Cathodic Protection with Film Galvanizing
July 04, 2019
Where steel is; is also corrosion. The corrosion-related cost runs annually in billions. Indeed, countries like USA, Great Britain, Germany and other industrialized Nations spend more than $10 billion a year for corrosion protection. And that corrosion costs significantly more than the sums spent worldwide on prevention industrial coatings.
A common method to protect steel against corrosion attack is to coat the structure with different zinc-rich primers, mid- and topcoats. On the one hand they contain anticorrosive pigments and additives, on the other hand they can react with surface humidity, which leads to changes in the zinc film wherein a protective barrier is formed between the air (oxygen) and the metal surface. Many of these systems are effective, but if the coating is cracked or has other defects - for example, by damage during work, construction or transport – then this passive barrier is broken and its anti-corrosion effect is almost completely gone.
In coastal areas, a coating can be fully intact but applied too thin (unprotected edges, not completed properly). This can lead to chloride ions at the surface, which penetrate the paint film and cause short-term corrosion. It is therefore important that the paint specifications are to be followed strictly   in order to avoid additional costs and defects. These costs are mostly five to ten times the original cost because of additional pre-treatment and other site costs - and even between 20 to 30 times higher for offshore structures.


The above-mentioned situations made sure that the thermal galvanization has been widely used for decades in order to protect steel. Hot galvanizing eliminates many problems that can occur with surface treatment by applying a paint. This being said, it is clear that it also has its own hot-dip galvanization parameters that must be followed in order to obtain a good result. Some metals such as cast iron may not be hot dipped in a liquid zinc bath because at a temperature of 450 degrees the structure will crack.
In Belgium, a film galvanizing system called ZINGA has been developed in collaboration with University of Ghent. The idea was originally for hot dipped structures that could not be dismantled, such as pylons and bridges to restore them, (in situ). Compared with hot dipping film galvanizing provides equivalent cathodic galvanic plating protection but it is applied in the same way as a traditional paint system (liquid, cold galvanization). This can applied with the paint brush, paint roller or spray gun. The film galvanizing system contains zinc powder (granular) resin and solvent. The method quickly proved to work well for repairs, and was able to repair the zinc layer on hot galvanized structures or to increase it, in order to make its anti-corrosion protection last longer.


Due to the good results of such repairs, some companies used the same method for new pieces to avoid the cracking of the zinc layer due to torsion of thin steel, or because of the fact that hot dipping was not possible or too expensive in some countries. Steel could now be galvanized by applying Zinga on the wharf and at normal environment temperatures. Film galvanising acts as cathodic /galvanic protection and as barrier protection. When Zinc comes into contact with the steel, a current that protects the steel occurs. The Zinc content of the dry layer is at least 96% (with a purity of 99.995%). The system can be applied at a humidity degree up to 95% (no drops of water may be on the surface) and at temperatures up to minus 15 ° C. The zinc layer dries within 10 to 15 minutes (at 20 ° C) and a second layer can be applied after one hour. Thanks to these properties, film galvanizing is often applied in the offshore sector and on pillar supports in ports and bridges.

Kalvoya BRIDGE

The Zinga film can be recoated, if desired, for aesthetic reasons, but it is not a requirement. An example of the proper functioning of the system Zinga is the Kalvoya Bridge near Oslo. The entire structure and the steel suspension cables were protected in 1985 with a layer of 120 microns ZINGA. In 1995, 5kg Zinga was used to carry out some local repairs on mechanically damaged areas. In February 2010, twenty-five years after the first application, the average thickness was still 100 to 110 microns. Nearly thirty years after the construction has been applied for the first time with Zinga, a refurbishment has been considered. With this refurbishment, no sandblasting was required before a new Zinc coating should be applied. A surface preparation with a high pressure steamer (700 BAR) was enough. Pull Tests also showed that nearly thirty years old Zinc film had a pull capacity of 11 Mpa.
The experience with ZINGA since 1970 shows that it is possible to provide all kinds of steel and iron structures with a maintenance-free cathodic protection for many years under the condition that the surface to be applied has been sandblasted and cleaned. It has also shown that it is not necessary to sandblast again after a few decades when the coating of zinc has to be renewed.