Galvanizing
When galvanizing , steel is provided with a thin layer of zinc to protect it from corrosion .
In contrast to non-metallic coatings, the zinc layer provides active protection against corrosion by acting as a sacrificial anode to the more noble iron (see: Voltage series ). The cathodic effect of the zinc coating prevents corrosion of the iron up to a distance of about 5 mm, so that imperfections in the zinc layer and exposed cut edges are also protected. However, the resulting bimetal corrosion causes accelerated removal of the adjacent zinc layer.
Galvanizing process
Process for applying a coating of zinc to components on steel :
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Hot-dip galvanizing ( hot-dip galvanizing )
- Piece galvanizing results in layer thicknesses of 50 to 150 µm
- Strip galvanizing ( Sendzimir galvanizing ) results in layer thicknesses of 5 to 40 µm, typically 10 to 20 µm
- Zinc spraying ( electric arc spraying or flame spraying with zinc wire or zinc powder), layer thicknesses of 80 and 150 µm
- Sherardizing (diffusion galvanizing), layer thickness controllable between 10 and 80 µm
- Zinc flake / binder systems, layer thicknesses of 4 to 5 µm per application, typical total thickness of 8 to 15 µm
- Electro-galvanizing (electrolytic galvanizing; in Switzerland: promatize), layer thickness controllable from 2.5 to 25 µm, but mostly only 10 µm is achieved
- mechanical galvanizing, layer thickness up to 50 µm
- Film galvanizing by applying a metallic zinc coating to iron or steel without exposure to heat. A zinc film with at least 96% zinc is applied.
The Federal Court of Justice ruled in 1969 (BGH, 12.03.1969 - I ZR 79/67): “By galvanizing, traffic understands that a pure zinc coating is applied, because that corresponds to the usage of the language and is also the result of all galvanizing processes, even if they are In other respects the course was different. ”Every galvanizing process is based on the application of a pure zinc coating. Accordingly, coatings such as zinc dust paints cannot be called galvanizing. Zinc-containing coatings can only guarantee active corrosion protection if they contain enough zinc particles to create a continuously conductive film. To achieve this, the binder content of the product usually has to be reduced to such an extent that the adhesion and resistance of the product suffer compared to other anti-corrosive coatings.
Hot-dip galvanizing
In hot-dip galvanizing, steel is immersed in a melt of liquid zinc, the temperature of which is around 450 ° C. A distinction is made between
- batch galvanizing, in which differently shaped steel parts (e.g. stair elements or railings) are immersed in the zinc bath and
- the continuous endless process for semi-finished products such as sheet metal, which is also called strip galvanizing or Sendzimir galvanizing .
The zinc layer thickness for batch galvanizing is usually between 50 and 150 micrometers and for strip galvanizing between 5 and 40 micrometers. Due to the greater thickness of the zinc layer, individually galvanized components usually have a service life of more than 50 years.
During hot-dip galvanizing, an alloy is formed at the interface between steel and zinc. Due to the effect of heat, larger (sheet metal) parts can warp during hot-dip galvanizing. A wall thickness of at least 1.5 mm is recommended. Sheet metal folds can be glued through the zinc layer, which increases the rigidity of the construction. Threads and precisely fitting openings usually have to be cut after hot-dip galvanizing. The coating of hot-dip galvanized surfaces requires special measures, as some paints cannot prevent the formation of an oxide layer on the surface of the zinc, which makes adhesion more difficult.
Spray galvanizing
Galvanizing is a variant of flame spraying in which zinc powder or a zinc wire is melted by an (oxygen- acetylene ) flame or arc and sprayed onto the workpiece using compressed air. The still liquid zinc forms a porous layer on the workpiece, which has anti-corrosion properties similar to those produced by hot-dip galvanizing. Due to the large inner surface of the porous layer, subsequent painting requires unusually large amounts of primer or filler.
In order to achieve sufficient adhesion, the surface of the workpiece is often roughened by sandblasting , which can lead to the warping of thin sheets. It is also difficult to achieve a uniform layer thickness, especially in cavities that are difficult to access. The coating often reaches a thickness of one millimeter. Compared to hot-dip galvanizing, the thermal load on the workpiece is low. The disadvantage is that cavities and places that are difficult to access (container interiors, folds, etc.) cannot be reached.
