Question: We are a manufacturer of musical instruments and have milled brass parts that need to be coated black. Inquiries to several electroplating shops resulted in very different statements. Black chrome plating is not possible as the parts have sharp-edged internal corners and chrome plating does not work.
We have now had some parts galvanized black. The electroplating shop also applied an intermediate layer of nickel. The main problem is the adhesive strength on the CuZn39Pb3. We would need suggestions for black coatings that adhere to the brass so that we can then select a suitable electroplating shop.
Answer: You use leaded, so-called "free-cutting brass". This is a precipitation alloy. The lead is not evenly distributed in the alloy, like copper and zinc, but is concentrated at certain points.
Lead can form poorly soluble salts, such as lead sulphate (PbSO4), during pre-treatment. In this case, sparingly soluble means that it is practically insoluble and sticks to the surface. It is usually rubbed off drum goods, but there is no 100% guarantee of this. If it sticks, it is then electroplated over and adhesion problems arise in these areas. To put it bluntly, this creates a lead detector.
This mainly occurs when pickling or decapping with sulphuric acid. Regardless of which coating you choose: You must pay attention or ask whether the electroplating company uses sulphuric acid during pre-treatment. Inform the company of the lead-containing alloy, as lead-free brass is very often pre-treated with diluted sulphuric acid and the difference is not visually recognizable.
Another reason for adhesion problems could be lead smearing, which can mainly occur with a higher lead content and with turned brass parts. The localized areas are normally covered with lead. However, if the lead is smeared over a large area (we are talking about an area of a few square millimeters), the coating may flake off. The defect pattern can therefore give a clear indication of whether it is more a lead sulphate problem or a general lead problem due to smearing. The flaking may be visible in the milling direction.
Pre-treatment
Patterns of blackened surfacesThetype and intensity of cleaning depends primarily on the alloy, the degree of soiling and the duration of storage. The latter has an effect on the corrosion and incrustation of deposits caused by upstream processes. For example, residues of polishing pastes or dehydration of preservatives. Fresh soiling can be removed relatively easily. Longer storage and more severe corrosion can lead to corrosion scars, which can only be removed mechanically by removing the scarred surfaces.
Aqueous, mildly alkaline cleaning solutions with surfactant combinations are best suited, as they usually have a hydrophobic effect on the metal surface. If a higher alkalinity is required, this can lead to an attack on the brass surface. To avoid this, silicates (sodium silicate) are added to the degreasing agent. With cleaners containing silicates, it is essential to follow up with a fluoride-containing decapping agent to remove the silicates from the surface (see below).
Industrially manufactured brass is very rarely pickled nowadays. In exceptional cases, it is chemically fired, whereby a distinction is made between pre-firing, bright firing and matt firing. In the case described above, we do not believe that any of this is necessary, provided that stress corrosion cracking is not an issue. The tendency to stress corrosion cracking can be significantly reduced with the help of firing, pickling and electropolishing [1].
We recommend a diluted hydrochloric acid (2-5 %) or a fluoride-containing solution, which usually consists of a mixture of sodium fluoride and sodium hydrogen sulphate.
The electrolytic degreasing is switched cathodically and serves as a final cleaning to remove the last particles on the surface. If there is still lead sulphate on the surface, this is torn off at the appropriate intensity. After E-degreasing, the parts are decapitated once again and can then be electroplated or dyed. The following list is not intended to be exhaustive, but aims to fulfill the above requirements.
Coatings
Black nickel coatings are usually very thin layers of nickel, which in turn are nickel-plated. Depending on the electrolyte, the result can range from an anthracite shade to a deep black, whereby the former is often also referred to as "black nickel" by electroplating companies. There is definitely a need for clarification here with regard to the color tone. To maintain the black color, the layer is usually painted, as it is not very abrasion-resistant despite its relatively high hardness.
Electroless nickel black is perfect for coating all areas evenly, including the corners. However, there is a risk of color differences.
Galvanize and passivate black. The process you have already tested can be carried out with or without an intermediate nickel layer and produces very good visual results. This also applies to zinc alloys such as zinc-iron or zinc-nickel. When passivating, care should be taken to ensure that the electroplating process does not use conventional chromating, as your parts are no longer RoHS-compliant due to chromium(VI) [2]. Depending on the coating (alloy or not) and use (abrasion, hand perspiration, weathering), corrosion problems may occur after some time.
Coloring process
There are several methods of coloring brass. The most common method is with potassium polysulphide (sulphur liver). Here, 10 g/L at room temperature is sufficient. Depending on the desired intensity and alloy, the solution can be modified with copper sulphate and sulphuric acid [3]. For the longevity of chemically colored surfaces, it is recommended to treat them with clear lacquer.
Combinations
It is possible to first apply an electroplated layer, which is then colored. The most common method is copper plating. Pure copper layers can be dyed black with excellent reproducibility. Depending on the requirements, other colors are also conceivable, such as a so-called old coloring.
However, we advise against silver plating with the intention of coloring it black using sulfur liver, unless you are aiming for a look that resembles old silver.
Literature
[1] U. Heuberger; A. Pfund; C.J. Raub: Einfluß galvanischer Schutzschichten auf die Spannungsrißkorrosion, Galvanotechnik 8/1989, Eugen G. Leuze Verlag
[2] https://de.wikipedia.org/wiki/RoHS-Richtlinien
[3] O.P. Krämer; T.W. Jelinek: Recipes for metal coloring, Eugen G. Leuze Verlag
[4] B. Gaida; B. Andreas; K. Aßmann: Technologie der Galvanotechnik, Eugen G. Leuze Verlag
