1.What is the working principle of the passivation layer?
Passivation is a chemical conversion treatment performed on the zinc plating layer to form an extremely thin (typically only tens to hundreds of nanometers) and dense protective film. The main functions of this film are:
Isolation: It isolates the reactive zinc surface from the external corrosive environment (such as oxygen, moisture, and salt in the air), acting as a physical barrier.
Inhibition of electrochemical reactions: Zinc corrosion is an electrochemical process. The passivation film can greatly inhibit the anodic or cathodic reactions in this process, thus significantly slowing down the corrosion rate.
2.How well does the passivation layer resist salt spray?
Salt spray testing is a standard method for rapidly assessing the corrosion resistance of materials by simulating harsh environments such as the marine atmosphere or de-icing agents. The unit is "hours," indicating the time it takes for white or red rust to appear.
Zinc coils without passivation: In a standard neutral salt spray test, they may only last a few hours to tens of hours before white rust appears.
Zinc coils with passivation: Salt spray resistance time can be significantly increased, several times or even tens of times.
3.What are the key factors affecting the salt spray resistance of the passivation layer?
Passivation Type: As shown in the table above, this is the most crucial factor. Trivalent chromium passivation is currently the best choice balancing performance and environmental friendliness.
Passivation Film Thickness and Uniformity: The thicker, more uniform, and denser the film, the better its barrier effect and the longer its salt spray resistance time.
Quality of the Zinc Plating Layer Itself: The passivation layer is built on top of the zinc layer. If the zinc layer itself has porosity, impurities, or uneven thickness, it will directly affect the quality and continuity of the passivation film.
Passivation Process Control: This includes the concentration, temperature, pH value, and processing time of the passivation solution, as well as the subsequent drying temperature and time, all of which affect the performance of the final passivation film.
4.What are the types of passivation?
Chromium-free passivation: Environmentally friendly, contains no heavy metal chromium. Performance varies considerably.
Trivalent chromium passivation: currently the mainstream technology, environmentally friendly and with good performance.
Hexavalent chromium passivation: a traditional technique with excellent performance, but highly toxic and has been banned.
5.What effect does the passivation layer of galvanized coil have on improving its salt spray (and corrosion) resistance?
The passivation layer of galvanized coils is specifically designed to improve their salt spray (and corrosion) resistance. By forming a dense protective film, it effectively increases the salt spray resistance time of galvanized coils from tens of hours to hundreds of hours, making it a key process for meeting the anti-corrosion requirements of the automotive, home appliance, and construction industries.