Why does stainless steel also rust?

Why does stainless steel also rust?

When the rust spots appear on the surface of the stainless steel tube, people are very surprised that "stainless steel is not rusty, rust is not stainless steel, and it may be a problem of steel." In fact, this is a one-sided misconception about the lack of stainless steel. Stainless steel is also rusty under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, that is, rustless property, and also has the ability of corrosion resistance in acid, alkali and salt medium, that is, corrosion resistance. However, its corrosion resistance is changed with the chemical composition, protection state, use condition and environmental medium type of the steel itself. For example, 304 steel pipes, in the dry and clean atmosphere, have excellent corrosion resistance, but move it to the seaside area, and in the sea fog containing a large amount of salt, it will soon rust; and 316 steel pipes are good. Therefore, not any stainless steel can be corroded and rust free under any environment.

Stainless steel is a thin, sturdy and stable chromium oxide film (protective film) formed on its surface to prevent the continuous infiltration and oxidation of oxygen atoms, which has the ability to resist corrosion.

Once for some reason, the film is constantly destroyed, the oxygen atoms in the air or liquid are constantly infiltrated, or the iron atoms in the metal are constantly separated out to form a loose iron oxide, and the metal surface is constantly corroded. There are many forms of destruction of the surface film.

1. Stainless steel surface deposits other metal elements of dust or heterogeneous metal particles attachment.In wet air, the condensate between the attachments and the stainless steel will connect the two to a micro battery, which causes the electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.

2. The surface of stainless steel adheres to organic juice (such as melon, soup, phlegm, etc.), which forms organic acids in the presence of water and oxygen, and the corrosion of organic acids to the metal surface for a long time.

3. The surface of stainless steel adheres to acid, alkali and salt substances, such as alkaline water and lime water splashing, which cause localized corrosion.

4. In the polluted air (such as the atmosphere containing a large amount of sulphide, carbon oxide and nitrogen oxide), condensate water can form sulfuric acid, nitric acid and acetic acid solution, causing chemical corrosion.

To ensure that the metal surface is permanently bright and not corroded, we suggest:

1) The surface of decorative stainless steel must be cleaned and scrubbed regularly, removing attachments and eliminating external factors that cause modification.

2) 316 stainless steel should be used in the seaside area, and 316 can resist seawater corrosion.

3) The chemical composition of some stainless steel pipes in the market can not meet the corresponding national standards, less than 304 of the material requirements. Therefore, it will also cause rust. This requires users to choose products of reputable manufacturers seriously.

Ps:

1. 316 material stainless steel (18Cr-12Ni-2.5Mo), because of the addition of Mo, its corrosion resistance, atmospheric corrosion resistance and high temperature strength are very good, which can be used under harsh conditions, processing hardening excellent (no magnetic). Applied in the production of equipment, chemical, dyestuff, paper, oxalic acid, fertilizer and other production equipment in the sea water; image, food industry, coastal area facilities, rope, CD rod, bolt, nut.

2. 304 material (18Cr-8Ni), as a kind of widely used steel, has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; punching, bending and other heat processing properties, no heat treatment hardening (no magnetic, then temperature -196 to 800 C). Applied to household items (1 and 2 types of tableware, cupboards, indoor pipelines, water heaters, boilers, bathtubs), auto parts, medical appliances, building materials, chemistry, food industry, agriculture, andship parts.