English
I. Functions and Impacts of Degreasing
Degreasing is a basic pre-treatment process for hot-dip galvanizing. Oil contaminants on workpiece surfaces originate from mineral oils, animal and vegetable fats used during processing, storage and transportation. Many manufacturers tend to overlook degreasing, yet its quality directly determines the performance of subsequent processes and the final quality of galvanized products:
1. Hinders pickling and reduces rust removal speed;
2. Affects the formation of flux film and impairs fluxing quality;
3. Incomplete degreasing leads to skipped plating on workpieces and deteriorates coating quality.
II. Classification of Oils and Fats
Saponifiable oils and fats: animal and vegetable fats, which undergo saponification reactions with alkalis and are water-soluble.
Unsaponifiable oils and fats: mineral oils (engine oil, gasoline, etc.), insoluble in water and incapable of saponification reactions with alkalis.
III. Classification and Characteristics of Degreasing Methods
1. Organic solvent degreasing: immersion / spraying; capable of dissolving various oils and fats without corroding workpieces; disadvantages: flammable, toxic, high cost and incomplete degreasing. Suitable for pre-degreasing of complex workpieces and heavily oil-contaminated parts.
2. Chemical degreasing: immersion, spraying; simple equipment and low cost, yet long treatment time. Applicable to ordinary workpieces.
3. Electrochemical degreasing: cathodic / anodic electrolysis; high degreasing efficiency and capable of removing deep-seated oil contaminants; workpieces are prone to hydrogen embrittlement, and a DC power supply is required. Used to remove etching residues.
4. Physical mechanical degreasing
Wiping method: manual wiping, flexible operation yet high labor intensity and low efficiency; suitable for local light oil contamination.
Burning method: heating at 300~400℃ to burn off oil contaminants, carbon residues are likely to remain, and this method is prohibited for some workpieces.
Sandblasting / shot blasting: completes degreasing and rust removal in one step, not applicable to thin-walled small parts.
Workpieces with heavy oil contamination can adopt combined degreasing processes (e.g.: chemical degreasing + electrochemical degreasing); most steel structural parts have light oil contamination and require simple degreasing schemes.
IV. Common Degreasing Processes for Hot-dip Galvanizing
1. Hot alkaline solution method (mainstream)
Chemical degreasing, NaOH concentration 100~150g/L, operating temperature around 85℃, immersion time 1~20min. It can remove both saponifiable and unsaponifiable oils and fats. For light oil contamination, low-concentration surfactant solution at 60℃ can be used instead. Hot water rinsing is required after degreasing to prevent alkali from being carried into the pickling tank.
2. Cold alkaline solution method: degreasing at ambient temperature, mainly using phosphates. Suitable for workpieces with light oil contamination, and the rinsing step is critical.
3. Other auxiliary methods: manual wiping of local oil contaminants, shot blasting, furnace heating to decompose oils; hot alkaline degreasing can be combined with electrolytic degreasing for special workpieces.