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Introduction – Hot dip galvanizing is painted for aesthetics or to capitalize on the synergistic effects of conventional paints to provide a protective barrier over the galvanizing, improving the longevity of the system. Unfortunately, conventional coatings do not always adhere well to galvanizing. This is often due to the presence of zinc salts, especially zinc hydroxide, on the surface, which is an alkaline corrosion product that generally forms on the surface of new galvanizing within a day or two. If zinc hydroxide is present, it needs to be removed, together with other loose oxides, for coatings to adhere properly. In time, the zinc hydroxides, in the presence of water and carbon dioxide, form insoluble zinc carbonate, which is generally acceptable for painting provided it is intact. The formation of zinc oxides, zinc hydroxides, and zinc carbonates on galvanizing is described by stages:
- First stage – newly galvanized steel with small amount of zinc oxide
- Second stage – partially weathered galvanized steel with zinc oxides and zinc hydroxides
- Final stage – weathered galvanized steel with zinc carbonate and some zinc oxides, but rarely zinc hydroxides
While it is accepted that zinc carbonate can be painted, field methods are not available to determine whether zinc carbonate or zinc hydroxide (unacceptable for painting) is present. Another problem occurs when the galvanizing is water quenched or treated with a chromate coating as part of the galvanizing process, which can also be detrimental to coating adhesion. Because of the many questions regarding galvanizing, it is good practice to clean and etch the surface prior to painting.
Preparation of galvanizing for painting – The requirements for preparing hot dip galvanized surfaces for painting are dependent on the condition of the galvanizing. Galvanization is the process of applying a zinc coating that is anodic to the steel substrate, protecting the substrate as it is exposed to the environment. As galvanized surfaces weather they go through three distinct stages as noted above. Each stage is characterized by the chemical composition of the oxidized zinc, and the surface preparation required for painting must be tailored to address the conditions present in each stage. The stages of the galvanized surface and the treatment methods for each are more fully described in ASTM D6386 – 16a, Standard Practice for Preparation of Zinc (Hot Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting. In addition, the surface requirements of the proposed coating system such as profile, must be considered.
Passivation can interfere with coating adhesion – Hot dip galvanized (HDG) steel is often passivated as part of the galvanizing process to reduce wet storage stain (white rust). Although passivation wears away during the weathering process of the galvanizing, if the components are to be painted the specifications should preclude passivation such as chromate conversion. If the status of the surface regarding passivation is uncertain it should be tested prior to coating. SSPC-SP 16, Brush Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals, Section 4.3.3 provides a procedure for determining the presence of passivating treatments using copper sulfate. If present, it needs to be removed before painting, typically using mechanical methods such as blast cleaning.
Water quenching of the galvanizing can also be a concern as it leads to the formation of zinc salts that must be removed as described below in the Second Stage.
Surface Preparation at the First Stage – Newly Galvanized Steel
Newly galvanized steel is characterized as predominantly “zinc metal with a small amount of zinc oxide.” This condition can last up to two days (48 hours) from when the steel is removed from the galvanizing kettle. It should not have received any post galvanization treatment such as water quenching (which leads to the formation of oxides) or chromate conversion coating. The two-day time limit is subjective and could be lessened depending on a variety of conditions. There should be no visible zinc salts (zinc oxide or zinc hydroxide) on the surface, which is a sign of partial weathering.
It is possible to apply coatings to newly galvanized steel with little preparation. The surfaces to be coated should be free of grease, oil, salts, zinc oxide or hydroxide, and other foreign materials. Cleaning with solvents, alkaline cleaners, hand and power tools as appropriate is discussed in detail in ASTM D 6386. The surfaces should be smooth and free of imperfections such as drips and dross inclusions. Note that the galvanizing may be acceptable according to ASTM A123, but still be unacceptable for painting due to the presence of imperfections such as globules that require removal. Hand or power tool cleaning can remove imperfections and provide a uniform surface. A surface profile may need to be created for adhesion of certain coatings systems but care should be taken to avoid removing the galvanizing while profiling the surface. The thickness of the zinc coating should be measured before and after the cleaning process to ensure the specified zinc thickness complies with ASTM A123, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, or the project specification, prior to coating application. While it is possible to apply coatings after solvent cleaning as described above, often the additional steps noted below for partially weathered galvanized substrates are undertaken to provide better adhesion for the subsequent coatings.
Surface Preparation at the Second Stage – Partially Weathered Galvanized
Partially weathered galvanizing is characterized as a surface composed of “mainly zinc oxides and zinc hydroxides with some zinc carbonates.”1 This stage is generally between two days and two years after galvanizing and is the stage of HDG steel most often being painted at fabrication shops. This stage presents particular challenges for painting.
