Electroplating or galvanizing is a surface treatment that consists of depositing a metal onto a substrate (metallic or not) through chemical or electrolytic reduction for protection, improved conductivity, and better suitability for welding on the treated surface. Other applications include improving appearance, binding non-conductive particles to the electrodeposited layer, abrasion resistance, improving surface hardness, temperature resistance, among others.
Electrodeposition or electroplating:
According to the Rosseti Dictionary of Chemistry, we can define electroplating as the technology responsible for transferring metal ions from a given solid surface or liquid medium called an electrolyte to another surface, whether metallic or not. This process uses electric current and is called “electrolysis”.
Electroplating was developed by Galvani, a physicist and chemist who studied electricity.
Electrodeposition is related to surface coating; it is the process used in silver plating, nickel plating, chrome plating, etc.
The object to be coated during electrodeposition must be connected to the negative pole of the generator, forming the cathode. For example, we can apply a gold or silver bath (gilding and silver plating) to a ring made of aluminum. The ring will be the cathode, connected to the negative pole of the generator; connected to the positive pole there should be a gold sheet.
These electrodes must be immersed in an aqueous solution of a gold salt, for example, gold(III) nitrate (Au(NO3)3].
Au3+ + 3e- → Au negative pole (half-reaction at the cathode – reduction) Au → Au3+ + 3e- positive pole (half-reaction at the anode – oxidation)
We could also use an inert electrode (platinum) at the anode, the aluminum ring at the cathode, and an aqueous solution of Au(NO3)3. The deposition of gold on the ring does not originate at the anode; the gold is present in the solution, and it is advisable that it be concentrated:
H2O → 2 H+ + ½ O2 + 2e- positive pole (half-reaction at the anode – oxidation) Au3+ + 3e- → Au negative pole (half-reaction at the cathode – reduction)
If we want to chrome-plate an iron bumper, it must be connected to the negative pole of the generator, forming the cathode. The positive electrode must be a chromium bar, or else the electrode must be inert and the electrolytic solution must contain a chromium salt (Cr3+). Industrially, the process of chrome plating automobile bumpers is carried out in three stages: • Copper plating • Nickel plating and • Chrome plating. This is intended to provide greater adhesion to the chromium, preventing the bumper from peeling and losing the chrome plating.
In the electroplating process, the reactions are not spontaneous; it is necessary to supply electrical energy for electron deposition (electrolysis) to occur. Electroplating is therefore an electrodeposition process in which the object that will receive the metallic coating is connected to the negative pole of a direct current source and becomes the cathode. The metal that will provide the coating is connected to the positive pole and will be the anode. The object to be coated must conduct electric current. If the object is plastic, which is not a good conductor, a surface treatment will make it conductive. There is also the problem of adhesion of one metal to another. For the metal film to bond to the base, in addition to perfect cleaning and degreasing, it is necessary to know the nature of the metals.
Galvanizing Galvanizing or electroforming is the entire electrolytic process that consists of coating the surfaces of metal parts with other, more noble metals. This coating is generally applied to protect the part from corrosion and/or as an aesthetic/decorative finish.
Galvanizing consists of using two electrodes immersed in an electrolytic solution connected to a direct current source or alternating current, converted to direct current and rectified. The part to be coated must function as the cathode, i.e., it must be connected to the negative pole of the electric current source. The anode, connected to the positive pole of the source, may be made of an inert material (graphite, lead, stainless steel, platinized titanium, etc.) or made of the metal with which the part is to be coated. The electrolytic solution must contain, as the electrolyte, a salt that contains cations of the same metal.
When the anode is made of the metal with which the part is to be coated, the electrolytic process occurs with a transfer of metal from this electrode to the part through the electrolytic solution.
Example: Coating a part with silver, using a silver anode (silver plating):
Anode: Ag → Ag+ + e –
Cathode: Ag+ + e – → Ag
The anode metal oxidizes. The cation formed goes into the solution, and the cation in the solution is reduced at the cathode, adhering in metallic form.
When the anode is an inert material, the discharge of water from the solution occurs there. The cation in the solution is reduced at the anode, also adhering to the part.
Example: Coating a part with chromium, using an inert anode, lead (chrome plating):
Anode: H2 → 2 H+ + 1/2 O2 + 3 e-
Cathode: Cr3+ + 3 e- → Cr
When using an inert anode, the concentration of cations must be higher because there is no replenishment of them by the anode.
Depending on the metal with which the part or object is coated, galvanizing receives special names: gilding (gold), chrome plating (chromium), silver plating (silver), nickel plating (nickel), and others.
The galvanizing process, or the application of a zinc coating, can be carried out in the following ways:
Zinc plating or Electrolytic Galvanizing; (cold)
Hot-Dip Zinc Plating; (or by fire)
Thermal Spraying Zinc Plating – metallization
We divide electroplating technology into two main parts:
The first and most widely used is deposition by means of electrolysis. That is, through electric current, a difference in electrical potential is created, thus favoring the deposition of a metal onto another surface, usually metallic and with opposite polarity.
The second form, more elaborate and therefore less widespread, is chemical deposition, which consists of depositing dissolved metal ions in the form of their salts, which, due to a difference in chemical potential, tend to deposit onto other surfaces, metallic or not, generating deposits with characteristics often superior to electrolytic deposits in terms of cohesion and deposit density.
ELECTROLYTIC ZINC PLATING/GALVANIZING
White zinc plating;
Yellow zinc plating;
Black zinc plating;
Black oxidation.
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