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Hot dip galvanizing
of steel.


Galvanizing

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The best steel
protection.


Protection


1.The chemical composition of steel

The recommended limits for silicon content in the steel are below 0.04% or between 0.15% and 0.22%. Attention should be paid when silicon content in the steel is 0.05%, because the curve in this range rapidly turns upwards, which can lead to an excessively thick zinc coating layer and a grey look.

Impact of silicon content in the steel on the coating weight and looks

Zinc coating weight, g/m²

grafik ru
 

Silicon content in the steel, %

2.Surface condition
If before the galvanization the steel surface is very rusty and after mechanical or chemical cleaning it is uneven, after the galvanization all imperfection will still be visible.

3.Previously coated steel
Steel products are often coated with preservative oils or colorants. If so, the colour must be removed before galvanization by abrasion, while the oils should be removed chemically. Special attention to these aspects must be paid in case of hollow steel products in order to ensure the quality of the final product.

4.Type of hardwareIf the metal fabrication has rivets, brazing or cast iron or non-metalic parts, it may cause serious problems in the galvanization process like low quality coating or even an explosion in the zinc bath.
Only steel elements may be hot dip galvanized!
High strength steel may cause problems during the heat treatment when coming into a contact with the molten zinc.

5.Quality of welding
Weld slag may leave adverse effect on the quality of galvanization. Around imperfectly welded joints gray areas may develop affecting the appearance of the surface and the anti-corrosive qualities. Also air bubbles may appear around the joints. Abrasion before galvanization may prevent such defects.

6.Very thick steel products
In case of very thick steel products (more than 100mm) problems may arise, because, when submerging the metal in the hot zinc bath a congealing effect takes place, which may adversely affect the preflux coating and the final result.

7.Ventilation and drainage holes
Proper distribution of these holes very much affects the galvanization process and quality, especially in case of hollow metal fabrications.

8.Transportation
In case of heavy metal elements, during the galvanization chains are used and after the process chain-marks may appear which is unavoidable.

9.White rust
Very common condition for galvanized surfaces, characteristic to recently galvanized steel. Manifested in the form of pale, uneven spots on galvanized surfaces; can be easily washed away. These are the by-products of zinc oxidation that appear, if freshly galvanized steel comes into contact with water and carbon dioxide. The white matter are zinc reaction products – zinc hydroxide and zinc carbonate. The galvanization quality is secured by the zinc oxide layer that forms rather slowly (approx. in 2 weeks’ time) and is stable. The white spots should be rinsed with water or scrubbed with a brush. A freshly galvanized product should be places in a dry, well-ventilated area. If there are large damages to the surface after the mechanical processing, the corrective measures must be taken by covering cleaned areas with epoxy zinc colours so that the thickness of the colour layer would be no less than 100 microns.

10.Flow downs and bleeds
You may see flow downs on galvanized products that appear when the elements are taken out of the bath and the zinc is rapidly cooling down. This cannot be avoided; however, the flow downs can be managed during the mechanical processing stage after the galvanization. During the chemical processing stage when the steel element is processed with hydrochloric acid, in cases the steel product has some additional details, e.g. flat iron, welded on, the hydrochloric acid reaction by-products may remain under these details and appear after the galvanization in a form of brown, rusty flow downs. This condition is called bleeding and it is not considered a serious defect, as it can be prevented by mechanically scrubbing and rinsing the area.


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