The phases of the galvanizing process

Like other surface protection processes, hot dip galvanizing consists of two main phases: the preparation of the surface to be treated and the actual galvanizing. To obtain a quality galvanizing, the surface of the steel to be galvanized must be perfectly clean, so that no presence of oxide and other contaminants can interfere in the contact of the iron in its metallic form with the molten zinc of the bath. moment of immersion. That the surface of the steel is perfectly reduced and metallic is an indispensable condition for the metallurgical reaction to form the galvanizing coating to take place.

The preparation (pre-treatment) of the surface of the steel products consists in the removal of impurities, dirt and oxide layers, through two successive phases that exploit the chemical action of the degreasing baths, first and acid pickling, then.  The formation of the protective coating occurs during immersion in the molten zinc. In this phase, a coating layer is developed on the steel surface by intermetallic reaction between the solid phase and the liquid phase (between zinc and iron of the steel).  In fact, the zinc diffuses inside the steel surface, forming a series of zinc / iron alloys with a zinc content increasing towards the outside.

The times of immersion and stay in the galvanizing bath are very different from situation to situation. In some cases, special attention is also required during the extraction procedure. These circumstances make it difficult to automate the immersion phase in molten zinc. However, new technologies offer the possibility of automating the chemical pre-treatment, which obviously is easier to implement in the case of more specialized productions, such as, for example, applications in the sector of road fences, guardrails, poles and sound-absorbing panels.

The “different” hot dip galvanizing

The treatments called hot galvanizing in the different types of systems all use the same metallurgical process, the same intermetallic reaction to obtain the protective layer, in order to obtain different types of galvanized products. General hot dip galvanizing is a batch process, i.e. suitable for the treatment of batches, with discontinuous production. Generally, this is surface treatment carried out on behalf of third parties mainly on pre-assembled iron and steel products for which galvanizing is very often the last stage of production.  This applies to carpentry of any weight and shape, to centrifuged small parts and to the most disparate steel products, but also to items intended for specific uses such as, for example, motorway barriers and pylons for running power lines.

Screws, bolts, nails and other small parts that need to comply with stringent dimensional tolerances or that need to have controlled surface conditions, can be processed in general galvanizing, but are subjected to centrifugation after extraction from the bath, to remove excess zinc. .  The centrifuge determines a more regular appearance of the coating and allows coupling to the threaded elements after galvanizing. The standards obviously provide for lower coating thicknesses than the other articles treated in general galvanizing. For technical details, see the end of this chapter. The galvanizing coating obtained on hot-dip galvanized pipes, wire and sheet metal is similar to that obtained in general galvanizing, but different technological and plant engineering solutions are used for the production.

Pipes

For the pipes, given the uniqueness of the product, semi-automatic systems are used, with process phases similar to general galvanizing. The really automatic phase is that of the passage of the tubes in the galvanizing tanks after their positioning on the racks with manual operation. Peculiar is the process step which involves blowing after extraction from the zinc bath, using jets of water vapor essentially to free the inner lumen of the tubes from excess zinc after extraction.

Wire and sheets

For hot-dip galvanized wire and sheets there are substantial differences with respect to general galvanizing as regards the surface preparation, the determination of the residence time in the zinc bath and the methods of immersion (or passage) and extraction from the zinc.

The galvanizing of coil strips and sheets takes place continuously. The surface preparation of the sheets is not generally carried out through the action of fluids, but through thermal treatments capable of burning and removing the residues of oily emulsion, and of obtaining the reduction of oxides in an inert atmosphere. Furthermore, for the galvanizing of sheets and strips, a certain determination of the thickness of the zinc layer is possible (or easier), obtained both by using mechanical systems (such as rollers), and by checking the composition of the galvanizing bath and of the coil steel.

Production phases of general galvanizing

In general galvanizing, the pieces to be treated must be suspended from the crosspieces (sling bars) in order to be handled and moved from one treatment bath to another. Galvanizing, especially in the case of third party processing, is usually the last stage in the finishing of pre-assembled steel products characterized by extreme variability in mass, geometric shape and size. It is therefore necessary to resort to different loading methods on the crossbeams: suspension equipment is used, the shapes of which are often the result of the galvanizer’s special experience as well as the type of items most frequently handled. Common is the use of hooking by means of intertwined iron wire, but also a disparate range of tools.

