How to avoid pitting on the surface when polishing stainless steel pipes

To avoid surface pitting when polishing stainless steel pipes, the key is to eliminate the sources of pitting (residual impurities, abrasive damage, and damage to the passivation film). This requires strict control in three stages: pre-treatment before polishing, process control during polishing, and cleaning and protection after polishing. The specific measures are as follows: 

1. Pre-treatment before polishing: Thoroughly remove surface contaminants

○ Perform degreasing cleaning first: Use acetone, ethanol or a dedicated stainless steel cleaner, combined with ultrasonic cleaning to remove lubricants, oil stains and metal debris left from cold working, to prevent these impurities from embedding into the surface and causing pitting during polishing.

○ Remove surface defects: For pits, rust spots and welding spatter on the surface of the tube, first manually sand them smooth with 80-120 mesh sandpaper; if there is oxide scale, acid pickling and passivation treatment must be carried out first to remove the oxide scale before polishing to prevent oxide scale particles from scratching the surface and causing pitting.

○ Clean the polishing environment: The polishing workbench and fixtures should be made of stainless steel to avoid iron filings from carbon steel tools falling onto the surface; operators should wear clean dust-free gloves to prevent sweat and oil on their hands from contaminating the surface of the tube. 

2. Polishing Process Control: Preventing Abrasive Damage and Overheating Oxidation

- Select dedicated abrasives strictly: Only stainless steel-specific polishing paste should be used; mixing with carbon steel abrasives or chlorine-containing abrasives is strictly prohibited. Gradually switch to finer abrasive grades in the sequence of "coarse polishing → medium polishing → fine polishing", and replace the polishing wheel with a clean one after each process to prevent coarse abrasive particles from remaining and scratching the surface.

- Control polishing parameters: The polishing wheel speed should be moderate. For tubes with a diameter of less than 50mm, the speed should be controlled at 1500–2500r/min; for tubes with a diameter of more than 50mm, it should be reduced to 800–1500r/min. Excessively high speed can cause overheating of the surface, damaging the passivation film and forming oxidation pits. The applied pressure should be uniform and gentle to avoid local pressure that could cause abrasive particles to embed into the surface.

- Maintain lubrication and cooling during polishing: During polishing, spray an appropriate amount of anhydrous ethanol or dedicated polishing liquid to provide lubrication and cooling, reducing dry friction between the abrasive and the tube surface and preventing oxidation spots caused by high temperatures from turning into pits. 

3. Post-polishing treatment: Thorough cleaning + Repairing the passivation film

- Multi-stage cleaning to remove residues: After polishing, the surface is first rinsed with high-pressure pure water, then placed in an ultrasonic cleaner with neutral detergent for 10-15 minutes to thoroughly remove any remaining polishing paste and metal powder. If these residues are not completely removed, they may cause pitting and the formation of spots during subsequent use.

- Timely passivation protection: Immediately after cleaning, passivation treatment is carried out by immersing the tubing in a 20%-30% nitric acid solution at room temperature for 20-30 minutes to repair the damaged passivation film during the polishing process and form a dense protective layer, preventing the appearance of spots due to incomplete passivation film.

- Drying and storage: After passivation, the tubing surface is dried with clean compressed air or allowed to air dry in a dust-free environment to avoid water stains that could cause spots. During storage, the tubing should be wrapped in moisture-proof paper and placed in a dry and well-ventilated area.