General information

Before this question we have talked about characteristics, utility, systems and different types of protection. We also considered difficulties connected with the type of used steel.
In this chapter we will talk about how to avoid the dangers in HDG, how to facilitate the operations associated with the HDG, how to save and improve the final product quality.

This means that the warning comes in conjunction with the design.
At this stage we can see what type of protection will be applied.
In the case of hot-dip galvanizing there should be included measures relating to: materials, the morphological characteristics of the final product, size, movement capabilities, production techniques, economy, reliability, process and auxiliary operations.

Many ferrous (glandular) metals can be exposed to hot dip galvanizing: mild steel, cast steel, steel of hot and cold rolling and also high-strength steel.  They are suitable for hot dip galvanizing to get long-term protection.

Maximum results, from a technical and economic point of view are achieved through a close collaboration of designer, manufacturer and producer of galvanizing.
The measures mentioned here are general suggestions in order to facilitate HDG operations and improve the final result quality, providing maximum protection against the corrosion.

The design characteristics of galvanizing products

Conditions for hot dip galvanizing concern some fundamental aspects: type of steel, susceptibility to hot-dip galvanizing process, hauling kike in hot-dip galvanizing and after that, possible deformation and rupture, advantages of reliability, welded and threaded products, marking materials.

Choice of steel

Selection is usually based on the destination of the product. Take into account everything said here about the interaction of components with hot-dip galvanized steel. And therefore differences in the thickness of coatings and related costs.

Predisposition

Pollution in the form of grease, oil, tar, paint and welding crusts removed by degreasing, sandblasting.  Even if these interventions make treatment more expensive, they are carried out with a great care because sometimes it happens that the contamination is not detected at first time and appear after hot galvanizing.
Such contamination is not removed by etching and cause the appearance of black open spots after the hot-dip galvanizing. In this case, you may need to re-galvanize product with an increase of costs.

Welding slag removed by the sandblasting or processing of straight bit. The processing is necessary because the scale of welding is not affected acid. In order to get rid of scale it is preferable to use a continuous weld thread. Remains of rolling and calamine are removed with the help of sandblasting.

Sizes and hauling

It is preferably to design structures that consists of components that can be easily moved or mounted after the hot-dip galvanizing, to provide a hole in a product for lifting or eyebolts (ringbolts) for fixing of the possible adaptations. The possibility of hot-dip galvanizing is really limited: one of the obstacles may be the bathroom’s size that is too small for the product. An inconvenience often solved by successive immersions.

Mobility

Details that need to be mobile for articulated joints or locking devices installed after galvanizing. If it is impossible, the freedom of parts movement can be guaranteed if the details are executed with a gap of 1 mm on each side. for example, a hole of 2 mm wider than the rod.

Structures that can be deformed: cases and precautions

Usually hot-dip galvanizing does not cause deformation, if the design and production principles are correct. When there is a deformation during the galvanizing of steel products, the reason usually in an early design stage.

Very rarely it is can be caused by manipulation of galvanizing equipment.
Most of the deformation generally occurs due to the voltage liberation when the steel is heated to temperatures for galvanizing (usually 445-465 C).
Internal voltage can also be present in the semi-finished steel and can vary from batch to batch. They usually occur because of technology products for the zinc production.

Cases are very different but they all have a very simple logical basis by which they are easy to remember.
The design should follow some basic principles and use mentioned system to minimize the risk of deformation and unaesthetic appearance.

- When the steel parts (that differ in thickness) are joined together in these cases you can see the deformation.  The heating in the zinc bath is heterogeneous and the parts may be repainted.
- It is necessary to avoid long and thin structures.
- To avoid conditions combinations of surfaces and different materials.
- Symmetric-sections (I-beams, pipes) have a lower tendency to deformation in compare with asymmetric cross-sections (profiles). Similarly, cylindrical tanks are less susceptible to deformation in compare with to rectangular or elliptical tanks. However, the smaller the steel thickness the higher a deformation risk.
No need to weld just rolled steel with steel that strongly attacked by rust or iron. These materials are processed differently, which is impossible if they are welded with each other. The zinc coating will be uneven and has different surfaces.
- Do not weld a boiling or appeased by aluminum steel with steel capped by silicon. Zinc coating will have a different thickness and type of surface will be uneven.

Internal stresses in steel

Most steel has internal voltage (stress); hot-dip galvanizing may release or modify the interaction of such stresses causing possible deformation.

Strain (stress) during the production

There are may be internal stresses in bending, drilling, cutting or welding. The last causes an extreme temperature differences in small areas of the product and then a significant residual stresses.