Sherardizing - diffusion galvanizing
During sherardizing, the metal parts to be galvanized are heated in batches with zinc powder in closed, rotating drums. Zinc evaporates at temperatures of 320 ° C to 500 ° C and combines with the base material by diffusion to form an intermetallic phase that grows out of the base material. Uniform, temperature-resistant, hard and abrasion-resistant zinc-iron alloy layers are formed. The process is suitable for parts with complex geometries and cavities, as well as for inexpensive coating of mass products. As the process is dry and no aqueous or caustic pretreatment is required, hydrogen embrittlement of the parts to be coated can be excluded. The matt-gray surface offers good paint adhesion and, in conjunction with passivation or duplex systems, achieves excellent corrosion protection.
Zinc flake / binder systems
Since the beginning of the 1970s, dispersions of small zinc and mostly aluminum flakes have been applied in a dip / spin process, dried and baked at 180-350 ° C. With the classic zinc flake coating, a layer thickness of around 4–5 µm is achieved in one coating process, and the layer is not pore-tight. Therefore it is usually coated twice and in some cases also sealed with silicate or overcoated with an organic topcoat.
Galvanic zinc plating
The workpieces are not immersed in a zinc melt but in a zinc electrolyte; the workpiece to be galvanized is hung in the solution as a cathode . An electrode made of the purest possible zinc is used as the anode . With electro-galvanizing, the zinc application is proportional to the strength and duration of the current flow, whereby - depending on the workpiece and anode geometry - a layer thickness distribution is created over the entire workpiece. If cavities are to be galvanized, the anode would have to be inserted into the cavity. In the galvanic bath, there is a risk of hydrogen embrittlement , which can be a disadvantage, especially with hardened steel components.
Due to the different layer thicknesses, the corrosion protection is usually less durable than with other galvanizing processes. Electro-galvanized workpieces that are exposed to direct weathering for long periods of time should be given an additional synthetic resin coating (see duplex system ). Galvanized sheet metal is well suited for the subsequent powder coating, as the surface is uniform and has almost no surface structure (flowers). Workpieces that are subject to high loads, such as screws, on which a plastic coating would peel off, are often then subjected to chromating to improve corrosion protection .
Small areas can also be treated on an outpatient basis by soaking a tissue or sponge with an electrolyte and placing it between the zinc and the workpiece. The zinc must be connected to the positive pole and the iron workpiece to the negative pole of a power source.
Mechanical galvanizing
For hardened parts that are extremely sensitive to hydrogen embrittlement , mechanical coating is prescribed in some specifications. Here, in a mixer, zinc dust with glass balls is practically hammered onto the parts to be coated without the effect of heat. Since it is not an electrolytic process, no hydrogen is produced that could penetrate the steel part. Depending on the part geometry (e.g. hexagon socket), the abrasion resistance may be somewhat limited. The high-gloss surfaces of the electroplating cannot be reached. Layer thicknesses of up to 50 µm can be produced.
This form of galvanizing is often used for disc springs or fastening clips.
Film galvanizing
Machining
If the zinc layer is damaged during subsequent processing of galvanized objects, the protective effect can be impaired.
Metallic connections can be made using inert gas soldering (MSG soldering), laser soldering and certain welding processes .
When re-galvanizing weld seams, DIN EN ISO 1461 (layer thickness) and DIN EN 1090-2 (preparation) must be observed. Flame spraying and zinc dust coating are possible. Zinc sprays are used for less protective requirements.
Post-treatment of galvanized surfaces
Galvanized steel parts are very well protected against corrosion (red rust ) by the zinc layer . However, the zinc layer itself is exposed to corrosion, often through contact corrosion. The influence of the maritime climate can also quickly lead to zinc corrosion ( white rust ).
Additionally applied passivation layers and seals delay the corrosion of zinc or zinc alloy coatings and thus also improve the resistance to base material corrosion.
Chromating processes have been developed especially for electro-galvanized parts , which differ in the degree of corrosion protection and color. Some chromate layers contain toxic chromium (VI). Chromium (VI) -free processes such as thick-film passivation have recently been developed.
