Why does stainless steel rust? When people notice brown rust spots on the surface of stainless steel tubes, they often express surprise, assuming that stainless steel is immune to rust. They tend to think there must be an issue with the steel itself.

However, this viewpoint stems from a misunderstanding of stainless steel. In reality, stainless steel can indeed rust under specific conditions.

Stainless steel possesses the remarkable ability to resist atmospheric oxidation, earning it the "stainless" moniker. However, it can still corrode when exposed to environments containing acids, alkalis, and salts.

This corrosion resistance varies depending on the chemical composition of the steel, its state, usage conditions, and the type of environmental exposure.

For instance, 304 steel pipes exhibit excellent corrosion resistance in dry, clean atmospheres but will quickly rust when placed in coastal areas exposed to salt-laden sea spray. On the other hand, 316 steel pipes perform exceptionally well in such conditions.

Therefore, it's essential to understand that not all stainless steel remains corrosion-resistant and rust-free in every environment.

Stainless steel relies on a thin but dense and robust protective film on its surface, primarily composed of chromium-rich oxide. This film prevents the continuous penetration of oxygen, which is crucial to oxidation.

As long as this protective film remains intact, stainless steel retains its resistance to rust and corrosion.

However, if this film is repeatedly damaged, iron atoms from the steel can precipitate, and oxygen from the air or liquids can penetrate, leading to the formation of loose iron oxide and continuous corrosion of the stainless steel surface.

There are various ways in which this protective film can be damaged, some of which are encountered frequently in daily life:

1. Dust and foreign metal particles can accumulate on the surface of stainless steel. When these particles come into contact with humidity, they create a microcell that initiates an electrochemical reaction, damaging the protective film—a phenomenon known as electrochemical corrosion.

2. Organic residues like food stains can adhere to stainless steel surfaces. In the presence of water and oxygen, these residues can form organic acids, leading to corrosion over time.

3. The surface of stainless steel can also come into contact with acids, alkalis, or salts (e.g., lime water on decorative walls), causing localized corrosion.

4. In polluted air containing nitrogen oxide, sulfide, and carbon oxides, condensation can form sulfuric acid, acetic acid, and nitric acid, resulting in chemical corrosion.

All these situations can compromise the protective film on stainless steel surfaces, initiating rust and corrosion. To maintain the lasting brightness and rust resistance of stainless steel, it is advisable to:

1. Regularly clean and scrub the surface of stainless steel locks to remove adhered substances, eliminating external factors that promote rust and corrosion.

2. In coastal areas, consider using 316 stainless steel, as it has superior resistance to seawater corrosion.