What is stainless steel ？
Definition of Stainless Steel
Stainless steel is a type of steel that contains less than 2% carbon (C) and more than 2% iron.
In the stainless steel manufacturing process, alloying elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti) and molybdenum (Mo) are added to improve its performance and make it corrosion resistant or rust free. This is what gives steel its name: “stainless”.
Why does stainless steel have different qualities?
The stainless steel manufacturing process involves adding different types and amounts of alloying elements to produce varying levels of performance.
To distinguish between these different levels, stainless steel is categorized into different grades.
The table below lists common alloying elements used in decorative stainless steel and is provided for reference purposes only.
Stainless steel chemical composition chart (percentage%)
| Steel grades | W | Yes | Mn | P | s | Cr | No |
|---|---|---|---|---|---|---|---|
| 304 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 18-20 | 8-10 |
| 301 | ≤0.15 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 16-18 | 6-8 |
| 202 | ≤0.15 | ≤1.00 | 7.5-10 | ≤0.05 | ≤0.03 | 17-19 | 4-6 |
| 201 | ≤0.15 | ≤1.00 | 5.5-7.5 | ≤0.05 | ≤0.03 | 16-18 | 3.5-5.5 |
What kind of stainless steel doesn't rust easily?
The 3 main factors that affect stainless steel rust:
Alloy element content
Generally, if the chromium content is 10.5%, stainless steel is less likely to rust.
The higher the chromium-nickel content, the greater the corrosion resistance.
For example, 304 stainless steel with a nickel content of 8 to 10% and a chromium content of 18 to 20% will not normally rust.
The manufacturing process of stainless steel can impact its corrosion resistance.
Large stainless steel factories with advanced casting technology, equipment and techniques have better control over alloying elements, impurity removal and billet cooling temperatures, resulting in more stable and reliable product quality. As a result, the stainless steel produced by these factories is less likely to rust.
In contrast, small steel mills with poor equipment and technology often have difficulty removing impurities during the smelting process, making their stainless steel more prone to rust.
External environment
Stainless steel in a dry, well-ventilated environment is less likely to rust.
However, in conditions of high humidity, continuous rain or environments with high acidity or alkalinity, stainless steel is more susceptible to rust.
Even 304 stainless steel can rust if the surrounding environment is poor enough.
Is non magnetic stainless steel better? Isn't magnetic stainless steel the same as grade 304?
Many customers go to the market with a small magnet in their pocket to test whether stainless steel is magnetic, believing that non-magnetic stainless steel will not rust.
However, this is a common misunderstanding. The magnetic property of stainless steel is determined by its structure and not by its resistance to rust.
Different solidification temperatures during the manufacturing process result in different stainless steel structures such as ferrite, austenite and martensitic. Ferrite and martensitic stainless steel are magnetic, while austenitic stainless steel has good mechanical properties and weldability, but is not as corrosion resistant as ferrite stainless steel.
The market also circulates low-nickel, high-manganese stainless steel from the 200 and 300 series, which are non-magnetic but have lower performance compared to high-nickel 304 stainless steel.
Furthermore, 304 stainless steel may exhibit slight magnetic properties after going through drawing, annealing, polishing and casting processes.
Therefore, using magnetic properties to evaluate the quality of stainless steel is not a scientific method and may lead to misunderstandings.
Why does stainless steel also rust?
Many customers go to the store with a small magnet in their pocket to determine whether stainless steel is magnetic, assuming that non-magnetic stainless steel does not rust.
However, this is a false understanding. The magnetic property of stainless steel is determined by its structure and not by its resistance to rust.
When customers see brown rust stains on the surface of stainless steel, they are often surprised, thinking that “stainless steel does not rust, if it rusts, then the quality of the steel must have a problem”.
In reality, this is a one-sided and misinformed view of stainless steel.
Stainless steel can still rust under certain conditions. It has the ability to resist atmospheric oxidation, but its corrosion resistance in acidic, alkaline and saline environments can change based on its chemical composition, state of interaction, conditions of use and type of environmental medium.
For example, 304 stainless steel has excellent anti-corrosion properties in a dry and clean atmosphere, but if it is moved to a coastal area, it will rust quickly in the salt-rich sea fog.
Therefore, not all types of stainless steel can resist corrosion and rust at all times.
Stainless steel forms a very thin, thin and stable layer of chromium oxide on its surface, which prevents oxygen infiltration and oxidation and resists corrosion. But if this protective film is continuously damaged, air, liquid and oxygen atoms will continuously infiltrate and separate the metallic iron atoms, causing the surface to rust continuously.
There are many ways the protective film on stainless steel surfaces can be damaged, including:
- Electrochemical corrosion: Occurs when stainless steel surfaces contain metallic powder or other metallic particles that, in humid air, form a microbattery with condensed water, damaging the protective film.
- Organic acid corrosion: This happens when stainless steel surfaces are contaminated with organic juices (such as melons or noodle soup), which form organic acids in the presence of water and oxygen and eventually corrode the metal surface.
- Local corrosion: occurs when stainless steel surfaces are contaminated with acidic, alkaline or saline substances (such as alkaline water or lime water spray), leading to localized corrosion.
- Chemical corrosion: This happens in polluted air (with high levels of sulfur, oxides and oxidized oxygen) when sulfuric acid, nitric acid and acetic acid form in condensed water and cause chemical corrosion.
How to prevent stainless steel from rusting?
The conditions mentioned above can damage and corrode the protective film on the stainless steel surface.
To maintain a shiny, rust-free metal surface, we recommend:
- Regular cleaning and scrubbing of the stainless steel surface to remove contaminants and prevent external factors from causing rust.
- Use 201 or 202 stainless steel in non-industrial, non-corrosive environments as these types are more prone to rust in coastal areas.
- Use of grade 304 stainless steel in coastal areas as it is resistant to seawater corrosion.
How to deal with stainless steel rust?
a) Chemical Methods:
Using pickling paste or spray to help remove rust and reform the chromium oxide film can restore stainless steel's corrosion resistance.
It is critical to properly wash stainless steel with water after pickling to remove any contaminants and acid residue.
After all processes, use polishing equipment to repolish the stainless steel and seal it with polishing wax.
For small, light rust stains, a 1:1 mixture of gasoline and motor oil can also be used to remove them.
b) Mechanical Methods:
Sandblasting, cleaning with glass or ceramic particles, sanding, cleaning and polishing.
It is possible to erase previously applied materials, polish materials or cover polluted areas by mechanical means.
However, all forms of pollution, especially exotic iron particles, can be a source of corrosion, especially in humid environments.
Therefore, it is best to clean the metal surface by mechanical means when it is dry.
Please note that mechanical cleaning can only clean the metal surface and does not change its anti-corrosion ability.
We recommend using polishing equipment to repolish the surface after mechanical cleaning and sealing it with polishing wax.
