Which passivation method for galvanizing provides better corrosion resistance?

Zinc (Zn), specific gravity: 7.14 g/cm3, melting point 419°C, potential -0.76V. Steel rusts easily in air and water, whereas the corrosion rate of zinc in the atmosphere is only 1/15 of that of steel in the atmosphere. Galvanized steel sheets use a thin layer of zinc coating to protect the steel from corrosion. Zinc does not change easily in dry air, but in humid air, a very thin film of basic zinc carbonate forms on its surface; this film protects the underlying zinc from further corrosion. The galvanized layer and the steel plate form an iron-zinc microcell. Even if the galvanized layer is partially damaged and a small area of the steel substrate becomes exposed, the steel substrate acts as a cathode and is thus protected. Hot-dip galvanized steel sheets have a thicker galvanizing layer that is free of pores, and they also possess an iron-zinc alloy layer, which makes them more resistant to corrosion. The galvanizing layer of electro-galvanized steel sheets is more corrosion-resistant than that of other types of electro-galvanized coatings.

Ammonium chloride for pickling chromium slag

Zinc has little passivation effect, but it can be significantly passivated in chromate solutions, forming a zinc chromate protective film

The zinc layer obtained through electroplating is relatively pure, and it corrodes more slowly in atmospheres containing acids, alkalis, etc.; chromate passivation is generally used

In terms of corrosion resistance, the galvanized passivations in order are military green passivation, black passivation, colored passivation, blue-white passivation, and white passivation. Corrosion resistance is good at typical film thicknesses; the military green passivation layer, due to its thicker thickness, is prone to damage upon formation, and it is generally not used without electroplating

Zinc plating is often used as a protective layer for steel due to its low cost and attractive appearance. However, it is prone to corrosion in the air, especially in humid environments, resulting in the formation of white, porous corrosion products that affect its appearance; therefore, passivation treatment is necessary to improve its corrosion resistance. In current chromium-based passivation films, chromium exists in the +3 and +6 valence states; hexavalent chromium is a highly toxic carcinogen that poses serious risks to both human health and the environment. With the rapid development of modern industry and science and technology, there are increasing demands for the quality of protective coatings. Traditional galvanizing coatings and hexavalent chromium passivation techniques can no longer meet the requirements for high corrosion resistance and environmental sustainability in industrial production and scientific fields. Therefore, seeking trivalent chromium or chromium-free passivation films that better meet environmental requirements and offer higher corrosion resistance has become a key focus of research for electroplating professionals.

After passivation, the acid potassium chloride galvanized layer has the best corrosion resistance, the alkaline cyanide-free galvanized layer ranks next, while the alkaline cyanide galvanized layer has the worst resistance. In a sodium chloride solution with a mass fraction of 1%, the order of resistance of the passivation film on the galvanized layer is: acidic potassium chloride galvanizing > alkaline cyanide-free galvanizing > alkaline cyanide galvanizing. The good corrosion resistance of the acidic potassium chloride zinc plating passivation film is likely related to the low amount of organic impurities mixed in the resulting coating.

Passivation is not a perfectly accurate term; it refers to a conversion film obtained through chemical methods. There are two types of passivation: one is chromium-based passivation, and the other is chromium-free passivation (including fingerprint-resistant types). Chromium-free passivation can be further divided into spray passivation and roll-coating passivation. For spray passivation, Chemital’s products are of good quality; they offer a 3+chromium passivation solution. As for roll-coating passivation, Paka’s fingerprint-resistant and chromium-free passivation solutions are excellent, but they are too expensive, so they are used rarely. 6+ chromium passivation involves measuring the chromium content; 3+ chromium and fingerprint-resistant passivation involve measuring other components, followed by further analysis such as that of SiO2. In fact, passivation occurs when the passivating agent reacts with the galvanized substrate at around 50 degrees Celsius (within an oven), resulting in the formation of a passivation film. Some passivators have a self-healing capability; that is, when a small amount of the passivation layer is scraped off, it regrows on its own to protect the substrate. Galvanizing passivation actually tests the quality of the passivation film formed on the galvanized surface; Chapter 11 of Part 12 of the \"Comprehensive Handbook on Hot-Dip Galvanizing, Electro-Galvanizing, and Innovative Production Processes for New Alloys\" provides a detailed explanation of this.

It is generally done through chromic acid passivation; of course, there are many different processes for chromic acid passivation, and the choice depends on the specific circumstances