Basics of electroplating

This overview covers key terms and processes used in electroplating.

Basics of electroplating

Electroplating:

An electrochemical process for the deposition of metal layers on an electrically conductive substrate.
Uses an electrolytic cell to transfer metal ions from a solution to a substrate.

Electrolyte:

A conductive liquid containing metal ions to be deposited.
Examples: Copper sulfate solution for copper deposition, nickel sulfate solution for nickel deposition.

Anode:

The electrode at which oxidation takes place.
In electroplating, often the metal that is to be deposited (e.g. a copper anode for copper deposition). Chromium is an exception. Chromium anodes must not be used with chrome plating solutions (based on trivalent chromium), as this can produce highly toxic hexavalent chromium (chromium VI)!
If no anodes are available from the electrolyte material, the use of inert anodes such as platinum (platinized titanium anode) or graphite is an option.
Graphite anodes have the disadvantage that the resistance in the anode can increase significantly, making it unusable. Although graphite anodes can be used universally, we do not recommend them as they do not dissolve chemically, but particles enter the bath and cloud it due to the development of oxygen at the anode. As the process progresses, these particles are also deposited and the surface produced becomes darker. Metal anodes are therefore preferable.

Cathode:

The electrode at which the reduction takes place.
The substrate onto which the metal is deposited.

Current source:

A direct current source that supplies the necessary energy to drive the electrochemical reaction.

Important terms and processes

Electrolysis:

The process in which chemical reactions are initiated in the electrolyte solution by applying an electric current.

Reduction:

A chemical process in which an atom or ion gains electrons.
In metal deposition, a metal ion (e.g. Cu²⁺) is reduced to a metal atom (Cu).

Oxidation:

A chemical process in which an atom or ion loses electrons.
In metal deposition, the anode is often oxidized to release metal ions into the solution.

Current density:

The current per unit area of the electrode.
An important parameter that influences the quality and speed of metal deposition.
The cathodic current density is important for the quality of the coating on the workpiece (cathode). Each electrolyte has an optimum current density range within which deposition is achieved with good results. If the current density is outside the parameters, the coating can become matt.
On the anode side, there is the anodic current density. This is particularly important for the stability of the electrolyte. As much metal should dissolve as is deposited at the cathode (workpiece).
Ideally, the anode dissolves as quickly as the metal is deposited at the cathode, so the electrolyte would last a particularly long time. In practice, however, there is a deviation.
For example, acidic zinc electrolytes are enriched faster than the metal is deposited, which leads to clouding of the electrolytes after a longer period of time.
In the case of nickel, the anode dissolves more slowly and the electrolyte slowly becomes poorer and poorer in nickel ions. In this case, suitable nickel salts could be added to increase the content again. However, nickel salts may not be sold freely due to the hazard classification. To improve anode solubility and reduce passivation, chloride ions are also added to the electrolyte by the manufacturer.

Overpotential:

The additional voltage requirement above the theoretical equilibrium potential necessary to drive the electrochemical reaction.
Affects the efficiency and properties of the deposited metal layer.

Bath composition:

The chemical composition of the electrolyte that influences the properties of the deposited metal layer.
Additives such as brighteners, wetting agents and buffer solutions are often used to improve the coating properties.

Types of electroplated coatings

Galvanizing:

Deposition of zinc on steel or iron for corrosion resistance.

Nickel plating:

Deposition of nickel for decorative purposes or as a base for further plating.

Gold plating:

Deposition of gold for decorative purposes or for electrical contacts due to its excellent conductivity and corrosion resistance.

Silver plating:

Deposition of silver, often for electrical contacts or to improve conductivity.

Copper plating:

Deposition of copper, often as an intermediate layer or for conductor tracks in electronics.

Important parameters and control

pH value:

The acidity of the electrolyte solution, which influences the efficiency and quality of the deposition.

Temperature:

The operating temperature of the electrolyte bath, which affects the reaction rate and layer properties.

In order to achieve the best possible results, the working temperature of the respective electrolyte should be taken into account. Many electrolytes already work optimally at room temperature. This means that no external heating medium is required.

In general, it can be said that almost no electrolyte works well below 15°C, so it is important to pay attention to the temperature if you notice problems with the electrolyte. The temperature of the workpiece must also be taken into account - especially with pin electroplating.