The treatment process of passivating stainless steel pipes mainly involves forming a dense passivation film on the surface of stainless steel through chemical or electrochemical methods, thereby improving its corrosion resistance. Passivation treatment usually includes steps such as cleaning, acid washing, passivation, rinsing, and drying. The following are common processing techniques for passivating stainless steel pipes:
1. Pre treatment (cleaning and acid washing)
Before passivation treatment, the surface of the stainless steel pipe should be cleaned and pickled to remove oil stains, oxides, and other pollutants. This is a key step in ensuring the uniform formation of passivation film.
Cleaning: Use neutral cleaning agents or solvents to clean the surface of stainless steel pipes, removing oil stains, lubricants, and other organic matter.
Acid washing: Acid washing can remove surface oxides, rust, metal impurities, etc. The commonly used pickling solutions are:
A mixed solution of nitric acid (HNO ∝) and (HF).
A mixed solution of sulfuric acid (H ₂ SO ₄) and nitric acid (especially suitable for removing oxide layers and welding marks).
Acid washing time and temperature: Acid washing is usually carried out between room temperature and 50 ° C, and the acid washing time depends on the degree of contamination of the pipe, usually ranging from a few minutes to 20 minutes.
2. Passivation treatment
Passivation treatment is a process of immersing stainless steel pipes in a solution containing oxidants to form a stable oxide film (mainly chromium oxide film) on the surface, thereby improving the corrosion resistance of stainless steel. Common passivation methods include chemical passivation and electrochemical passivation.
Chemical passivation:
Chemical passivation uses an oxidant solution, and common oxidants include:
Nitric acid (HNO ∝) solution.
Chromium sulfate (CrO3) solution.
A mixed solution of hydrogen peroxide (H ₂ O ₂) and ammonia water (NH ∝· H ₂ O).
The concentration and temperature of passivation solution need to be controlled according to different types of stainless steel and actual requirements, generally between 30 ° C and 70 ° C.
The passivation time is usually around 15 minutes to 30 minutes, depending on the degree of surface contamination of the pipe and the concentration of the passivation solution.
Electrochemical passivation:
Electrochemical passivation typically uses an electrolyte containing oxidants such as molybdate or nitrate, and places stainless steel tubes into an electrolytic cell at a certain voltage. The electrolysis process can enhance the density and uniformity of the surface passivation film, thereby improving the corrosion resistance of stainless steel pipes. The process parameters for electrochemical passivation include:
The type of electrolyte (usually nitric acid, sulfuric acid, etc.).
Electrolytic current density, processing time, and temperature.
3. Rinse
After completing the passivation treatment, it is necessary to rinse the stainless steel pipe to remove any residual acid or passivation solution on the surface, in order to prevent them from continuing to corrode the surface.
Rinse with plenty of clean water until there are no residual acidic substances.
For chemical passivation, deionized water or distilled water can be used for rinsing to avoid impurities in the water affecting the passivation film.
4. Drying
The cleaned stainless steel pipes need to be dried to prevent moisture from causing corrosion or leaving water marks.
Can be dried using hot air or natural wind.
If conditions permit, an oven can be used for heating and drying.
5. Post processing and quality inspection
The passivated stainless steel pipe needs to undergo surface inspection to ensure that the passivation film is uniform, dense, and free of obvious defects. Common inspection methods include:
Acid drop test: Detect whether the surface is uniformly passivated by adding acidic solution dropwise.
Metallographic analysis: Check the thickness and quality of the passivation film.
Corrosion resistance test: Evaluate passivation effect through methods such as salt spray testing.
After passivation treatment, if it is necessary to further improve the surface quality of the pipe, other post-treatment processes such as bright treatment, polishing, etc. can be carried out.
summary
The treatment process for passivated stainless steel pipes includes preliminary cleaning and acid washing, passivation treatment, rinsing, drying, and quality inspection. The purpose of passivation process is to enhance the corrosion resistance of stainless steel and extend its service life. Different types of stainless steel may require different passivation solutions and treatment conditions, so suitable processes need to be selected based on specific materials and applications.