Why paint hot-dip galvanized steel

Hot dip galvanizing alone is able to offer a satisfactory duration of corrosion protection for the products, in order to avoid maintenance for several tens of years or avoid it altogether during the entire period of use.

In constructions, for which durations of several decades tend to be required, the longest possible, the persistence of the protection can be further increased by applying a duplex system, which consists of a painting performed on the steel surface. hot dip galvanized. This association of the two protective systems, with the choice of the most suitable paint, is used to increase resistance to corrosion even when a particularly aggressive environment must be contrasted.

Beyond the duration, the duplex system can be adopted when it is necessary to give color to the structures to increase their visibility through the chromatic contrast. Application examples are signage and road signs. The application of colored paints can also be an important safety measure to increase the visibility of high power towers, telecommunication installations and airport structures from aircraft, for which an alternation of red and white bands is generally used. A similar need for highlighting occurs for the identification of pipes for the transport of flammable fluids, where the color serves to highlight the layout of the supply lines.

Conversely, the painting of galvanized works may be required in situations in which it is necessary to obtain camouflage, of structures such as for military installations or for the reduction of environmental impact in particularly valuable landscapes. The last and most banal reason for the use of a duplex system, certainly the most frequent, is of an aesthetic nature and meets the needs of those who require the duration of the protection offered by galvanizing without giving up a different color of the product for mere aesthetic preferences. In this case, it is always possible to choose between countless shades of color, possible with duplex systems as with any other metal coating.

Durability of a duplex system

By applying the paint layer, additional protection of the galvanized surface is obtained. The organic coating prevents direct contact with the corrosive environment and the erosion of atmospheric agents, thus keeping the thickness of the underlying zinc coating intact. This condition persists until the paint performs its protective action, that is, until it deteriorates due to degradation due to aging or mechanical damage, detaching itself from the substrate.

The painting, due to its permanence in an aggressive environment, tends to lose elasticity and become covered with cracks. On the other hand, the burning of the coating can also be a consequence of accidental impacts, surface abrasion due to wear or scratches. The cracks are real grooves in the coating that can meet unprotected steel, in the case of simple painting on steel as it is, or zinc in the case of steel protected by zinc plating.

In the first case, the corrosion of the steel proceeds quickly, causing the detachment of the paint in a short time. In the second case, the zinc corrosion products, which have a not excessive volume and are more compact than rust, have a sealing effect, interposing between the aggressive agent penetrated through the crack and the galvanized surface. In this way, corrosion is considerably slowed down and the protection lasts much longer than the sum of the protection durations that would have been carried out by the two treatments ideally applied separately. Even the porosity or the presence of ionic conduction paths, which characterize paints more or less incisively, are not a problem if the hot dip galvanizing layer is underneath the coating.

The synergy that can be found between hot dip galvanizing and painting in the duplex system was investigated by Dutch researchers, who derived the following empirical formula from the experiments conducted:

DSistema Duplex = [1,2÷2,5] (DZincatura+DVerniciatura)

The duration of the duplex system is defined as the time interval between the application of the system and the rusting of 5% of the protected steel surface. The variability of the multiplicative coefficient in the formula depends on the nature of the organic resin chosen and its interactions with the particular corrosive environment.

As stated above, only from the moment in which the overall deterioration of the paint occurs, when its complete detachment occurs, does the galvanizing layer begin to wear out at the speed determined by the aggressiveness of the atmosphere.

At this stage, it is still possible to intervene with very simple painting restoration interventions, given that, in most cases, it is sufficient to pass a metal brush or any other mild abrasive on the surface to remove the deteriorated paint. Subsequently, you can proceed to apply a new protective layer. Note that these operations do not affect the steel substrate in the least, which in any case is totally protected and immune from corrosion due to hot dip galvanizing. Indeed, the relative ease with which maintenance of the duplex system is achieved is due to the fact that there is no need to worry about the condition of the steel and no particular treatment, other than normal cleaning, is required for its restoration. In this way, even when maintenance is required, it is very economical and effective.

Quality of galvanization and surface preparation

The quality of the duplex coating is affected by the quality of the galvanizing. Any defects present in the zinc coating whether caused by surface conditions of the steel base also interfere with the final quality of the duplex system. The pieces to be painted must therefore be inspected and care must be taken to repair, if possible, any imperfections such as the presence of swellings, drops and spikes on the edges, coating defects in the attachment points of the material. Any repairs to the galvanizing coating must be carried out in compliance with the criteria of UNI EN ISO 1461 and following the fundamental principle of minimizing the removal of zinc from the coating. For example, the small and sharp points that are the result of steel chips under the coating, caused by lamination, can be removed by sanding the surface with sandpaper, but without consuming the galvanizing. Uncoated or damaged areas (if less than 10cm and maximum 0.5% of the surface area, according to UNI EN ISO 1461) must be repaired with a method compatible with the organic coating system to be applied. The choice must be made between metallization, the use of high zinc content paints and the coating carried out with low melting point zinc bars.

