Advice on galvanizing
Design for hot dip galvanizing
Requirements for the articles to be galvanized
It is necessary to follow a few simple rules to be able to take advantage of hot dip galvanizing as much as possible and satisfy yourself and your customers.
Given the simplicity of the process, it is very simple to prepare the items for hot dip galvanizing. If you are not an expert or if you have particular needs, it is advisable to establish a close collaboration relationship between technician, builder, steel supplier and galvanizer right from the design stage, in order to achieve the best results. The necessary adaptations are small and perfectly tolerable in the vast majority of cases for any type of product. The advantage that comes from the collaboration with the galvanizer is immeasurable. In this way, the best treatment performance is ensured, obtaining the required duration and structural integrity against corrosion attack, also avoiding the aesthetic compromise of the works caused by rust.
Negligible effects on the heat treatments of steel
Due to the temperatures of the galvanizing bath (lower than the eutectoid temperature) to which the articles to be galvanized are brought, and due to the short immersion and cooling times, changes in the microscopic structure of the steel do not occur. Even in the case of hardened steels, heating to a temperature of about 440 – 460 ° C, for the usual treatment times of hot galvanizing, corresponds to a mild tempering with a practically irrelevant effect on the mechanical properties. Specifically, the tensile strength and the yield point of the steel remain virtually unchanged.
In cases of surface hardening treatments such as cementing or nitriding, the galvanizing process removes and alters the characteristics obtained. These treatments are not usually used for structural steels used for construction purposes. Therefore, in general, the field of use of hot dip galvanizing does not interfere with applications that require such treatments.
The steels commonly used in the construction sector do not undergo significant changes in their mechanical characteristics during galvanizing operations. However, some types of steels due to an uncommon (high) nitrogen content, such as high hardness and low resilience steels, susceptible to aging, may lose their original ductility and become brittle during the galvanizing process, especially if previously cold worked or welded. This aging is mostly caused by hydrogen that develops during the pickling phase, and is accelerated during hot dip galvanizing by the increase in temperature. In such cases, precautions must be taken that can be decided on a case-by-case basis, by consulting the galvanizer or an expert of the Italian Galvanizing Association in the design phase.
How to avoid deformation
The pieces must be hot-dip galvanized, they must withstand the effect of heating. If the pieces are not well designed, or if some simple precautions are not followed, expansion and deformation can be caused. & Nbsp; The main causes of these problems are attributable to the following situations:
- Different heating and cooling speeds of complex structures with components of different thickness (for example thin sheet metal panels with heavier profile frames) galvanizing performed in two or more times, which determine, at the same time, the creation of warmer areas and other colder ones
- Residual internal stresses from processes prior to hot-dip galvanizing.
In fact, in all steel structures there are, to varying degrees, latent tensions of modest entity which normally remain in equilibrium and do not create problems during the galvanizing process. The phenomenon obviously depends on the entity of the tensions and on the characteristics of the material. withstand stress without deformation. The elasticity limit decreases with temperature. If the condition for which the internal stresses are so high as to exceed this limit at the temperature of the galvanizing bath is realized, the product tends to find a new equilibrium condition in which the stresses themselves are compensated by the deformations. The characteristics of the stress state (value, distribution and direction inside the piece), the rigidity of the construction, the type and thickness of the material used, determines inane the extent of the deformation. The intrinsic characteristics of steel return to their original values when the piece cools.
In zinc linen, in the processing stages prior to immersion in the zinc bath, there is no means to highlight the stress states that can give rise to deformation in the products. Prevention must therefore be carried out in the mechanical construction workshop, putting in place the appropriate techniques to avoid, as far as possible, problems during the transient reduction of resistance induced by the hot dip galvanizing process. It is necessary to pay attention that is not dissimilar from the exercise of the good rule of the art. It is, for example, a good idea to avoid forcing the components to be assembled to bring them into the positions envisaged by the projects. the galvanizing phase. In these circumstances, especially in the case of sheet metal, normalization should be carried out after processing, keeping the steel for a certain time at a temperature of 600-650 ° C, in order to release the internal tensions before the construction of the product. / p>
Of particular importance among the causes of stress is welding, especially welding with inhomogeneous filler material. Problems can be avoided by performing the welds in a symmetrical position with respect to the main axis of the piece and reducing them to the bare minimum. When it is convenient, even the individual profiles or simpler structures could be galvanized, following the assembly only later by means of mechanical connections, such as screws or bolts. If this is not possible, the necessary welds should be performed as close as possible to the main geometric axis of the section, in a symmetrical and equidistant position with respect to it and, as far as possible, simultaneously. Asymmetrical sections, in fact, run a greater risk of deformation, especially if made up of several elements joined by welds of considerable thickness, placed on one side only and distant from the main axis of symmetry.
