The thickness of hot-dip galvanized layer is the total thickness of the zinc and zinc-iron alloy coating on the steel surface, expressed in km. Hot-dip galvanized steel pipe is the only galvanized steel pipe that can be used for water transportation cold drawn seamless steel tube. It has not been eliminated or controlled now because the galvanized steel pipe has a thick galvanized layer and will not be rusted for a long time. In other cases, the safety is high, and the thickness of the steel pipe galvanizing is carried out in accordance with national standards. Let us look at the specific standards.
According to the main surface area of the hot dip galvanized parts, the thickness of the coating measured by the test should meet the following requirements:
For parts with a major surface area greater than 2 m (ie large pieces), the average thickness of the plating measured in all basic measuring surfaces (see 3 – 7) of each part in the sample shall not be less than Table 2 or Table The minimum value of the corresponding average coating thickness in 3;
Note: The length of corrosion protection of the hot dip galvanized layer is roughly proportional to the thickness of the coating. For parts that are serviced under extremely harsh conditions and/or require longer service life, the coating thickness requirements may be higher than those specified in this standard. However, the thickness of the galvanized layer is limited by factors such as the chemical composition of the substrate, the surface condition of the part, the geometry of the part, and the hot dip plating process parameters.
1. Formation mechanism of hot-dip galvanized coating
Hot dip galvanizing is a metallurgical reaction process. From a microscopic point of view, the hot dip galvanizing process is two dynamic balances: heat balance and zinc-iron exchange balance. When the steel workpiece is immersed in molten zinc liquid at about 450 ° C, the workpiece at normal temperature absorbs the heat of the zinc liquid. When it reaches 200 ° C or higher, the interaction between zinc and iron gradually becomes obvious, and zinc penetrates into the surface of the iron workpiece. As the workpiece temperature gradually approaches the temperature of the zinc liquid, an alloy layer containing different ratios of zinc to iron is formed on the surface of the workpiece to form a layered structure of the zinc coating layer.
Different alloy layers exhibit different growth rates over time. From a macroscopic point of view, the above process is characterized in that the workpiece is immersed in zinc liquid, and the zinc liquid surface appears to be boiling 150×150 mild steel square pipe. When the zinc-iron reaction is gradually balanced, the zinc liquid surface gradually calms. When the workpiece is raised with a zinc liquid surface and the workpiece temperature is gradually lowered to below 200 ° C, the zinc-iron reaction stops, the hot-dip galvanized coating is formed, and the thickness is determined.
2. hot-dip galvanized coating thickness requirements
The main factors affecting the thickness of zinc coating are: matrix metal composition, surface roughness of steel, active silicon and phosphorus content and distribution in steel, internal stress of steel, geometry of workpiece, hot dip galvanizing process. The current international and Chinese hot-dip galvanizing standards are divided according to the thickness of the steel. The thickness of the zinc coating and the local thickness should reach the corresponding thickness to determine the corrosion resistance of the zinc coating. For workpieces with different steel thicknesses, the time required to achieve thermal equilibrium and zinc-iron exchange balance is different, and the thickness of the formed coating is also different. The average thickness of the plating in the standard is an industrial production experience value based on the above-described galvanizing mechanism, and the local thickness is an empirical value required in consideration of the unevenness of the thickness distribution of the zinc plating layer and the corrosion resistance requirement of the plating layer. Therefore, the ISO standard, the American ASTM standard, the Japanese JIS standard, and the Chinese standard have slightly different zinc coating thickness requirements, which are similar.
3. The effect and influence of the thickness of hot-dip galvanized coating
The thickness of the hot-dip galvanized coating determines the corrosion resistance of the plated part. For a detailed discussion, please refer to the relevant data provided by the American Hot Dip Galvanizing Association. The user can choose a zinc coating thickness above or below the standard. For thin steel sheets with a smooth surface of 3 mm or less, it is difficult to obtain a thick coating in industrial production. In addition, the thickness of the zinc coating which is not commensurate with the thickness of the steel affects the adhesion of the coating to the substrate and the appearance quality of the coating. Excessively thick coatings can cause the coating to be rough and easily peeled off, and the plated parts cannot withstand collisions during handling and installation.
If there are more active elements such as silicon and phosphorus in steel, it is very difficult to obtain a thin coating in industrial production. This is because the silicon content in the steel affects the growth mode of the alloy layer between zinc and iron, which will make the phase zinc-iron alloy layer. Rapid growth and pushing the phase to the surface of the coating results in a rough and matte surface, forming a dark coating with poor adhesion.
Therefore, as discussed above, there is uncertainty in the growth of the galvanized layer. It is often difficult to obtain a certain range of coating thickness in actual production. The thickness specified in the hot-dip galvanizing standard is an empirical value generated after a large number of experiments. Taking care of various factors and requirements, it is more reasonable, the standard content of hot-dip galvanized layer thickness is here, I hope to help everyone.