Difference between galvanized steel coil and color coated steel coil
As a continuous production process of coiled materials, galvanized sheet can be divided into two methods: electrogalvanizing and hot-dip galvanizing. The method of electrogalvanizing is to coat the metal surface with a layer of Zn metal or Zn alloy by electrodeposition. Hot dip galvanizing, also called hot dip galvanizing, is to dip the protected metal products into the molten Zm metal to form a protective metal coating on its surface. Compared with electroplating, the hot metal coating is thicker; Under the same environment, its life is longer.
The corrosion of the hot dip galvanized layer on the steel surface is equivalent to the corrosion of pure zinc. The corrosion process of zinc in the atmosphere is similar to that of steel in the atmosphere. Chemical oxidation corrosion occurs, and electrochemical corrosion also occurs on the zinc surface with water film condensation.
In the neutral or weak acid atmospheric environment, the corrosion products of the galvanized layer of steel plate formed after corrosion are insoluble compounds (zinc hydroxide, zinc oxide and zinc carbonate), which will precipitate in the form of precipitation, forming a dense thin layer with a certain thickness, which is not easy to dissolve in water. It is also visible and can generally reach a thickness of 8um. This kind of film has a certain thickness, is not easy to dissolve in water, and has strong adhesion, so it can act as a barrier between the atmosphere and the galvanized sheet to prevent further development of corrosion. When the protective zinc coating is damaged and part of the steel surface is exposed to the atmospheric environment, zinc and iron form a micro battery, and the potential of zinc is significantly lower than that of iron. As an anode, zinc acts as a sacrificial anode to protect the steel plate substrate, preventing the steel plate
Color coated plate is to apply liquid paint on clean metal surface by brush coating or roller coating, and obtain paint film of different thickness after heating and curing. In addition to separating metal from corrosion medium, paint film can also passivate metal with the help of some pigments in the paint (such as lead, zinc chromate, etc.) to achieve permanent anti-corrosion effect.
From the perspective of anti-corrosion mechanism of color coated plate, organic coating (including top coating and bottom coating) is an isolating material, which separates the substrate from the corrosive medium to achieve the function of anti-corrosion.
However, from a microscopic point of view, there are many pinholes in the coating. Pinholes are pores like defects on the surface of the paint film, which are caused when the air trapped in the wet film escapes. The reason may be that the viscosity of the wet film surface and the interfacial tension of the bubble wall are too large. Generally, the average diameter of the pinhole is 10~10um. Water, oxygen and other ions in the environment can enter the film layer through the pinhole and form local batteries, causing corrosion under the film. The pinhole is inversely proportional to the thickness of the coating. That is, the thinner the thickness, the more pinholes.
The process (mechanism) of corrosion under the film is
(1) Water, ions and oxygen penetrate the organic coating.
(2) The conductive path between the base metal and solution is the coating part with low resistance
3) During the corrosion reaction, the concentration of ions increases below the conductive part of the organic coating, and solid corrosion products are generated.
4) The dissolution of iron reduces the pH near the reaction site.
5) The increase of pH value in the cathode area and the decrease of pH value in the anode area are harmful to the organic coating.