For prevention, therefore, it is useful to use parts with symmetrical sections, which hinder deformations, but also shapes that allow you to make a single rapid immersion in the zinc bath.
Dimensions, weight and geometry of the artefacts
From what has been said, the conclusion is drawn that in order to obtain the best results from galvanizing it is necessary to optimize characteristics linked to dimensions, weight and geometry of the pieces to be treated.
First of all, since each zinc linen is equipped with tanks of different sizes, in the case of particularly bulky pieces it is advisable to inquire about the width and size of the bathrooms. Any problems can be circumvented very often by studying suitable solutions during the design phase.
In general, it is always advisable to avoid bulky items, which could already be damaged during transport. Furthermore, with them, the processing phases are considerably lengthened and complicated, with often significant consequences on the costs and final quality of the galvanizing. The constructive geometry of the piece should preferably develop over two dimensions, possibly resorting to subsequent assembly and assembly operations of the separately galvanized parts.
However, many times the depth of the tanks and the loads that can be supported by the cranes in the galvanizing plants are greater than what the designer imagined. Information regarding the dimensions of the tanks is always easily available, so that structures can be created that allow the simultaneous optimization of galvanizing and installation times. In general, these are data that can be found very simply by visiting the website of the trusted galvanizer. Through the website www.aiz.it you can easily identify the zinc plating associated with the Italian Zinc Plating Association and the links to their websites.
If the pieces to be hot-dip galvanized are particularly long or bulky, it is possible to resort to several immersions, subjecting the individual sections to successive baths separately. However, this methodology involves an uneven heating of the parts of the product, which, in unfavorable conditions, can cause distortion, as already mentioned above. In fact, making the first dive, one part of the building element is inevitably at a temperature of 450 ° C, unlike the rest of the body which, when exposed to air, is subject to sharply lower temperatures, decreasing to room temperature over very short lengths. For an idea of the internal tensions that are caused in these articles, consider that the thermal expansion, or the elongation due to heating, at the temperature of 450 ° C against the ambient temperature of 25 ° C, is about 4-5mm per meter.
On the other hand, the problem is not particularly serious for long but thin structures, for solid core profiles, pillars and tie rods. These elements, unlike structures composed of plates and bracings, are generally homogeneous and their tensions can be distributed, spreading out over the whole structure so as not to create problems. & Nbsp; The risk of distortions becomes significant for elements with a height such as to request to turn the piece. If the steel structure is not so elastic as to resist the strong vertical stress, there may be some rather different dilations between the upper and lower part. In the event that the artifact is massive, if there are points where the stresses caused accumulate, it is also possible to overcome the σ of rupture and in certain critical points cracks can originate. , can be easily avoided through a preventive design, which adequately evaluates the relationships between temperature and expansion during the galvanizing process.
In any case, the galvanizing of large elements, to be galvanized in two or more operations, always causes variations in the thickness of the coating, due to the overlapping of zinc layers. of too different thickness, the longer duration of the immersion times required by the thicker parts, results in higher coating thicknesses even on the thinnest sections present in the product.
To prevent this inconvenience, it is advisable to design assembled structures whose components have thicknesses as uniform as possible and, in any case, whose thickness ratio is less than 5: 1. If this is not possible, it is advisable to use separate elements, which can subsequently be assembled by means of joints. appropriately choose the suspension points of the artifacts, to facilitate their transport, assembly and treatment. The choice should also always favor points that allow liquids and molten zinc to flow rapidly during the extraction of the piece from the tank. Finally, for pieces of considerable size and weight, it is essential to appropriately establish their number, distribution and positioning to avoid the possibility of deformation. at least one provision for hooking (for example a sleeve connection) on an edge of the bottom.
