Problems with acid cleaning of stainless steel surfaces

6.2 Formulas for pickling, passivation solutions and pastes 6.2.1 Pickling solution formula: 20% nitric acid (specific gravity 1.42), 5% hydrofluoric acid, and the remainder is water. The above are volume percentages. 6.2.2 Pickling paste formula: 20 ml of hydrochloric acid (specific gravity 1.19), 100 ml of water, 30 ml of nitric acid (specific gravity 1.42), and 150 grams of bentonite. 6.2.3 Passivation solution formula: 5% nitric acid (specific gravity 1.42), 4 grams of potassium dichromate, with the remainder being water. The above volume fraction of voids applies when the passivation temperature is at room temperature. 6.2.4 Blunting paste formula: 30 ml of nitric acid (67% concentration), 4 grams of potassium dichromate, along with bentonite (100–200 mesh), all mixed together until a paste is formed. 6.3 Acid Cleaning and Passivation Procedures 6.3.1 Acid cleaning and passivation can only be performed on containers or components that have undergone pretreatment first. 6.3.2 Acid pickling is mainly used for the overall treatment of relatively small, untreated components, and can be carried out by spraying. When the solution temperature is between 21–60°C, check every 10 minutes or so until a uniform white etched finish is achieved. 6.3.3 Pickling paste pickling is mainly suitable for large containers or local treatment. At room temperature, apply the pickling paste evenly and thinly over the equipment (to a thickness of about 2-3 mm). After waiting for one hour, gently brush it away using clean water or a stainless steel brush until a uniform white etched finish is achieved. 6.3.4 Passivation solutions are mainly suitable for the overall treatment of small containers or components; immersion or spraying methods can be used. When the solution temperature is between 48 and 60°C, inspections should be carried out every 20 minutes, while when the temperature is between 21 and 47°C, inspections should be done hourly, until a uniform passivation film forms on the surface. 6.3.5 Passivation paste is mainly suitable for large containers or local treatment. Apply the passivation paste evenly to the surface of the pickled container at room temperature (about 2–3 mm thick), and check after 1 hour until a uniform passivation film has formed on the surface. 6.3.6 Containers or components that have been pickled and passivated must have their surfaces rinsed thoroughly with clean water. Finally, an acidic litmus paper should be used to test any part of the rinsed surface to ensure that the pH value is between 6.5 and **; thereafter, the surface should be dried either by wiping or with compressed air. 6.3.7 After acid washing and passivation, containers and components must not be bumped, scratched, or have their passivation film damaged during handling, lifting, and storage. 6.4 Precautions 6.4.1 When preparing the solution, water should be added to an acid-resistant container in proportion, and then acid should be added slowly in the same proportion, to prevent splashing and injury caused by adding acid too quickly. 6.4.2 For acid pickling and passivation, the first application should be tested first on a small area of a component or on similar materials. 6.4.3 During the pickling process, operators must be meticulous and observe carefully to prevent excessive corrosion of the container surface. Effective measures should be taken to prevent carbon steel components from corroding. 6.4.4 After pickling, the stainless steel surface shall show no obvious signs of corrosion, no patches with uneven coloring, and the welds and heat-treated surfaces shall be free of oxidized colors. After pickling stainless steel pressure vessels, they must be rinsed thoroughly with water; no residual pickling solution is allowed. The passivated stainless steel surface should be rinsed with water, and once it is neutral, the water droplets should be dried off. 6.5 Blue Dot Test 6.5.1 Prepare a solution by adding 1 gram of potassium ferrocyanide K3, 3 milliliters of (65%–85%) nitric acid HNO3, and 100 milliliters of water (it is advisable to prepare it just before use). Then, after soaking filter paper in the solution, it is attached to the surface to be tested, or the solution is applied or dropped directly onto that surface; blue spots should appear within 30 seconds, and the absence of such spots indicates that the test is successful.

The pickling and passivation paste should come with instructions that detail the pickling time, etc.; simply follow those instructions. Furthermore, in practical operations, when the temperature of the workpiece is high (above 50 degrees Celsius), the pickling effect is improved, and the quality of pickling also becomes better.

Any formula containing chloride ions is outright rejected! !

I did as suggested above and also carried out surface treatment; I’m using acid cleaning paste now and applying it for 30 minutes, but it still doesn’t work. It’s still patchy, black in some areas and white in others; during inspection, the areas with a thicker coating appear white while those with a thinner coating appear black. What’s going on here? I would appreciate some guidance from those who are more experienced. Thank you!

Really? Is there nothing wrong with the materials? All the equipment that comes out of our company after washing is in excellent condition........

That’s really the case; we use pickling paste. Thank you in advance for the materials that can be shared from upstairs.

Could some expert provide some information? I’m in urgent need of it.

Could you provide some information? Email: wycwttxjx@sina.com Thank you.

Are there any methods for cleaning and passivating acetic acid storage tanks? It’s urgent!