One of the important issues in the design of connections is the question of how, how to protect steel fasteners - bolts, screws, nuts, washers - against corrosion. Corrosion protection of steel fasteners is mandatory, as, if the bolt or screw rusts and bursts, the integrity of the whole structure may be violated.
Hot dip galvanizing is most often used to protect steel fasteners from corrosion.. Hot-dip galvanizing ensures maximum durability of the protective coating in atmospheric conditions, in concrete, in soil and in water. It does not require maintenance for many years (cm. more details here).
Surface preparation of steel fasteners
As with any coating, the first and very important operation is surface preparation.. Hot dip galvanizing has a "built-in" quality control: the reaction between zinc and steel just won't go, if the surface is not clean enough.
The process of surface preparation of steel fasteners includes immersion in a series of working solutions.
- The first solution is alkaline (hot lye) to remove organic contaminants.
- From the wash, the fastener enters a bath with an acid solution for acid etching of the surface.. Scale and rust are removed in this bath.
- Third working bath – the last stage of surface preparation is fluxing.
Fluxing removes oxides and prevents further oxidation of the prepared surface prior to immersion in the molten zinc bath..
Surface-prepared steel fasteners are loaded into a perforated drum. Drum with fasteners completely immersed in molten zinc. After the metallurgical reaction is completed and the screws and nuts are completely coated with zinc, the drum is removed from the bath and the zinc flows out of it back into the bath. Then the drum is rotated around its axis, how to remove excess zinc from the fastener surface. This provides a smooth zinc coating surface and clean threads on screws and nuts..
Quality control is the final operation of hot dip galvanizing fasteners. With proper surface cleaning, this control is very simple.: if the cover looks good, and its quality is good.
Properties of galvanized steel fasteners
- Full and continuous coverage. Complete immersion of the product in molten zinc provides 100 %-corrosion protection with a continuous coating of uniform thickness, including corners and edges, internal cavities and threads.
- Coating thickness. Hot dip galvanizing on steel fasteners is usually between 40 to 85 μm.
- Cathodic protection. Unlike other barrier coatings, such paints, zinc is a sacrificial metal and corrodes to protect the underlying steel from it.
- Adhesion strength of the coating with the base. hotter zinc coating is extremely hard to damage, since its adhesive strength on fasteners is about 25 MPa.
- Hardness. Hot dip galvanizing includes zinc-iron layers, which are formed during the galvanizing process. The hardness of these layers is higher, than steel itself, on which they are located. These wear-resistant layers make the hot-coat highly resistant to damage when bolts and nuts are tightened..
- Temperature range. Galvanized steel fasteners work in a wide temperature range: from prolonged exposure to arctic climates to extreme temperatures around 200 ºС in production equipment.
- Colorability. To increase corrosion resistance and improve the decorative appearance, galvanized steel sheets are often additionally painted with special paints.. Before painting, the surface of the sheets is subjected to preparation.
New zinc coating is too smooth for paint application. Therefore, it is degreased and pickled with an alkaline solution.. Sometimes the zinc coating is subjected to a phosphate treatment to create a zinc phosphate conversion coating..
Old zinc coatings are already naturally "rough" and therefore require only degreasing or alkaline solutions or solvents.. Combination of hot dip galvanizing and painting provides long lasting protection against corrosion.
- Maintainability. Hot dip galvanizing is an industrial process, which does not depend on weather conditions and very rarely needs repair. but, if the need for repair arises, the zinc coating is easily restored with simple methods. One method for repairing zinc coatings is to heat the surface to at least 315 ºS, but not higher 400 ºS. Zinc is applied to the heated surface by rubbing the surface with a zinc rod or applying zinc powder to it.. Another method of repair is painting the surface with special zinc paints..
Bimetallic metal pairs
As with all steel products, it is very important for steel fasteners to be careful when using dissimilar materials.. When two materials are in contact, they form a corrosion cell, which is also called a bimetallic pair. In a bimetallic pair, metal, which is more anodic according to galvanic range will corrode to protect the other metal in the pair. This phenomenon must be taken into account when choosing the fastener material.. In addition to the relative arrangement of metals in the galvanic series, the relative area of the two materials is of great importance., in contact.
Galvanized bolt on bare steel sheet
for instance, zinc coating on galvanized bolt, which holds two large bare steel sheets together, very quickly collapsed and disappeared, since zinc is anode to steel, and the relative area of the bolt is small compared to the area of steel sheets.
Bare bolt on galvanized steel sheet
Otherwise, when the sheets are galvanized, and the bare bolt will not cause severe corrosion. In this case, although zinc is also anode to steel, its area on the steel sheets is much larger than the area of zinc on the bolt.. Therefore, the bolt will not corrode at all., and the zinc coating on steel sheets will slowly degrade.
It is always recommended to use galvanized fasteners to connect galvanized structures.. If there is a need to use dissimilar metals, then it is necessary to isolate the two metals from each other, gasket, which does not conduct electricity.
Increasing the size of bolts and nuts
Hot dip galvanizing increases the thickness of the steel by 50-200 μm. When designing holes, it is necessary to provide for both an increase in the thickness of the galvanized bolt, and reducing the hole in the galvanized part.