Stainless Steel: Things You Should Know (2018 Updated)

What’s Stainless Steel

What’s Stainless Steel

Stainless Steel Definition

Stainless steel is a type of steel, which refers to a steel containing carbon (C) under 2% and iron more than 2%.

In the process of stainless steel smelting, alloying elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo) etc are added in order to improve the performance of steel which made it corrosion resistance (i.e., not rust), which is what we call the stainless steel.

 

Why Stainless Steel Has Different Grades?

In the process of stainless steel smelting, different varieties and amount of stainless steel alloy elements will make different steel performance. In order to distinguish, we name it in different stainless steel grades.

The following “alloy elements” content table of the common decorative stainless steel is for reference only:

Stainless Steel Chemical Composition Chart (percentage %)

Steel GradesCSiMnPSCrNi
304≤0.08≤1.00≤2.00≤0.045≤0.0318-208-10
301≤0.15≤1.00≤2.00≤0.045≤0.0316-186-8
202≤0.15≤1.007.5-10≤0.05≤0.0317-194-6
201≤0.15≤1.005.5-7.5≤0.05≤0.0316-183.5-5.5

 

What Kind Of Stainless Steel Is Not Easy To Rust?

Stainless Steel Rust

Top 3 Factors Affecting Stainless Steel Rust:

  1. The content of “Alloy Element”

Generally speaking, if chromium content in 10.5%, the stainless steel is not easy to rust.

The higher the content of chromium nickel, the better the corrosion resistance. If the content of the nickel content of 304 stainless steel is 8-10% and the content of chromium reaches 18-20%, such stainless steel will not be rusted normally.

  1. The smelting process of stainless steel can also affect the corrosion resistance of stainless steel.

Large stainless steel factories with good smelting technology, advanced equipment and advanced technology have better control of the alloying elements, impurity removal, the billet cooling temperature, so they have stable and reliable product quality, therefore, the stainless steel made by them is not easy to rust.

On the contrary, some small steel plant has poor equipment and technology, in the process of smelting, impurities cannot be removed, the produced stainless steel will inevitably rust.

  1. External Environment

Stainless steel under environment with dry air and good ventilation is not easy to rust

However, if the air in high humidity, continuous rain weather, or the air in high acid alkalinity, the stainless steel is easy to rust.

For 304 grade stainless steel, if the surrounding environment is too poor, it will also rust.

 

Stainless Steel Without Magnetic Is Good One? It’s Not 304 Stainless Steel If With Magnetic?

Stainless Steel With Magnetic

Many customers go to the market to buy stainless steel with a small magnet in their pocket, if magnetic don’t attract it, then they will take it as good stainless steel.

Because they think if without magnetism, it will not rust.

In fact, it is a false understanding.

Stainless steel with or without magnetic structure is decided by it’s own structure. As a result of the different solidification temperature, the molten steel in the solidification process will form stainless steel with different structure, such as “ferrite stainless steel”, “austenite stainless steel” and ” “martensitic stainless steel”. Among them, ferrite and martensitic stainless steel have magnetic.

However, the austenite stainless steel has good comprehensive mechanical properties, and good weldable performance, but if only consider corrosivity,  ” ferrite ” stainless steel with magnetic is better than “austenitic”.

Currently,  the market circulation of less nickel high manganese content in the so-called 200 series, 300 series stainless steel are also without magnetic, but the performance is much worse than 304 stainless steel with high nickel. Instead, 304 stainless steel also has a slight magnetc after drawing, annealing, polishing and casting process.

Therefore, to judge stainless steel from with or without magnetic property is a kind of misunderstanding, also is not scientific.

 

Why Stainless Steel Also Rust?

When the surface of the stainless steel is brown rust (spot), people are surprised: “stainless steel will not rust, if rust it’s not stainless steel, maybe steel quality has problem.”

In fact, this is a one-sided erroneous view which lack of understanding of stainless steel.

Stainless steel also rusts under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, which is rustless, but also has the ability to resist corrosion in the medium of acid, alkali and salt.

But its corrosion resistance ability is changed with its chemical composition, state of interaction, conditions of use and the type of environmental media.

For example, the 304 stainless steel in the dry clean atmosphere has an excellent anti-corrosion capability, but if move it to the coastal area, it will soon get rusted in the sea fog which contains a lot of salt.

Therefore, not any kind of stainless steel at any time can resist corrosion, and not rust.

Stainless steel is formed on the surface of a layer of a very thin, fine and stable solid rich chromium oxide film (protective film), which will prevent oxygen to infiltration, oxidation, and resist corrosion.

But once this kind of film is continuously damaged, air or liquid and oxygen atoms will constantly infiltration or metallic iron atom separation out unceasingly and forming loose iron oxide, the metal surface will rust constantly.

There are many forms of damage to this surface film, the commonly seen in daily life are as follows:

1. Stainless steel surface containing other metal dust or heterogeneous metal makings attachments, in the humid air, attachments and the condensed water between stainless steel connect into a micro battery, which triggers the electrochemical reaction, therefore, the protective film is damaged. This is called the electrochemical corrosion.

2. The stainless steel surface adheres to organic juices (such as melon, noodle soup, etc.), forming organic acids in the presence of water oxygen, and for long it will generate the corrosion of the metal surface by organic acids.

3. The surface adhesion of stainless steel contains acid, alkali and salt substances (such as alkaline water and lime-water spray) causing local corrosion.

4. In polluted air (containing a large amount of sulphide, oxide, oxygen-oxidized atmosphere), sulfuric acid, nitric acid and acetic acid are formed in condensed water, causing chemical corrosion.

How to prevent stainless steel from rusting?

The above conditions can cause damage and corrosion of the stainless steel surface protection film.

Therefore, to ensure that the metal surface is permanently bright and not rusted, we recommend:

① It is necessary to clean and scrub the surface of the stainless steel regularly to remove the attachment and remove the external factors causing the rust.

② A type of 201 and 202 stainless steel in the market is easy to rust in the coastal area and it is suitable for use without industrial pollution or air corrosion.

③ The seaside area should use 304 material stainless steel, 304 material can resist seawater corrosion.

 

How To Deal With Stainless Steel Rust?

a) Chemical Methods:

Using pickling paste or spray to assist rust position to re-form a chrome oxide film which make it restore corrosion resistant ability. After pickling, proper wash with water is very important in order to remove contaminants and acid residue.

Stainless steel pickling pastes

After all process, use the polishing equipment to repolish and seal with polishing wax.

For local mild rust, a 1:1 gasoline and engine oil mixture can also be used to remove the rust stains.

b) Mechanical Methods:

Sandblast cleaning, cleaning with glass or ceramic particles, grinding, cleaning and polishing.

Stainless Steel Sandblast Cleaning

It is possible to erase previously cleared materials, polishing materials, or cover the polluted area by mechanical means.

All kinds of pollution, especially exotic iron particles, can be a source of corrosion, especially in humid environments.

Therefore, it is better to clean the surface of mechanical cleaning in the dry condition.

Mechanical method can only clean the surface of the metal, not change the anti-corrosion ability of the metal itself.

Therefore, it is recommended to use polishing equipment to repolish after mechanical cleaning and seal with polishing wax.

2017-12-26T15:22:37+00:00 Steel|

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