If a high-temperature workpiece after hot-dip galvanizing is directly exposed to air for cooling, the zinc layer will produce internal stresses due to sudden cooling, resulting in cracking or deformation of the coating. Water or air cooling slows down the cooling rate, stabilizes the structure of the zinc-iron alloy layer, and prevents plating defects caused by stress concentrations.
Author: Anna
I. Necessity of cooling treatment
Control of plating stress and structural stability
If a high-temperature workpiece after hot-dip galvanizing is directly exposed to air for cooling, the zinc layer will produce internal stresses due to sudden cooling, resulting in cracking or deformation of the coating. Water or air cooling slows down the cooling rate, stabilizes the structure of the zinc-iron alloy layer, and prevents plating defects caused by stress concentrations.
For example, zinc-aluminum-magnesium (ZAM) coatings need to be cooled in stages (air cooling followed by water quenching) to optimize the grain arrangement and improve the toughness of the coating.
Reduced production cycle time
Water cooling reduces the post-plating cooling time from hours to minutes, significantly improving the efficiency of the production line and making it suitable for high-volume industrialized production.
II. The need for passivation treatment
Enhanced corrosion resistance
Passivation treatment through chemical or electrochemical reactions in the zinc layer surface to form a dense oxide film (such as chromate passivation film), blocking oxygen, moisture and zinc direct contact, slowing down the oxidation and corrosion rate of zinc.
Experiments have shown that the salt spray test life of galvanized layers after passivation can be extended from 100 hours to more than 500 hours, which is suitable for high corrosion environments such as marine and industrial.
Inhibition of white rust generation
Unpassivated galvanized layers tend to react with carbon dioxide in humid environments to produce white zinc alkali carbonate (white rust). Passivation coatings inhibit such reactions and maintain a clean appearance of the coating.
For example, the “self-healing” properties of chromate passivate films allow them to re-form when the coating is partially damaged, preventing the white rust from spreading.
Meeting environmental and safety requirements
Traditional hexavalent chromium passivation has been gradually replaced by chromium-free passivation (e.g. trivalent chromium, silicone passivation) due to toxicity issues, which meets the requirements of the “Twelfth Five-Year Plan for Comprehensive Prevention and Control of Heavy Metal Pollution” and other regulations.
Chromium-free passivation to ensure corrosion resistance while reducing the harm to humans and the environment, becoming the mainstream technology in the industry.
III. Synergy between cooling and passivation
Process sequence optimization: passivation immediately after cooling avoids oxidizing the zinc layer by exposing it to air for too long and ensures uniform coverage of the passivated film.
Enhancement of coating performance: Cooling controls the crystalline quality of the coating and passivation strengthens the surface protection. The combination of the two gives galvanized workpieces both high mechanical strength and long-lasting corrosion protection.
Cooling and passivation after hot-dip galvanizing are key steps to ensure the performance of the coating.
Cooling: stabilizes the structure of the coating and improves productivity;
Passivation: enhances corrosion resistance, inhibits white rust and meets environmental requirements.
Both are essential to ensure the long-term reliability of galvanized products in complex environments.