In general:
1) Thick section should be welded  in a continuous seam. Thin sections and products from sheet can take advantage from intermittent welding, because in this case the heat source is quickly exited. Although more stress can occur from the initial point of welding.
2)  Product components for assembly should be designed with extreme care in order not to distort or make an efforts for parts, so they can be welded in the correct position of the assembly.
3) Welded products must be installed in such way that stresses in product were balanced and never in such a way that they are concentrated in the same area or in one direction.
4) The cross sections for structure components should vary as little as possible.

Also, keep in mind that:
Thick or thin sections absorb and lose heat in varying degrees and can shrink or grow in different ways.
The wide flat sheets without amplification have a tendency to waviness.  In these cases, the project must provide flanging and / or bending rigidity and support.
Frame around the flat panels (ordinary sheet or open type) should be galvanized separately because the frame will act as a narrowing element and will be try to cause deformation.

You should always consult with specialist to decide what the maximum size may have item for suitable galvanizing.
In standard symmetric products, even in double-immersion there is no deformation or they  can be detected in a small amount.
In tubular structures the size and location of holes for filling, output and overfall may have a decisive influence on the curvature.  It is also valid for the size and location of holes for hook especially on any hollow structure.

The stresses of manufacturing

An internal production stresses sometimes can be eliminated by treatment to relieve tension before hot-dip galvanazing.
In addition to good project performance prior consultation between the HDG producer, builder and designer (planner) is always the key to a success in the fight against the deformation.

Tubular or box materials

A special attention should be paid to structural engineering of tubular structures that are often used in zootechnics and agriculture. In the joints of pipes should be accurately controlled an open channel that allows for freely circulate fluids and chemical treatment of zinc. Thus, it is important to avoid areas that are closed, even partially, such as a clogged pipe.
Indeed, fumes that are developing inside in contact with the zinc bath temperature can reach pressures exceeding 100 atmospheres. It can cause explosions in the zinc bath and irreparable damage to the material, the yield of molten zinc and serious danger to the personnel involved treatment.

The diameter of holes

Correct drilling of holes in the pipes joints is also important because it facilitates the removal operation for galvanized parts with conveyances and allows it to exit semi-liquid zinc out of the hole.
Indeed, if the zinc will be set on the inside part of pipe its subsequent output access can cause the formation of crusts, deposits, incrustations, which are removed with difficulty, with the subsequent risk of a loss or injury to those who will have contact with it.

Thus, it is important to match the cross section to the hole through which should flow zinc. It is necessary to avoid delays, excessive growths or tip leakage.

Practical safety and security prerogatives:

Boilers, autoclaves, sealed product

Need to pay attention to the fact that the coupling (sleeve) and tank hole was welded at the level of  casing. Then you can avoid wasteful zinc expense and increase the cost.

The designer should always provide large and wide openings in accordance with the volume of tanks to allow rapid filling and output, and then faster processing, saving money and zinc.
In the tanks should be avoided connecting branch of pipes that are inside of the container body. Place connecting branch in the corners and along the diagonal, otherwise the container can not be completely free from acid and then from the molten zinc at the time of removal from the bath.


The right location of the holes has a great importance: then coating on the inner walls becomes more even and not remain zinc at the bottom.
For boilers, which require special working slope, is the principal location of the relevant couplings (never less than inch) in position diametrically opposite the center of gravity of the product. Thus there is an opportunity for zinc drain and air out that is trapped at the top of the boiler.
Without the air output, there may be formed an air bag with the interior of boiler which is not galvanized.

The external coil pipes and tubular and welded structures galvanizing

In tubular structures where galvanizing shall be made only from the outside it requires special techniques. Such structures (with entrance and exit) should be equipped with airtight plug with asbestos gasket from one side, and from the other side with tube output, fixed by welding to the frame structure without any vibration.

Assembled details galvanizing

When you need to galvanize structure formed from parts welded together, it is necessary to act with extreme caution. It is important that the designer takes into account the need for continuous sutures, which is important for the safety of the person who galvanizes the structure (because the temperature of the bath ranges from 440 to 460 C). It also guarantees the excellent quality of galvanizing.
In badly welded points there are penetrating acid. If it is outside, it will cause an explosion and ejection of molten zinc during process. In addition in these areas is a strong aggressive rust, because iron is naked and liquid zinc can not form an alloy in the defective points that is needed for protection.

Summary: the contact of plane with the plane should be avoided as soon as possible. If it is technically unavoidable, you must make an absolutely continuous welding seam, drilling a hole passing through both connected planes.
It is important that the designer avoids the connection profiles with very different thicknesses. Because due to temperature expansion and cooling occur at different speeds that leads to subsequent mechanical action of deformation.

