What is galvanizing and galvanizing
By Galvanizing steel is provided with a thin layer of zinc to protect it from corrosion.
There are a number of different methods of coating a steel component with a zinc or zinc alloy layer:
Because of the great differences between the processes, the term "galvanizing" is not very meaningful. The exact specification of the galvanizing process is sometimes omitted because a specialist can often guess the respective process from the context - a terrace railing is therefore almost always hot-dip galvanized.
Coatings by a Zinc spray or the like, do not belong to the galvanization, but represent the special case of a paint.
Hot-dip galvanizing is the coating of steel parts with a massive, metallic zinc coating by immersing the pretreated steel parts in a melt of liquid zinc, the temperature of which is approx. 450 ° C.
Special cases of hot-dip galvanizing:
- piece galvanizing
- Galvanizing of small parts
- Continuous strip galvanizing
Galvanic zinc plating
The workpieces are not immersed in a zinc melt, but in a zinc electrolyte. See: Electroplating.
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 the stresses of corrosion and zinc corrosion (white rust) can occur relatively quickly, especially in a maritime climate. The occurrence of zinc corrosion can be greatly delayed and slowed down through suitable post-treatment, whereby the entire corrosion protection is extended even further until the occurrence of base metal corrosion. Such post-treatments are part of the passivation process and can be used for both galvanized and hot-dip galvanized parts. Various chromating processes have been developed especially for galvanized parts, which differ in the degree of corrosion protection and color. Some of these chromate layers contain toxic chromium (VI). Recently, new chromium (VI) -free processes have been developed.
According to DIN EN ISO 12944-5, duplex systems are understood to be a "corrosion protection system that consists of galvanizing in combination with one or more subsequent coatings." 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. Conversely, damage to coatings does not have any adverse effects, as the galvanizing can withstand high loads due to its high resistance and abrasion resistance. As a result, typical under-rusting of coatings cannot occur. Due to this so-called synergistic effect between the galvanizing and the coating, the total protection period of a duplex system is between 1.2 and 2.5 times greater than the sum of the respective individual protection periods of galvanizing and coating.
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 under the action of heat. Since it is not an electrolytic process, no hydrogen is produced that could penetrate the steel part.
However, the adhesive strength and the decorative appearance are much worse than with galvanic zinc plating, which is why mechanical zinc plating is very rarely used.
Zinc flake / binder systems
For some years now, coating in a dispersion of small zinc flakes and sometimes aluminum flakes has been available as an alternative to mechanical galvanizing. They are applied in a dip / spin process, dried and, depending on the process, baked at 250-350 ° C. A layer thickness of about 4-5 µm is achieved with one coating process, and the layer is not pore-tight. This is why coating is usually carried out twice and recently an additional silicate seal.
Galvanizing is a variant of flame spraying, in which a zinc wire is melted by a flame or an electric arc and applied to the workpiece using compressed air. The still liquid zinc forms a porous layer on the workpiece that has been pretreated by sandblasting, which has similarly good corrosion protection properties as that produced by hot-dip galvanizing. This layer is very absorbent due to the high inner surface. Subsequent painting therefore requires unusually large amounts of primer or filler.
Advantages of the process compared to hot-dip galvanizing are that the thermal load on the workpiece is very low and distortion can be excluded even with large areas. The disadvantage is that cavities or places that are difficult to access (container interiors, folds, etc.) cannot be reached.
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