Hollow and tubular profiles, overlapping surfaces
Vent and drain holes
In the case of hollow steel structures it is essential that during the immersion the zinc can easily penetrate inside with the complete outflow of the air present in the volume and that, during the extraction from the bath, the superfluous zinc can drain easily. & nbsp; It is also essential that the aqueous degreasing, pickling and flushing solutions can drain easily from the pieces. A stagnation of even minimal quantities of liquid inside the pieces can cause explosions, even of a strong entity, if brought into accidental contact with the molten zinc. In that case, it causes a serious risk to the safety of the galvanizing operators. For parts with completely closed cavities that are difficult to inspect, it is therefore necessary to carefully plan and create well-sized vent openings.
The tubular sections must be provided with inflow and outflow holes whose diameter must be calculated based on the length and section of the bodies, i.e. the volume of air that will pass through them. Particularly relevant is also the position of the openings, which must be placed considering the orientation of the suspension, as close as possible to the welds of the nodes, in diagonally opposite positions with respect to each other and, if necessary, can be covered later in the assembly phase.
It is not uncommon for complex hollow bodies to be galvanized, such as containers of various kinds, equipped with internal reinforcements, end plates, dividing diaphragms, flanges etc. It is understood that it is essential to pay close attention to the creation of the openings. A puncture of insufficient dimensions or even in the wrong positions, determines a possible deterioration in the quality of the galvanizing. The consequence of a poorly executed drilling is often the escape of excess zinc in such a way as to favor the formation of burrs, crusts and roughness, even dangerous for the handling of galvanized products. In this case, it is necessary to proceed with their removal which is not always easy, which is always better to avoid.
Designers must indicate the positions in which to affix the holes and the dimensions they must have.
In case of doubt, the advice of the galvanizer or the expert of the Italian Galvanizing Association is often the best solution for designing the drainage of products with particular shapes. Whoever performs the galvanizing, in any case, inspects the structure and evaluates whether to carry out, with the customer’s authorization, an adjustment of the drilling. However, it is advisable to pay close attention to this aspect in the design and construction phase, to obtain the best result without running risks.
It is necessary that the vents, designed to allow the inflow and outflow of liquids and zinc from edges and corners, are positioned taking into account the preferential orientation that the parts will assume during the various process phases. In the event of insufficient venting, stagnant air bubbles could remain, which would prevent portions of the surface from coming into contact with the liquefied zinc, hampering the galvanizing process, with obvious damage.
As for the tubular elements, also for the profiles it is advisable to place openings and holes in pairs, ensuring variable dimensions based on the structural characteristics of the pieces, but, in any case, with diameters greater than 10mm for the holes and 14mm for the corner grooves. & nbsp; The location of the vents and drains is also important. Cases should be avoided in which small accumulations of zinc can occur which constitute an unnecessary weighting of the pieces and an increase in the cost of galvanizing, in the face of any deterioration in the functionality of the objects.
In the construction of artifacts using steel profiles, overlapping surfaces must be avoided, since pickling and flushing liquids can penetrate into the interstitial spaces, as already seen for poorly executed welds, which due to violent evaporation can cause explosions but also fluid retention highly corrosive pickling, which cannot move away from the interstice during galvanizing. During the cooling of the pieces, the acid condenses again and is released from the interstitial cavity, forming small rivulets capable of locally removing the zinc layer. This results in unsightly and dangerous oxidative effects. & Nbsp; Even if there is no acid present, condensation water can enter these cavities which solubilizes any salts that may be present. The consequent corrosion in the interface space can generate water dripping mixed with rust, which stain the galvanized product. continuous welding.
If the overlapping surfaces are very large, it will still be necessary to drill vent holes or discontinuous welding. This serves to release the pressure created by the humid air enclosed between the two floors. In these cases, it is necessary to seal the interstice immediately after galvanizing, in order to prevent the phenomena from occurring.
