Causes of rust in stainless steel

The rust phenomenon of stainless steel materials may have the following reasons:

(1) Chloride ions exist in the use environment

Chloride ions exist widely, such as salt/sweat/seawater/sea breeze/soil and so on. In the presence of chloride ions, stainless steel corrodes rapidly, even more than ordinary low carbon steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be wiped frequently to remove dust and keep it clean and dry. (In this way, he can be rated as "improper use".) There is an example in the United States: a company used an oak container to hold a solution containing chloride ions. The container has been used for nearly a hundred years and was planned to be replaced in the 1990s. , Because the oak material is not modern enough, the container leaked due to corrosion 16 days after the replacement with stainless steel.

(2) without solution treatment

Alloy elements are not dissolved into the matrix, resulting in low alloy content in the matrix structure and poor corrosion resistance.

(3) Natural intergranular corrosion

This titanium- and niobium-free material is prone to intergranular corrosion. The addition of titanium and niobium, together with stabilization treatment, can reduce intergranular corrosion. A high-alloy steel that can resist corrosion in the air or in chemically corrosive media. Stainless steel has a beautiful surface and good corrosion resistance. It does not need to undergo surface treatment such as color plating, but instead exerts the inherent surface properties of stainless steel. It is used in many A type of steel, commonly referred to as stainless steel. The representative performance is high alloy steel such as 13 chrome steel and 18-8 chrome nickel steel. From the metallographic point of view, because the stainless steel contains chromium, a very thin chromium film is formed on the surface, which isolates the oxygen from the steel and plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, steel must contain more than 12% chromium. For use where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, which can lead to intergranular corrosion (weld erosion) of stainless steel in some environments. Because it will damage the stainless steel surface and adhere to the iron powder to form the cause of rust.

In daily life, we sometimes find that the stainless steel of some flagpoles, bus shelters, light boxes and other facilities on the street has obvious rust pickling phenomenon. Since stainless steel is passivated, why does it still rust? There are two reasons for these situations. First, the content of chromium in the material is low, which belongs to inferior stainless steel. The second is that it is not stainless steel at all, but uses electroplating to deceive users. It is understood that many decorative materials are now treated with this electroplating process. Since the material is ordinary steel, when the electroplating layer is peeled off, it will naturally rust.

Stainless steel has the ability to resist atmospheric oxidation—that is, rust resistance, and it also has the ability to resist corrosion in media containing acids, alkalis, and salts—that is, corrosion resistance. However, the size of its anti-corrosion ability varies with the chemical composition of its steel itself, the state of mutual addition, the conditions of use and the type of environmental media. For example, 304 steel pipe has excellent anti-corrosion ability in a dry and clean atmosphere, but if it is moved to the seaside area, it will rust quickly in the sea fog containing a lot of salt; and 316 steel pipe is good performance. Therefore, it is not any kind of stainless steel that can resist corrosion and rust in any environment.

Stainless steel relies on a layer of ultra-thin, firm and stable chromium-rich oxide film (protective film) formed on its surface to prevent the continuous infiltration and oxidation of oxygen atoms, so as to obtain the ability to resist rust. Once for some reason, this film is continuously damaged, oxygen atoms in the air or liquid will continue to infiltrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will be continuously corroded. There are many forms of damage to this surface film, the most common in daily life are the following:

1. On the surface of stainless steel, there are deposits of dust or heterogeneous metal particles containing other metal elements. In humid air, the condensed water between the deposits and the stainless steel connects the two into a micro-battery, which triggers an electrochemical reaction. , the protective film is damaged, called electrochemical corrosion.

2. Organic juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, organic acids are formed, and organic acids will corrode the metal surface for a long time.

3. The surface of stainless steel adheres to substances containing acids, alkalis and salts (such as alkali water and lime water splashing from decoration walls), causing local corrosion.

4. In the polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide and nitrogen oxide), when encountering condensed water, it will form liquid spots of sulfuric acid, nitric acid and acetic acid, causing chemical corrosion.

The above conditions can cause damage to the protective film on the stainless steel surface and cause rust. Therefore, in order to ensure that the metal surface is always bright and not rusted, the following suggestions:

1. It is necessary to clean and scrub the surface of decorative stainless steel frequently to remove attachments and eliminate external factors that cause modification.

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

3. The chemical composition of some stainless steel pipes on the market cannot meet the corresponding national standards and cannot meet the material requirements of 304 stainless steel. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers.