English
## I. Purpose of Pickling and Rust Removal Mechanism
Steel workpieces have scale and rust such as FeO, Fe₂O₃ and Fe₃O₄ on their surfaces, which must be removed prior to hot-dip galvanizing. Otherwise, the fluxing effect will be reduced and skip plating will occur.
1. Rust (ferrous hydroxide and ferric hydroxide) can undergo direct dissolution reactions with acid.
2. Scale (Fe₃O₄, Fe₂O₃) is hardly soluble in acid at room temperature. A corrosion cell forms between the steel substrate and the scale. The iron substrate dissolves preferentially and releases hydrogen gas. Hydrogen reduces high-valent iron oxides to FeO, which is then further dissolved and removed. Cracks and pores in the rust layer facilitate pickling.
3. Hydrogen atoms generated from side reactions penetrate into the steel, causing hydrogen embrittlement and reducing the toughness and ductility of steel. Escaping hydrogen forms acid fume that pollutes the environment. Solutions: add corrosion inhibitors, wetting agents and fume suppressants.
## II. Common Pickling Methods
1. Hydrochloric Acid Pickling (the most widely used in the industry)
Common mass fraction: 5%~15%. Higher concentration and temperature lead to faster pickling speed. The suitable operating temperature is 18~21℃; pickling efficiency drops sharply below 15℃.
Pickling speed decreases significantly when iron ion accumulation reaches 150~200g/L, requiring full bath replacement.
Hydrochloric acid concentration can be determined by looking up tables according to solution density. The St. Andrew's cross rule can be used to calculate the mixing ratio of new and spent acid.
Testing: acid-base titration for hydrochloric acid content; iron ion concentration obtained by combining density with charts.
2. Sulfuric Acid Pickling
Concentration is controlled at 8%~15%. Heating to 50~60℃ is required to achieve ideal speed, with a maximum temperature not exceeding 75℃.
Advantages: low cost and convenient waste liquid recovery. Disadvantages: weak dissolving capacity at room temperature; high temperature tends to corrode the substrate and aggravate hydrogen embrittlement. Never pour water into concentrated sulfuric acid.
Rust removal capacity declines when iron ion content exceeds 60g/L; waste liquid discharge is recommended at 80~120g/L. Water washing requirements after sulfuric acid pickling are stricter than those for hydrochloric acid pickling.
3. Hydrofluoric Acid Pickling
Mostly used for castings with adhered sand as an alternative to sandblasting. It is highly toxic and volatile, so full protective gear must be worn during operation.
Weak hydrofluoric acid requires a long pickling time (up to 24 hours), while strong hydrofluoric acid only needs 10~30 minutes. Mixed acid of hydrochloric acid and hydrofluoric acid can also be adopted.
## III. Pickling Corrosion Inhibitors
1. Function: remove rust only, inhibit corrosion of the steel substrate, prevent over-corrosion and roughening of workpieces, and guarantee galvanizing quality.
2. Performance requirements: stable in high-temperature and high-concentration acid, no reduction in rust removal speed, easy control and treatment, low cost.
3. Mechanism: adsorb on the metal surface to form a protective film and suppress the anodic/cathodic reactions of electrochemical corrosion.
4. Key application points: there are upper limits for applicable temperature and concentration. Inhibition efficiency no longer rises once the critical dosage is exceeded. Increased temperature and aging of bath liquid degrade performance, so regular replenishment is required.
## IV. Precautions for Pickling Operation
1. Control acid concentration and replenish water to maintain the process range to prevent over-corrosion.
2. Keep the bath liquid clean and clean or replace the bath periodically.
3. Strictly control pickling temperature.
4. Apply proper agitation to improve pickling efficiency.
5. Use staged water washing; two-stage counter-current cleaning delivers better results to eliminate residual acid.
6. Maintain continuous operation; pickling cannot be suspended halfway.
7. Periodically remove sludge deposited in the bath.
8. Minimize pickling time on the premise of complete rust removal to reduce corrosion and hydrogen embrittlement.
9. Implement safety protection to avoid burns from splashing acid.
10. Install exhaust equipment in the pickling area to discharge acid fume and protect personnel and equipment.