Duplex system
Duplex systems are a corrosion protection system that consists of galvanizing in combination with one or more subsequent coatings. The coating can be carried out both as a wet and as a powder coating. Duplex systems with wet coatings are regulated in DIN EN ISO 12944-5, duplex systems with powder coatings are regulated in DIN 55633. Galvanizing and coating complement each other. The galvanizing is protected from atmospheric and chemical influences by the overlying coating. This increases the service life of the galvanizing, which is subject to constant wear when exposed to direct weathering.
Damage to a coating applied directly to steel has far more serious consequences, as an intermediate galvanizing prevents the coating from infiltrating and detaching from corrosion products from the iron.
Due to the so-called synergistic effect between the galvanizing and the coating, the total protection period of a duplex system is 1.2 to 2.5 times as great as the sum of the respective individual protection periods of galvanizing or coating.
Repair of damaged zinc coatings
If galvanized material is damaged or processed - for example by scratching, drilling, grinding or welding - the zinc coating must be repaired. Suitable repair methods according to DIN EN ISO 1461 are: thermal spraying with zinc, the application of suitable zinc dust coatings and zinc-based solders. In the case of zinc coatings applied by batch galvanizing, the layer thickness of the repaired area must be at least 100 µm. Zinc sprays are generally unsuitable for repairing damage to individually galvanized steel, as the required layer thickness of 100 µm is not achieved even if the damaged area is sprayed over several times.
Web links
- Information portal on corrosion protection through hot-dip galvanizing
- Sherardizing in practice / Sherart bv NL-Helmond
Individual evidence
- ^ Brockhaus ABC chemistry. VEB FA Brockhaus Verlag Leipzig 1965, p. 1484.
- ↑ a b Overview of galvanizing processes and table: Galvanizing is not galvanizing , industrial association for hot-dip galvanizing
- ↑ Lexicon - technical terms from the construction sector: Sendzimir galvanizing ( Memento of the original from April 10, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , LKG - engineering office for structural engineering
- ↑ arc spraying. Oerlikon, accessed on April 12, 2019 .
- ↑ Wire flame spraying. Oerlikon, accessed on April 12, 2019 .
- ↑ a b Lexicon - technical terms from the construction sector: Galvanizing ( Memento of the original from April 10, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , LKG - engineering office for structural engineering
- ↑ "Oldtimer Markt" magazine, special issue Oldtimer Praxis No. 62 - Rust Protection, p. 95, October 2018
- ↑ See also: Rust # Reduction of the potential difference in local elements
- ↑ Table "Service life of components for life cycle analysis of the assessment system for sustainable building (BNB)" , German Federal Ministry for Transport, Building and Urban Development
- ↑ "Oldtimer Markt" magazine, special issue Oldtimer Praxis No. 62 - rust protection, p. 92 f., October 2018
- ↑ See also: Rust # Reduction of the potential difference in local elements
- ↑ "Oldtimer Markt" magazine, special issue Oldtimer Praxis No. 62 - rust protection, p. 94, October 2018
- ↑ a b Gerhard Jokisch, Bruno Schütze, Werner Städtler in: Author collective: The basic knowledge of the engineer. VEB Fachbuchverlag Leipzig, 1968, pp. 991–1163, there p. 1048.
- ↑ "Oldtimer Markt" magazine, special issue Oldtimer Praxis No. 62 - rust protection, p. 94, October 2018
- ↑ With a piece of zinc a few square centimeters in size, a voltage of 12 volts and a current of at least 20 amperes, layers of several micrometers can be achieved within a few minutes. See: "Oldtimer Markt" magazine, special issue Oldtimer Praxis No. 62 - rust protection, p. 95 ff., October 2018
- ↑ a b Stéphane Itasse: Welding galvanized steel . Machine market. 11th August 2017.
- ↑ Worksheet for hot-dip galvanizing G.2 WET COATING OF HOT-GALVANIZED STEEL. In: Worksheet hot-dip galvanizing. Retrieved May 18, 2020 .
- ↑ Worksheet for hot-dip galvanizing G.3 POWDER-COATING OF HOT-GALVANIZED STEEL. In: Worksheet hot-dip galvanizing. Retrieved May 18, 2020 .
- ↑ Information on duplex systems , industrial association for hot-dip galvanizing; accessed in October 2016
- ↑ E.4 Professional repairs. In: Hot-dip galvanizing worksheets. Institute for hot-dip galvanizing, accessed on May 26, 2020 .
