for decorative purposes, but especially due to their sliding properties and high hardness for contact structural elements and sliding connections.

1.1.3 Nickel Layer

Nickel coatings are largely used for decorative corrosion protection. In this way fittings, vehicle parts and lighting elements – a few examples – are coated with a protective nickel layer. By nickel-plating plastic parts, metallic appearance, high reflection structure and minimal weight are reasonably combined with each other. In galvanic shaping, nickel layers also play an important role in wear resistance and corrosion resistance due to their hardness.

1.1.4 Zinc Coating

Protection of load-bearing parts from corrosion is a very important issue in the automotive industry. The surfaces of these parts are therefore galvanized. Subsequent passivation and sealing in special organic or inorganic solutions enhance the protective effect of the zinc coating. The effective corrosion protection thus obtained has led to an increasingly large number of galvanically zinc-coated and additionally processed parts being used in motor vehicles. Optically attractive passivation layers generally eliminate the need for painting of steel structural parts.

1.2 Layer Combinations

Layer systems consisting of copper, nickel and chromium (separated from metal or plastic materials in the stated sequence) have proven themselves as decorative surface protection and first made aesthetic design in automobiles possible. Besides this, such a layer combination also protects the metallic structural part against corrosion and wear and gives plastic properties that it only has to a limited extent. These include: Scratch resistance, hardness, rigidity, heat resistance, electrically conductive surface and decorative appearance. Additionally, the use of galvanized plastic materials has resulted in a significant reduction in weight in automobile production. A point particularly worthy of mention in this context is that electrical and electronic structural elements and machines, through galvanization of the plastic housing, have the capability of screening (shielding) against the reception and transmission of electromagnetic waves (electrosmog). 1.3 Alloy Layers In most cases, surface requirements arise that a single pure metal layer alone cannot fulfill. In such cases, surfaces are coated with alloy layers. Gold-Copper-Cadmium-Alloy layers distinguish themselves by their excellent corrosion resistance. Nickel-Cobalt-Layers are used in galvanic shaping in the production of crosshairs and binoculars due to their high strength. Nickel-Phosphorus-Alloy layers chemically separated with a phosphorus content between 8-15% (also called Chemical Nickel) yield precise contoured and flat parallel layers which have high corrosion and wear resistance and weld or solder very well. Zinc-Cobalt-Layers are used for decorative purposes. Zinc-Cobalt, Zinc-Iron and Zinc-Nickel layers distinguish themselves by excellent protective effect against corrosion and are increasingly being used in the automotive industry as a corrosion-protective layer.

1.4 Dispersion Layers

These protective layers consisting of metal and solid matter particles distinguish themselves by their excellent wear resistance and their ability to retain properties under harsh conditions. Nickel layers with 8-10 vol.-% Silicon Carbide are used to extend engine service life (for example in wood saws). They are also used in the internal coating of light metal cylinders of combustion engines; however, other layer systems have recently replaced them. Cobalt-Chromium Carbide dispersion layers are used for coating wear parts of aircraft turbines exposed to permanent stress up to 800°C. Chromium layers containing Al2O3 are used in the production of diesel engine piston rings. Single-crystal diamonds embedded in nickel as an electrolyte are processed as cutting edges of tools such as band saws, grinding discs or dental drills.

1.5 Conversion Layers

Conversion layers formed on the surface as a result of chemical reaction of the layer with special solution (for example chromate layers) increase the corrosion resistance of the structural part. This possibility is particularly relevant for layers of zinc and its alloys. Chromate layers formed in very thin coating layers can be transparent or blue, yellow and black, and thus serve different decorative purposes. They also provide an excellent adhesion base for organic layers and paint very well. Thus the optical properties of individual structural groups are configured more effectively. Special chromating-phosphating mixtures are used on aluminium and its alloys.

1.6 Anodic Layers

The wear resistance and corrosion resistance of parts made of aluminium and its alloys are substantially improved by anodic oxidation of the surface (anodizing coating). The resulting anodizing coating layers (aluminium electrolytic oxidation) with a thickness between 5-25 μm are very suitable for painting. Colour particles penetrating into the pores of the anodizing layer can produce colours ranging from unfading dark brown to black on aluminium surfaces. The same effect can be obtained by processing aluminium parts in self-colouring anodizing electrolytes or by processing with metal particles accumulating in the pores of the anodizing layer as electrolyte. The fields of application of anodized parts are: Building industry, aviation and space travel, electronics industry and computer industry, optics and medical technology.

1.7 Galvanic Shaping

Metal parts with complex shapes and surface structures can be produced by means of a special technique known as galvanic shaping. The dimensions range from tools for the production of micro-precision parts (for example watch bezels) to large-surface automobile and aircraft parts to cooling-channel rocket combustion chambers. For this purpose, models made of wax, plaster, plastic or metal are used as negatives of the part and are galvanically coated with metal layers of sufficient thickness. The electrolyte used must have good distribution properties so that metal can be separated in all depths and corners. Thus an exact mould of the surface is prepared. The galvanically shaped part is then separated from the model and processed. Other fields of application for galvanic shaping are mechanical engineering, graphic industry and dental technology.

Fields of Application

The applications so far have shown that galvanic technique facilitates a number of production methods and even makes some possible for the first time. Layers and coatings produced by galvanic technique have specific properties according to their purpose of use and meet different requirements in aesthetic or technical ways. Thus the service life and maintenance intervals of structural parts, machines and equipment are extended many times over. The information given in Table 3 regarding hardness, wear and friction coefficients in relation to some galvanically or chemically separated layers documents this. 1) Galvanic separation, 2) Chemical separation, 3) Determined with Taber Raser. Table 4 shows the important properties of layers produced by galvanic technique and provides examples for practical applications in different industrial sectors.

The examples in Table 4 have demonstrated that with galvanic coating methods, specific properties of a material can be well combined with a suitable layer and thus there will be an appropriate task distribution between the individual components of the compound system (structural part/layer).

For example, if the base material has tasks to fulfill that must meet requirements expected of the structural part due to its mechanical properties, the layer will provide sufficient corrosion resistance, hardness, wear resistance and desired surface brightness. Thus there is a distinction between the functional and decorative tasks of the layer. Functional or decorative galvanizing is generally referred to here. The objective of functional galvanizing should be to produce layers with resistance to corrosion and wear or to produce them with appropriate sliding properties and the ability to retain properties under harsh conditions or with good soldering or welding properties. Appearance should not be of great importance here, as such layers are subject to mechanical finishing under certain conditions. In decorative surface refinements, the aesthetic appearance of the surface is given priority. Good adhesion of the layer to the base material is desired. Due to the continuously increasing demands related to technological progress in modern industrial society and the continuously changing fashion consciousness, the clear distinction between functional and decorative galvanizing seems unsuitable in some areas. But ultimately what matters is that the properties of the structural part and the layer are fully compatible, taking into account economic and ecological aspects. İzzet Aydın / General Director / Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti