Structural characteristics of joints

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In hot-dip galvanizing, joints which have not been sealed through welding must generally be avoided since the acid that is used for treatment gets into the openings but the required zinc layer does not form. Those places will be the sources of corrosion on the product.

It must be taken into consideration that the air trapped between the joined surfaces will expand during galvanizing and may deform the product. Therefore, the sealed joint area of two planes must not exceed 70 cm2. If the design requires the welding of a larger surface a ventilation opening has to be provided. It is also not recommended to galvanize riveted or bolted pre-fabricated products as in that case the joint surface will not be galvanized and the joints may break during cooling.

 

When planning any openings it must be taken into consideration that the zinc layer will decrease the diameter of the opening; refer to the following table:

Dimensions shaft/spindle Recommended radial- clearance
in the opening
kuni 10 mm 1 mm
10-30 mm 2 mm
üle 30 mm 2-2,5 mm

 

When designing threaded parts the zinc layer has to be taken into account, therefore threaded openings have to have positive dimensions and bolts negative dimensions before galvanizing. Special threading dies and bores have to be used to process the openings; contact the dealer for more information.

 

You may also refer to the following table:

Recommended thread under-cut (diameter)
After galvanizing Before galvanizing
up to M22 0,40 mm
M24 0,45 mm
M27 0,50 mm
M30 0,55 mm
M36 0,60 mm
M36-M48 0,80 mm
M48-M64 1,00 mm

 

For small threaded openings, the thread is cut after galvanizing. For large threaded openings, the thread has to be over-cut after galvanizing. It is also possible to protect the threaded parts with tape or fireproof silicon before dipping into the zinc bath.

 

 

An important and oft-forgotten requirement for hot-dip galvanized products is the prevention of so-called pouches that form during dipping. If the product is raised out of the molten zinc without having openings for draining, pouches and corners will form in which hardened zinc and zinc ash particles will collect and get stuck, these have an impact on the appearance of the product and are unacceptable for regular clients. Such joints have to be designed with the necessary draining openings or with open joint ends.

During the design phase, it should be kept in mind that the product will be raised out of the zinc bath at a 25…30-degree angle to allow the zinc to drain and to prevent drops from forming. The design of the draining openings for excess zinc should be based on that consideration.

 

 

 

 

To have minimal internal stress on long beams, the lifting points should be positioned as follows:

If the lifting points are near the ends of long beams, they may cause deformation of the beam:

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.