There may be cases of particular construction elements that require hot galvanizing only for the external parts. This operation is more expensive because, although it requires lower quantities of zinc, it requires more complex procedures.
First of all, it is obviously essential to hermetically close any openings to avoid the influx of zinc inside the structure. In addition, the artifacts must be equipped with an appropriate duct to release the high pressure that would inevitably be created inside due to the heating of the air. A typical example is offered by tubular heat exchangers.
By immersing hollow structures in the zinc bath, they will naturally oppose a buoyancy thrust. Therefore, it is necessary to load these systems with additional weights, even of several tons, and verify that they are able to support the hydrostatic thrust of the molten zinc in the tank.
Finally, materials capable of resisting pickling acids and the temperature of the bath must be chosen for any sealing gaskets. i zinc. & nbsp; In the case of containers it is advisable to check that joints, flanges and supports are placed on the surface of the structure, so as not to create recesses or cavities that could cause stagnation of air with consequent defects in the galvanizing.
Threads and wire products
Steel wires for metal meshes are treated as semi-finished products in automatic galvanizing plants. Artifacts are rarely galvanized after construction. Some examples are found in the agricultural sector (bird cages) and in the construction of some types of nets. Some artifacts maintain an adequate consistency only if subjected to cold stiffening. When using steel wires that are susceptible to aging, cold reinforcement bends can cause the material to harden, which unfortunately becomes visible after hot dip galvanizing.
If wires of suitable material are used, zinc plating, on the other hand, provides an additional advantage in addition to corrosion protection. In fact, the molten zinc gives rise to a welding effect at the intersection points of the wires, increasing the solidity of the products. If there are frames or welds, unsightly distortions can be created. An interesting solution is to leave the corrugated wire mesh and the surrounding frame separate or partially unswelded during galvanizing, to weld or screw them only later.
Steel and cast iron castings
The galvanizing of cast iron parts deserves a separate discussion. This alloy certainly does not play the role of steel in construction, however, in recent times it has been, so to speak, rediscovered. & Nbsp; Unlike steel, cast iron is an alloy of iron and carbon that contains the latter element. in quantities greater than 1.8%. Similarly, the percentages of silicon and phosphorus are also generally higher. & Nbsp; In assessing the suitability for galvanizing, it is therefore particularly important to consider the chemical composition of the product to be treated. As for silicon, it must be taken into account, however, that the same reference limits as for steel do not apply. In fact, in cast irons, silicon is present alongside other chemical elements, and partly combined with them, and therefore does not give rise to a significant thickening of the coating.
The different types of castings are classified into steel castings, lamellar graphite (GGL) or gray cast iron, globular or spheroidal graphite (GGG) cast iron and malleable cast iron.
In the first case it is the castings of molten steel in shape. This type is moldable like cast iron, but exhibits superior mechanical characteristics. Having composition and characteristics corresponding to steel, it behaves in a very similar way even during hot dip galvanizing. and therefore can lead to the formation of thick, gray or gray-stained coatings. The two types are obtained through different types of heat treatments that give them characteristics of malleability and workability. & Nbsp; In GTS cast iron the carbon content is lower than in GTW, while that of silicon is higher. It is therefore evident, from the above, that for malleable cast iron with a black heart there will be a faster iron-zinc reaction, as opposed to white for which normal values will be obtained.
Both the surfaces of the steel castings and those of the cast iron castings to be hot-dip galvanized must be free of residues of sand, carbons and annealing cements, graphite and so on. These particles can withstand the normal pre-treatments performed in galvanizing plants, therefore they must be subjected to pickling in mixtures of hydrochloric and hydrofluoric acid (rarely available in galvanizing sites). processed, which would prevent positive results.
In general, it should be remembered that the typical roughness of cast iron can give rise to coatings of greater thickness than smooth steel parts. Finally, it is preferable to use hot galvanizing for small sized cast iron pieces sions. Within the parts of considerable size, tensions may in fact arise at the temperatures of the zinc melt, capable of causing more or less serious injuries. The greatest damage occurs, in particular, when walls and ribs with very different thicknesses are coupled into large objects.
