Hot-dip galvanization

Hot dip galvanizing is to apply a metallic zinc coating on iron or steel by immersion in molten zinc ( at about 450 ° C). It forms at the contact surface a resistant alloy layer of iron and zinc, and also a very adherent layer of pure zinc. Hot-dip galvanizing is one of several methods to galvanize around. In the hot-dip galvanizing galvanizing and continuous galvanizing distinction is made between ( discontinuous ).

Properties

The zinc layer is shining brightly in the fresh state in the rule. Over time to form as a result of corrosion of the zinc from a patina and dark and opaque in appearance. The patina is a weather-resistant protective layer of zinc oxide and zinc carbonate. Is the air and hence the carbon dioxide access restricted creates the undesirable white rust.

A hot-dip galvanizing provides both active and passive corrosion protection. The passive protection is provided by the barrier effect of the zinc coating. The active corrosion protection arises due to the cathodic action of the zinc coating: Compared with noble metals such as iron (see voltage range ) is used zinc as a sacrificial anode that protects against corrosion as long as the iron underneath until she is completely corroded.

Depending on the thickness of the zinc coating can be a galvanizing reach a different long term of protection. In Stückverzinkungsverfahren produced zinc coatings are characterized by thicker zinc coatings as produced in Bandverzinkungsverfahren zinc coatings. Therefore " batch-galvanized " coating reach a decades- long economic corrosion protection with no maintenance. According to the German Federal Ministry issued for Transport, Building and Urban Development Table " useful lives of components for life-cycle analysis of the evaluation system Sustainable Building ( NBB ) " batch-galvanized components achieve a useful life of generally more than 50 years. Notes on the term of protection of hot-dip galvanized coatings in different corrosivity categories are also Table 2 of EN ISO 14713-2.

The term of protection of hot-dip galvanizing can be further improved by an additional coat of paint ( " duplex system ").

Piecework

Under batch galvanizing Hot-dip galvanizing is the process of ( mostly) larger steel parts and structures. This blanks or finished workpieces after pretreatment are individually dipped into the molten bath. By diving internal surfaces, welds and inaccessible places are easily reached. Examples of batch galvanizing: safety barriers ( guardrails ), railings, exterior stairs, car trailers, truck trailers, parts for steel buildings, steel and glass buildings and parking garages.

Process steps for hot dip galvanizing

The procedures described below are only a sample explanation dar. According to the respective operating conditions to variances. In systems for hot dip galvanizing of small parts sometimes substantial changes in the process flow may occur.

Preparatory steps

Except in large industrial installations (such Karosserieverzinkung ) finds the hot-dip galvanizing usually take place in Lohnverzinkereien. For this purpose, must have been carried out a rough surface preparation, otherwise the galvanizing refuses to accept. The parts must be free of loose rust and old paint. If necessary, previous sandblasting is required. Hollow profiles must be drilled (10 mm and larger). From these holes all liquids must be able to drain away completely. Top inside corners should be rounded.

After delivery of the parts to be galvanized in the galvanizing plant and a necessary entrance test, the workpieces are first assembled into batches as possible identical or similar parts to prevent an economic galvanizing is possible. The parts are weighed before or after. In the so-called Schwarzverwiegung ( before galvanizing ) a predetermined percentage is calculated as zinc assumption added. The resulting weight is then included in the invoice. In the so-called Weißverwiegung ( after the galvanizing ), the final weight is multiplied by a fixed Eckpreis ( according to a material-based tariff group ). The product of weight and tariff group then gives the final invoice amount plus the VAT.

A Differenzverwiegung not done mostly.

Degreasing / rinsing

Parts which have residues of fats and oils are cleaned in a degreasing bath. As degreasing agent usually come aqueous alkaline or acidic degreaser for use. After an alkaline degreasing followed by a brief immersion in a water bath to avoid the entrainment of degreasing agents with the material being galvanized.

In an acidic degreasing, dilute hydrochloric acid is usually used as a base. This can be dispensed to the rinsing after degreasing, if in the subsequent pickling hydrochloric acid is also used as the operating medium.

Pickle

The next step is a pickling treatment for the removal of species-specific impurities (eg rust and scale ) of the steel surface. Pickling is usually carried out in dilute hydrochloric acid. The duration of the pickling process is governed by the rusting of the work to be galvanized and the concentration of the stain. Pickling bath to be operated normally at room temperature, but it can also occur to 40 ° C bath temperature.

After pickling done two rinses in water to minimize the spread of acid and salt residues as far as possible with the material being galvanized.

Pickling galvanized parts

After a defective galvanization or after a design change to a part of the galvanized zinc coating may be removed. For this purpose, but should not the above pickling bath be used, but a special pickling with dilute hydrochloric acid. Here, there is a chemical reaction in which hydrogen ( ) forms. This gas is bound in a foam film. The fiddling with ignition sources should be avoided in the vicinity of the bath, as this can lead to an explosion. The dezincified parts are then fluxed and re- galvanized.