The surface preparation

The steel surface of the rough pieces in general, at the entrance to the galvanizing plant, has patches and layers of contaminants, such as rust and calamine, oxidation products and other foreign substances, such as soaps, oils, paints, welding and melting, together with other residues, produced by previous processing. For their removal, steel is generally subjected to chemical degreasing and pickling treatments. uses mechanical cleaning. This happens, for example, in the presence of residues of paints, enamels, welding slags, anti-spray silicone resins (sometimes used in welding). For their removal, grinding,  brushing or sandblasting are used, instead or in addition to the normal cycle.

Upon visual inspection, which is normally carried out at the entrance of the zinc-coated material, these contaminants are not always immediately detectable, perhaps because on areas of limited extension or difficult to identify. However, if they are not carefully removed, they cause defects in the protective layer, even very evident ones. In fact, in the areas affected by their presence there is the lack of development of the galvanizing layer, and, conversely, the formation of showy blackish burns, due to the presence of the burnt residues of the organic substances brought into contact with the zinc of the molten bath. To avoid these problems it is good to protect the surfaces to be galvanized from contamination. It is always necessary to remember not to mark the pieces to be galvanized with paints or markers with non-water-soluble enamels.

Degreasing

With the degreasing baths, the elimination of oils (refrigerants, antioxidants, lubricants) and fats deposited on the surface during the production and assembly of steel products is obtained. The alkaline degreasing baths contain sodium hydroxide, carbonates, phosphates, silicates, surface surfactants that remove oil and fats from the surface of the metal through the emulsion. The concentration, the temperature of the bath and the immersion time of the pieces determine the efficiency of the degreasing bath. The normal operating temperature range for heated degreasing baths is 30 – 70 ° C. The bathrooms can have different compositions. Indicatively, they may consist of a 1 – 10% sodium hydroxide solution, with the addition of other alkaline reagents such as soda, sodium silicate, condensed alkaline phosphates and borax as well as specific surfactants, emulsifying and dispersing agents.

The alkaline degreasing treatment is followed by washing to avoid the entrainment of alkaline substances inside the acid pickling baths. In this case, in fact, the alkaline degreasing agents, with their neutralizing action, would shorten the time of use. The acid degreasing baths are obtained starting from strong inorganic acids (such as hydrochloric acid and / or phosphoric acid) diluted with additives, emulsifiers and corrosion inhibitors. Acid degreasing agents usually form stable oil emulsions, which can create some difficulties in the maintenance of the bath due to their skimming and separation. There is, however, a certain advantage in their use, consisting in the possibility of omitting the subsequent washing step with a consequent reduction in water consumption.  

The energy required to determine the heating of the baths to the operating temperature comes from fuel oil or gas, depending on the local conditions of each plant. In many cases, the degreasing systems are heated through the recovery of heat from the exhaust fumes of the galvanizing furnace, by means of coils for heat exchange. The maximum duration of activity of the degreasing solution is usually about 1-2 years but, in some particular cases, it can exceed 5 years.

Pickling

The next process step consists in acid pickling, which serves to remove encrustations, oxides and rust that may be present on the pieces to be galvanized. The most used pickling agent is diluted hydrochloric acid, but it is also possible to use other inorganic acids such as sulfuric acid and, in very special cases, hydrofluoric acid (for example, pickling of cast irons). A general galvanizing plant usually includes a series of pickling baths with varying acid concentrations. Hydrochloric acid is normally purchased in a solution of approximately 28 – 33% and diluted up to 12 -16%. To prevent excessive pickling of the steel pieces and to protect the tanks, pickling inhibitors are added to the bath which reduce aggression towards the metal. Evaporation inhibitors can also be used. During the operation of the tanks, the iron content of the pickling bath increases while the amount of free acid decreases, making it necessary to occasionally top up the bath by adding fresh acid. Pickling becomes impossible in the presence of excessive concentrations of FeCl2, but usually the pickling bath is replaced or regenerated before reaching this condition. Pickling at the plant with an open pre-treatment section is carried out at room temperature. In this case, the average consumption of acid is about 20kg per ton of product, but it is highly variable depending on the surface conditions of the steel: a realistic range is 10-40 kg / ton.

For temperatures above ambient (but not over 40 ° C), it is possible to apply closed tunnel systems, in which the pre-treatment tanks are enclosed in a more or less isolated space, to form a sort of extractor hood. . The use of these heated systems makes possible shorter treatment times and requires a smaller amount of acid. Furthermore, by raising the temperature of the pickling bath, it can be used at relatively high concentrations of FeCl2, for example 175-200g / l with the bath at 35 ° C. A disadvantage is that the emissions and vapors generated by the acid tanks are higher in relative terms and, therefore, are collected and, in some cases, purified by suitable abatement equipment.

Cleaning

Post-pickling washing is a very important step in the galvanizing process, as it prolongs the life of subsequent treatment baths, reduces waste production and increases the reuse of by-products. After degreasing and pickling, the assembled steel is washed-immersed in water baths, which are sometimes heated. The washing has the purpose of preventing the entrainment of acid, iron and iron salts from the pickling inside the flushing baths and beyond, up to the galvanizing tank. The effect of the dragging of dissolved iron would consist in an increase in the generation of mattes and, consequently, in an increase in the consumption of zinc, with the same coating thickness applied to the pieces.

Fluxing

Fluxing represents a peculiar process stage for hot dip galvanizing. For the realization of this phase there are two different technological solutions:

• In dry galvanizing, by far the most used process, the products are immersed in a flux solution consisting of salts such as zinc chloride (ZnCl2), ammonium chloride (NH4Cl) and double salts of the type ZnCl2 · NH4Cl · 2H2O (in solution there is sometimes also a small quantity of potassium chloride – KCl – which limits the emission of fumes), so that the concentration of the salts of the solution reaches an optimal value. Dry galvanizing necessarily involves the subsequent drying and preheating phase.
• In wet galvanizing, the same flushing agents form a layer of molten salt, more or less thick, floating on the surface of the galvanizing bath. The steel parts to be galvanized are passed through the flux layer inside the zinc bath. At the end of the immersion (after the metallurgical reaction of the zinc with the iron), the layer of molten salt is moved from the surface of the bath and collected on one side by means of blades, to allow the extraction from the zinc bath without further contact with the flux. In this case it is not necessary to carry out a drying and preheating phase.

This system is old and destined to fall into disuse, because it does not allow optimal productivity of the galvanizing tank.

Flushing: what is it?

To understand what flushing is really used for, it is necessary to observe what happens in the galvanizing tank during the subsequent immersion phase: in molten zinc, at temperatures above 200 ° C, the ammonium chloride of the flux decomposes into ammonia (NH3) and hydrochloric acid (HCl), which produces an additional pickling effect and eliminates any residual oxide from the pickling phase or that forms on the way on the pieces to the galvanizing tank. In addition, the flushing salts lower the surface tension in the bath, allowing for better wettability between molten zinc and the steel surface. In the case of dry galvanizing, the thin film of salts (zinc chloride in particular) helps protect the metal surface of the steel from oxidation, until the product is immersed in the molten zinc, preserving the parts at high temperature, not still immersed, from the interactions with the atmosphere. This is especially important when the drying time is long.

The total concentration of flushing salts (the sum of zinc chloride, ammonium chloride and double salt) and the ratio of zinc chloride to ammonium chloride are both very important. Ammonium chloride in a good flushing bath often accounts for 40 – 60% of the flushing salt. Ammonium chloride ensures fast drying and better removal of iron oxides from the surface of the pieces, but also causes more smoke, foaming (ash) and mattes during the coating process. When the pre-treatment of the manufactured articles is insufficient, a greater quantity of ammonium chloride is required. In dry galvanizing, the flushing tank is kept at a temperature of 25-50 ° C. The pH value of the flushing baths is normally kept approximately 4.5. The pH of the flushing bath can however vary between 1 and 5. In order not to detect variations in acidity, in some cases, salts are used which constitute a buffer system. In order not to detect variations in acidity, in some cases, salts are used which constitute a buffer system. Generally, the flushing bath is kept in optimal conditions through continuous regeneration systems.

