Most steels can be satisfactorily hot-dip galvanized. However, reactive elements in the steel, in particular, silicon (Si) and phosphorus (P), have an effect on the result. An appropriate selection of composition can therefore give more consistent quality of coating with regard to appearance, thickness and smoothness. The prior history of the steel (e.g. whether hot rolled or cold rolled) can also affect its reaction with the zinc melt. Where aesthetics are important, or where particular coating thickness or surface smoothness criteria exist, specialist advice on steel selection should be sought prior to fabrication of the article.
The influence of silicon and phosphorus on steel reactivity
During steel production, silicon or aluminium is added to remove oxygen. These steels are known as “killed steels”. Steel may also be produced without these additions and are then called “rimming steels”. However, such steels are not so common today because of their lower quality.
Choice of steel
Since the type of steel, primarily the silicon content, has a large influence on the coating thickness in hot- dip galvanizing it is very important that the constructor or manufacturer are well aware of this in order to get the right result in every individual case.
Aluminium-killed steels suitable for galvanizing have a low silicon + phosphorus content, below 0.03 weight percent.
Aluminium killed steel with lower reactivity than expected
Aluminium killed steels also contain low levels of silicon, which is important for the reactivity. In recent years aluminium killed steel with so-called ultra-low silicon content, below 0.01%, and aluminium content above 0.035% has become more common. These steels have many positive properties when it comes to cutting and formability, however the low silicon content in combination with the high aluminium content makes the zinc layers thinner than stated in the hot dip galvanizing standard EN ISO 1461. If the galvanizing is performed in a nickel alloy bath, which is common today since nickel is considered to add several positive properties, the reactivity is further decreased, with thinner layers as a result. A deviation from the standard for such steels can be agreed between customer and galvanizer. If a deviation cannot be accepted, this type of steel must be blasted before galvanizing.
Risk of Sandelin Effect
As described above, it is important that the silicon + phosphorus content in the Aluminium killed steels is kept below 0.03 % if a layer of good quality should be formed. Research studies have shown that cold and hot rolled steel can behave somewhat differently in hot dip galvanizing, which is discussed below. When the finnish requirements on the galvanized surface are “normal”, an aluminum killed steel steel with silicon + phosphorus content < 0.03 % is adequate and gives acceptable surface finish on both cold and hot rolled steel.
If the appearance of the galvanized surface is of high importance, for example in architectural applications, the following expression shall be used for cold rolled steel:
Si < 0.03 and Si + 2.5 x P < 0.04 weight percent
For hot rolled steel the silicon content is even more critical, but the phosphorus content is of less importance, and the following expression could be used :
Si < 0.02 and Si + 2.5 x P < 0.09 weight percent
Silicon-killed steels suitable for hot-dip galvanizing have a silicon content above 0.14%. The reactivity, and thus the thickness of the layer, increases with increasing silicon content.
Higher coating thicknesses
If the hot-dip galvanized steel should be used in a more corrosive environment the Swedish appendix to EN ISO 1461 gives advice and instructions. The suggested silicon content is then > 0. 22%, and the coating thickness increases with increasing silicon content. If the corrosion environment is extreme, thick coatings should be used, or galvanizing in combination with paint, a so called duplex system.
Effect of heat treatment
Cold rolled steels are usually annealed after rolling. During this heat treatment silicon near the steel surface may be oxidized. This socalled ”internal oxidation” means that the free silicon content, that influence the reactivity during galvanizing, is lower than the mean value of the steel chemical analyses. Ann ealing steel with a silicon content in the range 0,15-0,21 weight percent can easily decrease the free silicon content so it falls within the Sandelin range. The steel is then very reactive and get a thick, brittle coating with poor adherence to the steel surface. The depth of the oxidized zone depends on temperature, time and atmosphere during the annealing. The oxidized zone is usually removed during pickling, but in some cases it is deeper, and it may be necessary to blast the steel before galvanizing.
The influence of silicon and phosphrous content when galvanizing cold- and hot rolled steel:
| Silicon +|
|Cold rolled steel||Hot rolled steel|
|Si+P < 0,03|| Acceptable surface finish in most cases. Shiny coating. If the appearance of the galvanized surface is very important, the following expression should be used : |
Si < 0,03 and Si+2,5P < 0,04
| Acceptable surface finish in most cases. Shiny coating. If the appearance of the galvanized surface is very important, the following expression should be used :|
Si< 0,02 and Si+2,5P < 0,09
|0,03 < Si+P ≤ 0,14||Not suitable||Not suitable|
|0,15 ≤ Si ≤ 0,21|| Thicker coatings than in standard. |
Internal oxidation may change reactivity.
|Thicker coatings than in standard.|
|0,22 ≤ Si ≤ 0,28|| Significant thicker coatings than in standard. |
| Significant thicker coatings than in standard. |
|0,29 ≤ Si ≤ 0,35||Thick coatings that may be brittle. Grey appearance.||Thick coatings that may be brittle. Grey appearance.|