Galvanized steel is widely used to control atmospheric corrosion. To improve the corrosion resistance or decoration of galvanized steel, it is often left to be painted with a topcoat.
However, the main problem with galvanized steel is that it is difficult to obtain good adhesion. This is because, in an air-polluted environment, the zinc corrosion products on the surface of the galvanized layer contain some water-soluble substances that reduce the performance of the paint film.
Besides, zinc has a very high activity and will interact with certain components in the paint. The reaction caused the paint to fall off in just a few months. Due to the above-mentioned problems, galvanized steel is usually used as a difficult-to-paint substrate, so it is necessary to perform surface treatment on the galvanized layer and specially select paint for galvanized steel.
Chemical properties of zinc coating
When the galvanized layer is exposed to the atmosphere, the initial chemical reaction takes place, namely, zinc produces zinc oxide. However, the initial stage within about 48 hours after galvanizing usually does not affect the adhesion of the paint. This time can be longer or shorter, depending on the storage and exposure conditions.
In addition to the formation of zinc oxide and zinc hydroxide, the formation of a mixture of zinc chloride and zinc sulfate is also common in the early stages of exposure. The surface film formed in the early stage is loose and powdery, so it cannot be used as a good paint primer. Although the surface film formation may not be visible to the naked eye, it can cause damage to the paint film.
The galvanized layer is exposed to the atmosphere. After 8 months to 2 years, the final surface layer is formed. The time required depends on the harshness of the environment. The final surface is a basic zinc carbonate film, which is basically inert and closely adheres to the galvanized layer. When the outer surface of the galvanized layer is completely converted into zinc carbonate, it can be successfully painted on the surface. This inert, slightly rough surface layer only needs to be pressure cleaned before applying the topcoat.
As can be seen from the above, the surface of the galvanized layer has the following three different development stages:
(1) Pure zinc-within 48 hours after galvanizing.
(2) Generate zinc oxide and zinc hydroxide (and other substances) for 48 hours to 1 year.
(3) Zinc carbonate is finally produced from 8 months to 2 years.
In the second stage, when zinc oxide and zinc hydroxide are formed on the surface of the galvanized layer and other contaminants are present, it is the most dangerous to apply a top coat on it and the paint film will be damaged.
Coating a varnish film on the surface of the zinc, conducting a long-term salt spray test or exposure to the ocean-atmosphere, after inspection, it can be found that a large amount of soluble zincate is generated under the paint film, which is due to the cathodic reaction under the paint film. One, sometimes the pH of the alkali generated under the paint film is greater than 12, and when the pH is 9, it can react with zinc, thereby consuming the alkali, thus promoting the cathode reaction.
The anode reaction is balanced with the cathode reaction, and the cathode reaction accelerates the anode reaction, so zincate is formed endlessly, and a layer of zincate is formed on the paint film and zinc surface. The liquid layer destroyed the paint film.
Surface pretreatment of galvanized steel before painting
Although the galvanized layer has good corrosion resistance and protection properties, in some cases it is still necessary to apply liquid paint or powder coating on the surface of the galvanized layer. The main purpose is to obtain a certain decorative and camouflage effect. The double protection of organic coating and galvanized layer in the corrosive environment can make the service life of steel structure longer.
Generally speaking, there is no time limit for applying paint on galvanized steel structures. It can be painted soon after galvanizing or after the aging of the galvanized layer is over.
The surface treatment of galvanized steel structural parts is best carried out within a short period of time after galvanizing because, during this time, the surface of the galvanized layer has not been contaminated by oil, grease, or other impurities. At this stage, any one of the following four methods can be used to treat the surface of galvanized steel:
① Twash method (TWash is a treatment agent that can stabilize the oxide on the surface of galvanized steel structural parts. Modified water-based zinc phosphate solution containing a small amount of copper salt).
② Phosphating primer method.
③ Comprehensive aging method (aging 8 months to 2 years).
④ Light shot peening method.
The purpose of surface cleaning of galvanized steel before painting is to remove the contaminants on the surface, and the loss of the galvanized layer should be as little as possible. Commonly used methods for surface cleaning of galvanized steel include alkaline cleaning, ammonia solution cleaning, and solvent cleaning. Some cleaning agents will react with some coatings. Therefore, when choosing a cleaning agent, contact the relevant company to have a comprehensive understanding of the performance of the selected material. After the surface is cleaned, the surface must be cleaned with hot water and then painted after drying.
Surface treatment of galvanized layer
In the first stage after galvanizing, the surface treatment can be omitted before painting, or only a simple surface treatment, such as wiping with a brush or a clean cloth. In addition, in the final stage of the formation of zinc carbonate, there is no need for surface treatment or only a simple treatment.
Although stable zinc carbonate can be formed after 8 months, it usually takes 1 to 1.5 years after galvanizing to represent the safe period for the formation of stable zinc carbonate.
