Although copper materials have a decorative inherent colour, there is often a desire for other surface colours. In the course of time, copper develops a natural patina which, starting from bright, reddish copper, can run through dark brown to light green. These and other shades can also be produced artificially. These very thin colour coatings on copper materials are created by reactions of certain chemicals, mostly in aqueous solutions, with the metal surface. Copper coatings come in different varieties. While enamel coatings are mostly applied for decorative reasons, lacquer and synthetic resin coatings – often transparent – are used to preserve the natural colour of the materials or the chemical colourings. Other coatings are used to improve corrosion protection. Metallic coatings are common, for example by electrolytic (galvanic) coating, electroless (chemical) deposition and the application of hot-dip coating.
Surface treatments of various copper materials
It is imperative that the legal aspects of these treatment procedures be examined before they are applied.
Copper is a material that is particularly easy to work on the surface and to refine. Grinding is done on discs that are coated with the abrasive grit or completely interspersed. Grinding with belts and brushes has also proved successful. Polishing is done on cloth or felt wheels with paste or liquid polishing agents. Copper can be polished chemically or electrolytically. Acid solutions with additives are used for this. Sulphuric and amidosulphonic acids as well as amidopersulphate solutions and diluted hydrobromic acid are used for cleaning pickling. If the surface does not show any scale, it is better to use chemical brightening instead of pickling. Metallically pure copper surfaces can be treated with a chromating solution to give them a slight shine; this also creates a temporary passivating protection against tarnishing and weak corrosion influences.
Tin and tin-lead coatings promote the solderability of the copper. Nickel coatings achieve decorative effects. Before galvanic chrome plating of the copper, nickel plating is usually applied. Nickel-cobalt coatings lead to specific magnetic properties, but also to wear-resistant surfaces. Nickel-manganese coatings are more heat-resistant than nickel coatings.
Functional copper components can be coated with almost all metals, alloys and hard materials using numerous processes, e.g. CVD, PVD, ion plating, in a vacuum or molten, electrolytic or mechanical. Diffusion zones of alloys with copper are created by thermal processes. In arts and crafts, copper is often chemically coloured. In the building industry, copper sheets for solar collectors are blackened by chemical and galvanic processes and supplied artificially patinated for roofing.
Enamelling is also a popular finishing process in the arts and crafts and metalware industries. Enamel also becomes a tried and tested coating for copper when it is necessary to protect plant components, e.g. for the chemical and pharmaceutical industries, against chemical attack. Lacquering with clear lacquers preserves the natural colour of the copper for a long time, often for years. Coloured copper parts are also painted.
Finished parts made of copper-zinc alloys are often only treated mechanically, chemically or electrochemically because of their attractive inherent colour. Almost all copper-zinc material grades can be well polished mechanically and shined chemically and electrochemically. The alloy hardening associated with increasing zinc content results above all in excellent mechanical polishability. In the case of a/ß alloys, the ß-phase may influence the polishing appearance. Cleaning and degreasing is carried out in organic solvents (pre-cleaning), in alkaline and acidic cleaners. The cleaning process is intensified and accelerated with the help of electrochemical processes and ultrasound.
Pickling is carried out in hydrochloric or sulphuric acid solutions, firing and burnishing in nitric acid-based solutions. These and other processes are also available on the market in the form of ready-to-use solutions and concentrates as well as salts. However, the use of nitric acid is in sharp decline due to the formation of nitrous gases (NOx) and associated restrictions.
The natural inherent colour of copper-zinc materials is retained for a long time if the surface is painted colourless. For interior applications (indoors), colourless zapon lacquers provide sufficient protection. Coloured lacquers lead to special surface effects. They are used, for example, to tint surfaces in the direction of bronze and gold colours.
Arts and crafts in particular like to use surface finishing by chemical dyeing or enamelling. Chemical and electrochemical colouring processes offer a versatile range of green, brown, grey and black tints. Copper-zinc alloys with max. 10 % zinc can be enamelled. Preferred etching quality, e.g. for dials, is CuZn36. Coatings of other metals and numerous alloys that are to perform decorative and functional tasks are electroplated on copper-zinc surfaces without current (Ni). The electroless process is gaining importance in nickel plating. With proper pretreatment , single-phase a- or b-copper-zinc alloy can be electroplated perfectly. This must be taken into account in the range around 37 % zinc, because depending on the heat treatment, a heterogeneous structure may be present there. For pre-treatment, copper-zinc alloy containing lead must not be pickled in sulphuric acid, because the lead sulphate that is produced can lead to the formation of bubbles or even to the flaking off of the coatings. In this case, pickle with diluted nitric acid or better with 10 – 20 % fluoroboric acid. However, fluoroboric acid slightly etches the surface, so that mechanical polishing may be necessary.
It is often more difficult to pre-treat CuZn20Al2As, CuZn28Sn1As etc. for electroplating, which must be additionally brushed after pickling. During pretreatment – in order to counteract a risk of dezincification or stress corrosion cracking – attention must be paid to the risk of overpickling in the case of certain alloys (e.g. CuZn39Pb3).
