Technical Sheet #13
Hot-dip galvanizing typically increases the weight of black (ungalvanized) steel by 3 to 12%. The weight increase is determined by the combination of a series of factors; including the following: (average) wall thickness of the steel structure or object, steel composition of the composite parts of the structure or steel object, composition of the zinc bath, surface condition of the steel, temperature of the molten zinc, mass of the object to be galvanized, and immersion time in the zinc bath.
THICKNESS
The reason is obvious: the smaller the wall thickness of the steel, the lighter the object is. The surface area of the object relative to its weight is large. The larger the surface area to be galvanized and the smaller its weight, the greater the weight increase.
The reverse is also true: for the same steel composition, one can expect a thicker coating (and thus a greater weight increase) on thicker-walled steel. The requirements as stated in the International Galvanizing Standard EN ISO 1461 explicitly take this into account.
COMPOSITION OF THE STEEL
Certain additives to the steel and their combination can greatly influence the “reactivity” of the steel and the molten zinc. The most influential steel additives are Si, P and Mn. For example, the influence of the amount of Si present in the steel is known to cause the so-called “Sandelin effect” (see Technical Data Sheet 18).
Certain percentages of the aforementioned additives make the steel very reactive with respect to molten zinc, which leads to large coating thicknesses and thus to large weight increases of the steel during hot-dip galvanizing. In such cases, thicknesses can be as much as 2.5 times greater than the layer thickness on non- (or little) reactive steel with the same wall thickness.
COMPOSITION OF THE ZINC BATH
To reduce the influence of reactive steel on the coating thickness of zinc, zinc alloys have been developed, for example, with nickel, aluminum, tin and bismuth. The use limits the additional weight gain of steel types that are reactive compared to unalloyed zinc. The percentages of these substances in the total zinc bath composition are very small, together about 1.5%. The zinc bath composition is continuously monitored by the galvanizing companies affiliated with Zinkinfo Benelux.
TEMPERATURE OF THE ZINC BATH
Assuming a “normal” temperature galvanizing plant, the zinc bath is always 450°C (usually +/- 1 or +/- 2°C). At this temperature, the zinc is liquid, high enough to bring the object up to process temperature and not high enough to affect the steel galvanizing bath. Practically every galvanizer thus uses 450°C as the zinc bath temperature. Should the temperature be higher, it would damage the galvanizing bath and be a drain on the budget in terms of energy costs. A lower temperature produces a somewhat more viscous zinc runoff resulting in zinc thickening, zinc points.
SURFACE CONDITION OF THE STEEL
As a rule, the rust and mill scale present on the black steel are removed by pickling. If, instead of pickling, one cleans the steel by blasting, one gets a steel surface with a larger specific area and a different surface structure. This gives zinc coatings that are more uniform and can be up to 20 μm thicker than on the same steel that has only been pickled.
MASS OF THE OBJECT AND IMMERSION TIME
The time an object is immersed in the zinc bath (immersion time) somewhat determines the coating thickness and very much depends on the steel composition, surface condition and, of course, the weight of the object. This is because the object will be heated to 450°C and then cooled. If the object is very massive and heavy, the heating takes a long time but the cooling will also take a longer time. From 200°C, the zinc-iron reaction will already start. It stops after galvanizing again at 200°C. The period of the diffusion reaction is so long that a greater zinc coating thickness can be expected.
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 structures against corrosion – Part 2: Hot dip galvanizing
TECHNICAL SHEET 18
Influence of chemical composition on zinc coating formation
