What Causes Stainless Steel Flanges to Rust?

Stainless steel flanges are favored for their attractive appearance, corrosion resistance, and durability, as they are less prone to damage.

However, brown rust spots appearing on the surface of stainless-steel flanges can be surprising. People may wonder why "stainless steel," which is supposed to resist rusting, shows signs of rust. Is there an issue with the material itself?

What Causes Rust on Stainless Steel Flanges?

In reality, viewing stainless steel in this manner is one-sided and incorrect. Under certain conditions, stainless steel can indeed rust.

Stainless steel exhibits resistance to atmospheric oxidation, known as stainlessness, and corrosion resistance in environments containing acids, alkalis, and salts. However, the degree of its corrosion resistance is contingent upon factors such as its chemical composition, structural state, operating conditions, and the type and variability of environmental media.

For instance, 304 stainless steel demonstrates outstanding corrosion resistance in dry and clean atmospheres. However, when exposed to sea spray abundant in salt, such as in coastal regions, it is prone to rusting rapidly. Conversely, 316 material exhibits superior performance in such conditions. Thus, it's important to recognize that not all stainless steels offer universal resistance to corrosion and rust across all environments.

Stainless steel's protective coating can also be compromised.

Stainless steel resists rust and corrosion by forming a thin, robust, and stable chromium-rich oxide film on its surface, known as a protective film. This film acts as a barrier, preventing further penetration of oxygen atoms and subsequent oxidation.

Once this film is continuously damaged, oxygen atoms from the air or liquid can infiltrate, or iron atoms within the metal can precipitate out, forming loose iron oxide and continuous rusting of the metal surface. This protective film can be compromised in various ways.

The following are the most common in daily life.

50. When dust or foreign metal particles containing other metal elements adhere to the surface of stainless steel, and when humidity is present in the air, condensation between the adhesion and the stainless steel can create a microcell. This can trigger an electrochemical reaction, leading to damage of the protective film, a phenomenon known as electrochemical corrosion.
50. When organic matter juices, such as squash, noodle soup, saliva, etc., adhere to the surface of stainless steel and come into contact with water and oxygen, they form organic acids. This can result in longer-lasting metal surface corrosion compared to corrosion caused by organic acids alone.
50. The adhesion of acidic, alkaline, and salty substances to the surface of stainless steel, such as from decorative walls exposed to alkaline water and lime water spray, can lead to localized corrosion.
50. When stainless steel is exposed to polluted air containing sulfides, carbon dioxide, and nitrogen oxides and encounters condensation, it can form sulfuric acid, nitric acid, and acetic acid droplets. This leads to chemical corrosion. 

All of the aforementioned factors can compromise the protective film on the surface of stainless steel, leading to rust corrosion. Hence, it is crucial to maintain the metal surface in a permanently bright and rust-free condition.

Our suggestions:

50. The decorative stainless steel surface requires regular cleaning and scrubbing to remove contaminants and mitigate external factors that may initiate corrosion.
50. Coastal regions ought to utilize 316 stainless steel due to its ability to withstand corrosion from seawater.
50. Certain stainless-steel tubes on the market fail to meet the chemical composition standards outlined by their respective national regulations and do not adhere to the specifications of 304 materials. Consequently, they are prone to rusting, underscoring the importance of users meticulously selecting products from reputable manufacturers.

Conclusion

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