Essential Knowledge of Pickling Passivation
When designing pressure vessels, stainless steel is a commonly used material. For products that require resistance to corrosion or specific operating conditions, pickling and passivation are essential processes.
Stainless steel pipe fittings
Why pickling and passivation are necessary?
During the processing of stainless steel, black and yellow oxide scales can form on its surface. To improve both the appearance and corrosion resistance of stainless steel, it undergoes pickling and passivation after processing. This treatment removes the oxide scales produced during welding and high-temperature processing, giving the stainless steel a bright, shiny appearance. Additionally, it forms a chromium-rich oxide layer on the surface, preventing further oxidation and enhancing corrosion resistance. This significantly improves the surface quality and extends the lifespan of stainless steel products.
Standard steps of pickling and passivation
The primary steps involved in pickling and passivation of stainless steel include:
Pickling: This step is designed to remove impurities and contaminants from the surface of stainless steel and to brighten the surface.
Passivation: This process creates a dense oxide layer on the stainless steel surface, which significantly enhances its corrosion resistance without altering its natural color.
Methods of pickling and passivation
Depending on the application method and the characteristics of the workpiece, several pickling and passivation methods can be used:
Immersion Method: Suitable for small workpieces such as stainless steel hardware, pipes, and valves. This method is effective for large-scale, continuous operations.
Application Method: Using stainless steel pickling paste, this method is ideal for on-site application, particularly for treating weld seams, welding discoloration, and hard-to-reach corners on large stainless steel products.
Spraying Method: Suitable for fixed locations and enclosed environments, this method is used for single products or equipment with simple internal structures.
Pipeline Circulation Method: This method is used for pickling and passivation of welded stainless steel parts through a circulation system.
Additionally, stainless steel pickling and passivation liquid is a specially formulated chemical that combines both pickling and passivation functions. It can effectively remove rust, weld stains, and grease while passivating the stainless steel workpieces, greatly enhancing their corrosion resistance. This liquid typically does not contain nitric acid or strong inorganic acids, making it environmentally friendly, simple to use, economical, and practical.
Cold galvanizing threaded flanges
Detection of pickling and passivation
For detecting the effectiveness of stainless steel pickling and passivation, several standards are used. Besides the common visual inspection method, the internationally recognized blue point test is often employed.
In China, the standard GB/T 25150-2010, “Testing Method for Quality of Passivation Film on Austenitic Stainless Steel in Chemical Cleaning of Industrial Equipment – Blue Point Method,” is widely used. This standard outlines the test conditions, methods, and steps for evaluating the quality of the passivation film on austenitic stainless steel equipment and components during chemical cleaning. It is applicable to passivation film quality testing for austenitic stainless steel equipment that comes into contact with strong oxidizing media, and can also be referenced for other situations.
By adhering to these guidelines and standards, Haihao Group ensures that our stainless steel products maintain the highest quality and durability, providing our customers with reliable and long-lasting solutions.