About painting galvanized steel

Galvanized steel painting, which is applied outside buildings, is an old and proven practice. but, known examples of the rapid destruction of such a color on a galvanized surface due to incorrect painting technology. Below are the recommendations of the Australian Zinc Coating Manufacturers Association, which are designed to prevent such cases.

Why paint galvanized steel

When painting galvanized steel, there are three main goals::

  • Improving the decorative appearance of the coating
  • Extended coating life
  • Increase the chemical resistance of the coating in environments, which are harmful to galvanized steel, eg, in an environment with pH from 6 to 12.

When used outdoors away from the sea coast and areas of heavy industrial pollution, hot dip galvanizing naturally has a high durability.. service life. In this it differs from paints and other organic materials., which are degraded by solar radiation. Therefore, in most atmospheric conditions, a zinc coating with a thickness of more than 300 g / m2 (42 μm) gets little from the paint layer, apart from aesthetics and color.

Under normal interior conditions and especially under conditions of impact or intense friction, if issues of color or gloss are not prevalent, zinc coating is best left unpainted.

A large amount of zinc coating is randomly painted with conventional latex paint or primed with solvent alkyd paint.. The choice of primer is critical and requires clear guidance from the paint manufacturer.. In particular, the use of an alkyd primer in direct contact with the zinc coating leads to the risk of peeling of the paint due to its saponification.

It is important to note, that since the paint layer is very thin, usually from 70 to 120 µm in three-layer coatings, then some areas of the zinc coating with increased zinc thickness, eg, on the edges, may remain visible, just like the texture of painted wood. Therefore, paints of increased thickness are usually applied to zinc coatings..

Requirements for the quality of painting galvanized steel are usually lower, than normal coloring. Consistent adherence to proper surface preparation and proper priming are essential to ensure a reliable paint finish on galvanized steel..

The paint protects the zinc

Hot zinc plating is inherently very durable. Therefore, it rarely needs to be painted to increase its service life.. Moreover, in highly corrosive areas (C3-C5) painting galvanized steel can actually accelerate the corrosion of the zinc base and reduce the overall life of the product compared to zinc coating without additional painting. The positive coloring effect of the zinc coating is achieved under the following conditions:

  • the right choice of paint;
  • uniform paint application;
  • proper surface preparation of zinc coating;
  • timely maintenance of the paintwork.

However, in conditions, when the zinc coating is subjected to slow corrosive action in a corrosive environment, applying a suitable coating, which isolates zinc from the environment, can increase the service life of zinc coating. therefore, in industrial areas near the seas, painting the zinc coating can significantly increase its service life. Even in favorable outdoor conditions, in which zinc coating can serve for many decades, it will be reasonable to color those parts of it, who are exposed to the rain, to increase even longer the service life of the building structure.

Paints for zinc coating

For painting galvanized steel, paints from liquid epoxy and polyurethane to powder coatings are used.. Most paints have special restrictions on how, how they should be superimposed and then reach the finished state. These restrictions must be strictly followed. for instance, below 8 ºС latex paints usually do not dry, and most two-layer paints require a fairly quick application of the second layer.

Powder paints require curing (polymerization) in the oven and it goes beyond 15-20 minutes, what can be convenient in terms of logistics. Besides, powder paints are usually applied by electrostatic method, which gives a more uniform coverage on difficult surfaces, which is difficult to achieve with conventional liquid paints. However, one must remember, that powder coatings require careful surface preparation.

Surface preparation and paint thickness

With an increase in the corrosivity of the environment, in order to increase the effectiveness of the protective paint coating on galvanized steel, it is necessary to increase its thickness with increasing. In very corrosive environments, it becomes difficult to ensure a strong bond between paint and zinc.. Therefore, in this case, you need to look for other methods of protection against corrosion..

Another prerequisite is the use of a suitable product to prepare the surface of the zinc coating for painting.. Most of the problems with painted zinc coatings arise precisely from the “wrong” primer or insufficient thickness of the paint layer..

Zinc coating and rain

If galvanized steel is protected from rain, then its surface is never cleaned of salts and other pollutants, that fall on her from the air. Therefore, galvanized structures under the veranda or in covered outdoor areas corrode in 3-5 times faster, than the same surfaces, that are washed by the rain. In such cases, painting the galvanized surface will give a noticeable increase in the service life of the protective coating..

Even in the most favorable conditions, it is recommended to paint galvanized surfaces to increase the service life., that are not washed by the rain, up to paint layer thickness 75 μm. This additional coating isolates the zinc coating from the effects of contaminants., that accumulate on the outer surface of building structures.

In some cases, galvanized construction, which is exposed to the environment, may be exposed to frequent or continuous rainwater in areas of contact with the ground. In such situations, local application of paint to galvanized steel will protect it from premature corrosion.. Naturally, the coloring of the surface should go beyond the boundaries of this "underground" surface with a margin.

Zinc coating and chemicals

Zinc coating is recommended for use in environments with a pH value of 6 to 12. Outside this range, the service life of the zinc coating will be unacceptably short.. This includes exposure to strong acids and alkalis., salts of strong acids and weak bases and vice versa. Contact of zinc coating with some organic substances may contribute to their accelerated destruction..

A layer of paint on the zinc coating provides its protection, like in a corrosive atmosphere, and in the atmosphere of aggressive chemicals. The paint only protects the zinc coating from mild to moderate chemical attack.. In extreme conditions it is necessary to use materials, which have a natural inertness to various chemical influences, eg, stainless steel or plastics.

Source: https://www.gaa.com.au/index.php?page=painting-galvanizing