The galvanizing process for malleable iron pipe fittings primarily employs hot-dip galvanizing, aiming to coat the cast iron surface with a dense zinc layer for excellent rust and corrosion protection. This process is widely used in construction, water supply, and fire protection. The core process can be divided into the following steps:
Step 1: Pre-treatment – Cleaning and Activating the Surface
This is crucial for determining the quality of galvanizing. The pipe fittings first undergo high-temperature degreasing to remove residual grease and dirt from the casting process. They then enter an pickling tank (usually using hydrochloric acid) to thoroughly remove surface rust and oxide scale, exposing the pure metal substrate. After cleaning, they are immersed in a flux (usually a zinc ammonium chloride solution) to form a protective film, preventing re-oxidation before entering the zinc bath and enhancing the wettability of the iron base with the molten zinc.
Step 2: Hot-Dip Galvanizing – Forming an Alloy Coating
The treated pipe fittings are immersed in molten zinc maintained at a temperature of 440-460℃. The iron substrate undergoes a series of complex metallurgical reactions with the liquid zinc, forming an iron-zinc alloy layer. Immersion time needs to be precisely controlled (usually several minutes) to ensure uniform alloy layer growth. Afterward, the fitting is slowly lifted out of the molten zinc, leaving a layer of molten zinc on the surface.
Step 3: Post-treatment – Cooling and Finishing After leaving the zinc pot, the fitting is immediately centrifuged to remove excess molten zinc, ensuring a uniform coating on the inner wall and threads and preventing zinc nodules. It is then rapidly cooled in a water-cooling bath to solidify the coating. After cooling, necessary finishing is performed, such as removing small zinc nodules or burrs, and inspecting the appearance, thickness, and adhesion of the coating.
Final Results and Characteristics Through this process, the malleable iron fittings acquire a robust coating consisting of an alloy layer and a pure zinc layer. This coating not only provides long-term physical barrier protection but also, due to zinc’s “sacrificial anode” electrochemical properties, provides cathodic protection to the iron substrate, continuing to prevent corrosion even if the coating is locally damaged. This process endows malleable iron fittings with excellent durability and significantly extends their service life.
Post time: Dec-04-2025