Fluxing

The rinse is followed by a flux bath. Task of the flux is carried out at the reaction with the molten zinc a final intensive precision cleaning the steel surface and dissolve the oxide layer of the zinc surface, and to prevent re- oxidation of the steel surface during the waiting period for a short time until the galvanizing process. Similarly as for example solder, which also are used in the flux control, the flux is increased, the wettability between the steel surface and the molten zinc. The flux usually consists of an aqueous solution of chlorides, most commonly a mixture of zinc chloride ( ZnCl2 ) and ammonium chloride ( NH4Cl).

Dry

In most cases is followed by a drying station, in which the fluxing agent film is dried by means of heat. For heating degreasing and drying ovens, the waste heat from the heat the galvanizing is shared in many galvanizing. The heating / drying of the components has the sole meaning, adhering water to evaporate (moisture), so that undesired reactions (formation of water vapor) in the 450 ° C hot zinc are avoided. A relaxation of the components ( avoiding thermal distortion ) may, at the prevailing dry bulb temperature of max. 90 ° C not be technically and physically.

Galvanize

After the flux treatment or drying be galvanized is immersed in the liquid zinc bath. Zinc has a melting temperature of about 419 ° C; the operating temperature of a galvanizing bath is in most establishments 440-460 ° C, in special cases, at more than 530 ° C ( high temperature galvanizing ). The zinc content of the melt is in accordance with DIN EN ISO 1461 at least 98.5 %.

After immersion of the be galvanized in molten zinc, the parts remain in the zinc bath until they have accepted its temperature. After the flux is now " scoured ", the surface of the zinc bath of oxides and flux residue is cleaned before being galvanized is then withdrawn from the zinc bath.

When galvanizing forms as a result of mutual diffusion of the liquid zinc on the steel part with the steel surface a coating variously composed of iron -zinc alloy layers. When pulling out the dip galvanized items still remain on the top alloy layer yet - also referred to as pure zinc layer - layer of stick zinc, which corresponds in its composition of molten zinc.

Cool

The parts are cooled in air or in a water bath.

Plastering or deburring

Now the slings or Anbindedrähte be removed and refinished which devoted splices conforming to standards. Should ungalvanised areas indicate the components, these may be reworked within narrow limits and according to established rules. As maximum size allowed for a single defect are 10 cm ², where all defects located on a component in all of the area must not exceed 0.5 % of the component surface. Follow-up work can be done by arc spraying or by a zinc dust coating. Which of the above methods is used, is the component and the technical conditions in the galvanizing dependent.

In a last step, the finished galvanized material is weighed, as a rule, the galvanized weight forms the basis for determining the price of the company. Before delivery, the quality of the hot-dip galvanizing is controlled. If necessary, nor done a plastering the parts, that is tine tips and impropriety on the zinc coating to be removed.

After-treatment

The requirements for the appearance of hot dip galvanizing to take. The freshly galvanized surface as long as possible to keep the shine and slowly and evenly form of white rust and thus longer maintain high optical quality of the galvanizing. There are post-treatments in terms of DIN EN ISO 1461, edition March 1999 " Hot dip galvanized coatings applied to steel Zinc coatings ( galvanizing ). ; Requirements and tests ", and the national supplement I, available that maintain the luster of a freshly galvanized surfaces longer and delay the formation of white rust. This extends the excellent quality impression of a freshly galvanized surface, increases the value of the surface and reduced customer complaints, which are due to white rust and reduced shine. Advantageously, chrome-free air-drying, water-soluble paints that can be applied in a dip application. Most post-treatments are also an excellent primer for subsequent powder coating or wet painting.

Result

An important criterion for the quality of hot-dip galvanizing is the thickness of the zinc coating in microns ( micrometers, a thousandth of a millimeter ). The standard EN ISO 1461 " Hot dip galvanized coatings applied to steel Zinc coatings ( galvanizing ) " (previously standardized in Germany in the DIN standard DIN 50976 ) are the minimum values ​​of the required coating thicknesses specified, as they are to deliver depending on the material thickness when hot dip galvanizing.

Minimum thickness of zinc coatings in accordance with EN ISO 1461:

In practice, the layer thicknesses are significantly higher than the minimum thickness specified in EN ISO 1461. As a rule, produced by hot dip galvanizing zinc coating has a thickness of 50 to 150 microns.

Hardware Ever tines

For small steel parts, the galvanizing is often too expensive. In this case, the small parts are filled as bulk material in a metal basket and the metal cage is completely immersed in the molten zinc. Compared to the batch galvanizing the following differences:

• The bath temperature is in this case set over 530 ° C.
• As an additional step after galvanizing small parts are usually spun by rotating the metal basket. This is called spin goods.
• By centrifugation, the zinc layer thickness is reduced. The reduction of the zinc - coating thickness is especially important for parts with thread, or parts with exact dimensional tolerances. Thread be without centrifugation often no longer accessible.
• The color of finished galvanized spin goods is usually less bright shiny than the color of individually galvanized parts, but rather gray. This is a consequence of the centrifuging, because pure zinc is centrifuged from the surface.

