Stainless steel flanges are widely loved for their aesthetic appeal, corrosion resistance, and durability. However, people may be surprised when brown rust spots appear on the surface of stainless steel flanges. They wonder: why would “stainless steel” rust, and if it does, can it still be considered “stainless steel”? Does it indicate a lack of quality in the material?
In reality, this is a misconception about stainless steel because it can indeed rust under certain conditions.
Stainless steel has the ability to resist corrosion in media containing acids, alkalis, and salts, known as corrosion resistance. It also possesses the ability to resist atmospheric oxidation, known as stainlessness. However, the extent of its corrosion resistance varies depending on the chemical composition of the steel itself, the operating conditions, and the type of environmental media. For example, 304 stainless steel exhibits excellent corrosion resistance in dry and clean environments, but when exposed to coastal areas, it can quickly rust in the salt-laden sea mist. On the other hand, material like 316 has better performance. Therefore, it can be concluded that not all types of stainless steel can remain rust-free in every environment.
The extremely thin, robust, and dense chromium-rich oxide film formed on the surface of stainless steel grants it the ability to resist corrosion. However, if, for some reason, this thin film is continuously damaged, oxygen atoms from the air or liquid can infiltrate, or iron atoms from the metal can continuously dissociate, leading to the formation of loose iron oxide. Consequently, the metal surface undergoes continuous rusting, and the protective film of stainless steel is also compromised.
Here are some common corrosion scenarios in daily life for stainless steel:
1.Accumulation of dust on the stainless steel surface, including adherents containing other metal particles. In humid air, the connection of adherents and stainless steel, facilitated by condensation water, forms a microbattery, triggering electrochemical reactions that break down the protective film. This is known as electrochemical corrosion.
2.Adhesion of organic substances, such as juices from fruits and vegetables, soup, or phlegm, to the surface of stainless steel. In the presence of water and oxygen, these substances can form organic acids, contributing to corrosion.
3.Stainless steel surfaces can accumulate substances containing acids, alkalis, and salts (such as alkaline water used in wall decoration or splashes of lime water), leading to localized corrosion. Additionally, in polluted air containing high levels of sulfides, carbon oxides, and nitrogen oxides, condensation can form sulfuric acid, nitric acid, acetic acid, causing chemical corrosion.
Exactly, all the mentioned scenarios can damage the protective film on the stainless steel surface and lead to rusting. Therefore, to ensure that the metal surface remains permanently bright and rust-free, it’s recommended to clean and scrub the stainless steel surface to remove adherents and eliminate external factors. In coastal areas, using 316-grade stainless steel is advisable as it can resist corrosion from seawater. Some stainless steel pipes in the market may not meet the required chemical composition standards, failing to meet the specifications of materials like 304, which can also lead to rusting. It’s crucial for users to carefully select products from reputable manufacturers.