As a continuous coil production process, galvanized sheet can be divided into two methods: galvanizing and hot-dip galvanizing. The galvanizing method is to electrodeposit a layer of zinc metal or zinc alloy on the metal surface. Hot dip galvanizing,
also known as hot dip galvanizing, is to impregnate protected metal products in molten Zm metal to form a protective metal coating on its surface. Compared with electroplating, the hot metal coating is thicker and has longer service life under the
The corrosion of hot-dip galvanizing layer on steel surface is equivalent to that of pure zinc. The corrosion process of zinc in atmosphere is similar to that of steel under atmospheric conditions. Chemical oxidation corrosion occurs, and electrochemical
corrosion occurs on zinc surface with water film condensation.
In neutral or weak acidic atmospheric environment, the corrosion products of galvanized steel sheets formed by corrosion are insoluble compounds (zinc hydroxide, zinc oxide and zinc
carbonate). These products will precipitate in the form of precipitation, forming a compact thin layer with a certain thickness, which is not easy to dissolve in water, appendage and layer, generally up to 8um thickness. This kind of film has certain
thickness, is not easy to dissolve in water, and has strong adhesion. Therefore, it can act as a barrier between atmosphere and galvanized sheet and prevent further development of corrosion. When the protective galvanizing layer is damaged and some surface
of steel is exposed to atmospheric environment, zinc and iron form micro-batteries. The potential of zinc is significantly lower than that of iron. Zinc acts as an anode to protect the plate substrate from sacrificing the anode and prevent the corrosion
of the steel plate.
The color coated plate is a liquid coating, which is coated on the clean metal surface by brushing or roller coating. After heating and curing, varying thickness of the paint film can be obtained. In addition to separating metals
from corrosive media, the function of paint film can also passivate metals with the help of some pigments in paint (such as lead, zinc chromate, etc.) to achieve permanent anticorrosive effect.
According to the anticorrosion mechanism of color coated
steel sheet, organic coating (including surface coating and bottom coating) is a kind of insulating material, which separates the substrate from the corrosive medium in order to achieve the anticorrosion effect.
However, from the microscopic point
of view, there are many pinholes in the coating. Pinholes are porous defects on the surface of the paint film. They are caused by the escape of air trapped in the wet film. The reason may be that the surface viscosity of the wet film and the interfacial
tension of the bubble wall are too large. In general, the average diameter of pinholes is 10-10 um. Water, oxygen and other ions in the environment can enter the film through pinholes and form local batteries, resulting in 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 submerged corrosion is
(1) Water, ions and oxygen infiltrate into organic coatings.
(2) The conductive path between matrix metal and solution is the coating with lower resistance.
3) When the corrosion reaction proceeds, under the conductive part of the organic coating, the concentration of ions increases and solid corrosion products
4) The dissolution of iron reduces the pH value near the reaction site.
5) the increase of pH value in cathode area and the decrease of pH value in anode area are harmful to organic coating.