Drying and preheating

The drying is obtained in a special oven, located near the galvanizing oven. In fact, the oven partially recovers the heat from the combustion fumes, although in most cases additional burners are required to allow this appliance to function. The complete drying of the product allows to reduce splashes and ejections of metal when the piece is immersed in the zinc bath. The benefit obtained in this phase is twofold, since the product to be galvanized is pre-heated and the conditions of the layer formation reaction improve, reducing and optimizing the residence times in the zinc bath.

Immersion in molten zinc

The pieces that come out of the drying ovens are slowly introduced into the molten zinc tank. The set of pieces suspended from the crosspieces is called lowered During the immersion of the descent, the surface of the iron slowly frees itself from the flushing salts which, reacting with the bath due to the high temperatures, slag. The pieces, perfectly reduced and free of oxide layers, are wetted by the zinc and the metallurgical reaction to form the protective metallic coating takes place on the surface. surface of the bathroom, from where it is removed with a necessarily manual shoveling operation. 

It is in this phase that the steel on the surface reacts with the zinc, forming a coating consisting of a series of layers of iron-zinc alloys with different composition and crystalline structure, topped by a layer of zinc with the composition of the bath that is deposits by dragging, as already mentioned. The immersion time in the bath varies from a few minutes for relatively thin steel artifacts, up to 15 minutes for objects assembled with thicker profiles. The pieces to be galvanized must be left in immersion until they reach the bath temperature and the layer formation reaction is completed.  From a practical point of view, the galvanizing that took place coincides with the subsiding of the surface of the bath by the characteristic swirling motion of the zinc, determined by the development of gases and vapors.

Some artifacts with a particular shape or mass may require longer immersion times. Objects assembled with thicker steel sheets possess greater thermal inertia. This implies that they take longer times to reach the bath temperature. The immersion phase is therefore the longer the thicker the material. This, all other conditions being equal (for example, with the same steel composition), given the longer residence time at high temperatures, means that greater thicknesses of steel correspond to greater thicknesses of the coating. The molten zinc in the tank is usually kept at a temperature of 440-460 ° C. For special applications, the galvanizing bath can be managed at higher temperatures, up to 530 ° C and beyond. In this case we speak of high temperature galvanizing. This is required in the treatment of some categories of steel and some specific types of components. 

The galvanizing tank

The galvanizing tanks are made of low carbon steel, made of a particular alloy to be little reactive with zinc. In fact, otherwise, the walls would become thinner due to the reaction with the zinc and puncture in the points where the aggression was most violent. Gas or, rarely, oil burners are used to heat the tank. The tank is built to withstand the high hydrostatic loads and thermal stresses generated during heating to operating temperatures.

The free surface of the zinc is at the level of the floor or a few tens of centimeters higher, to allow the operators to remove the foam from the bathroom, during the extraction of the descents. For the ceramic tanks, mostly used for the galvanizing of small parts, heating with immersion burners or hood heaters is used when the zinc temperature is higher than 460 ° C.

The extraction of the artifacts

After the layer formation reaction takes place, extraction from the bath takes place. It is a delicate operation in which, with the help of suitable movements, the drainage of the liquid metal is favored. During extraction, an additional zinc coating is formed on the surface of the pieces which has the same composition as the molten metal bath and gives the freshly treated steel a bright appearance. After extraction, when the pieces are still suspended on the bath, the last residues of excess zinc are eliminated, hitting or, in some cases, shaking the artifacts.

The iron / zinc reaction of formation of the layer continues even after extraction up to a temperature of about 200 ° C, at which diffusion phenomena now occur at a negligible speed. The galvanized pieces are then cooled in air or water and inspected. We then carry out the weighing to determine the weight of the coating and evaluate its cost. Finally, the quality of the coating is assessed, which must comply with regulatory requirements regarding the minimum thickness values (measured in μm or, more rarely, in g / m2) and the appearance requirements. Small surface imperfections are repaired and the artifacts are unhooked and prepared for shipment.

