Detailed process of the core process of galvanized round pipes
1. Hot-dip galvanizing process (industrial mainstream, accounting for more than 80%)
Hot-dip galvanizing is a typical process of "pipe making first, galvanizing later". The process is complicated but the coating performance is stable. It is suitable for most structural and transportation galvanized round pipes (such as municipal, chemical, and construction fields). The specific steps can be divided into three stages: pre-treatment, hot-dip galvanizing, and post-treatment:
(1) Pre-treatment: The key to ensuring uniform bonding of the coating
The core goal of pre-treatment is to remove oil, rust, and scale on the surface of the steel pipe to prevent impurities from affecting the adhesion of the zinc layer. It is the "basic process" that determines the quality of galvanizing and mainly includes 4 steps:
Degreasing (degreasing): Place the steel pipe in an alkaline solution (such as sodium hydroxide solution) or solvent to remove oil stains such as engine oil and anti-rust oil on the surface of the steel pipe during pipe making and transportation through chemical dissolution or emulsification. If the oil stains remain, it will cause the subsequent zinc layer to "leak" (partial zinc layer).
Pickling (Rust Removal): After degreasing, the steel pipe is immersed in a hydrochloric acid (commonly used) or sulfuric acid solution to dissolve surface rust (Fe₂O₃) and scale (Fe₃O₄), revealing a clean metallic surface. The pickling time must be controlled (usually 10-30 minutes). Too long a pickling time can lead to "over-corrosion" (pitting on the surface), while too short a time can result in incomplete rust removal.
Rinsing (Neutralization): After pickling, residual acid will remain on the surface of the steel pipe. Immediately rinse with clean water and then immerse the pipe in a weak alkaline solution (such as sodium carbonate) to neutralize the residual acid. This prevents continued acid corrosion and subsequent "bubbling" or "delamination" of the zinc coating.
Activation (Plating) After rinsing, the steel pipe is immersed in a "plating flux" (usually a mixed aqueous solution of zinc chloride and ammonium chloride). This forms a thin salt film on the surface that is easily soluble in molten zinc. The salt film can isolate the air and prevent the steel pipe from re-oxidation before entering the zinc pot. At the same time, it reduces the interfacial tension between the zinc liquid and the surface of the steel pipe, helping the zinc liquid to adhere evenly.