Stainless steel 304, as a widely used material, has found applications in multiple fields such as construction, food processing, chemicals, and medical equipment due to its excellent corrosion resistance, heat resistance, and mechanical properties. However, the surface treatment of stainless steel 304 not only affects its aesthetics but also directly relates to its corrosion resistance and service life. This article will introduce in detail the surface treatment methods for stainless steel 304, including pickling, mechanical polishing, electrolytic polishing, sandblasting, chemical plating, and post-welding treatment.

I. Pickling Treatment

Pickling treatment is an important step in the surface treatment of stainless steel 304, primarily used to remove oxides, welding burns, and contaminants from the surface, restoring its original bright appearance. Pickling treatment typically uses a mixed solution of nitric acid (HNO3) and hydrofluoric acid (HF), commonly in a ratio of 10% nitric acid and 2% hydrofluoric acid, with the remainder being water. The specific steps for pickling are as follows:

  1. Surface Cleaning: Thoroughly clean the stainless steel surface with warm water and a neutral detergent to remove oil and dust.
  2. Pickling Solution Preparation: Adjust the solution concentration and temperature according to actual conditions, generally conducted at room temperature.
  3. Pickling Process: Apply the pickling solution uniformly to the stainless steel surface or immerse it in a pickling tank, ensuring all areas to be treated are covered. The pickling time is generally 10-30 minutes, with the specific time adjusted according to the severity of surface dirt and oxide scale.
  4. Rinsing and Neutralization: After pickling, immediately rinse the surface with plenty of water to ensure all pickling solution is thoroughly removed. Then use an alkaline neutralizing solution (such as sodium hydroxide solution) for neutralization treatment to remove residual acid solution and prevent corrosion.

Pickling treatment not only improves the smoothness of the stainless steel surface but also lays a good foundation for subsequent treatment steps such as passivation.

II. Mechanical Polishing

Mechanical polishing is a method of reducing the roughness of the stainless steel 304 surface through mechanical grinding to make it smoother and more uniform. Mechanical polishing usually consists of two stages: coarse grinding and fine grinding:

  1. Coarse Grinding: Use 60 to 80 grit sandpaper for initial grinding to remove larger defects and irregularities.
  2. Fine Grinding: Use 120 to 240 grit sandpaper for fine grinding to further smooth the surface.

After mechanical polishing, the stainless steel surface can achieve a certain degree of smoothness, but there may still be some minute scratches. Therefore, mechanical polishing is often followed by electrolytic polishing or chemical polishing to further improve surface smoothness.

III. Electrolytic Polishing

Electrolytic polishing is an electrochemical method to remove microscopic irregularities on the stainless steel surface, enabling the stainless steel 304 surface to have lower roughness and a brighter appearance. The specific steps for electrolytic polishing are as follows:

  1. Pretreatment: Clean the stainless steel surface to remove oil and dust.
  2. Electrolytic Polishing: Place the stainless steel as the anode in the electrolytic polishing solution and remove microscopic irregularities through electrolysis.
  3. Rinsing and Drying: After electrolytic polishing, rinse the surface thoroughly with plenty of water and dry it completely.

Electrolytic polishing not only improves the smoothness of the stainless steel surface but also enhances its corrosion resistance.

IV. Sandblasting Treatment

Sandblasting treatment removes oxides and contaminants from the surface of stainless steel 304 by injecting sandblasting particles at high speed, while also creating a decorative texture on the surface. The specific steps for sandblasting treatment are as follows:

  1. Surface Cleaning: Clean the stainless steel surface to remove oil and dust.
  2. Sandblasting Treatment: Use sandblasting equipment to inject sandblasting particles at high speed onto the stainless steel surface to remove surface dirt and oxides.
  3. Subsequent Treatment: Depending on requirements, further treatment such as mechanical polishing or electrolytic polishing can be performed on the sandblasted surface to improve smoothness.

Sandblasting treatment not only cleans the surface but also achieves different surface textures and decorative effects by adjusting the size of the sandblasting particles and injection pressure.

V. Chemical Plating

Chemical plating forms a layer of metal coating on the surface of stainless steel 304 through chemical reactions, improving its decorative properties and corrosion resistance. The specific steps for chemical plating are as follows:

  1. Surface Pretreatment: Clean the stainless steel surface to remove oil and dust, and perform activation treatment to improve coating adhesion.
  2. Chemical Plating: Place the stainless steel in the plating solution and form a layer of metal coating on the surface through chemical reactions.
  3. Rinsing and Drying: After plating, rinse the surface thoroughly with plenty of water and dry it completely.

Chemical plating not only enhances the decorative properties of the stainless steel surface but also improves its corrosion resistance, extending service life.

VI. Post-Welding Treatment

During welding of stainless steel 304, due to welding heat and flux, the surface may develop weld marks, oxide layers, and slag. Therefore, post-welding treatment is also an important aspect of stainless steel 304 surface treatment. Post-welding treatment typically includes the following steps:

  1. Surface Cleaning: Clean the surface using suitable stainless steel cleaners and soft brushes or cloths to remove welding flux, dirt, and other contaminants.
  2. Mechanical Grinding: Use sandpaper, grinding wheels, or grinding machines for initial grinding to remove weld marks and oxides.
  3. Pickling Treatment: Use a pickling agent (such as dilute hydrochloric acid or phosphoric acid) to treat the surface to remove residual oxides and welding slag.
  4. Polishing Treatment: Use polishing wheels and polishing paste to polish the ground areas, restoring surface gloss.
  5. Passivation Treatment: Use a passivating agent containing nitric acid or phosphoric acid to treat the surface to form a protective oxide film, enhancing corrosion resistance.

Post-welding treatment not only removes weld marks and oxide layers produced during welding but also improves the corrosion resistance of stainless steel, maintaining the smoothness of the appearance.