Zinc Rich Coatings

In recent years, a number of paints have been developed that will deposit a film of metallic zinc having many properties in common with zinc coatings applied by hot-dip galvanizing, electroplating, thermal spraying, and mechanical plating. Such paint films will protect the underlying steel sacrificially if they contain 92 to 95% metallic zinc in the dry film and if the film is in electrical contact with the steel surface at a sufficient number of points. They are effective where steel is subjected to high humidity and water immersion. Under normal conditions, zinc-rich coatings are long lasting and most effective where a regular maintenance program may be difficult. In applications in which steel is immersed in brackish or salt water, zinc-rich coatings, along with a suitable top coat, should be used. Most zinc-rich paints are of the air-drying type although oven-cured primers containing a high content of zinc dust are available.

The type of zinc dust used is a heavy powder, light blue-gray in color, with spherically shaped particles having an average diameter of approximately 4 mm. Such powder normally contains 95 to 97% free metallic zinc with a total zinc content exceeding 99%.

Surface Preparation. Zinc-rich primers must be applied over clean steel surfaces to provide the metal-to-metal contact essential to successful performance of the coating. Abrasive blasting is the most effective method of cleaning steel. Although white metal blast-cleaning (NACE No. 1) is preferred, near-white SSPC-SP-10 or Commercial Blast Cleaning SSPC-SP-6 is acceptable (Ref 52).

Where the zinc is supplied as a separate component, it should be added slowly to the vehicle with constant agitation. After a homogeneous mix is obtained, the primer may be applied with air spray. Airless spray may also be used, but the nozzles may wear quickly. Because zinc settles rapidly, continuous agitation of the paint is essential during application, and fluid lines should be kept as short as possible.

To obtain a wet coat, the gun should be kept within 30 cm (1 ft) of the surface. Uneven film thickness due to brushing or rolling may result in mudcracking in the thick portions. Zinc-rich primers should be applied at a dry film thickness of 0.06 to 0.08 mm (2.5 to 3.5 mils).

The Nature of the Zinc-Rich Coating. Depending on the binder, zinc-rich coatings fall into two classes: organic and inorganic. The inorganic solvent-base types are derived from organic alkyl silicates, which become totally inorganic upon curing. Each offers particular protection characteristics, and each requires different preparation of the steel surface. The following comparisons should be helpful in selecting the most useful binder system.

The organic zinc-rich coatings are formed by using zinc dust as a pigment in an organic binder. This binder may be any of the well-known coating vehicles, such as chlorinated rubber and epoxy. The zinc dust must be in sufficient concentration so that the zinc particles are in particle-to-particle contact throughout the film. Thus, zinc provides cathodic protection. In the case of the organic binder, there is no reaction with the underlying surface other than for the organic vehicle to wet the steel surface thoroughly and to obtain mechanical adhesion.

Organic zinc-rich coatings do not require a white blast preparation of the steel surface, although a commercial blast should be included if the application is heavy service. For mild-service applications, the organic coating can be applied to a well-hand-cleaned surface, even if traces of rust are present.

Some proponents feel that maintaining proper humidity during surface preparation, application, and curing is not necessary. Because this type of coating is more flexible than inorganic coatings, exacting surface preparation for bonding to a substrate is not required. Finally, although organic coatings are more compatible with top coats, they are somewhat less abrasion resistant than the inorganic types.

As to the advantages of these coatings, organic zinc-rich coatings require less critical surface preparation, allow greater variation in application techniques, are less sensitive to varying climatic conditions during application and curing, and are more flexible and more resistant to chemical environments. Their disadvantages include flammability, blistering, harmful solvent effects, sensitivity to atmospheric influences, and relatively low heat resistance.

For better resistance against continuous exposure to salt water and to acid or alkali chemical fumes, zinc-rich coatings should be top coated with organic topcoats to provide a totally organic system, with optimum intercoat compatibility. A top coating may also be applied to provide color or to prevent gradual erosion of the zinc coating. Although zinc-rich coatings vary in application characteristics, they can be applied by brush or spray, and depending on the specific formulation, one coat can vary in thickness from 0.05 to 0.2 mm (2 to 7 mils).

Inorganic Zinc-Rich Coatings. Many inorganic zinc-rich coatings use water solutions of alkali silicates as vehicles. Others use phosphates, silicones, and modifications of these groups.

Self-cured coatings are two-component materials consisting of zinc dust and a vehicle; they are mixed immediately before application. Postcured coatings are three-component materials that consist of zinc dust, the vehicle to be mixed with it before application, and a curing agent that is applied on top of the coating.

The inorganic zinc coating forms its film and its adhesion to the steel surface by methods quite different from those of the organics. The coating system is a chemically reactive system, and the chemical activity is similar for either the water- or the solvent-base inorganic. Zinc is the principal reactive element in the inorganic coating systems and is primarily responsible for the development of initial insolubility. Depending on the formulation, other metal ions may be present in the system that also react and aid in the insolubilization of the coating. The silicate vehicle can also react with the underlying iron surface to form a chemical bond with the iron or steel substrate.

Inorganic zinc-rich coatings commonly require a white metal blast as preparation for the steel surface. Because inorganic coatings generally have limited flexibility and tend to break or crack upon bending or impact, careful preparation of the steel surface is required to ensure a good bond between the coating and the steel. However, despite the difficulties of preparation, these inorganic coatings are unaffected by solvents, oils, petroleum products, aliphatics, aromatics, ketones, and alcohols. They do not chalk, peel, or lose thickness over long periods of time. Also, they are easier to weld through and have excellent abrasion resistance and surface hardness.

Inorganic zinc-rich coatings offer good conductivity; good adhesion to clean steel; excellent resistance to weather, sunlight, and variations in temperature; resistance to radiation, heat, and abrasion; and reduced undercutting. Conversely, these coatings require unusually good surface preparation, display a lack of distensibility and adhesion to some metals other than steel and zinc, require moderate temperatures and atmospheric humidity for cure, and exhibit unsatisfactory durability under conditions of continuous immersion in electrolytes, and a lack of resistance to strong acids and alkalies.

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