Thermal galvanization and electro-galvanization

Steel corrosion represents the partial or total destruction of the material under the action of various environmental factors. In order to extend the life of these materials, anti-corrosion protection is carried out. The most frequently used processes for this are thermal galvanizing and electro-galvanizing.

What are the differences between thermal galvanizing and electro-galvanizing

Thermal galvanizing is performed by immersing the product in molten zinc, at a temperature of 445-460˚ C. The objects are kept in the zinc bath until the metal reaches the zinc temperature.

Before the actual galvanizing, the following steps are performed:

• An acid solution is used to remove unwanted elements from the unprotected surface of the steel (degreasing);
• A solution with a low concentration of acid is applied to remove the rust from the desired surface (stripping);
• The action of the remover is stopped, by washing;
• A heated solution of ammonium zinc chloride is applied to remove the oxides and to prevent further oxidation of the material;
• Before being immersed in the molten zinc, the object is dried.

Hot-dip galvanizing is usually applied to mild steels, weak steel alloys, cast iron and cast steel. Zinc plating in this way has a long life. It is recommended for large objects. Hot-dip galvanized products are recyclable.

Electro-galvanization process

This process consists in depositing a layer of zinc through electrolysis. In the electrolytic bath, through which electric current flows, there is a cathode (the metal object that will be galvanized) and an anode (zinc). With the help of the electric current, the dissociation, transport and deposition of the metal ions from the anode (zinc) to the cathode (the metal object) is carried out. The zinc galvanizing process can be carried out in a variety of aqueous solutions, such as: alkaline cyanides, non-alkaline cyanides or acid salt chloride.

It is necessary to clean the objects that will be electro-galvanized, to remove rust, dirt, oil, etc. An alkaline detergent is used to ensure a good quality zinc galvanization and for the zinc layer to remain intact for a long period of time. The metal is soaked in an alkaline bath, for 5-10 minutes, at about 150 ˚ F . Then an electric charge is applied, which leads to the release of oxygen or hydrogen in the solution, thus achieving a cleaning at the micro level. Pickling follows, to remove the layers of oxides and scales from the metal surface. The type of metal and the thickness of the scales present decide the type of acid, the immersion time and the temperature required for this process. After galvanizing, the metal is washed with water, after which it is dried.

Hot-dip galvanizing and electro-galvanizing – what are the advantages of each process

Electro-galvanizing is a very cost-effective process, usually used for small objects (screws, fasteners, car parts, etc.). Zinc is usually used to galvanize iron or steel objects, but other metals can also be subjected to this process.

The main advantages of electro-galvanization are cost efficiency and ease of application. Its few limitations include low durability in seawater and the fact that the zinc layer does not form easily in the internal areas of the object.

Thermal galvanizing is, most often, used for objects used outside. While electro-galvanization lends itself well to those used inside. Also, the zinc layer applied by hot-dip galvanization is several times thicker than the one applied by electro-galvanization, thus being more resistant to corrosion. Objects that have been electro-galvanized can have a smooth and shiny appearance, being preferred for aesthetic reasons.

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