Passivation of Stainless Steel: Understanding and Implementation

The Theory Behind Passivation

The corrosion resistance of stainless steel is primarily due to the high concentration of the Cr element in steel. This element forms a dense passive chromium-rich oxide film on the steel surface, which acts as a barrier to corrosion. However, various contaminants (dirt, grease, scale, free iron, etc.) can compromise this film, reducing its resistance and making passivation essential.

The Techniques for Passivation

A. Dip Passivation

Dip passivation, also known as Immersion Technique, is a process in which stainless steel parts are fully immersed in a passivating acid bath, usually at a specified temperature. This process creates a thin, transparent oxide film that protects the steel from corrosion. While effective, this method has its limitations. In particular, it requires an acid bath, the shape and size of which can limit the size and shape of the parts that can be treated. As a result, it's generally used for smaller parts that can be fully immersed in the bath.

B. Pickling Paste Passivation

Pickling Paste Passivation, also known as the Paste Method, involves applying a paste containing the passivating acid to the steel surface where it forms the protective oxide film. This method is particularly suitable for large stainless steel structures that cannot easily be immersed in an acid bath. Although this method is easy to implement and quite flexible, it creates a more demanding working environment and requires a high level of precision from the operators.

C. Spray Passivation

Spray Passivation, also known as the Spray Technique, involves spraying the passivating acid onto the steel surface. This method can be particularly effective for singular products or equipment with simple internal structures. Spray passivation can cover large areas quickly and is useful where the material or structure cannot be easily immersed. However, it requires specific conditions to be effective, including an enclosed environment to prevent the acid spray from dispersing.

Each passivation method has its unique advantages and suitability. The choice of method will depend on the specific requirements of the stainless steel product or structure to be treated. Understanding the implications of each method is crucial to ensure the optimal passivation process.

Precautions

Before passivation, it's vital to clean the stainless steel surface of any oil or other contaminants. During the pickling and rinsing stages, ensure all steel is completely wetted with no dry areas. Moreover, the control of chloride ion concentration and waste liquid treatment must meet environmental protection requirements. After passivation, quality control is an essential step.

Quality Inspection

After passivation, quality inspection is an important step. This often includes tests for visual inspection, chemical testing, etc. Appearance testing looks for a uniform, silver-white, shiny surface with no significant traces of corrosion. Chemical tests, such as the blue stain test, check the integrity of the passivation film. Note that chemical tests can destroy the passivation film, so precautions must be taken.

In summary, these are the major forms of stainless steel passivation and the precautions that need to be taken. This information will hopefully be of assistance.