The "rust-proof gene" of stainless steel pipes: from chromium-nickel alloy to passivation film, revealing the underlying logic of "stainless steel"

The core of stainless steel pipe's ability to resist corrosion lies in its unique "material defense system". When the chromium content is ≥10.5%, a dense Cr₂O₃ passivation film (thickness is only 0.0001mm) will automatically form on the surface of the steel pipe, like a "molecular shield" to prevent oxygen from contacting iron. This article analyzes this mechanism from a microscopic perspective:

Why is the anti-rust ability of 304 stainless steel pipe (containing 18% chromium and 8% nickel) far superior to that of 201 stainless steel pipe (low chromium and low nickel)? The salt spray test shows that 304 steel pipes can remain rust-free for 500 hours in a 5% sodium chloride solution, while 201 steel pipes can only maintain rust-free for 200 hours;
The role of nickel: nickel can stabilize the austenite structure and enhance the toughness of the passivation film, allowing 304 steel pipes to remain plastic at low temperatures (-196°C), making them suitable for liquid nitrogen delivery pipelines;
The "enhanced buff" of molybdenum: 316 stainless steel pipes have a passivation film with 2%-3% molybdenum added, which increases the ability to resist chloride ion corrosion by 3 times, making them the "standard configuration" for desalination pipelines and chemical reactors.
According to the case of a coastal power plant: ordinary carbon steel pipes rusted through in 3 months, but after being replaced with 316 stainless steel pipes, there was no corrosion for 5 years. Although the initial cost was 3 times higher, the overall life cost was lower - proving that "rust-proof performance determines long-term value."