In any case, the roughness of hot dip galvanizing cannot be considered a defect in itself. In a Dutch technical standard (NEN 525) a criterion was established that determines the acceptability if, from a distance of 3 meters from the surface of the duplex system, non-uniformities do not appear when observed with the naked eye with normal sight.

Obtaining a quality duplex system passes through the observance of application procedures that involve, first and foremost, particular attention to surface preparation. The purpose of this pre-treatment is the removal of any type of pollution from the zinc surface, including grease, dirt and zinc corrosion products, to promote adhesion of the coating.  To achieve optimal results, it is essential to carefully and adequately prepare the surface to be treated, based on the age and condition of the zinc coating.

In the past it took one or two years to paint a galvanized artifact. In this way, it was intended to allow the zinc layer to react with the carbon dioxide present in the atmosphere to form, as seen above, a protective film based on basic zinc carbonates. Effectively, this increases the gripping effect between the galvanized surface and the paint, since the latter is able to incorporate the inert products that have layered on the galvanizing.

Nowadays, due to atmospheric pollution, after a not necessarily long period, the galvanized surface could have been attacked by pollutants capable of deteriorating it, or have reacted with the atmosphere in a different way from point to point, or, again, they could have formed hygroscopic and water-soluble zinc corrosion products. In this case, it would be necessary to remove a relevant layer from the surface to be painted through an energetic mechanical cleaning accompanied by a degreasing phase and phosphoric-based surface conditioning with subsequent washing and drying operations.

In general, it is advisable to avoid that the galvanized surface is contaminated by atmospheric agents or other substances during transport and assembly, therefore, it is advisable to paint the product immediately after galvanizing, whenever possible. In this case, the duplex system is completely carried out at the zinc-coated factories equipped with a painting department. However, even the newly galvanized artifact must be subjected to adequate preparation. Surface oxidation, the permanence of processing residues and the contacts during the handling of the pieces, inevitably create a certain degree of contamination even if the product does not leave the zinc linen.  

In the more frequent case in which the painting must be carried out with liquid products, to remove the pollutants it is sufficient, as mentioned, to use a wire brush, subsequently treating the surfaces with degreasing chemical substances based on weak ammonia solutions and / or phosphoric, followed by accurate washing with water and towel. There are, in fact, several reliable methods of removing contaminants from the surface and preparing it to accept paint. All, however, can be traced back to the two methods mentioned. We report in Tab. 10.1 different preparation procedures.

Alternatively, general mechanical cleaning can be accompanied by light sandblasting. This system is in most cases the one that offers the greatest guarantees.

Sandblasting deserves particular attention: it must be performed with very regular movements, so as not to generate surface unevenness. The operation must be done with great skill to prevent the zinc layer from being damaged or removed in unacceptable quantities. Fine-grained, sharp, inert, non-metallic products are used. Typically, silica sand, fine copper slag or corundum powder is used. The surface conditions after sandblasting must be such as to have a slight roughness that promotes adhesion.  The normal appearance is with a smooth matte finish Notwithstanding that slightly different blasting conditions may equally provide acceptable results, a good procedure involves:

• Grain size of the blasting product up to 0.5mm
• Air pressure up to 3 bar
• Minimum sandblasting distance 60 cm
• Blasting angle (blast-surface jet) 50 ÷ 70 degrees.

The action of sandblasting should not remove more than 10µm of zinc. This appears to be a fair compromise between the need for surface preparation and the inevitable abrasive action of sandblasting.

The application of the paint on the sandblasted pieces usually takes place without intermediates. However, it is also possible to apply thin layers of specific products for surface conversion. These chemical compounds are able to improve the adhesion of the organic coating to be applied and generally also improve the corrosion resistance characteristics. There are on the market industrial pre-treatments of metal-organic composition with calcium, nickel, manganese, or inorganic such as chromating (which can also be free from hexavalent chromium) or zinc-phosphating. Some of them are no-rinse, that is, without the need for post-application washing and can be applied simply by immersion in solution baths just after extraction from the molten zinc bath.

Paints and methods of application

First of all, attention must be paid to the type of product that is applied, in order to provide the appropriate adhesion and resistance to the organic coating.