Tin coatings are used to improve solderability or are applied when surfaces come into contact with food and beverages. In addition to pure tin coatings, coatings of tin-copper and tin-nickel alloys are also used.
Nickel and chromium are the most important coating metals for copper-zinc materials. Fittings and sanitary accessories made of copper-zinc alloys are hard-chrome plated.
Precious metals and precious metal alloys are mainly applied by electroplating. They are used to perform specific functional tasks or to improve corrosion resistance. In the jewellery and watchmaking industry, coatings of silver, gold and platinum metals are used. Components made of platinised copper-zinc material are used in the chemical and process engineering industry.
Copper-tin (tin bronze)
Blasting is used to achieve functional and decorative surface structures while simultaneously strengthening the surface. The quality is determined by the abrasive material, shape and grain size. Levelling of the surface, matting, silk-gloss polishing and strengthening are the predominant effects produced by blasting on wrought copper-tin alloys.
Grinding can be done by hand using wheels that rotate on grinding stands. These are portable grinding wheels driven by hand motors. Sheet metal is preferably sanded with abrasive belts using the freehand and contact method. Larger surfaces are processed by belt or pendulum grinding on table machines, which can also be fully automated.
Mechanical polishing is also done by hand on cloth wheels. Common devices are polishing stands and hand machines. Mass-produced parts are processed on automatic polishing machines. Polishing greases, waxes and emulsions are used to achieve a better polishing effect of the discs and to avoid overheating the surface area exposed to the polishing work. There are polishing aids with formulated inhibitors that prevent oxidation and tarnishing of the freshly polished surface over a longer period of time.
Especially bulk parts made of copper-tin alloys are suitable to be ground, polished and smoothed by tumbling. The polished material is mixed with ceramic or mineral grinding or polishing media, the chips. In addition, an aqueous treatment solution is added, which contains cleaning, grinding, polishing, deoxidising and possibly also passivating agents.
Pickling is used to remove scale – such as that produced by annealing – as well as oxide layers of other origins. Generally, pickling solutions of diluted sulphuric acid as well as pickling mixtures with contents of sulphuric and nitric acid are introduced. These solutions also contain additives to inhibit the pickling attack on the metallic surface as well as for brightening and passivation. The light-coloured tin oxide is more difficult to remove than copper oxide. Partial dissolving of the metallic edge zone in diluted nitric acid is necessary.
Coating surface refinement plays a special role for copper-tin alloys. In hot-dip tinning, the parts are immersed in a molten tin. Diffusion tin plating is done by dipping the parts into molten salts containing SnCI2 at about 400°C (reaction temperature).
Flame spraying is used to apply coatings of metals, alloys, hard, super and special alloys as well as refractory materials and ceramics.
Galvanic coatings of silver, gold, nickel, chromium, cadmium, tin or lead-tin alloys can be easily applied. Generally, a nickel layer is applied beforehand as a so-called diffusion barrier, which serves as a carrier for further layers. Due to the high corrosion resistance of the carrier material, there is then no need to fear any disadvantages even if the layers are not absolutely pore-tight.
As protection against tarnishing, painting with clear lacquer is common. The prerequisite for good adhesion of the paint film is proper pre-treatment of the metal surface.
Copper-aluminium alloys can be polished well mechanically. Both galvanic metal coating and hot-dip tinning are problematic because the completely dense coating of the metal surface required for reasons of corrosion resistance is very difficult to achieve. The aluminium oxide components of the outer material zone are difficult to remove and hinder adhesion of the applied coating. The application of metal and alloy coatings by galvanic processes and from melts therefore requires special measures. For this reason, pre-treatment processes have been developed that lead to perfect results in electroplating and hot-dip tinning. Small components can also be coated with numerous metals, alloys and cermets after vapour deposition processes (PVD and similar processes).
Copper-nickel alloys can be polished well mechanically and electrochemically. For pickling to remove the very resistant oxides formed during oxidising annealing or hot forming, hot 15 % sulphuric acid with the addition of about 2 % sodium nitrate, nitric acid or sodium dichromate is effective as a pickling solution. Warm hydrochloric acid (1:1) with added bichromate is also suitable. Pickling treatments can be avoided by quenching at higher temperatures in water with 2 % alcohol.
For bright burning, pre-burning with a solution of 1000 ml nitric acid (38 Bé), 1000 ml water and 60 to 90 g common salt at 25 to 35°C can be used for all alloys. After hot rinsing, briefly immerse in nitric acid (1:1), then neutralise in diluted ammonia solution, rinse and dry. During chemical surface treatment, the relevant environmental regulations must be observed. Since copper-nickel alloys are corrosion-resistant in numerous media, electroplating is hardly necessary for corrosion-chemical reasons. On the other hand, coatings are applied to take over functional tasks, e.g. silver.
