The presence of Si in the steel articles to be galvanized results in the formation of a thick matte gray surface layer and poor coating adhesion. The fastest and thickest coatings usually occur when the Si content is between 0.04% and 0.12%, it also depends on phosphorus levels.
In the presence of more than 0.26% Si in the steel, the Fe-Zn alloy layers grow rapidly under the Effect of Sandelin Curve. Delta and Zeta phases form at the interface, making the pre-existing Zeta phase matte by pushing it through the Zinc towards the surface. The matte gray layer, on the other hand, increases the iron loss and causes a thick and brittle layer to form. The coating on Si-killed steel is thicker than on unkilled steel. These layers can be localized or spread over the entire surface. When the silicon on the surface is not distributed homogeneously, it causes the coating to be rough. These layers increase Zinc consumption. Coatings formed in this way are mostly Zinc-Iron alloys and, although very protective, they have little resistance to mechanical damage during use.
There are Several Ways to Reduce the Effect of Si, Which Causes Thick Coating to Form:
- It can be pressed by galvanizing at high temperatures such as 530-560°C in ceramic baths.
- If more than 0.035% Phosphorus is present in the steel, the reaction rate between Fe-Zn can be reduced by galvanizing at 430°C to reduce the effect of Si and Sandelin Curve, but since the freezing temperature of Zinc is 419°C, the risk of zinc solidification arises from working at temperatures close to the freezing point.
The Real Solution to Get Rid of the Effect of Sandelin Curve is to Use Nickel in Zinc Bath
- With 0,05% Nickel in Zinc bath, Zinc consumption is reduced by limiting the growth of the Fe-Zn alloy coating layer, preventing the matte gray and rough appearance of the material.
- It allows to obtain a homogeneous and glossy surface appearance.
- It gives elasticity (ductility) to the coating.
SmartNi nickel tablets use 30% less Nickel compared to Zn-Ni alloy ingots and nickel powder. Compared to other nickel tablets, since it creates more heat shield during combustion, it provides better dissolution in zinc and less precipitation in dross.
Adding SmartNi to the zinc bath is also a pretty simple and straightforward process. The tablets readily react with the molten zinc upon addition, as burning is facilitated by the layer of wax & chemicals on the tablet.
ANI METAL is one of the leading plant suppliers in hot dip galvanizing industry, provides supply of turnkey galvanizing plants and engineering supports as well. Contact now to get more information!