Regardless of the surface conditions, alkyd enamels (those commonly used for wood or for covering normal rust inhibitors) cannot be used for painting galvanized steel as a base coat, in direct contact with galvanizing. These can be used after a suitable primer has been applied to the surface, capable of ensuring compatibility between zinc and topcoat. The fact that products suitable for galvanizing must be used does not mean that the painting phase of the duplex system involves higher costs than any other painting due to the cost of the paint. The appropriate paints for galvanizing are, in fact, in any case cheap and easy to find, for both domestic and industrial applications. The costs are just a little higher than normal paints. Obviously, the overall cost depends on the result to be obtained and on the quality of the materials used, but this is a general consideration that is universally valid. To obtain a duplex system with acceptable characteristics, just be sure to use a product specifically prepared for application on galvanizing, which can be obtained simply by consulting the technical data sheets, verifying the suitability not only for the surface but also for the conditions of use. Together with the adhesion, for a long time the duplex system must ensure resistance and impermeability to humidity.

In the case of painting cycles that involve the simultaneous use of different products for different coats, it is advisable to contact the same manufacturer, who can provide all the information necessary to establish compatibility. It will be up to the applicator to follow the right procedures indicated by the manufacturer of the specific paint to be applied. It is not possible, in fact, to dictate a single recipe, given the wide range of products on the market.

Solvent-based paint applications are also possible with high solids, or with water or powder. As previously mentioned, the wide and varied offer on the market of paint products suitable for galvanizing allows in many cases to directly paint the surface of the product previously cleaned of impurities, preferably made slightly rough with a light sandblasting, as previously stated. The paints used obviously must not interact with the zinc, compromising adhesion, for example, with the formation of condensation water, or saponification reactions or the production of gaseous or even corrosive compounds. In any case, if you do not want to give up the final effect of the paint, this can often be avoided by applying a base coat compatible with both zinc and subsequent coats of paint on the zinc.

For liquid solvent-based paints, the experience gained over the years leads to consider more reliable cycles that include respectively:

• Acrylic resin and chlorinated rubber (for example, chlorinated rubber)
• Epoxy resins, as long as they are free from amines, in combination or not with polyurethane resins with a low content of isocyanates, to finish
• Epoxy phenolic or epoxy phenolic resins
• Combinations of acrylic and alkyd resins, as long as the alkyd is not in direct contact with zinc.

Good results are also obtained with water-based paints of similar composition (acrylic resins, modified acrylics, styrene-acrylics).

For powder coating, quality coatings are made with very accurate technological cycles. The application is industrial type with the use of fluid beds or electrostatic spraying and care of the coating (polymerization phase) obtained in static ovens at 150 ° C or, in any case, for temperatures not exceeding 175-180 ° C (or according to the instructions of the manufacturer of the powder) to avoid the appearance of craters or delamination of the zinc layer itself. An alternative method for the crosslinking of certain paints is constituted by IR (infrared) irradiation. Available formulations include epoxy, polyurethane powders or combinations thereof. It is always advisable to form a conversion layer that separates the zinc and the powder film in order to avoid any reaction between the zinc and the topcoat components. In particular, it is necessary to avoid painting untreated galvanized steel, on whose surface there may be residual moisture or salts. If so, the zinc demonstrates strong reactivity. Ideal is the use of passivating agents capable of transforming the metal surface into non-metallic zinc phosphate or chromate.

A typical application cycle of powder coatings includes a chemical conversion treatment with the following outline:

• Alkaline degreasing
• One or two washes in water
• One or two washes in water
• Washing with demineralized water
• Chromating or other compatible passivation
• Washing with demineralized water
• Dry in the oven at 60-90 ° C
• Dust deposition
• Polymerization (baking or irradiation).

The application cycles are generally specific with respect to the type of paint used, whether powder or liquid. The manufacturer’s prescriptions must therefore be strictly observed. However, some general application criteria may be indicated Organic coating system, whatever its nature, should be applied immediately after chemical or mechanical pretreatment.   After sandblasting, the coating must be laid within 12 hours, but if the relative humidity is greater than 70% this time is limited to 3 hours to prevent the formation of zinc corrosion products.

Standardization of duplex systems

The zinc surface must comply with the requirements of the UNI EN ISO 1461 standard, which also establishes that the galvanizer must be informed in advance that an organic coating must be applied. At the same time, it is necessary to agree on the most appropriate preparation of the substrate for the coating in cases where the applicator is different from the galvanizer. Subsequent painting is regulated by UNI EN ISO 12944 and UNI EN 13438 for powder coatings. A useful reference on the best procedures to be adopted is undoubtedly the UNI EN 15773 standard, also from the point of view of the behavior that the customer must have, even different specifications can be indicated by the paint manufacturer.

The standards also establish a series of tests of the coating system concerning the thickness, the appearance, the adhesion, the hardness, the porosity, the elasticity, the resistance to blows, to the impact, to the scratch and the differences in color and brilliance. I test possono essere di tipo meccanico, non-meccanico e climatico, e non sono dissimili dai test effettuati per ogni altro sistema di verniciatura.