Exclusion of individual parts
There may sometimes be a need to keep certain portions of the steel construction zinc-free, which have specific functions. Among the different examples we can mention bolts and threaded parts in general, parts with close tolerances, through or blind mounting holes, surfaces on which after galvanizing a welding must be performed. & Nbsp; Based on the different types of pieces and individual needs It is possible to resort to different procedures, in all cases, however, it is advisable that these operations be carried out by the customer before sending the products to the zinc-coated linen shop. around the piece several turns of fabric adhesive tape, normally available on the market. Although the high temperature of the zinc bath burns the tape, its residues are sufficient to preserve the area in question.
At the end of the galvanizing procedures, the remains of the tape must be removed, for example, using a wire brush. causing irregularities in the coating. In this case, it is advisable to use special paints which, as happens to the belt, are destroyed in contact with the heat of the molten zinc, but the residues still prevent the deposition of the zinc coating. As in the previous case, at the end of the processing the remains must be brushed. for threaded parts, since the paints would not distribute themselves uniformly on them, thus creating an uneven protection. If you intend to exclude blind holes or internal threads from galvanizing, it is advisable to use gypsum, putty or plasticine, which are normally found on the market.
By adding small doses of water, these products increase their plasticity and can be easily molded and pressed into the openings to be protected, trying to avoid the formation of air bubbles. After hardening they will prevent the penetration of the molten zinc. As is evident, once the galvanizing procedures have been completed, the residues will be eliminated. & Nbsp; To protect the internal threads, an alternative solution is to screw in a suitable screw previously greased with silicone grease. During galvanizing, the screw is welded from the zinc to the rest of the product, therefore it is necessary, after extraction from the bath, to act with a light flame to remove the screw. The operation is, however, very delicate and must only be performed by experienced personnel. & Nbsp; Sometimes threaded holes and blind holes are closed by introducing wooden plugs that are carbonized by the molten zinc and, in any case, avoid the galvanizing of the covered parts.
All the protection procedures that can be adopted inevitably lead to an increase in overall costs. However, they make local grinding or treatment of the zinc coating with flame unnecessary, otherwise necessary to free the parts from the zinc.
Assembling the artifacts
Up to now, structures and individual steel elements subjected to galvanization have been considered once the processing phase is finished. & nbsp; Starting from semi-finished products & nbsp; Sometimes, it can be useful to produce these construction components from previously hot-dip galvanized semi-finished products. p >
Mainly for this purpose square or round section tubular profiles are used, available in a wide range of thicknesses and sizes (usually lengths from 6 to 12m), and hot or cold rolled solid steel profiles, also present on the market in various sizes. The semi-finished products are hot-dip galvanized in partially or fully automated systems. In this way a high quality and homogeneous coating is obtained at low costs. With these production processes it is possible, in fact, to obtain large continuous productions. In addition, technological systems capable of improving the surface appearance of the galvanizing are easily applied. For example, during the extraction of the pieces from the bath, it is possible to resort to the forced drainage of the zinc by means of compressed air and / or steam.
Waiting to be employed, the semi-works The pre-galvanized products must be placed in storage to be preserved from the so-called white rust. This risk can be prevented by depositing the material in a dry place and inserting spacers between the layers of the stacks to facilitate air circulation. Covering with plastic sheets can, on the other hand, cause a considerable formation of condensation, which is very harmful for the protection of zinc. , nailing, brazing and gluing, which require relatively longer machining operations than non-galvanized profiles. During these operations, the zinc protection can be damaged.
In addition, it is necessary to check that any iron filings fallen on the galvanized surfaces during the processing phase do not create rust stains. In fact, due to humidity, areas of intense red-brown color are formed around the iron particles. .
