Does stainless steel rust?
In general, stainless steel does not rust, but if used or maintained improperly, or in a harsh environment, it will rust or tarnish, for example, the photo above shows a stainless steel shackle on a cruise ship corrodes due to long time. prolonged exposure to the marine environment, the surface shows rust and brown spots.
The reason stainless steel doesn't rust is because it contains chromium (Cr) and nickel (Ni). These two metal elements will form a dense oxide film with oxygen, this oxide layer is also called a passivation film, which increases corrosion resistance. When the surface of stainless steel is scratched, it will instantly form a new oxide film, thus preventing further rust and corrosion.
Why does stainless steel rust or stain?
Stainless steel rusts because the oxide film on its surface is continually destroyed, therefore it loses its rust-proof effect, resulting in the continuous oxidation of iron.
What causes stainless steel to rust?
Halides are one of the most serious factors that cause stainless steel to rust, as they can destroy the passivation film. Halides include chloride, fluoride, bromide, iodide and some interhalogen compounds. Among them, chloride is the most common in nature and in everyday life. For example, chlorine ions (Cl-) are widely present in salt, sauce, physiological saline solution, bleach, disinfectant, salt lake water and sea water, etc. over time it will rust or tarnish.
Other factors that can cause stainless steel to rust include: prolonged exposure to inorganic acids such as nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, etc. hydroxide (NaOH), etc.
Different grades of stainless steel have different corrosion resistance
Different types of stainless steel have different rust resistance due to different Cr-Ni content. Other chemical elements that affect corrosion resistance are: carbon (C), phosphorus (P), sulfur (S), molybdenum (Mo), titanium (Ti), copper (Cu), niobium (Nb), etc.
According to the organization methods, they can be divided into 4 classifications:
- Austenite: series 200 (essentially Cr-Mn alloys) and 300 (essentially Cr-Ni alloys), e.g. 201, 302, 304 (18/8 stainless steel), 310, 316, 317, 321, etc.
- Ferrite: essentially Fe-Cr alloys, 400 series, such as 430
- Martensite: essentially Fe-Cr-C alloys, series 400, such as 410, 420, 440, etc.
- Duplex stainless steel: including 2204, 2205, 2507, etc.
In terms of chloride corrosion resistance, the rust resistance rating of common types of stainless steel is as follows:
904L > 2205 > 316L > 316 > 304L > 304 > 302 > 201 > 430 > 440.