Once the surface becomes partially weathered, the zinc oxides and hydroxides are not conducive to the adhesion of coatings and must be removed. Painting over these compounds or the byproducts of passivation are often the cause of coating failure on HDG substrates.
ASTM D6386 lists several ways to prepare partially weathered HDG surfaces, but it must first be determined if the surface was passivated by chromate conversion. Hexavalent chromium treatment may be removed by weathering, sanding or sweep blasting. If the surface tests negative for chromate treatment or it is documented that it received no treatment, the next step is to check for the presence of wet storage stain. This white, grey or black discoloration of the galvanized surface is oxidation as a result of moisture and insufficient air flow to dry the surface and allow the zinc hydroxide to convert to zinc carbonate. This conversion forms the patina common to galvanized surfaces and protects the surface from further corrosion.
Light wet storage stain may be removed by hand or power tool cleaning. Scrubbing with a soft bristle brush and ammonia can also be effective. More severe cases may require scrubbing with an acidic solution in the 3.5 to 4.5 P.H. range. Following the washing process the components should be thoroughly rinsed to remove the solution, and then dried completely. Hot air drying is often used to prevent additional oxidation from forming on the wet surface. Brush blasting is not recommended to remove wet storage stain as it could remove too much zinc from the galvanized surface. Heavy wet storage stain can itself result in the breakdown of much of the zinc galvanized surface. In all cases the thickness of the galvanized coating should be measured after cleaning to ensure that sufficient thickness remains according to ASTM A123 or the project specification.
Other processes commonly used to prepare HDG for overcoating include acid etching wash primers, phosphate conversion coatings, and acrylic passivation treatments. These treatments may be selected as appropriate for certain coating systems. Therefore, coordination between the specifier, galvanizer and paint shop is required to select the proper treatment for the chosen coatings. These processes typically involve application via spray, scrubbing with synthetic pads or brushes and may also serve to clean the surface of grease and oils. Some require rinsing and/or forced drying following application.
Abrasive blast cleaning is also used to prepare HDG for overcoating. SSPC-SP 16 is commonly specified for preparing unpainted or previously painted surfaces, providing a surface “free of all visible oil, grease, dirt, dust, metal oxides (corrosion products), and other foreign matter” and calling for a minimum uniform profile on unpainted surfaces. It contains provisions for testing for passivation treatments and removal of wet storage stain. Selection of abrasive media and lower air pressure to minimize reduction of galvanized zinc thickness, as well as measurement of the thickness before and after blast cleaning is addressed.
Other methods of preparation include the use of phosphoric acid/detergent blends that clean and etch the surface, leaving a phosphate conversion coating on the surface that can improve the adhesion of subsequently applied coatings.
Following completion of surface preparation, the galvanized components should be painted as soon as possible, at a minimum within the same shift. Once cleaned, the surfaces are as susceptible to oxidation as when they first came out of the galvanizing kettle, and a delay in coating may compromise the coating’s adhesion.
Surface Preparation at the Final Stage – Weathered Galvanized
Weathered galvanize is characterized as “mainly water-insoluble zinc carbonates, some zinc oxides, and rarely, zinc hydroxides.”1 Galvanized surfaces are believed to reach the weathered stage in 1 to 2 years, but this can vary due to exposure or protection from the elements, the service environment and other factors. The weathered surface should have reached a dull, uniform patina. Much older surfaces may begin to show rusting where the zinc coating has completely worn away. This may require spot repairs as needed. Preparation of weathered galvanized may be as simple as removal of surface contaminants and zinc salts by pressure washing. But to be certain that more extensive cleaning is not required, it is best to conduct test patches to confirm that the adhesion is acceptable before painting. If unacceptable, then more aggressive cleaning may be needed using the methods discussed in Stage 2.. As always, the surface requirements of the proposed coating system must be taken into account.
Conclusion – Hot-dip galvanizing can be successfully painted provided the surface is prepared or treated to overcome the detrimental effects of quenching or chromate treatment, or the presence of zinc salts on the surface. Whether painting for aesthetics or to provide greater protection to the substrate than is provided by either galvanizing or paint alone, proper surface preparation is needed to achieve the desired results.
Other topics in this galvanizing series:
Topic 1 – Inspection of the galvanizing process
Topic 2 – Problems with galvanizing reactive steel
Topic 3 – Measurement of galvanizing thickness
Topic 4 – Preparation of galvanizing for painting
Topic 5 – Coatings for galvanizing
Topic 6 – Inspection of surface preparation and coating application
 ASTM D6386 3.2
 SSPC-SP 16
1 ASTM D6386 3.2