Large stainless steel storage tanks require pickling and passivation; how is the quality of pickling and passivation tested?

Blue spot testing of stainless steel after acid washing and passivation is a common surface quality inspection method, primarily used to detect whether there is any residual free iron or other contaminants on the stainless steel surface, thereby assessing the effectiveness of the passivation process. The following are the detailed instructions for blue dot detection: 1. Detection principle Blue dot detection is based on the principle that potassium ferrocyanide (K₃) reacts with free iron ions to produce prussian blue (a blue precipitate). If there is free iron or other iron contaminants on the stainless steel surface, blue spots will appear after adding a potassium ferrocyanide solution. 2. Inspection steps: Surface cleaning: Clean the stainless steel surface with alcohol or ** to ensure there is no oil or dust present. Reagent preparation: Mix a 1% potassium ferrocyanide solution with a 1% hydrochloric acid solution in a 1:1 ratio. Reagent addition: Use a dropper or cotton swab to apply or drop the mixed solution evenly on the stainless steel surface. Observe the reaction: Let it sit for 1-2 minutes and check whether blue spots appear on the surface. Result evaluation: No blue spots indicate the absence of free iron on the surface, suggesting good passivation. Blue spots: This indicates the presence of free iron or other contaminants on the surface, indicating that the passivation process was not thorough enough. 3. Precautions: Reagent concentrations: Ensure that the concentrations of potassium ferrocyanide and hydrochloric acid are accurate to avoid deviations in the test results. Reaction time: The reaction time should not be too long, as this may lead to misjudgments. Environmental cleanliness: The testing environment should be free of iron dust or other contaminants to avoid interfering with the results. Safety precautions: Wear gloves and goggles when handling to prevent the reagents from coming into contact with the skin or eyes. 4. Significance of testing: Blue spot testing allows for a rapid and intuitive assessment of the pickling and passivation effectiveness of stainless steel, ensuring that the surface quality meets the required standards. For applications that require high corrosion resistance (such as in the food, pharmaceutical, and chemical industries), blue spot testing is an important quality control method. In summary, the detection of blue spots after stainless steel pickling and passivation is a simple and effective method for assessing surface quality, with the effectiveness of passivation being determined through the reaction between potassium ferrocyanide and free iron. When operating, attention must be paid to reagent concentration, reaction time, and environmental cleanliness to ensure accurate and reliable test results.

After pickling and passivation of large stainless steel storage tanks, due to their large surface area, the blue spot method is inconvenient and time-consuming to apply. Therefore, the mercury salt test (mercury sulfate test) or copper sulfate test is generally recommended to evaluate the quality of passivation. These two methods can cover large areas quickly and provide accurate detection results. At the same time, auxiliary judgment can also be made through surface curing conditions and visual inspection. For projects with special requirements, it is recommended to contact a professional third-party testing agency for more detailed testing. .

Blue dot inspection is also a form of random sampling; there is no need for large-scale testing. It is sufficient to randomly inspect typical areas and those with inconsistent colors