The utmost attention must also be paid during the blanking, bending and chamfering with a very narrow radius of the semi-finished products, since the coating could suffer some damage not bearing the stresses applied. & nbsp; In case of damage, the repair of the surfaces must be evaluated in based on individual situations and may require some effort. & nbsp; To restore the zinc on damaged points, the use of thermal metallization is recommended. If this is not practicable, the affected surfaces can be painted with products containing at least 90% zinc in the pigment, creating coatings with a maximum thickness of 100mm. It is important to treat only the damaged parts, minimizing the extension to adjacent areas. It is therefore evident that subsequent galvanizing processes may involve further restoration operations with consequent costs. However, using already galvanized semi-finished products avoids the introduction of tensions that can cause deformations during galvanizing.
It should therefore be assessed from time to time whether it is more advantageous to use pre-galvanized semi-finished products or finished structures subsequently subjected to integral galvanization.
Welding of galvanized parts
A rule for general galvanizing products is to try to avoid welding galvanized parts as much as possible. In fact, it happens that the zinc in the vicinity of the weld evaporates (sublimes) leaving the area in the vicinity of the bead uncovered. The zinc coating has interference in the welding process, so you cannot always simply use the techniques that can be used for uncoated steel. The degree of interference due to the zinc coating depends on the thickness, composition and structure of the coating itself
In welding galvanized parts, the main problems are:
- Increased spatter during welding
- Greater formation of fumes
- Greater formation of porosity in the weld metal
- Less penetration
- Risk of intercrystalline cracking in the weld metal.
If you can’t help but weld the structures after they have been hot-dip galvanized, for example, for the assembly of particularly bulky parts, you must plan for an appropriate repair of the layer through metallization, zinc-rich paints or other systems, such as the use of built-in bars.
The welding methods conventionally used for non-galvanized steels can also be used for galvanized steel with some adaptations. It is a question of paying attention to the way in which the zinc is moved away from the weld site. Care must be taken so that the sublimation of zinc does not cause blowholes and the formation of porosity in the bead. This effect can be reduced or completely eliminated by a reduced welding application rate, which allows the zinc vapors to move away from the surface portion involved in the weld. Moreover, research conducted in the sector has shown that steel maintains its mechanical characteristics unchanged even after galvanizing. & Nbsp; Among the various methodologies, manual arc welding is preferred, which offers multiple advantages. It is in fact easier to check the final result. It can be considered a good idea to rub the edges of the pieces involved in welding with the electrodes in order to anticipate the removal of the zinc for sublimation before carrying out the actual operation.
For the treatment of sheet metal with a thickness up to 3mm, it is preferred, instead, to resort to gas welding which however destroys a larger area in the zinc coating. & nbsp; Resistance welding procedures find, finally, greater use in the joint of hot-dip galvanized thin sheet metal.
Automatic open arc or gas protected arc welding can also be performed. This last procedure, called GMA (Gas Metal Arc) welding, generally uses CO2; the MIG (Metal Inert Gas) method uses argon, however, it is preferable to mix 20% CO2 and 80% argon, which allow to achieve better results from an aesthetic point of view. short arc, numerous sparks are produced which attach themselves to the steel. It is therefore advisable to spray the parts to be welded with appropriate sprays that facilitate the removal of residues by brushing.
In butt or V-profile welding, it would be advisable to increase the distance between the parts to be welded, to facilitate the removal of vapors and avoid the formation of porosity. If the arc method is used, it is suggested to slightly increase the welding current to stabilize the arc affected by the emission of vapors. During the welding of the corner joints, in particular positions, the drops can interfere in the arc and get stuck in the nozzle of the welding gun, interrupting the wire feed. Droplet particles can also adhere to surfaces close to the weld and affect their appearance.
The penetration into the joint is reduced due to the fact that the zinc coating at the edges of the joint interferes with the energy balance of the electric arc, reducing the arc voltage and the current. & nbsp; The intercrystalline cracks in the welded metal, due to the penetration of zinc, they are more frequent in T-joints, of material with a thickness greater than 13mm, joined with coated electrode welding, and over 6.5mm for CO2 welding.
