Stainless steel inevitably goes through annealing, normalizing, melting, welding and other processing processes during the production process, and black oxide scale often appears on the surface. Oxide scale not only affects the appearance quality of stainless steel, but also adversely affects the subsequent processing of the product. Therefore, it must be removed by surface treatment methods such as pickling and polishing before subsequent processing. Stainless steel oxide scales are dense oxides such as NiO2, Cn2O3, FeO Cr2O3, FeO Cr2O3Fe2O3, etc. The difficulty of removal varies depending on the stainless steel grade, part specifications, and manufacturing conditions. Improper surface treatment will not only waste manpower but may also cause pitting corrosion and scrapping of the stainless steel. . Stainless steel pickling and passivation. There are currently few research reports. The author introduces some simple and feasible stainless steel surface treatment methods in production as follows.


        1. Stainless steel pickling

        The main purpose of pickling is to remove the oxide film produced by high-temperature operations (such as heat treatment, welding, forging, casting, etc.) on stainless steel to expose the original surface of the stainless steel. Stainless steel parts with different shapes and specifications sometimes require different pickling methods, such as dipping, scrubbing, pickling, pickling, etc.

        Pickling and passivation of stainless steel include the following steps

        Acceptance before pickling - oil removal - water washing - pickling - water washing - dust removal - water washing - passivation - water washing - neutralization - water washing - drying - hydrogen removal. When the oil stain on the surface is serious, organic solvents such as gasoline should be used to clean it. When the oil stain is less, chemical degreasing, electrochemical degreasing, etc. can be used.

        (1) Chemical oil removal: sodium hydroxide 60~80g/L, sodium phosphate 20~40g/L, sodium carbonate 20~40g/L, sodium silicate 3~10gL. The total alkalinity is 65~100g/L, the temperature is 70~90℃ and the time is until the oil is removed.

        (2) Electrochemical oil removal: sodium hydroxide 30~50g/L, sodium phosphate 20~30g/L, sodium carbonate 20~30g/L, water glass 3~5gL. The total alkalinity is 40~60g/L, the temperature is 60~80℃, the current density is 3~10A/dm2, the cathode is 3~5mn, the anode is 1-~5mn, and the electrode material is steel plate or nickel-plated steel plate.

Large-sized stainless steel products such as composite panels and large pressure vessels cannot be put into a degreasing tank for degreasing. They can be wiped with a rag or mop dipped in an aqueous solution of room temperature degreasing agent, washing powder, etc. to remove the degreasing. Large batches, small size and light weight parts are better degreased by rollers. Put these parts into the drum and add appropriate abrasives and degreasers for degreasing. The effect is good, the cost is low and the operation is convenient. But it is not suitable for thin parts that are easily deformed. The standard for degreasing and cleaning is: wash with water after degreasing, and the water film should not crack for 30 seconds.


        2. Select pickling liquid according to the material of stainless steel. Austenitic stainless steel has a better pickling effect with a more corrosive solution, while martensitic stainless steel has a better pickling effect with a less corrosive solution. Different shapes and specifications of stainless steel parts sometimes require different pickling methods.

        2.1 Dip: Smaller parts can be immersed in a pickling tank.

        Martensitic stainless steel pickling can use the following processes:

        (1) Nitric acid 20%(), hydrofluoric acid 8%~10%, balance of water, 3~1min at room temperature

        (2) Nitric acid 140~150g/L Phosphoric acid 110~120g/L, water balance 5~10min at room temperature

        2.2 Scrubbing: If large-sized stainless steel workpieces, such as composite plates and large pressure vessels, cannot be put into the pickling tank for pickling, you can use an acid-resistant cloth dipped in the solution and wipe it repeatedly for pickling. The welds and heat-affected zones can be coated with solution and scrubbed with water sandpaper. The flux and slag contained in the weld can be picked out with tools such as steel needles.

        2.3 Pickling with pickling paste

        For large-sized workpieces such as composite panels and pressure vessels, pickling paste can also be applied for pickling. Commonly used recipes are as follows:

        (1) Nitric acid 10%~15%(), hydrofluoric acid 15%~20%, water 60%~70%.

        (2) 5% nitric acid, 10% sulfuric acid, 40% hydrochloric acid and 45% water. Add clay to the two solutions and mix them into a paste.