Views: 0 Author: Site Editor Publish Time: 2025-09-03 Origin: Site
Many people choose stainless steel products because of the "stainless" in their name, believing it will never rust. Therefore, when they discover brown rust spots on their stainless steel sinks, spoons, or outdoor railings, they are often surprised and confused: "Did I buy fake stainless steel?"
In reality, stainless steel isn't completely rust-proof, but it does have excellent corrosion resistance. Whether or not it rusts depends on the material, the environment in which it's used, and how it's maintained. To understand this, we need to first understand the principles behind stainless steel's "stainlessness."
Ⅰ. Why is stainless steel “rust-proof”?
The key to stainless steel's "rust-proof" properties lies in a single metallic element: chromium (Cr).
Ordinary steel, when exposed to air, reacts chemically with oxygen and moisture, forming a loose, porous rust (iron oxide), which continuously corrodes the metal underneath. Stainless steel, however, typically contains over 12% chromium.
When chromium comes into contact with oxygen in the air, it quickly forms an extremely thin, dense, strong, and protective film of chromium oxide (Cr₂O₃) on its surface. This film is called the "passivation layer."
This passivation layer:
1. It is extremely stable: It effectively isolates the metal substrate from the external environment.
2. It has self-healing properties: If the surface is scratched or damaged, the exposed chromium will re-combine with oxygen to form a new passivation film, repairing the protective layer.
It is this invisible "golden bell" that gives stainless steel its corrosion (rust) resistance.
Ⅱ. So, why does stainless steel rust?
When this passive film is continuously damaged and fails to heal itself, the underlying iron substrate begins to rust. Common causes include the following:
Surface deposits: The most common culprit
Salt: Salt-rich air in coastal areas, or residual salt and sweat from tableware, absorb moisture and damage the passive film, leading to pitting.
Acidic and alkaline chemicals: Strong acids and bases (such as cleaning agents, renovation cement, and limewash) corrode the passive layer.
Organic liquids: Liquids like vegetable soup, noodle soup, and spit, exposed to humidity, can form organic acids over time, corroding the surface.
Dust and iron filings: At construction sites or near steel mills, airborne iron particles can adhere to the stainless steel surface. These iron filings themselves rust and "graft" their rust onto the stainless steel, forming loose rust. This rust is usually easily removed.
Mechanical wear: Damages the protective layer. Scratching with hard objects like steel wool and metal brushes can severely damage the passive film. If subsequently exposed to moisture, it is highly susceptible to rust.
Chemical Damage: The Threat of Chloride Ions
Chloride ions (Cl⁻) are the natural enemy of the passive film, penetrating it and causing localized corrosion (usually pitting).
Common sources of chloride ions include table salt (sodium chloride), bleach (sodium hypochlorite), some cleaning and disinfecting agents, and seawater.
Material Issues: Not All Stainless Steels Are Created Equally
Stainless steel is a large family of grades, primarily categorized as 201, 304, and 316, with increasing corrosion resistance.
304 stainless steel (also known as 18/8 stainless steel, containing 18% chromium and 8% nickel): The most common stainless steel in everyday life, used in sinks, tableware, and appliances, offers excellent corrosion resistance.
316 stainless steel: Based on 304, molybdenum is added, significantly enhancing resistance to chloride ion corrosion. It is commonly used in the marine, chemical, and medical industries, as well as in coastal areas, and is also known as "marine steel."
In some harsh environments (such as coastal areas and chemical plants), if low-grade 201 or nickel-free "stainless iron" is mistakenly used, the risk of rust will be greatly increased.
Ⅲ. How to Treat and Prevent Rust on Stainless Steel?
1.Treating Existing Rust:
For surface rust: Use a dedicated stainless steel cleaner, toothpaste, or white vinegar to clean it, then rinse with clean water and wipe dry with a soft cloth.
Remember: Avoid using hard objects like steel wool to scrub, as this can cause further scratches. Use a sponge, soft cloth, or nylon brush instead.
2.Preventing Rust is More Important:
Keep it Clean and Dry: Wipe it frequently to remove surface debris, especially soy sauce, oil, and salt, and keep it dry.
Choose the Right Material: Choose a stainless steel grade based on the intended use. For example, 316 stainless steel is recommended for outdoor decoration in coastal areas.
Proper Cleaning: Avoid using cleaners containing chlorine bleach. Rinse thoroughly with clean water and wipe dry.
Regular Maintenance: For important items, apply stainless steel maintenance oil regularly to strengthen the protective film.
Conclusion
In short, stainless steel isn't "rust-proof," but rather "rust-resistant." Its strong corrosion resistance comes from the protective chromium film on its surface. Once this film is damaged, it can still rust.
The rust you've encountered on stainless steel is most likely caused by other metal particles or corrosive substances adhering to the surface. Proper use, cleaning, and maintenance can completely prevent rust and keep your stainless steel products looking like new for a long time.