Examples of the small component Ever tines: screws, nuts and fasteners.

Economic importance of Stückverzinkens

By galvanizing around 1.9 million tons of steel protected against corrosion in Germany per year. This corresponds to a turnover of approximately 700 million euros. About 85 percent of the tonnage is refined contract basis. Among the customers of galvanizing plants mainly include steel and metal companies, production parts manufacturers and the steel trade. About half of the hot-dip galvanized tonnage is used in construction. Other important areas of use are industrial equipment, transportation, automotive, agriculture, road equipment, supply and disposal as well as small parts. Typical products are safety barriers ( guardrails ), balcony railings, stairs with grate, car trailers, truck trailers, steel structures (eg steel halls, Modern steel and glass architecture, parking garages ).

Continuous galvanizing ( Sendzimir )

The Continuous galvanizing is also known Sendzimir (named after Tadeusz Sendzimir ) known. Continuous galvanizing consist of the inlet part, the actual treatment part and the outlet part.

The starting material is usually unannealed cold rolled steel strip ( 0.4 to 4 mm thick, from 400 to 1800 mm wide), which to a coil ( Bund) is wound. The band length of such coils can be up to 3000 m. In the inlet part of the steel tape is unwound from the coil. Two uncoilers and a welding machine enable the manufacture of endless belts, so that the system can be continuously driven.

The actual processing part consists of a continuous furnace, the molten bath, a device for adjustment of the zinc coating and cooling. The continuous furnace is divided into the direct fired preheat zone, the indirectly heated reduction and retention zones and the cooling zone. In the preheating zone, the strip is rapidly heated to about 450-650 ° C. This portion is also referred to as oxidation zone. Here was an oxidative cleaning of the belt, for example by burning off of the emulsion residues from the cold rolling. In the reduction zone and holding the tape is in a reducing protective gas atmosphere is heated ( about 25 to 28 % forming gas of hydrogen in nitrogen ) at about 800 ° C and held at this temperature. The strip is continuously annealed, be carried recrystallization of the steel the desired mechanical properties of the base material adjusted. In addition, in the preheating iron oxides formed are reduced. The subsequent cooling zone is connected through a lock ( trunk ) with the weld pool. In the cooling zone, the strip is cooled under inert gas to a temperature near the melt temperature.

The obliquely downward band dips in the molten bath (bath temperature about 460 ° C for zinc ) A is deflected by a roller in the bath upwardly and leaves the molten bath again. On emerging from the bath, the band rips a dependent of the belt speed metal crowd with it, far exceeding the desired coating thickness ( Edition) is. The excess coating metal is still liquid in the so-called scraped Düsenabstreifverfahren with a directed jet of air from a wide slot die on the front and back. The so- adjustable and controllable coating thickness depends on the tape speed, the blow-off pressure and the nozzle distance from the tape from ( For further information see: electromagnetic strip stabilization ). Thereafter, the belt is cooled by air with a jet cooler, wherein the metallic coating solidifies on the strip surface. The crystal structure thus formed and their appearance depends on the type of coating and the terms of the solidification process.

The hot-dip galvanized steel strip can still re-rolled if required in the hot dip galvanizing line ( trained ), directed and possibly chemically treated ( phosphate, passivated) are. Thereafter, the tape is rewound into a coil.

The belt speeds in continuous hot-dip galvanizing, depending on the tape thickness up to 220 m / min. , Thus achieve a hot-dip galvanizing tons capacity of over 2,000 tons of galvanized steel strip per day. From the heat of the tape to the emersion of the tape from the liquid zinc must be taken to ensure that not the slightest traces of oxygen reach the steel strip, otherwise not wet the zinc, the steel surface and a galvanizing is no longer possible ( zinc roll off ). Steel bands can be hot-dip galvanized in such good quality that they even in automobiles for skin panels (roof, fenders, ... ) are used. For this, the coated strip is cut to a plate and deep-drawn in several steps. The zinc adheres so well that it does not flake off in this deep drawing. The thus molded part ( fender, hood, refrigerator door, ...) is painted and installed.

Zinc baths for the continuous galvanizing contain ( in addition to zinc ), about 0.2 % aluminum to prevent the formation of ZnFe phase and to promote the formation of zinc flowers. Since ZnFe phases are brittle, this would break the following forming a galvanized sheet, the zinc would lose their grip and peel off. The oxidation of the surface of the molten zinc is slowed by air by aluminum, which reduces the slag formed on the surface of molten zinc. In addition, due to the small amount of aluminum in the zinc bath later zinc layer brighter and shinier.

Galfanbäder contain ( in addition to the zinc ) or 4 to 5 % of aluminum and small amounts of lanthanum and cerium. Galfan only little used today.

In addition to aluminum, among other elements in varying quantities are sometimes added to the zinc:

• Lead, which prevents a hard layer of zinc is formed on the bottom of the tub,
• Silicon.