Finishing

During the extraction of the pieces from the tank, it may happen that the fluid zinc, dripping, hardens forming light thickenings or drips. Generally, if these defects are of modest size, they do not create problems for the product.  However, if the galvanized parts need to be assembled or mounted precisely, in case of difficulty, the extra thicknesses and small accumulations can be removed by abrasion. In any case, imperfections must be eliminated when they are sharp, as during transport, assembly or use, they could cause injury to operators or detach in impacts, leaving small steel surfaces uncovered.  To remove such thickenings it is preferable to operate manually with a file, with the help of an angle grinder and a flexible disk. In this way, it is easier to check that the coating is not removed completely, leaving the steel unprotected. Drips can, on the other hand, be eliminated using a blowtorch. A light flame is able to locally melt zinc without burning it. Once fluid, the superfluous zinc drips off on its own or, if this does not happen, it can be easily removed with a wire brush or metal spatula.  

Similarly, if you galvanize a steel structure to which threaded elements are welded (not centrifuged), it is necessary to remove excess zinc from the thread to make it efficient again. As for the male threads, the most effective, simplest and fastest method is to melt the coating with a flame and brush off the fluidized zinc. For female threads, the protection of the threads to prevent contact with zinc must in any case be carried out upstream of the zinc plating.  In the presence of hinges and joints, the solidification of the zinc stiffens these parts, preventing them from moving. Also in this case it is useful to use a blowtorch to melt the zinc coating. Once the zinc is made fluid, the hinges, once again movable, must be kept in motion until the coating has solidified. This operation guarantees the preservation of the mobility of the joints even after the solidification of the zinc. Therefore, when possible, these joints should be moved immediately after extraction from the bath, without waiting for the coating to cool, thus avoiding the need for zinc remelting.

After galvanizing structures such as frames or frames with corrugated wire nets or grids, it is possible that in some parts zinc residues with the consistency of a thin membrane remain. If they cause any problems, in relation to specific uses, to eliminate them it will be sufficient to use a wire brush  The use of the wire brush is also sufficient to remove any residual foam (ashes) that may be deposited on the surface of the product due to a defect in the shoveling operation. The inclusion of ash is harmless for the protection offered to steel. It intervenes when galvanizing is already formed and affects the outermost layers of the coating, from where it can be removed without damage. Quite different is the case of the inclusion of flushing salts (of different morphology and well characterizable), which must be removed with accuracy and must be followed by a repair treatment of the layer, using paints with a high zinc content, spray or metallization or zinc coating with low melting point alloy bars. Same fate for small areas that for various reasons should be uncovered.

A quality product is such that it does not show intolerable defects. The repaired areas cannot exceed in total 0.5% of the surface of the building and each single area to be repaired cannot exceed 10cm2 in extension, in accordance with UNI EN ISO 1461.

Galvanizing of centrifuged small parts

Small objects to be galvanized that require centrifugation, such as accessory nails, fittings, bolts, nuts, screws, washers etc. they are subjected to the same chemical pretreatments of surface cleaning and immersed in the zinc bath, collected in perforated baskets. Once galvanized, the basket is extracted from the zinc bath and placed in a centrifuge, where the excess zinc is eliminated, in order to avoid droplets and irregularities and respect the standard dimensional tolerances. Generally, cooling is carried out in water which avoids the gluing of the pieces to each other.

The centrifuge can cause the partial removal of the last layer of pure zinc which, as mentioned above, tends to settle on the pieces extracted from the tank. The result is a more opaque surface than other galvanized products, also because for the galvanizing of small parts, the temperatures of the molten zinc are generally higher, around 460-470 ° C. This determines a faster reaction, which persists beyond the extraction until it affects the pure zinc layer, thus causing the emergence of zinc-alloys. On the other hand, the lower viscosity of zinc at high temperatures allows for greater ease of cleaning. Sometimes, for particular uses, temperatures up to 550 – 560 ° C are applied. For these temperatures (already over 470 ° C) ceramic galvanizing tanks are used Such high temperatures can compromise the mechanical properties of high strength bolts, beyond class 8.8. Furthermore, in this case, the pickling prior to galvanizing can induce hydrogen embrittlement, so that beyond that class, galvanizing is not recommended. To overcome this drawback, thicker bolts of the lower strength class or a greater quantity of lower strength bolts can be used.   

During centrifugation, excess zinc is removed quite easily from the threads of the bolt screws. Bolts with a diameter greater than 8 mm can be effectively hot-dip galvanized and satisfactory results are sometimes achieved even for smaller sizes. For the threaded parts it is necessary to leave a tolerance between male and female equal to four times the thickness of the coating (usually around 40 μm).

