Frequently asked questions - Hot dip galvanizing


What is hot dip galvanizing?

Galvanizing is a process for corrosion protection of steel that consists of specific surface preparation and dipping it into a bath of alloyed molten zinc at 450 ° C for 3-10 minutes. Following this process, on the steel surface a coating is formed due to the reaction between Fe and Zn having a metallurgical bond with the substrate, consisting of several layers of intermetallic compounds with different concentrations of zinc. This coating has a very good resistance to corrosion, abrasion, impact, thermal shock a.o.


Does hot dip galvanizing need special surface preparation?

Galvanizing requires no special preparation of the surface, this being achieved in the galvanizing process. Welding slag, paint and grease abundance though can not be removed through the specific surface preparation steps and must be removed before being sent to the galvanizer.


What are the usual turnover times from galvanizing?

Most of corrosion protection systems depend on adequate temperature or humidity conditions to be correctly applied. Galvanizing is a process whose aplication does not depend on environmental factors, can be done 24/7, 365 days a year, rain or good weather. Hot dip galvanized steel can be galvanized, delivered on site and installed in the same day but the usual processing times are between 3 and 5 days depending on the quantity and typology of parts.


What is the lifespan of hot dip galvanized products?

Hot dip galvanized steel has good corrosion resistance in most types of atmospheres. Usually the life of hot dip galvanized steel exceeds 70 years depending on the thickness and atmospheric corrosivity.


Does the coating present sufficient adhesion to the substrate and abrasion resistance?

The metallurgical bond that the coating forms with the substrate layer at the immersion in the molten zinc confers an excellent adhesion and abrasion resistance to the coatings obtained by hot dip galvanizing. Zinc intermetallic layers formed during the galvanizing process have a higher hardness than steel substrate.


Do hot dip galvanized parts need special storage and transport precautions?

A hot dip galvanized metal object can be stored, transported and used in any natural weather conditions, in air, in water, in soil or on soil. In any of these circumstances the role of corrosion protection of hot dip galvanization is not affected. Instead, the appearance of coating might change. White spots may appear or different shades, zinc flower, shiny or matte surfaces etc. All these aesthetic changes of the hot dip galvanized surfaces occur due to the triggering of the mechanism by which zinc protects the steel and prevents rusting (oxidation).


Why do shiny areas and matt areas appear on the same product?

The aspect of the hot dip galvanized steel immediately after galvanizing can be bright and shiny, zinc flower, mat gray or a combination of the previous. After a period of exposure in the atmosphere, the surface aspect is uniform. The most common cause that generates a different aspect of the surface is the chemical composition of the steel. For specific concentrations of silicon content thicker coatings are obtained that determine differences in appearance. Suggested silicon content must be either less than 0.04 % or between 0.15 % and 0.25 %. Steels whose silicon content is not in this range are considered reactive steels and will form thicker zinc coatings whose appearance will be dull gray or mottled unlike the bright and shiny appearence obtained in general. Differences in aspect can also appear be due to the difference between the cooling rate of the different sections of products (different thickness) or as a result of the processing of the materials during fabrication.


What is the price difference between galvanizing and painting?

Contrary to preconceived opinions, depending on the type of product and the condition of its surface, the initial cost of hot dip galvanizing is often lower than that of painting. If we consider the cost of the entire product life cycle including maintenance costs, hot dip galvanizing is the most economical method of corrosion protection. Painting requires maintenance and reapplication at regular time intervals, which on long term proves extremely costly.


Why are the air and venting holes necessary?

The main purpose of the air and venting holes is to allow therelease of air and gas retained in the inner hollow sections and to enable solutions used in the surface preparation process and molten zinc to freely circulate on all surfaces of the product both interior and exterior and then drain back into the bath. Due to the high temperatures that occur in the galvanizing process and rapid warming there is a significant risk of explosion at the immersion in the galvanizing bath when liquids remain locked inside parts.


What are the proper sizes for the air and venting holes?

The minimum size of the holes to avoid extended zinc leakage at the withdrawal of the product from the bath or blocking in in the interior is 8mm. For products with larger inner hollow sections, larger holes (12mm +) must be executed. This will ensure a better appearance of the surface, the immersion time in the zinc bath can be better controlled like this.


Why certain parts deform after hot dip galvanizing?

Galvanizing process takes place at a temperature of 450 ⁰ C. This is a temperature at which partial stress relieving of the induced stresses during fabrication can appear that sometimes lead to deformations. Deformations might also occur due to section differences between materials welded together that cause various expansions and contractions in the material at the immersion in zinc.


Can we paint over hot dip galvanizing?

Duplex system, as painting over hot dip galvanized steel is known, is an extremely efficient corrosion protection system. Proper surface preparation is essential to avoid possible adhesion problems. Depending on the age of the zinc layer the required surface preparation is established. Hot dip galvanized steel can be painted successfully in a wide range of colors.