What are the considerations to be taken during the design of steel structures to be galvanized?
As its name suggests, steel is dipped in a molten zinc bath during the hot-dip galvanizing process. Although the process appears simple, hot-dip galvanizing is different from that of other corrosion protection coating systems or paint, and therefore has special requirements during the design phase in order to ensure a high quality zinc coating. ASTM A385, Standard Practice for Providing High Quality Zinc Coatings (Hot-Dip) specifies design recommendations that should be followed during the manufacture of steel to be galvanized.
Hot-dip galvanizing is a versatile method that can provide high quality coatings for most iron-containing materials. This includes carbon steel, cast steel, cast iron, hot rolled steel, cold rolled steel, stainless steel and even steel exposed to the elements. However, there are certain steel attributes that can affect the characteristics of the galvanized coating.
- Carbon levels should be less than 0.25%
- Manganese levels should be less than 1.3%
- Phosphorus levels should be less than 0.04%
- Silicone levels should be less than 0.04%, or between 0.15% and 0.22% (see Sandelin Curve)
- Silicone equivalent should be less than 0.04%, or between 0.15% and 0.22%
Silicone equivalent = silicone content + [2.5 x phosphorus content]
Accelerated growth of galvanized coating may give a matte gray finish, but this is mainly seen as a cosmetic problem. Other than the rare cases where the coating is too thick, the performance of steel corrosion protection is not affected by its appearance.
While it is important to keep the steel’s chemistry in mind when selecting steel for your galvanizing project, it is much more important to communicate with your galvanizer throughout the design process. As is the case with most design considerations, your galvanizer has more experience with the matter at hand and may be able to offer solutions that are not mentioned here. If the galvanizer is notified before the project begins, reactive steel galvanizing can be done with good results.
Source: AGA - American Galvanizers AssociationReturn to news