What Is The Pre-treatment of Zinc Plating on Zn-al-mg Steel?

The surface of steel materials and components is prone to the generation or attachment of foreign substances such as oxide skin, oil stains, processing debris, and dust during processing and storage, making it unsuitable for direct hot-dip galvanizing. Therefore, proper surface treatment is required. The quality of pre-treatment for  Zn-al-mg Steel components has a significant impact on the quality of the galvanized layer and should be highly emphasized.

The process steps for pre-treatment of galvanizing are: degreasing → rinsing with water → acid pickling for rust removal → rinsing with water → solvent assist plating → drying.

1. Degreasing and Oil Removal

This process is mainly used to remove oil and other non-water-soluble substances adhering to the surface of the workpiece to prevent interference with rust removal, solvent main plating, and hot-dip galvanizing effects. Several oil removal methods are introduced below:

① Chemical degreasing method ② Electrolytic degreasing method ③ Acid degreasing method ④ Ultrasonic degreasing method ⑤ Biological degreasing and oil removal method

2. Inspection of Degreasing Cleaning Effect

The cleanliness requirements of metal materials after cleaning are determined based on the nature of the components or processing requirements. The cleanliness inspection generally uses the following three methods, which are simple and feasible:

① Visual inspection method ② Wipe test method ③ Water film rupture method

3. Paint Stripping

When the paint film on the surface of steel components is damaged due to poor coating, transportation, or aging during use and needs to be hot-dip galvanized again, the old paint should be removed. Common methods for paint stripping include mechanical, flame, and chemical methods.

4. Acid Pickling for Rust Removal

In hot-dip galvanizing of steel components, commonly used acids include hydrochloric acid and sulfuric acid. The purpose of acid pickling is rust removal without corroding the steel substrate. Excessive acid pickling can roughen the surface of steel components, affecting the quality of hot-dip galvanizing. Therefore, acid pickling usually involves the addition of acid inhibitors. In a corrosive environment, adding a small amount of this substance can effectively inhibit the corrosion of steel components. Points to note for acid pickling:

① Control the concentration of the acid pickling solution ② Maintain cleanliness of the acid pickling solution ③ Control temperature ④ Proper stirring ⑤ Pay attention to the rinsing process

5. Rinsing with Water

Water rinsing after alkaline and acid pickling is mainly used to remove impurities adhering to the workpiece after alkaline and acid pickling to prevent cross-contamination and entry into the next main plating process. There are two ways to rinse with water:

① Immersion rinsing ② Spray rinsing

In the production process, the practice of recycling cleaning water is adopted. Conductive measuring instruments are installed on the water tanks after acid pickling to monitor the acidity level of the water. When the acidity reaches a certain value, the solenoid valve opens automatically, pumping the water into the cleaning tank after alkaline pickling and injecting clean water into the tank after acid pickling. This method minimizes water consumption, reduces pollution, and lowers water usage.