How is hydrofluoric acid used for acid pickling of stainless steel?

How is hydrofluoric acid used for acid pickling of stainless steel? During welding or polishing, low-chromium high-temperature oxide scale is generated on the surface of stainless steel. The color is usually yellow to blue. Due to the low chromium content, it is prone to corrosion. In order to obtain a surface rich in chromium and maintain the corrosion resistance of stainless steel, steelmakers will perform acid pickling treatment on the stainless steel surface to remove manufacturing residues formed by carbon steel contamination, polishing, and heat tempering. Next, the editor will share the method of acid pickling for stainless steel: First, we prepare the acid pickling agent. The formula (the percentage content of the formula is by volume; during the preparation process, water is added first, followed by acid, and hydrochloric acid is added first, followed by nitric acid) has three major categories: 1. Immersion acid solution: nitric acid (density 1420kg/m³) 20%, hydrofluoric acid 5%, and the rest is water. 
2. Brushing and washing acid solution: 50% hydrochloric acid + 50% water. 
3. Acid paste: 20ml of hydrochloric acid (density 1190kg/m³) + 100ml of water + 150g of bentonite. 
Acid pickling process 1. Acid pickling and immersion: Applicable to small workpieces. Immerse the workpiece in the brushing tank for 25-40 minutes, then remove it and rinse it with clean water. 
2. Acid pickling and brushing: Applicable to large components, use a brush or cloth to repeatedly brush and clean until the workpiece turns white and shiny, and then rinse it with water. 
3. Acid paste pickling: Applicable to large components, apply acid paste to the surface of the workpiece structure, let it stay for a few minutes, and then rinse it off with clean water. 
Acid pickling can remove the oxide layer on the surface of the workpiece, forming a colorless and dense oxide film on the brushed surface, effectively improving the corrosion resistance of stainless steel.