Pretreatment in the Hot-Dip Galvanizing Production Process

The surface of steel materials and components may accumulate impurities such as rust, oil, processing debris, and dust during processing and storage, making it unsuitable for direct hot-dip galvanizing. Therefore, proper surface treatment is essential. The quality of pretreatment for galvanizing significantly impacts the quality of the galvanized layer and should be given high priority.

The process steps for pretreatment before galvanizing are degreasing → rinsing → acid pickling for rust removal → rinsing → solvent-assisted plating → drying.

Pretreatment in the Hot-Dip Galvanizing Production Process
Pretreatment in the Hot-Dip Galvanizing Production Process

I. Degreasing and Oil Removal

This step is primarily used to remove non-water-soluble substances like oil adhered to the surface to prevent interference with rust removal, solvent plating, and hot-dip galvanizing effects. Various degreasing methods include:

  • Chemical degreasing method
  • Electrolytic degreasing method
  • Acidic degreasing method
  • Ultrasonic degreasing method
  • Biological degreasing and oil removal method

II. Inspection of Degreasing and Cleaning Effects

The cleanliness requirement for metal materials after cleaning is determined based on the nature of the components or the requirements of the processing technology. Inspection of cleanliness is usually performed through three simple methods:

  • Visual inspection
  • Wiping method
  • Water film rupture method

III. Removal of Old Paint

When the paint film on the surface of steel components is damaged due to poor coating, transportation, or aging during use and requires re-galvanizing, the old paint should be removed. Common methods include mechanical, flame, and chemical methods.

IV. Acid Pickling for Rust Removal

In the hot-dip galvanizing of steel components, commonly used acids are hydrochloric acid and sulfuric acid. Acid pickling aims to remove rust without corroding the steel base. Excessive acid pickling can roughen the steel surface, affecting the quality of hot-dip galvanizing. Corrosion inhibitors are often added during acid pickling to prevent excessive corrosion.

Precautions for acid pickling:

  • Control acid pickling solution concentration
  • Maintain cleanliness of acid pickling solution
  • Control temperature
  • Stir appropriately
  • Pay attention to the rinsing process

V. Rinsing

After alkaline washing and acid pickling, rinsing is crucial to remove impurities adhered to the workpiece. This prevents cross-contamination and ensures a clean surface for the next process. There are two main methods of rinsing:

  • Immersion rinsing
  • Spray rinsing

Some domestic enterprises adopt a practice of recycling cleaning water. They install a conductivity meter in the acid pickling water tank to monitor acidity levels. When the acidity reaches a certain value, an electromagnetic valve automatically opens, transferring the water to the rinsing tank after alkaline washing and introducing clean water into the acid pickling rinsing tank. This reduces water consumption to a minimum while achieving low pollution and water conservation effects. Contact us for more