The problems described above can be eliminated or reduced to acceptable levels by taking the following measures:
- Weld with a distance between the joint surfaces of 1.5mm for CO2 welding and 2.5mm for welding with coated electrodes. In this way the number of pores is reduced, penetration is increased, avoiding the risk of intercrystalline cracking (zinc penetration)
- Welding with minimal displacement
- Waving with the electrode along the joint so that as much zinc as possible is burned in front of the melting pool. This reduces the number of pores and the risk of intercrystalline cracking
- Chamfer the vertical plate of a T-joint to obtain a double bevel or single bevel groove. This eliminates the intercrystalline cracking and reduces the number of pores, regardless of whether the edges of the joint are galvanized or not.
To reduce the risk of intercrystalline cracking when welding thick T-joints, electrodes with a low silicon content should be chosen. In general, systems that produce slag that solidify more slowly are preferred, as they allow more time for the vapors to escape. For construction steels and welds that do not involve particular needs, it is advisable to use, for example, medium diameter electrodes coated with rutile or rutile cellulose. both sides. In this case it is advisable to resort to burning (removal by evaporation) of the zinc, since mechanical removals such as grinding and filing can leave annoying residues.
As is evident, the zinc coating is removed locally in all welding procedures. We repeat that in all these cases it is advisable to provide an adequate protection restoration system, based on paints rich in zinc, zinc coating or spray metallization. Paints with a high zinc content use two-component epoxy resins, polyurethanes or one-component ethyl silicates (inorganic zinc) as binders.
A drawback related to the welding of galvanized parts derives from the fact that the high temperature makes the zinc incandescent, from which gray-whitish vapors of zinc oxide are released which negatively affect the health of the operator and affect the quality of the work carried out. Inhalation of zinc oxide causes the so-called metal fume fever. In any case, it is a passing effect with symptoms very similar to those of the flu. Since there is no risk of zinc accumulation The organism, no permanent effects on people affected by this exposure are reported in the medical literature.
If you weld inside rooms, it is good practice (in any case even if you weld steel without zinc plating) to provide for the extraction of fumes to prevent operators from inhaling the metals and welding fumes. There are various types of hoods and aspirators available on the market and gas torches equipped with an integrated aspirator. no particular caution.
Bolting, the best assembly system
In general it is recommended that hot-dip galvanized components, intended to make up more complex structures, are assembled, when possible, with bolting. In this case, the coating is not damaged, as happens with welding. Naturally, the bolts used must be protected from corrosion as well as the steel structure to which they are applied. For this, as long as they are hot-dip galvanized elements. It is not convenient to use galvanized bolts with an electrochemical process, because their protection is not lasting. The thickness of the zinc layer is, in fact, much lower.
Care must be taken in the case of high-strength steel bolts. Since hot-dip galvanizing of classes higher than 8.8 is not recommended, the connecting element must be designed in such a way as to use lower-class bolts with a larger section, or alternatively a greater number of bolts with the same section.
Friction joint components can also be hot dip galvanized. Initially the coefficient of friction between the galvanized surfaces is rather low, however, thanks to a phenomenon of cold welding between the two parts in contact, it increases as soon as the sliding begins. If the latter is to be avoided, it is possible to increase the friction coefficient by subjecting the surfaces to metal brushing.
Unlike the US legislation which provides for hot-dip galvanizing among the few protection systems for surfaces in contact, Eurocode n ° 3 only mentions spray galvanizing and painting with zinc-based products. ensure the necessary tightening tension (homogeneously in all bolts) as well as to avoid friction causing wear of the galvanized surfaces of the threads, it is necessary to adequately lubricate the bolts, for example, with molybdenum disulfide, which has proved extremely effective .
ISO 14713 standard
Published in Italian in June 2001, the Italian and international standard UNI EN ISO 14713 provides guidelines and recommendations for the design of products to be hot-dip galvanized in the informative appendix A. The recommendations refer to the preparatory phases of the surface, to procedures related to particular design considerations (such as in the case of large pieces, pieces for which defects related to air pockets may originate and advice to avoid them, etc.), to the design characteristics and tolerances to be foreseen both for surfaces flat and threaded. exclusively of external surfaces.
Therefore, in addition to the guidelines on corrosive environments, also for the design aspects, the standard is an indispensable tool for the technician, designer of steel products intended to be hot-dip galvanized.