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Why does rust on the surface of stainless steel casting?

When the surface of the stainless steel casting appears brown rust spots (points), we everyone are surprised that stainless steel is not rusted, otherwise is not stainless steel. Maybe there is a problem with the steel property. In fact, this is a one-sided wrong view and lack of understanding of stainless steel. Stainless steel will rust under certain conditions. Stainless steel has the ability to resist atmospheric oxidation and corrosion in the medium containing acid, alkali, salt, which are stainlessness and corrosion resistance. But the ability of its corrosion resistance is depended on its chemical composition, status of interaction, working conditions and the type of environmental media. Such as 304(OCr18Ni9, CF8, SCS13) steel grade, there is absolutely excellent anti-rust ability, in the dry and clean atmosphere. But it will rust soon, if will be moved to the coastal areas which the sea fog contained a lot of salt. But the 316 (0Cr18Ni12Mo2Ti, CF8M, SCS14A) steel grade performed well. How is get the ability of stainlessness and corrosion resistance of Stainless steel?  It forms a layer of very thin and strong fine stable chromium oxide film (protective film) on the surface of stainless steel casting to prevent the oxygen atom to continue to penetrate and oxidize, Once this protective film has been constantly destroyed and the oxygen protons or liquid in the air will continue to infiltrate and the iron atoms in the metal will continue to separate out to forming loose iron oxide, the surface of steel casting will also be constantly corroded. There are many reasons for damaging this kind of surface film and there are several common ones in daily life as below: 1. electrochemical corrosion. There are dust contained other metal elements or heterogeneous metal particles on stainless steel casting surface. In the humid air, the dust attachments and stainless steel condensate together form into a micro battery and triggered an electrochemical reaction and the protective film is damaged. 2. stainless steel casting surface adhere organic object (such as engine oil, machine oil etc.) which under the condition of  water and oxygen constitute organic acids and it corrode the metal surface long time 3. stainless steel casting surface adhere contains of acid, alkali, salt (such as alkaline water, stone ash water splash) and cause local corrosion.4. In the polluted air (such as the atmosphere containing a large number of sulfide, carbon oxide, nitrogen oxide), when condensed water, it forms sulfuric acid, nitric acid, acetic acid liquid point and cause chemical corrosion. The above circumstances can cause the destruction of the protective film on the surface and corrode the stainless steel casting. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, we suggest: 1. We must process the Sand blast to stainless steel casting with fine steel shot, you’d better use 0.3mm diameter of steel shot. 2. we must clean completely the oxide of surface of casting through the process of acid pickling 3.we must clean completely acid liquid with warm water and cold water for several times until it have no any point. Then blow-dry. 4. The seashore area should be made of 316 stainless steel grade which can resist the corrosion of sea water. In other words, we must use suitable steel grade in different condition.5. The chemical composition must meet the corresponding national standards which requires users to carefully choose the products of reputable manufacturers.
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