Electroplating is the electrochemical deposition of metal precipitates (coatings) on objects.
The deposition of metals in electroplating is based on an electrolyte through which a current is passed. The electrolyte (an electrically conductive fluid) consists of, among other things, metal salts of the metal to be deposited (such as nickel, copper or zinc). In the electrolyte, the metals exist in the form of positively charged ions (e.g., Ni2+, Cu2+ or Zn2+), provided these metals are not complexed. The material to be plated is connected cathodically (negative pole) and is therefore called the cathode. To create a closed circuit, the opposite pole (positive) is required, which is called the anode. Once the current is applied, the ions begin to migrate to the opposite pole. The positive metal ions (cations) migrate to the negative cathode (workpiece) and by accepting electrons are reduced to the metal. The longer the process takes and the higher the electric current, the thicker the coating will become.
Fundamental distinction between decorative and functional plating. In detail, an electroplated coating can fulfil one or several of the following functions:
– Corrosion protection
– Improvement of conductivity
– Improvement of bonding
– Primer for additional layers
– Wear protection
– Production of micro and macro structures
– Improvement of solderability
The areas of application for electroplated coatings are incredibly diverse, which is why electroplating has a ubiquitous presence in everyday life and has become indispensable. Areas of applications are
– Components for the aerospace industry
– Watches, jewellery, consumer goods and decorative objects
– Electronic circuits
– Sanitary fittings
– Machining, cutting and forming
– Façade building units
– Structural elements
– Corrosion protection (screws, pipes, nails, fasteners, etc.)
– CDs / DVDs (stampers, masters for pressing)
With the electrolytical pad-plating method, metal can be applied to almost any conductive metal without the use of a bath. The following technical equipment is required for this application technology:
Rectifier, anode holder, graphite anodes with absorbent pads, electrolytes.
The workpiece to which the metal is to be applied is connected to the negative pole of the rectifier. The positive pole is connected from the rectifier to the anode holder and thus to the anode. The anode with the absorbent pad is immersed in the electrolyte.
The metal precipitate is formed by moving the anode on the cathode (workpiece) under the action of direct current.
The structure and composition of the electrolytes enable deposition rates that are 5-10 times faster than in a conventional bath (calculated per anode surface).
Due to the deposition technique, metallic precipitates formed by means of a pad have special physical properties:
– Good adhesion with little or no porosity
– No or very low hydrogen embrittlement
– Controlled hardness and low loss of fatigue resistance (in the case of friction alloys)
Small areas of 1 cm2 to 1 m2 and larger can be plated. Approx. 20 pure metals and various binary and ternary alloys can be deposited. Alloys are obtained by mixing certain electrolytes (alloys are currently not being used for plating SF 6 components). The thickness of the coating can vary between 1 micron and several tenths of a millimetre. The coating thickness can be controlled very accurately.
With a sandblasting system, metals that are difficult to galvanise can undergo mechanical pre-treatment. Metallic coatings can thus be applied to difficult materials such as silicon-containing aluminium alloys, titanium, zirconium and stainless steel. During this pre-treatment process, the dense and sometimes thick oxide layer is removed mechanically.
Pre-treatment of metals (general Information)
In order to create an adherent electroplated surface, it must undergo sufficient pre-treatment. The surface must be free of greases, oils, oxides and other bond breaking agents. This is achieved chemically, electrochemically or mechanically, for example with the sandblasting process.
Since each alloy, particularly special alloys, reacts differently to the pre-treatment, the processes have to be adapted to the relevant basic material to create a good base for the plating. Faults on the surface such as pores also have an adverse effect on the plating.
Other influential factors are crystallisation, so-called “precipitates”, a Beilby layer and all types of smudges and dirt on and inside of the surface. These can increase the effort involved in further Treatment.
Problems, which are attributed to the pre-treatment process, often occur in conjunction with process changes and fluctuations that take place prior to electroplating and affect the quality of the surface. General examples for this are:
– Modification of mechanical surface treatment (example: change from turning to milling)
– Use of other oils
– Fluctuations in the thermal treatment processes