Painting of galvanized surfaces

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It should be noted that the primary aim of hot-dip galvanizing is to provide anti-corrosion protection and, to a lesser extent, a decorative coating. To get an aesthetically nicer looking surface galvanized surfaces are usually painted, which also increases the products lifespan.

The painting of galvanized surfaces has some distinctive traits due to the low coarseness of the surface and the chemical compounds that form on the zinc coating. This requires thorough preparation of the zinc surface before painting. Both mechanical and chemical prepping methods are used. The most common method is a light treatment through sandblasting, but this requires extreme caution, and the following instructions should be considered. The most suitable powerwashing media is natural sand or aluminium oxide (under no circumstances should steel shot be used). The grain size of the material should be 0.2-0-5 mm, the air pressure 0.2-0.3 MPa, powerwash distance at least 500 mm, and the angle 30-60 degrees. The zinc will peel off the metal surface if the pressure is too high, the grain size too big, the blasting angle perpendicular, and the distance too close.

 

Chemical cleaning of different galvanized surfaces.

  • Pure galvanized surface – washing with an ammonia solution and then powerwash, washing with different washing solutions (phosphates etc.) from paint producers according to the instructions of the producer
  • Dirty, smeared surfaces – same treatment, increase the concentration of the solution when necessary, brushless
  • White rust (mould) covered surfaces – washing with a vinegar solution and then powerwash, washing with different washing solutions from paint producers according to the instructions of the producer, sandblasting with fine grain sand

Choosing the paint:

Not all paints are suitable for a galvanized surface. Nowadays, all manufacturers of industrial paints have their own recommended paint types for galvanized surfaces, and the manufacturer’s instructions and requirements should be followed during use. Dual component paints are the most common type of paints for hot-dip galvanized surfaces. In practice, epoxy paint is used for the first layer and the first layer should be relatively thin (the paint should be heavily diluted to speed up the removal of gasses from the paint layer). When necessary experience in the painting of galvanized surfaces is lacking, paint producers should be consulted and asked for instructions.

Frequently Asked Questions

Bath sizes and recommended dimensions of components:

length 14.0 m- component 13.8 m
width 1.6 m- component 1.55 m
depth 3.0 m- component 2.8 m

  • Max lift weight – 8 tons

For any technical inquiries please contact the sales department.

Galvanizing time depends on the volume, size and type of the material. Generally, the galvanizing process lasts for about half a day, although one must consider the time of year as well as industry demand. Please contact us at the beginning of the design process, so we can give you advice on different construction and production methods. Many delays are caused by inadequate construction and the lack of knowledge concerning the galvanizing process. For further information please contact our sales department.

The international Galvanizers Association announced that today, a typical galvanized coating can reach a lifetime of up to 50 years in most environments, and up to 25 years in an urban/marine environment.

Dark or light grey – it is still galvanised.

Dark grey coatings provide the same protection as light grey coatings – and sometimes even better. If rimmed steel or killed steel with aluminium is hot-dip galvanized, layers of alloy are formed which are in turn covered in zinc with a blue-grey shine. On some occasions, zinc may form irregularly oriented crystals, creating an “opalescentâ€? effect, which is not an indication of good or bad hot-dip galvanizing, nor does opalescence affect the corrosion resistance of zinc coatings. During production, silicone is sometimes added to steel as a deoxidizing agent which accelerates the reaction between steel and molten zinc. If the steel component is removed from the galvanizing bath while it is still hot, the reaction may continue and transform the surface zinc layers (entirely or in part) into alloys of zinc and iron. Compared to the light grey colour of zinc, zinc-iron alloys have a darker grey colour and are more abrasion resistant. Normally, the coatings of zinc-iron alloys are thicker and therefore grant a longer lifetime than coatings of rimmed steel or killed steel with aluminium. In any case, zinc-iron alloys are at least as corrosion resistant as zinc; because of their thickness they may be more resistant to self-corrosion in acidic industrial environments. These thick coatings may be more sensitive to mechanical damage, especially when handled recklessly, so they must be handled with care. Spots of iron oxide may appear on surfaces with dark grey coating even in mild conditions if the surface is not completely dry. This is just a surface effect and does not develop into nodular or flaky rust: the galvanized coating is not broken and continues to protect the steel.

Flawlessly galvanized steel with a long corrosion-free life may sometimes reveal spots of rust or changes in colour. This may give a false impression of an unsuccessful coating and is visually unacceptable on some occasions. The main reasons for spots and colour changes are listed below with suggestions for avoiding further problems or dealing with a problem that has already occurred. Colour changes in galvanized coatings caused by rust may occur as a result of one or more of the following factors.

  • Direct contact between galvanized components and unprotected or insufficiently protected steel (e.g. slices of galvanized steel, secured with unprotected, galvanically covered or painted steel bolts).
  • Iron dust and iron residue on galvanized surfaces originating from other operations or sources.
  • Water runoff from unprotected or insufficiently protected steel constructions, e.g. unpainted areas of painted steel constructions.
  • During the etching process, hydrochloric acid may penetrate the weld area due to pinholes or intermittent welds. Water may sometimes accumulate in residual salts, causing “leakingâ€? in weld areas. This effect is usually limited to a small area, stops after a short while and does not damage the coating.
  • Rusting of areas that have been welded after galvanizing and left uncovered or covered insufficiently.
  • Spots may appear on galvanized material if water is running off of other materials, metals such as copper and certain hardwoods such as oak in particular; in fact, every time water can dissolve material from a surface and precipitate them to galvanized steel.

Avoidance

If possible, all components of the constructions should receive equal corrosion protection. The thinner zinc coatings used in steel mesh, sheets, wire and pipes do not often last as long as the coatings of hot-dip galvanized components that have been galvanized in accordance with the BS/IS EN ISO 1461 standard which concerns all constructions that have been galvanized after production. If possible, the welding should be continuous and slag-free in order to minimize the preservation of etching residues. Design the constructions so that water cannot run off of other materials onto galvanized steel. Runoff from insufficiently protected steel and copper should certainly be avoided. If welding is required after galvanization, the welded areas should be cleaned thoroughly and the zinc coating should be restored with zinc dust paint or repair mastics sold by the brand, in accordance with the BS/IS EN ISO 1461 §6.3 standard. Corrective changes Colour changes and spots do not affect the lifetime of the coating. However, you can clean the corresponding areas to improve the appearance of the construction. Generally, using a steel brush or scouring powder will remove the spot and leave a flawless galvanized coating.

Small galvanized areas may be damaged by operations such as cutting or welding performed after galvanizing. Because of the protector in zinc, small local defects (up to 5 mm in diameter) tend to be fixed on their own and do not significantly affect the lifetime of surfaces. Nonetheless, it is recommended to renew the coating for aesthetic purposes on both smaller and larger surfaces, using one of the following techniques.

  • Brush the surfaces thoroughly with a steel brush and apply several layers of zinc dust paint to form a coating at least as thick as the initial galvanized coating.
  • Brush the surfaces thoroughly with a steel brush, heat the bare surface area to 300 °C with a blowtorch and apply a specific soldering rod with alloy of zinc. Applying zinc dust paint is usually the easiest option, but if colour matching is important, the zinc alloy soldering rods are more suitable.

For any technical inquiries, please contact the sales department.

If the client brings constructions with hidden holes and claims that all nessesary holes are made according to the instructions then they must send confirmation letter or on site write a letter to our stockkeeper what confirms that information. If we have an accident in the production because of the missing holes then this company is financially responsible.