Pickling and Passivation for Stainless Steel Pressure Vessels

The pickling and passivation process for stainless steel pressure vessels is a crucial procedure to enhance their corrosion resistance and extend their lifespan. This process not only removes surface impurities and oxides but also forms a dense passivation layer on the surface, improving the corrosion resistance of stainless steel pressure vessel. It is widely used in industries such as chemical, food, and pharmaceuticals. Below is a detailed overview of this process, including steps and requirements.

Cleaning Preparation

The first step in the pickling and passivation process is cleaning. The purpose of this step is to remove all surface impurities, such as oil, dust, welding slag, and oxides. Incomplete cleaning will directly affect the effectiveness of the pickling and passivation treatment.

1. Preliminary Cleaning

Preliminary cleaning is usually done by rinsing with clean water to remove loose dust and impurities from the surface. This step helps prevent impurities from being hardened or adhering to the surface during the pickling process, ensuring the best treatment results.

2. Deep Cleaning

After rinsing, the surface is further wiped using sandpaper or a steel brush dipped in gasoline or carbon tetrachloride, with special attention given to the weld seams and heat-affected zones. This step ensures that oil, rust, and other contaminants are completely removed, preventing any impact on the pickling and passivation quality.

3. Surface Inspection

After cleaning, the surface quality must be inspected to ensure that no welding slag, spatter, or other residues remain on the weld seams, and that the steel surface is free of deep scratches, grinding marks, or other defects. If issues such as scratches or dents are found, fine grinding wheels should be used, and polishing may be needed to ensure a smooth and clean surface for the pickling and passivation process.

Pickling Process

Pickling is the first step of the pickling and passivation treatment. The main purpose is to remove oxides, rust, welding spatter, and other impurities from the stainless steel surface, while also enhancing its appearance. Pickling not only cleans the surface but also provides an ideal base for the formation of the passivation layer.

1. Selection and Control of Pickling Solution

The concentration, temperature, and time of the pickling solution must be carefully controlled. A common pickling solution consists of a mixture of sulfuric acid and hydrochloric acid. The acidity should be controlled between 10% and 20%, as either too high or too low acidity will affect the pickling process. The temperature should be kept between 60°C and 80°C to effectively remove surface oxides without excessively corroding the metal.

2. Pickling Method

During the pickling process, the pressure vessel can be treated by either immersion or spraying. Immersion usually lasts for 10-30 minutes, depending on the surface condition. If spraying is used, the pickling solution must be evenly applied to the surface to ensure uniform coverage.

3. Precautions

It is essential to control the time and temperature during pickling to avoid local over-corrosion or pitting. After pickling, the vessel should be thoroughly rinsed to remove all pickling solution, preventing any residual solution from reacting with the passivation solution and affecting the passivation process.

Passivation Process

Passivation is the subsequent treatment after pickling. Its purpose is to form a dense oxide layer on the surface of the pressure vessel, improving its corrosion resistance. The formation of the passivation layer depends on the choice of passivation solution and control of treatment time.

1. Selection of Passivation Solution

The choice of passivation solution depends on the material and surface condition of the stainless steel. For austenitic stainless steel pressure vessels, passivation solutions based on nitric acid are commonly used. Nitric acid is effective in removing any remaining oxides from the pickling process and forms a dense oxide film on the surface.

2. Passivation Time and Temperature Control

The passivation treatment generally lasts 20-30 minutes, depending on the concentration of the passivation solution and the condition of the vessel surface. The temperature is typically controlled between 30°C and 50°C, ensuring good passivation quality while avoiding excessive corrosion of the metal surface.

3. Post-Passivation Rinsing

After passivation, the pressure vessel must be thoroughly rinsed to ensure no passivation solution remains on the surface. This is to prevent any impact on the vessel's subsequent use. The rinsing should be done with clean water until the rinsing solution reaches a neutral pH, ensuring no adverse effects on future operations.

Safety and Operational Precautions

During the pickling and passivation process, operators must take appropriate safety precautions to avoid exposure to pickling and passivation solutions, which can be harmful to the skin and respiratory system. Operators should wear rubber gloves, protective goggles, acid-resistant shoes, and protective masks. Proper ventilation is essential, especially in enclosed environments, to ensure safe operation.

Quality Control and Inspection

After the pickling and passivation treatment, a thorough quality inspection of the pressure vessel must be conducted. The inspection includes:
Surface color and uniformity: The treated surface should be a uniform silver-white color with no noticeable corrosion marks.
Weld seams and heat-affected zones: There should be no oxidized color or uneven discoloration on the weld seams and heat-affected areas.
Quality of the passivation film: The surface should have a dense passivation layer without any peeling.

Conclusion

The pickling and passivation treatment of stainless steel pressure vessels is a complex and meticulous process. Through cleaning, pickling, and passivation, the corrosion resistance and lifespan of stainless steel are significantly enhanced. Each step requires strict control to ensure that the final result meets the required standards. As the demands for stainless steel materials continue to rise, the application of pickling and passivation technology will become more widespread and essential in various industries.