It is obvious that the metal sheet  undergo a  rapid expansion and cooling, while the reference profile is in 3-4 times thicker and  undergo  much slower process of expansion and contraction and in this case distortion is inevitable. In contrast, when you galvanize parts separately and assemble them only after galvanizing with galvanized screws or rivets deformation problems don’t occur. We must remember that the right choice for the shape of product for HDG that thicker than 3 - 4 mm and may contribute to galvanizing and provide almost complete rescue from uncorrectable deformities.

Reinforced profiles require HDG for separate elements or they should be welded with a continuous suture. But you should never weld reinforcement gussets or shelves completely. Especially it happens when they located asymmetrically on the structure. It is necessary to provide adequate openings for out of zinc during the process, and for taking material. For this aim openings should be made for suspension components and they should be provided on the shelves of symmetric I-sections and their diameter shall not be less than 14 mm. For C-profiles holes should be located on the surface sided to shelves in order to avoid possible side-stacking parts.

It is important to pay attention on the holes position due to the length of details, especially if it is a very flexible profile and structure.
Indeed, the holes should be located far enough from the ends to avoid excessive bending of parts due to own weight. For 8 m length holes should be 1.5 m from ends and for the length from 10 to 15 m openings must be not less than 2.5 m from ends.

Acids stagnation zone

Avoid areas of possible stagnation acids. Design structure in a such way to form narrow gaps. Use a butt weld more than overlapping welding. The same can be said about any air type or compounds which may contain substances detainees because of the slope components during the process.
If you have to resort overlapping welding it is better to weld around the joint. If the contact area is greater than 70 square meters it is necessary to provide ventilation holes to avoid the risk of explosion.

There shouldn’t be any pores in joints that might be a trap and entrap acid. If the joints are made from the corner on both sides ends should also be welded in such way that the acid could not get into a possible air gap.
If it’s possible use a welding method without the formation of scale such as welding with CO2. If you are using coated electrodes gently remove the scale from seams because it is not removed by etching and causes the appearance of black spots after hot dip galvanizing.


When the acid penetrates into the gap, it can not be removed from there. Zinc is not flowing as fast as the acid and can just plug the hole.
After a time period acid will create a hole in the zinc coating over the hole. Then the rust-colored liquid will flow to the underlying surface, destroying their appearance.
Corrosion in these gaps can be so severe that sometimes can cause a drawdown structures.

Welding: systems, materials, and consequences

Hot-dip galvanized steel can be welded using all taken to the black plate methods and processes. Zinc coating provides the interpenetration during the welding process, so that data relating to the welding of steel without coating are not always valid. The degree of interpenetration depends on the thickness, composition and coating structure.
During the process of welding galvanized parts you can have following problems:

- Increase of splashes during the welding
- Higher fumes formation
- Higher metal porosity in a welded metal formation
- Less penetration
- The risk of intercrystalline cracking in a weld metal

Welding of steel without the coating causes the appearance of smoke that contain varying amounts of iron oxide, ozone, hydrogen, nitrogen monoxide, fluorides and other compounds.
When galvanized steel is welded or cut with a welding torch there may be also added zinc oxide in the list of substances.
Zinc oxide – is a white voluminous compound, clearly visible in the welding fumes.

Zinc oxide effect

Inhalation of the new formed zinc oxide can cause a fever from metal fumes. The symptoms resemble the symptoms of influenza: fever, chills, more intense salivation, headache, and sometimes nausea and vomiting. However, zinc does not accumulate in the body, as for example, lead and cadmium. It removes from the body with waste products. Thus, the symptoms of fever from metal fumes caused by zinc, disappear within a few hours. Permanent side effects are unknown.
There provides a proper welding fumes removal in non-galvanized and galvanized steel in order not to subject welder to risk of fumes welding inhalation. In general, open-air welding can be carried out without special precautions regarding fumes welding.

During the welding of corner joints in the special provisions, the drops can get into and get stuck on the arc welding nozzle ticks interrupting the supply of wire. Drops particles can alsoa here to the surface near the weld and affect on their appearance.
Penetration in compound decreases due to the fact that the zinc coating on the edges interfere electric arc energy balance reducing the arc voltage and current.

Intercrystalline cracks in welded metal (the result of  zinc penetration) are more common in the compounds in T-from from material thickness greater than 13 mm and welded with coated electrodes and  thickness greater than 6.5 mm with welding in CO2.