Therefore, surface treatment is the most important in the dangerous stage of formation of unstable mixtures from 2 days to 1 year after galvanizing. During this period of time, it is necessary to comply with the requirements of completely removing zinc oxide or zinc hydroxide on the surface of the galvanized layer or conducting chemical conversion treatment before applying any paint.
1. Removal of zinc oxide or zinc hydroxide
This treatment can be achieved by spray treatment or chemical cleaning. The better and simpler method is to perform spray treatment, that is, to use a slight sandblasting treatment on the surface of the galvanized steel, so that the friction of the sprayed abrasive removes the pollution film and exposes the new pure zinc, and makes the surface of the galvanized layer.
A certain degree of roughness is formed to make it have excellent adhesion with paint. This method requires skilled blasting operators. Sandblasting is too light to remove the contaminated film; too deep sandblasting will remove too much zinc, and zinc will come off, forming scales on the surface.
The choice of sandblasting abrasive is also very important. Hard abrasives will grind off too much zinc; soft abrasives will only polish the surface, but cannot remove contaminants and provide a certain degree of roughness to effectively bond the paint.
Automatic sandblasting equipment with steel abrasives should be avoided, because steel particles will be embedded in the relatively soft galvanized layer, and the embedded particles will cause rust after the topcoat is applied, and ooze out of the paint film surface. After removing the abrasive, the surface of the galvanized layer should be blown off the dust with clean compressed air and painted immediately, otherwise, it will quickly regenerate zinc oxide.
Another method of surface removal treatment is to use vinegar or some weak acid solution for chemical removal and then rinse thoroughly with water. If the rinsing is not complete, the weak acid will remain on the metal surface and affect the adhesion of the paint.
In addition, the use of weakly alkaline high-pressure water for cleaning is also effective. But this chemical removal cannot provide a rough surface that is conducive to paint bonding.
2. Chemical conversion treatment
The surface of galvanized steel is passivated with a chromium-containing passivation agent. The oxidation of hexavalent chromium, zinc, and carbon dioxide in the air generates zinc carbonate. Since zinc carbonate is more stable than zinc, the life of the zinc coating can be extended from about 2 years to about 7 to 8 years.
After passivation, the zinc surface will have a slight yellow color that is invisible to the naked eye. After this passivation, the galvanized surface does not need to be painted again, which is much cheaper than painting.
Painting Process of galvanized steel
1. Thoroughly clean the galvanized steel, and let it dry for several hours after cleaning.
2. Polish the entire galvanized steel surface, especially rough places such as crevices and dents.
3. Wipe the entire surface with a sticky cloth.
4. Apply primer to cover the entire surface of the metal. It’s best to start at the top and then work on it step by step.
5. Let the primer dry for several hours.
6. Choose metallic paint, apply it, and let it dry.
How to choose a topcoat for galvanized steel?
It is important to choose a top coat for the surface of galvanized steel because when the metal surface under the paint film is corroded, the alkali generated by the cathodic reaction accumulates under the paint film, sometimes the pH is as high as 14, while zinc can only be resistant to media with a pH of about 10.
In addition, many topcoats are also prone to saponification under the above-mentioned high alkalinity. Therefore, topcoats that are resistant to atmospheric corrosion and not easy to saponify should be selected.
The surface of galvanized steel after light sandblasting or chemical cleaning is very effective when it is cleaned and immediately coated with epoxy topcoat and polyurethane topcoat because polyurethane paint is resistant to ultraviolet rays in most industrial environments. Epoxy paint has very high adhesion, and this thermosetting paint film is excellent in chemical and physical properties.
Alkyd-based paints should be avoided, because this paint is prone to saponification at the interface between the zinc and the coating film, resulting in poor adhesion.
There are also topcoats based on chlorinated rubber-acrylic and vinyl-acrylic terpolymers.
Maintenance of galvanized steel structure coating
The coating on the galvanized steel structure should be maintained regularly so that the galvanized layer under the coating can always be effectively protected.
(l) The coating is powdered or the surface is dirty.
Rinse the surface with a detergent solution and apply a coating. There is no need to use shot blasting for surface pretreatment before painting. The coating should match the original coating. It is best to use the same type of coating as the original coating. The weathered coating can be restored to its original luster by applying a layer of polypropylene paint.
(2) The coating is cracked and peeled off.
Coatings with weak bonds must be cleaned up before repair. Light shot blasting is a good way to remove the loose coating, and the loss of the zinc coating is not much. The repair paint should be matched with the original coating and the bare zinc coating.
(3) The steel has been corroded.
Before coating repair, rust must be cleaned up. Manual cleaning, power tool cleaning, or shot blasting can be used. Similarly, the repair paint should be matched with the original coating and the bare zinc coating.
