The electrode is made from a base and a coating.
The coating is very similar in certain aspects to a varnish, but deeply different both in composition and application method.
Its function is that of making possible the electrochemical reactions requested by the process it’s used in.
The coating needs a support which sustains it and which supplies it with the necessary mechanical structure which constitutes the electrode.
Among many materials tested, the choice of the support, or substrate as you say in scientific language, has fallen on Titanium.
Titanium has in fact the features required in order to receive the coating, to resist the chemically aggressive environments of the processes and it’s suitable for the current’s conduction.
Another advantage of Titanium is that it can be worked in a wide range of shapes: sheets, nets, tubes, wires, complex manufactured products etc.
The electrode was born from the combination of coating and Titanium, which has afterwards taken different commercial names such as MMO etc...
The main feature is that it remains unaltered in time and that only the coating wears out very slowly. This makes it so that an electrode which has lost all or part of the coating during its life can be regenerated with subsequent applications an undefined number of times.
The electrode can be used, depending on the process it’s destined for, both as cathode and anode.
Used as cathode, for example in the membrane chlorine – Alkali process, the base is in Nickel, whereas the coating maintains its noble metals-based composition.
It’s a very thin layer, a few microns, which covers the titanium’s surface in the areas where the anodic reaction takes place.
It’s composed by oxides, in variable percentages, of noble metals belonging to the Platinum family.
In particular Ruthenium and Iridium are the main elements used, mixed with others within the same family: Palladium, Rhodium, Platinum.
The Titanium family’s oxides are part of the coating’s formula too: Titanium, Tantalum, Niobium; which have the duty to make the structure more compact and adherent to the Titanium base and at the same time provide a protective function against the aggressiveness of some solutions the electrodes need to work in.
The formation of the coating takes place through the application of a solution of the above described salts’ elements and a subsequent high temperature thermal treatment to create the oxide.
Each application or “coat” is a cycle during which a fraction of the prearranged coating’s quantity settles. The number of cycles is subject to the final noble metal’s load, which, in turn, is in relation to the type of final use it’s designed for.
There are many formulations or “recipes” for coating, each of which is specific for its final use.
You can distinguish between two families: anode and cathode coating.
Among the anode coatings you can distinguish between: coating for Chlorine evolution, coating for Oxygen evolution or mixed coating.
The key component or the “catalyst” of the coating for Chlorine evolution is Ruthenium Oxide; for Oxygen evolution it is Iridium Oxide.
Membrane cell cathode coatings are based on Ruthenium Oxide or Platinum, mixed with Palladium, Rhodium.
A coating for each process
As we’ve seen the main use of the coating concerns anodes, as cathodes are often used with materials of different composition.
There are two great families which distinguish the anodic reaction and they are:
The production of Chlorine (Cl2) from solutions containing Chlorides (Cl-) according to the electrochemical reaction:
Cl- + e- ----> Cl2
the production of Oxygen (O2) from acid solutions containing mainly Nitrates, Sulphates according to the electrochemical reaction:
2H2O ----> O2 + 2e- + 2H+
The most important industrial applications Chemical Newtech refers to are:
Anodes for Chlorine:
Chlorine – Caustic Mercury Cells
Chlorine –Alkali Diaphragm Cell
Chlorine-Alkali Membrane Cells
Electrolysers for Hypochlorite production
Electrolysers for Chlorate production
Electrochlorinators for swimming pools
Metal Winning from Chlorides
Anodes for Oxygen:
- Decorative plating
- Hard plating
- Rhodium plating, Gilding
- Copper plating, Nickel plating
Anodes for cathodic protection