The nuts, on the other hand, are usually galvanized before the thread is made. The thread of the nut is not coated, but the cathodic protection is exercised by the coating of the male thread, once the nut and screw are tightened, and is sufficient to guarantee protection against corrosion. In fact, once the bolt has been tightened, electrical continuity is guaranteed for electrochemical protection. In addition, the zinc corrosion products of the screw thread fill the empty space and seal the coupling.   This must be done in order to ensure that the male and female threads are properly coupled, so that the load on the threads themselves is carried entirely by the steel substrate and the zinc does not improperly exert mechanical seal.

Galvanizing of pipes in dedicated plants

The plants that produce galvanized steel pipes are very similar to general zinc plating since the pipes undergo the same surface preparation and immersion in molten zinc. One difference is in the technique in which the tubes are passed through the zinc by means of a semi-automatic drag system. In fact, the pipes, coming from chemical degreasing, pickling and flushing treatment, are placed on a rack and manually sent to a transport system that automatically takes them immersed in tanks with a specific configuration. The residence time inside the tank is regulated by the speed with which the pipes cross the bathroom in the cross section of the tank.

Compared to general galvanizing, the galvanizing process of the pipes includes an additional subsequent stage called blowing: inside the pipes just extracted from the tank, high pressure water vapor is blown in order to obtain the uniformity of the internal layer and the removal of the excess zinc and ash, which would otherwise clog them.

Galvanizing of wires and sheets

The plants that produce hot-dip galvanizing on wire or sheet from coil (strip) are based, on the other hand, on fully automatic continuous processes.  For the production of galvanized wire, a process similar to general galvanizing is applied in dedicated plants. In this case, by calculating the travel speed and the appropriate residence times in the tanks, completely automatic continuous productions are obtained. Excess zinc is removed by mechanical means.

Also the galvanizing of the continuous sheets, characterized by a substantial difference in the surface preparation, is based on the continuous passage of the metal strip through the molten zinc for an appropriate residence time. The surface preparation after degreasing the sheet consists of passing through a controlled atmosphere oven to allow the steel to obtain the required characteristics and reach the optimal temperature before immersion in the molten bath. In some cases (as in the case of the use of direct flame ovens) in the surface preparation of the sheets, degreasing and surface cleaning are not obtained by the action of fluids, but by heat treatments capable of removing the residues of oily emulsion by burning them and to obtain the reduction of oxides in an inert atmosphere. For the galvanizing of the sheets and strips it is also possible (or easier) to control the thickness of the zinc layer, which is obtained both by checking the composition of the bath and the steel of the coils, and through the use of mechanical rollers.

The pre-galvanizing surface treatment furnaces can be:

• Sendzimir oven: it is a type of continuous horizontal oven, consisting of:

1. Preheating zone, separated from the rest of the oven, in which the web is heated directly. Through oxidation it is cleaned of oily emulsion residues (oxidizing zone)
2. Reduction zone in which there is the reduction of oxides in a controlled reducing atmosphere (H2 / N2) and indirect heating maintenance zone for the transition of state (recrystallization) and normalization of steel, in an inert atmosphere
3. Cooling zone in which the strip is cooled to a temperature slightly higher than that of the molten zinc bath.

• Direct flame oven (D.F.F.): the direct flame oven (non-oxidizing type) can be divided into various sections:

1. Preheating section, with heat recovery of the fumes coming from the heating furnace
2. The heating furnace, in which the tape is placed in direct contact with the flame
3. Annealing section, where, through an electric heating or through radiant elements, the tape comes
4. Cooling section of the strip, divided into several parts having different cooling rates
5. Outlet section, from which the belt is sent to the molten zinc bath.

(This type of oven allows cleaning of the belt surface, which makes preliminary degreasing of the metal not always necessary)

• Radiant tube oven (R.T.F.): the sheet metal strip is passed inside the oven heated by radiant coils that allow the exchange of heat without contact with the combustion fumes. The atmosphere in the oven is controlled to create a reducing environment (H2 / N2). The maintenance area can be heated with radiant tubes or with electric resistances. This type of oven, in combination with a preliminary alkaline degreasing, allows to obtain very high quality standards and to improve the adhesion of the subsequent metal coating.