What are the methods for steel plate surface pretreatment?
Steel plate surface pretreatment is a key step before hot-dip galvanizing (such as SGCC steel plates), painting, welding, and other processes. Its purpose is to remove surface oil, scale, rust, and impurities, thereby improving the adhesion and uniformity of the plating or coating. Common pretreatment methods can be categorized as mechanical, chemical, and physicochemical.
1. Mechanical Treatment
This method uses mechanical force to directly remove surface contaminants or alter the surface condition. It is suitable for treating thick scale, heavy rust, or large workpieces. Common methods include:
Sandblasting/Shot Peening
Principle: Abrasives (such as quartz sand, steel shot, or aluminum oxide particles) are sprayed at high speed onto the steel plate surface using high-pressure airflow or centrifugal force. The impact and grinding action removes scale, rust, and impurities, while also creating a certain surface roughness (enhancing coating adhesion).
Features: High efficiency and wide applicability (complex shapes can be processed). However, dust generation requires dust removal equipment. The choice of abrasive (such as particle size and hardness) affects surface roughness. Pickling and Rust Removal (Mechanically Assisted)
Often combined with chemical pickling, mechanical agitation and high-pressure washing are used to assist in removing residual scale debris after pickling, preventing impurities from interfering with subsequent processing.
Manual or Power Tool Treatments
Such as sandpaper polishing, wire brushing (manual or electric), and angle grinders, these are suitable for small-area or localized treatments. While cost-effective, they also have low efficiency and poor quality consistency, making them suitable only for simple applications.
Shot Blasting
Similar to sandblasting, this method uses a high-speed rotating impeller to project steel shot onto the steel plate surface, resulting in a stronger impact force. It is suitable for thick steel plates or batches of workpieces, and can simultaneously remove rust and strengthen the surface (e.g., by relieving stress).
Chemical Treatments
Using chemical solutions to react with contaminants on the steel plate surface, dissolving or converting impurities, this is the most commonly used method in industrial mass production. These methods include:
Degreasing
Removes oils such as rolling oil, rust preventative oil, and lubricating oil from the steel plate surface to prevent interference with subsequent rust removal or coating adhesion. Common methods:
Alkaline degreasing: Using alkaline solutions such as sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃) to remove oil contaminants through saponification (for animal and vegetable oils) and emulsification (for mineral oils), it is suitable for industrial batch processing and is low-cost.
Solvent degreasing: Using organic solvents (such as gasoline, trichloroethylene, and alcohol) to dissolve oil contaminants is suitable for precision components or applications where alkaline degreasing is difficult, but solvents are volatile and pose environmental risks.
Emulsion degreasing: Mixing an organic solvent with water and an emulsifier combines the advantages of solvent dissolution and water cleaning, making it more environmentally friendly than pure solvent methods.
Acid pickling (rust and scale removal)
Acidic solutions are used to dissolve scale (such as Fe₃O₄, Fe₂O₃) and rust (FeO (OH)·nH₂O) on the steel plate surface, exposing the clean iron substrate. Commonly Used Acids:
Hydrochloric Acid (HCl): Rapid rust removal, mild reaction, and low risk of over-corrosion. Suitable for most steel plates (especially cold-rolled steel), it is the most widely used.
Sulfuric Acid (H₂SO₄): Requires heating (50-80°C) to improve efficiency and is low-cost, but can easily form ferrous sulfate precipitates on the surface, requiring cleaning. It is less effective on high-silicon steel (e.g., steel containing >0.5% Si).
Phosphoric Acid (H₃PO₄): Rust removal also forms a phosphate passivation film. Suitable as a pretreatment before painting, but expensive and slow, it is rarely used as a pretreatment for hot-dip galvanizing.
Improvements: Corrosion inhibitors (such as hexamine) are often added during pickling to prevent excessive acid corrosion of the steel substrate. Wetting agents (such as surfactants) are also added to improve acid penetration into the oxide scale.
Neutralization Treatment: Residual acid on the steel surface after pickling must be neutralized with an alkaline solution (such as sodium carbonate solution) to prevent the residual acid from corroding the substrate or affecting subsequent galvanizing/painting.
III. Physicochemical Treatment Methods
Combining physical action and chemical reaction to further optimize the surface condition, this method is commonly used in applications requiring high precision or special requirements:
Electrolytic Degreasing/Rust Removal
In an alkaline degreasing solution or acidic rust removal solution, the steel sheet serves as an electrode (anode or cathode). The bubbles (hydrogen or oxygen) generated by electrolysis remove oil stains or scale, while also accelerating the chemical reaction. This method is highly efficient and thorough, making it suitable for precision components.
Phosphating Treatment
A phosphate solution (such as zinc-based or manganese-based) reacts with the steel sheet surface to form a dense phosphate film (typically 1-10μm thick). This enhances the adhesion of subsequent plating or coatings and provides a certain degree of rust resistance. This method is commonly used for pre-treatment of automotive and home appliance panels before painting, but is less commonly used before hot-dip galvanizing (as it may affect the zinc-iron reaction).
Passivation Treatment
The clean surface after pre-treatment is treated with chromates or chromium-free passivating agents (such as silanes and titanates) to form a passivation film, temporarily preventing rust before subsequent processing (e.g., short-term rust prevention before galvanizing). IV. Pretreatment Process (Taking Hot-Dip Galvanizing as an Example)
Industrial production typically utilizes a multi-step process. The typical process is:
Degreasing (alkaline or solvent) → Water washing → Pickling (hydrochloric acid/sulfuric acid) → Water washing → Neutralization → Water washing → (Optional: Passivation) → Drying
This step-by-step process removes oil and scale, ultimately resulting in a clean, activated surface that ensures uniformity and adhesion during subsequent hot-dip galvanizing.