Usually people think that stainless steel is not easy to rust steel, in fact, a part of stainless steel, both rust, and acid resistance (corrosion resistance). Stainless steel pump is common anticorrosive pump on the market, the not rust and corrosion resistance of stainless steel is due to its rich chromium oxide film on the surface (passivation film), the formation of the membrane make metal dielectric isolation with the outside world, prevent the metal from being further corrosion, and the ability to repair itself, if once destroyed, the chromium in steel can with the oxygen in medium to regenerate the passivation membrane, Continue to protect. So stainless steel pump can be corroded, the following look at different types of stainless steel material knowledge;
This rust resistance and corrosion resistance are relative. The experimental results show that the corrosion resistance of steel in weak media such as air, water and oxidizing media such as nitric acid increases with the increase of chromium content in steel. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes from easy to rust, from corrosion resistance to corrosion resistance. Stainless steel rust, but also related to the use of the environment, different environment, to use stainless steel with different chromium content.
Stainless steel pump
1, stainless steel classification
There are five basic types of stainless steel: austenitic, ferrite, martensite, duplex stainless steel, precipitate hardened stainless steel.
(1) Austenitic stainless steel is not magnetic, the typical steel is added 18% chromium and contains a certain amount of nickel, to increase corrosion resistance, they are widely used steel.
(2) Ferrite has magnetic properties. Chromium is the main content of ferrite, accounting for 17%. This material has good oxidation resistance.
(3) Martensitic stainless steel is also magnetic, chromium content is usually 13%, and contains an appropriate proportion of carbon, they can be hardened by quenching and tempering.
(4) Duplex stainless steel with a mixture of ferrite and austenite structure, chromium content between 18% to 28%, nickel content between 4.5% and 8%, they have a good effect against chloride erosion.
(5) The conventional content of chromium in precipitated stainless steel is 17, plus a certain amount of nickel, copper and niobium, which can be hardened by precipitation and aging.
2. According to metallographic organization, it can be divided into:
(1) Ferritic stainless steel (400 series), for chromium stainless steel, mainly represented by Gr13,G17, GR27-30;
(2) austenitic stainless steel (300 series), chrome-nickel stainless steel, mainly representing 304,316,321, etc.;
(3) Martensitic stainless steel (200 series), chromium manganese stainless steel, high carbon content, mainly represented by 1Gr13, etc
3, why will stainless steel rust?
Reason for rust: Chromium is key
Stainless steel rust phenomenon, there may be the following reasons:
(1) There are chloride ions in the operating environment
Chloride ions are found in salt, sweat, sea water, wind, soil, etc. Stainless steel corrodes faster in the presence of chloride ions than ordinary mild steel. So the use of stainless steel environment has requirements, and the need to often wipe, remove dust, keep clean and dry. (This gives him an "inappropriate use.")
(2) Without solid solution treatment
Alloy elements did not dissolve into the matrix, resulting in low alloy content in the matrix structure and poor corrosion resistance.
(3) inherent intergranular corrosion
This titanium - and niobium-free material has a tendency to intergranular corrosion. The addition of titanium and niobium, combined with stable treatment, can reduce intergranular corrosion. In the air or chemical corrosion medium can resist corrosion of a high alloy steel, stainless steel is a beautiful surface and good corrosion resistance, do not have to be plated and other surface treatment, and play the inherent surface properties of stainless steel, used in many aspects of a kind of steel, usually called stainless steel.
On behalf of the performance of 13 chromium steel, 18-8 chromium nickel steel and other high alloy steel. From the metallographic point of view, because stainless steel contains chromium and the surface of the formation of a very thin chromium film, the film separated from the oxygen in the steel invasion of corrosion resistance. To maintain the corrosion resistance inherent in stainless steel, steel must contain more than 12% chromium.
For soldering applications. Lower carbon content results in less carbides precipitated in the heat affected zone near the weld, which may lead to intergranular corrosion (welding erosion) of stainless steel in some environments. Because it will damage the surface of stainless steel, and adhere to iron powder and rust.
In daily life, we sometimes find some flagpoles, shelters, light boxes and other facilities in the streets of stainless steel pickling obvious rust phenomenon, since it is stainless steel passivation, why will rust? Occurrence these circumstances, have two reasons, it is the content of chromium in material is low, belong to inferior stainless steel. Second, it is not stainless steel at all, but with electroplating Mongolian users.
Such as 304 steel pipe, in the dry and clean atmosphere, there is absolutely excellent corrosion resistance, but it will be moved to the coastal area, in the sea fog containing a lot of salt, will soon rust; And 316 steel pipe is good performance. Therefore, not any kind of stainless steel, in any environment can withstand corrosion, not rust.
The above situation can cause the damage of stainless steel surface protective film to cause corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, the following suggestions are made:
1. The surface of decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors causing modification.
2. Stainless steel 316 material should be used in coastal areas, which can resist corrosion of sea water.
3, some stainless steel tube chemical composition on the market can not meet the corresponding national standards, can not meet the requirements of 304 material. Therefore, it will also cause rust, which requires users to carefully choose the products of reputable manufacturers.