Zinc coatings

Zinc coatings are a type of coating that provide corrosion protection to ferrous surfaces. They contain large quantities of zinc metal powder dispersed in poly-siloxane, urethane, epoxy or butyl resins, or in the form of zinc silicate binder (very similar to the sodium silicates of mural paints and mastics, capable of cross-linking in a sort of glass transition).

After the coating is applied to the surface, the existing binder retains the zinc particles connected to the surface. The contact between two different metals will give rise to a galvanic cell capable of corrosion if subjected to an environment rich in electrolytes. The zinc particles act as the anode while the steel becomes the cathode in the galvanic cell. The action of the galvanic cell causes corrosion of the zinc, protecting the steel from electrolytic attack.

These coatings have the following properties:

• Excellent mechanical properties capable of resisting impacts and scratches.
• Offer galvanic protection even against holes, voids and other defects.
• Heat resistant up to 400°C.
• Thanks to their toughness and resistance, they can be used in joints and on bolted structures, even subjected to friction.
• Some zinc-rich paint formulations can be soaked in salt water.
• They are resistant to most solvents and can therefore be used for oil and other hydrocarbon tanks.

Single layer inorganic zinc silicate coating provides one of the most durable and economical paint systems available. Being a single layer system, it can provide significant cost and time savings: blasting and coating can often be done in just one day. As long as atmospheric conditions are correct, it will dry and cure quickly, so it can be transported or put into service more quickly than many conventional coating systems.

If applied correctly, it will provide many years of durability even in the harshest environments, such as offshore platforms and other marine applications. It is also widely used in many other industries where color and aesthetics are not required, such as power plants, refineries, bridges and other engineering structures, water and wastewater treatment plants. In fact, as long as the environment is not too acidic or alkaline, it can be used in virtually any atmospheric application. However, they cannot be used for applications where it is continuously in contact with water, such as water tanks or underground structures.

There are water-based coatings, ideal where there are limitations of volatile organic compounds (VOCs), and solvent-based coatings, which are easier to use but require moisture to cure.

Surface preparation

Typically the treatment of manufacturing defects such as uneven welds, weld spatter and sharp edges should always be the first step of any surface preparation, however, as zinc coatings provide galvanic protection and have very little shrinkage on drying, the treatment of edges and imperfections can be minimized, significantly reducing the costs and time required.

In parallel with sandblasting (a P2 treatment is usually sufficient), you must be sure of the removal of pollutants such as oil and grease (especially if you use water-based products, which are particularly intolerant to any oil).

The absence of salts on the surfaces to be coated is a less critical parameter with zinc coatings compared to other coatings. Since it is a porous coating that chemically adheres to the steel substrate, the formation of osmotic bubbles is not to be taken into consideration. Additionally, any salt present in the air after application can actually aid in the continued curing and hardening of the coating as it aids in the curing of the silicate.