The "zinc flower" of hot-dip galvanized pipe and the influence of various factors on the formation of zinc flower
The surface of the hot dip coating of seamless steel pipe will form a crystal pattern of the coated metal during the solidification process. This crystal pattern is particularly obvious on the galvanized seamless pipe, showing a beautiful zinc crystal appearance, so it has become an important feature of the appearance of hot-dip galvanized seamless pipe, often called "zinc flower". Zinc flowers occur when other metal impurities are present in the galvanizing solution. The size and shape of the zinc flower depends on the type and content of impurities in the zinc solution, common impurities include lead, aluminum, antimony, bismuth and tin.
Zinc flower usually refers to the appearance of the grain formed after the hot galvanized sheet is pulled out of the zinc pan and the zinc layer is cooled and solidified. The size, brightness and surface morphology of zinc flowers depend on a series of factors, but are mainly related to the composition of zinc layer and cooling method. The principle of producing zinc flowers is to try to reduce the solidification temperature of zinc liquid.
According to the diameter of the zinc flower, it can be divided into the following three categories:
(1) Large zinc flowers
Zinc flowers with a diameter greater than 3mm that can be distinguished by the naked eye are called large zinc flowers, and they are also called normal zinc flowers or natural zinc flowers. The internationally recognized best size is 8~12mm.
Lead and zinc are symbiotic deposits in nature. When smelting by fire process, the density of lead liquid is high in the lower part and zinc liquid density is low in the upper part, so it can be separated. However, the saturation concentration of lead in zinc liquid is 2%, so there is a large amount of lead in crude zinc. Hot galvanizing has a history of more than 150 years, when there was no modern refining technology, only the use of crude zinc for hot zinc plating production, because the zinc liquid is mixed with lead, so the hot galvanized sheet will appear zinc flowers.
Zinc flowers that are more than 3 mm in diameter and visible to the naked eye are called large zinc flowers, and some people call them normal zinc flowers or natural zinc flowers. The internationally accepted optimal size range is 8 to 12 mm.
(2) Small zinc flowers
Zinc flowers between 1 and 2 mm in diameter are called small zinc flowers. When the hot-dip galvanized seamless pipe is removed from the galvanized pool, if the water mist or zinc powder is sprayed to the unsolidified zinc layer, a water mist and a zinc powder particle will be formed, and then a crystallization center will be produced. This will cause the number of zinc flowers per unit area to increase and become smaller, so the surface of the galvanized seamless pipe will form small zinc flowers visible to the naked eye. Traditionally, the product produced by this process is known as small zinc flowers. However, because this process often leads to uneven distribution of zinc flowers, inconsistent surface color, and high production costs, the current application of this process is not widely.
(3) No zinc flowers
The phenomenon of no zinc flowers on the surface of the pipe is called no zinc flowers or zero zinc flowers, and this effect is achieved by adjusting the chemical composition of the zinc liquid. In the process of hot galvanizing, only a certain proportion of aluminum is added to the zinc solution to avoid the addition of lead, antimony and tin, and a product without zinc flowers can be produced. This process not only does not increase equipment investment, nor does it increase production costs, but also ensures a uniform surface appearance. As a result, this process has been widely used around the world.
(4) Finish zinc flowers
Large zinc flowers are not allowed to enter the finishing process. Small zinc flower or zinc-free flower products are processed through the finishing process, which is called finishing zinc flower.
There are many factors affecting the size of zinc flowers. When the composition of zinc liquid has the conditions for the formation of zinc flowers, the following factors affect the size of zinc flowers:
(1) Raw materials: the thicker the raw steel plate, the smaller the surface roughness, the larger the zinc flower.
(2) Cooling rate: the faster the cooling rate, the shorter the crystal growth time, the smaller the zinc flower.
(3) Number of crystal nuclei: The size of zinc flowers is closely related to the number of crystal nuclei of zinc flowers. When the surface of the steel base is rough or the surface cleanliness is poor, the zinc flower is relatively small. Zinc grains are formed on the steel base, and the more the crystal nuclei, the smaller the zinc flowers. This principle is used in the production process of small zinc flowers.
(4) Other factors (production environment, etc.)
(5) Influence of zinc layer thickness. The thicker the zinc layer, the longer the solidification lasts, so the larger the zinc flowers will be generated. When blowing hot dip galvanizing, if the blowing pressure is less than 0.02MPa, the coating is like not blowing, although the coating will appear waveform and fluctuation, but the surface zinc flower is very large. When the zinc layer is thin, the zinc layer solidifies quickly, and the zinc flowers can only form small zinc flowers because they have no time to grow up. Practice has proved that if the injection pressure is greater than 0.08MPa, there is almost no zinc flower on the surface.
In order to obtain small zinc flowers, water droplets with a diameter of less than 0.1mm are immediately sprayed onto the surface of the galvanized sheet when the zinc solution is near the freezing temperature to form fine and uniform crystalline nuclei. In order to get the finished zinc flower, then use less than 1% reduction to smooth rolling. Small zinc flowers are suitable for painting, and smooth zinc flowers are suitable for deep stamping.
The main factors affecting the uneven zinc bloom include: ratio of two alloying elements in the composition of zinc liquid, gas knife control technology, strip temperature, impurity content in gas and combustion air, strip surface cleanliness and furnace reduction state, roll nodding, strip scratches and surface oxidation of raw materials, strip into the zinc pot temperature, zinc liquid temperature, cooling temperature after plating, etc.
In nature, lead and zinc usually exist as symbiotic deposits. When smelting by fire, the liquid density of lead is larger and located below, while the liquid density of zinc is smaller and located above, so separation can be achieved. However, the saturation concentration of lead in the zinc solution is 2%, which results in a large lead component in the crude zinc. Hot dip galvanizing has more than 150 years of history, at that time there was no modern refining technology, only the use of crude zinc for hot dip galvanizing production, because the zinc liquid included lead, so the hot dip galvanizing pipe will inevitably appear zinc flowers. Therefore, there has been a misunderstanding for a long time that the hot-dip galvanized seamless pipe must have zinc flowers, and if there is no zinc flowers, it is not a real hot-dip galvanized seamless pipe. However, from the long-term use point of view, the performance of hot-dip galvanized seamless pipe without zinc flowers will actually be better.
In fact, zinc flowers only have ornamental value, no practical value, on the contrary, zinc flowers in corrosion resistance is not good, the surface will appear convex and concave level after painting. Therefore, the home appliance and automobile industry tends to choose small zinc flower (no zinc flower) products, large zinc flower zinc plated plate due to its good corrosion resistance, excellent processing performance, low production wood, beautiful appearance and other advantages. It should be widely used in construction, light industry and other industries.