The above problems can be eliminated or reduced to acceptable limits by means of these measures:
- Weld for a distance between the surfaces of the connection of 1.5 mm with welding in CO2 and 2.5 mm for welding with coated electrodes. Thus the number of pores reduces, penetration increases and avoids the risk of intercrystalline cracking (penetration of zinc).

- Weld. Offset should be reduced to a minimum.
- Shake with a long-electrode connection to make  more zinc burnt before melting bath. It reduces pores’ amount and the risk of intercrystalline cracking.
- Smooth vertical plate connection in the T-form in such way to make a groove with a double or single facet. The process eliminates intercrystalline cracking and reduces pores number, whether compound edges galvanized or not.

Choice of electrodes

To reduce the risk of intercrystalline cracking in the weld T-form joints the electrodes should be chosen with low silicon content.

Prevention rust appearance in welded seams

The weld seam should be protected from rust, as soon as the procedure over.
The outer surface is free from rust and easy to handle. You can proceed as follows: zinc-rich paint coating, if treatment is carried out as soon as the weld has cooled there is no need to clean by sand-blasting method. Oxides and loose scale can be removed with a wire brush.
If the time past and outer surface was covered with rust it must be cleaned by sandblasted method or grinding. Sandblasting treatment should be done very carefully not to damage the surrounding galvanization. You can use paint with a high content of zinc- mono-and two-component.
In practice, mono-component type may be preferred for this type of painting – on the seams of galvanized parts. The paint must overlap at least a layer of the same thickness as the adjacent zinc coating.

Details of thread

Zinc coating, the resulting hot-dip galvanized, in most cases, the thickness of other coatings, and the tolerance on the thickness provided as internal and external threads.
External thread structure for the assembly after galvanizing should be properly exempted from any excess of zinc to allow binding compounds. Alternatively, you can begin to protect the outer thread with a simple action: two or three layers tightly wrapped with insulating tape to isolate the tissue type can be threaded pipes, water heaters, boilers, tanks and can be removed after galvanizing with a movement of the brush. For internal threads can be used with appropriate caution antigalvanized paints.

Internal threads can alternatively be exempt from zinc as long as it doesn’t acquire the nominal size or cut after galvanizing with a large nominal diameter with a glance of cover external threads.
To prevent corrosion it is needs only external threads protection because protection of external thread protects both contact surfaces.

Fastening bolts

Fastening bolts is an excellent system for galvanized structures because it allows to galvanize simple components. Bolts and nuts, of course, must also be protected by hot-dip galvanized to ensure of the same level of protection against corrosion to the whole structure.

If the bolts have to be under stress with high torques (for example, friction in the joints) both contacting surfaces, external and internal thread must obtain the proper lubricant.
Usually, all the bolts starting with the diameter of 8 mm and above can be hot-dip galvanized without any problems. As a rule there is calculation that the theoretical minimum tolerance between inner and outer surface to achieve a good bond is 4.33 times the thickness of the coating.
In practice, the required tolerances for the diameter range from 0.3 to 0.7 mm. Standard stipulates that the nuts are cut with a larger diameter after galvanizing. However, in some countries threads or nuts should be cut out with a nominal diameter. In this case, the bolts must be made with a reduced diameter to compensate coating thickness.

High strength bolts

8.8 type bolts can be galvanized by usual galvanizing techniques. For high types (10.9 and 12.9) hot-dip galvanized can be with precautions to avoid possible phenomenon of hydrogen embrittlement increase.
Technical requirements of some countries (eg, BS 4393 ​​United Kingdom of Great Britain or American ASTM A490) is not recommended for hot-dip galvanizing for these two types of bolts. On the contrary, it is allowed in many other countries, including Italy, France, Japan, Australia and Germany.

Compounds by friction

Initially, the coefficients of friction for the contacting surfaces of galvanized rather low: about 0.15. When you start sliding friction increases with high speed and quickly lock is achieved due to the phenomenon of cold welding between two surfaces.
When small slip is allowed there is no need in any additional surface treatment. If it is not possible, the friction coefficient can be increased by staining or (better) roughening the surface. Thus you can achieve values about 0.30 - 0.45.

Additional operations and precautions

Material marking for galvanizing

Temporary markings shall be carried out only with water-based paints. You should avoid all drugs, which (if they are not removed) in the etching bath will counteract to galvanization in the places of coating.
Permanent marks can be made by lithographic letters or numbers on parts or attached to it plate. In any case, stamping should be deep enough so that it can be read, even after hot dip galvanizing.

Packaging, storage, transportation

Galvanizing gives a great strength of surfaces in any damage during the movement and transportation. In any case, it is better the necessary take measures to prevent white rust.