Technical Info Sheet #7
Before a steel object can be hot-dip galvanized, the steel surface must be cleaned and made suitable. This is done by chemically pretreating the objects in the galvanizing plant. This chemical pretreatment consists of degreasing (removing thin layers of grease and oil, cutting, punching and drilling oil), pickling (removing rust and mill scale) and fluxing (allowing steel surface to react with the liquid zinc).
material delivery
The unique properties of hot-dip galvanized steel originate in a chemical reaction that takes place between iron and zinc. This reaction is only possible if there is optimum contact between steel surface and zinc in the zinc bath.
Proper cleaning, before the materials are left in the zinc bath, is necessary to obtain a completely closed zinc layer. If the steel is not adequately cleaned, ungalvanized spots will develop, which are visible as black spots.
In most cases, the galvanizer’s chemical pretreatment is sufficient to achieve the required cleanliness of the steel surface. However, it is up to the client before galvanizing, to deliver the materials to the galvanizing plant sufficiently clean. One can also decide, in case of an aesthetic application for example, to have the material blasted before delivery to the galvanizing plant. One can also consider having the steel already blasted before starting the production of the object.
CONTAMINANTS THAT CANNOT BE REMOVED
Contaminants that cannot be removed or limited by chemical pretreatment are:
These contaminants must be removed before delivery to the galvanizing plant
. This is usually done mechanically (blasting, scraping, sanding, grinding). However, some of the above contaminants are difficult to detect (such as paint, welding sprays, silicone and sticker adhesive residues). The client should be well aware that the galvanizing plant cannot possibly inspect all materials and often does not have the ability to perform additional work.
DEEP RUSTED STEEL
Heavily corroded steel due to rusting is often best not galvanized. This is because the final surface may become pockmarked which may lead to comments about its appearance. In addition, there is a risk that the normal chemical pretreatment will not completely remove this rust, resulting in ungalvanized spots. It is advisable to discuss this with your contact person at the galvanizing plant.
WALSFOUTS
The construction profiles to be galvanized are brought into their final shape by rolling. During rolling, the steel surface may be disturbed. This is called over rolling, splinters and doubling. These distortions are not visible to the naked eye on untreated steel. After galvanizing, however, sharp points protruding from the surface are visible (see Figure 4). In case of aesthetic applications, it is desirable to take this into account when ordering the steel. After all, the galvanizing plant has no influence on the roughness caused by these deviations on the steel surface.
MASKING OF SURFACES WHERE NO ZINC COATING SHOULD BE FORMED
Some products must remain partially ungalvanized. Examples include screw threads, places where welding is required, and so on. These places can be smeared with a suitable covering agent or mechanically de-galvanized afterwards. Make arrangements for this with the galvanizing plant.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance.
EN ISO 14713 part 2
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in con-structions against corrosion – Part 2: Hot dip galvanizing
TECHNICAL DATA SHEET 18
Influence of chemical composition on zinc coating formation