Copper-nickel-zinc (nickel silver)
Surface treatment is very important for copper-nickel-zinc alloys to achieve decorative effects. They can be polished well mechanically and electrochemically. Warm, 10% sulphuric acid at about 60 °C is suitable for pickling. If a bright surface is to be achieved, the addition of a few percent nitric acid to the pickling is recommended. However, it should be noted that too high contents of strongly oxidising additives in the pickle and also too long pickling times result in rough surfaces.
Glossy surfaces are achieved by briefly dipping the pickled material in concentrated nitric acid and immediately rinsing thoroughly in water containing tartaric acid. Instead of nitric acid, a potassium dichromate solution containing sulphuric acid is also suitable.
The galvanic application of coatings of e.g. chrome, nickel, silver, gold, tin, tin-lead and zinc does not cause any particular difficulties. Such coatings are generally used to improve corrosion resistance in numerous areas of application as well as tribological behaviour. Electroplated metal and alloy coatings adhere well to copper-nickel-zinc alloys. Intermediate layers are not necessary for chrome and nickel plating. For silver plating, the alloys with less than 20 % Ni are better suited, because the adhesion of the silver coatings decreases with higher nickel contents. Silver coatings deposited galvanically on copper-nickel-zinc alloys with less than 20 % Ni adhere excellently, so that intermediate nickel plating is not necessary even when silver-plating tableware and decorative tableware. Numerous layers perform functional tasks, for example tin and tin-lead alloys.
For lacquering, which is intended to preserve the lustre of polished, shined or fired surfaces, suitable lacquers are applied to the degreased surface by brushing, dipping or spraying. For parts used in covered areas, colourless Zapon and nitrocellulose lacquers are common. Unprotected surfaces must be maintained with metal polishes. Copper-nickel-zinc alloys can be enamelled and chemically coloured. This is used in arts and crafts and in the metalware industry.
Tinning of copper materials
The main use (60-65 %) of copper materials is in electrotechnical applications. Almost all branches of industry use them for current-carrying components. Connectors and stamped grids in particular are exposed to a variety of environmental influences during use, as shown here using the example of an automobile:
- Temperature: -40 to 150 °C
- Electrical operating conditions
- Pollutant gases
- Fine dust
In order to ensure the durable functioning of the connectors, a number of material properties must therefore be considered and matched to the application. These complex requirements are met by plating copper materials. On the one hand, copper components are electroplated with precious metals – but tin plating is also a cheaper solution that is just as adequate for many applications. In terms of process technology, the alternatives are hot-dip tinning or electroplated tinning.
The hot-dip tinning of strips is standardised according to EN 13148, whereby the thoroughly degreased, dried and activated strip is passed through a bath of liquid tin, which has temperatures between approx. 250 and 290°C – the melting point of tin is only 232°C. After leaving the tin bath, the liquid tin is usually stripped off to the required thickness by means of air nozzles and the strip is cooled to solidify the tin. After leaving the tin bath, the liquid tin is usually stripped off to the required layer thickness by means of air nozzles and the strip is cooled to solidify the tin. The schematic structure of a hot-dip tinning line is shown here. In this way, coating thicknesses can be achieved which, with values between 0.7 and 20 μm, cover a wide range and thus open up numerous applications. Depending on the layer thickness, the focus is on different properties.
Pure tin is significantly softer than copper (HV (Sn) = 8), so that the real contact area becomes larger with increasing thickness of the tin layers due to plastic deformation of the contact surface, although this also requires higher insertion and extraction forces. The solderability and corrosion resistance is better with higher layer thicknesses.
Galvanic tin plating
With electrogalvanising, no temperature stresses are exerted on the base material. This can be useful for heat-sensitive materials that tend to soften during hot-dip tinning (such as ETP-Cu). Similarly, there is the possibility of multiple or selective plating and intermediate layers can be used to improve adhesion or reduce diffusion from the base material – nickel in layer thicknesses greater than 2 mm or copper/nickel layers are particularly suitable as barrier layers. As a rule, however, electro-tinned strips are limited to layer thicknesses < 4 μm due to the long deposition times and the associated high costs. In addition, it is often reported that the service life of the stamping tools is shorter than that of hot-dip tinned strips.
The essential difference between electroplating and hot-dip tinning is that no intermetallic phases are formed due to lower temperatures during coating. The diffusion of the copper atoms in electroplated tinning takes place mainly via grain boundaries, whereby residual stresses are formed in the solid, which can result in the formation of whiskers. Whiskers are hair-shaped single crystals that can “grow” several hundred micrometres out of the surface and pose a short-circuit hazard in electronic components.
However, the problem has been known for a long time and can be minimised, among other things, by reflow treatment – a subsequent heat treatment with brief re-melting of the tin – or via the prior deposition of copper-nickel barrier layers.
In principle, it is possible to apply a multitude of different colours to copper, brass or bronze by means of so-called dyeing. The variety of achievable optical effects and design possibilities has always represented an almost inexhaustible potential for copper materials. With chemical dyeing, it is never possible to achieve a completely uniform colour tone or appearance on large-area components.
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