How is stainless steel pickling performed?

Stainless steel pickling uses an acidic solution to selectively dissolve surface defects, achieving cleanliness. It adheres to the core principles of "adapting the acid, precisely controlling the temperature, and preventing residue and oxidation." The process consists of three steps: preliminary preparation, pickling, and post-treatment, ensuring both efficiency and substrate protection.

I. Preliminary Preparation: Laying the Foundation for Pickling

Acid Preparation: Choose based on the stainless steel material-for 304 stainless steel, use a mixture of 10%-20% nitric acid and 2%-5% hydrofluoric acid. For 316L stainless steel, increase the nitric acid concentration to 15%-25%. When preparing the solution, always add water first, then acid. Stir while pouring, and use a concentration meter to confirm compliance. Do not directly mix concentrated acids.

Workpiece Pretreatment: If the surface is oily, soak it in a 5%-8% neutral degreaser at room temperature for 10-15 minutes, or wipe it with a degreased cloth, then rinse with running water for 2-3 minutes, and finally dry it with a lint-free cloth. If there is noticeable welding slag, gently brush it with a stainless steel wire brush to remove it.

Equipment Preparation: Use a PP plastic or fiberglass acid-resistant tank, equipped with acid-resistant gloves, a protective mask, a thermometer, and a plastic scoop. The pickling area must be ventilated to prevent inhalation of acid mist.

II. Pickling Implementation: Precisely Control the Reaction

Workpiece Placement: Slowly place the pretreated workpiece into the acid tank, ensuring complete immersion. Gently shake the workpiece to remove any surface bubbles.

Parameter Control: Maintain the temperature between 40-60°C (304 stainless steel) and 50-70°C (316L stainless steel). Monitor the temperature with a thermometer. A temperature that is too low will prolong the pickling time, while a temperature that is too high will accelerate substrate corrosion. The pickling time is 10-30 minutes, with regular observation. When the surface oxide scale is completely removed and the workpiece appears uniformly silvery white, remove it immediately with a scoop. Exceeding the soaking time is prohibited.

III. Post-treatment: Preventing Residue and Secondary Oxidation

Immediate Water Rinse: After removing the workpiece, quickly place it in a running water bath for 5-8 minutes, focusing on areas prone to acid accumulation, such as welds and grooves. A neutral pH test indicates the surface is free of acid residue, preventing continued acid corrosion of the substrate.

Neutralization and Drying: If significant acid residue remains, soak the workpiece in a 1%-3% sodium carbonate solution for 5 minutes to neutralize the residual acid. Rinse once with pure water. Finally, wipe the surface dry with a lint-free cloth or dry with hot air below 60°C. Within 2 hours of drying, proceed to subsequent processes such as passivation. If immediate treatment is not possible, spray a thin layer of rust-proof oil, such as industrial vaseline, to isolate the workpiece from the air.

The entire pickling process requires strict control of acid concentration, temperature, and time. The key is to "remove defects without damaging the substrate." Safety precautions and waste disposal must also be maintained. The ultimate goal is a clean, defect-free stainless steel surface, laying the foundation for subsequent passivation or use.