Bonanomi has been offering the best precision mechanical products for more than 75 years, using suitably treated materials. The surface treatment of metals in fact enables their performance to be increased, making them more resistant.
Among the possible processes, in this article we delve into chromium plating, what it is and how it works, and we understand together its usefulness in precision mechanics.
What is Thick Chrome Plating?
Thick chrome plating, also known as hard chrome plating or electroplating, is a process of surface coating of chromium on a metal part. Deposition occurs by electroplating and, therefore, electric current is used to finalise the treatment.
Electroplating takes place inside tanks (or electrolytic cells), where the component is immersed in a specially formulated solution.
A variety of materials can be subjected to the process, including iron, steel, copper and brass.
How Metal Chrome Plating Works
The chromeplating of metals is based on electroplating, a technique first pioneered by Johann Wilhelm Ritter in the early 1800s. It is based on reduction and oxidation reactions, which result in the acquisition and loss of electrons. Let's take a closer look at how chromium plating works and what is the electrochemical principle governing the process.
Galvanic processes involve immersing the artefact in aqueous solutions, to which the salt of the metal to be deposited is added. In the case of chromium plating, this is usually chromic sulphate, a ternary ionic compound (consisting of a non-metal, a metal and oxygen).
Two electrodes are inserted in the electrolytic cell: the cathode and the anode. The first is the negative electrode, represented by the part to be covered; the other is the positive electrode, consisting of an inert metal.
With the aid of a current generator, a potential difference is imposed: this leads to an exchange of cations and anions between the two electrodes. The cathode gains electrons, through a reduction reaction; while the anode gives them up, as part of an oxidation reaction.
At the end of the process, cations are deposited on the cathode (i.e. on the metal component undergoing treatment), which in turn acquire electrons from the anode. Metal atoms are formed from the union of cations and electrons. These will progressively coat the object, forming the chromium coating.
As mentioned, electroplating is not specific to chrome plating, but is also used in other surface treatments, including nickel plating and galvanising.
The Advantages of Chrome Plating
Many of the advantages of chrome plating depend on the structure of hard chrome. This takes the form of a body-centred cubic lattice, which causes the metal to be very wear-resistant. It manages to demonstrate this property even under extreme conditions, e.g. when subjected to compression or abrasive action.
Furthermore, a very high level of hardness can be achieved, although it is advisable not to exaggerate: exceeding 800 HV means running into problems of structural fragility of the coating.
The passivation phenomenon that chromium undergoes spontaneously causes a protective oxide layer to form on its surface. This results in greater resistance to corrosion and scoring.
The advantages of the treatment are many, but attention must be paid to the thickness of the coating: its breakage leads to very rapid and intense deterioration.