What makes stainless steel stainless?
Stainless steel is stainless like normal steel. So are you wondering what is being done to the steel to prevent it from rusting?
What is rust?
The corrosion of iron causes the formation of a reddish brown, hard layer called rust. Corrosion is the oxide and hydroxide layers formed by the reaction of the metal with oxygen. Therefore, corrosion is also called metal oxidation. There are plenty of hydrogen and oxygen in nature. Therefore, metals are easily exposed to them and corrosion occurs.
The corrosion of iron is an electrochemical reaction, so the first layers of rust are made up of fine atoms, but the long duration of corrosion causes this layer to thicken and become more and more apparent. Rust reduces the strength of iron. So a rusted key cannot unlock. Losses from rust are so costly that humanity has had to seek a remedy to prevent rust.
What is Steel?
By combining two or more metals and non-metallic materials, a stronger and more durable composite can be created. This is called an alloy. Steel is also an alloy, so steel is not included in the periodic table.
Iron is a metal abundant on earth, but it is almost impossible to find it in its pure form. Iron atoms consist of layers that can slide even with a slight intervention. Therefore, it is not used as a construction material. To use as a building material, iron is turned into an iron-carbon alloy, that is, steel. This composite is extremely strong and durable and forms the backbone of the constructions.
Although iron is converted into a strong building material such as carbon and steel, the abundance of iron in the steel still leads to low corrosion resistance of this material. Therefore, if a structure comes into contact with moisture or water for a long time, it will deteriorate. Therefore, an element was needed to make steel strong against highly reactive oxygen. More precisely, an element was needed to react with oxygen before iron. This element is chrome.
What is stainless steel?
Stainless steel is not just an alloy of iron, carbon and chrome. It also contains manganese and certain types of nickel molybdenum. Although there is a widespread opinion that iron corrosion, that is, rust is very harmful to health, it is not actually harmful. When aluminum corrodes, the resulting layer of aluminum oxide protects it from further exposure to oxygen molecules. In this way, corrosion does not continue. The same is true for chrome.
Stainless steel contains about 10% chromium. Chromium reacts with oxygen. and forms a hard and thin layer to prevent corrosion from moving deep into the steel. In fact, stainless steel is not as durable as other forms of steel, but it is advantageous to make objects in contact with water made of stainless steel. At the same time, chrome makes these tools shine.
Why Does Stainless Steel Rust?
1) Rusting by reacting:
Stainless steels can react with various elements due to improper production stages and improper use, and can lead to corrosion in this reaction. An example of this type of corrosion is the following:
-Stainless steel coating with a wrong element and steel to react
2) If wrong heat treatment is made:
Stainless steels may cause stainless steel to react after heat treatment if heat treatment is carried out at the wrong times and at the wrong temperatures under the wrong conditions. This reaction can lead to both blackening and rusting of stainless steel. The main reason for this event is that the protective layer on the surface of the stainless steel will deteriorate with heat treatment and lose its feature. In order to prevent this event, heat treatment at the right temperatures, at the right temperatures and at the right times should lead to the physical and mechanical properties of the material not to be lost. In these and similar events, methods such as heat treatment at extremely high temperatures, excessive heating, incorrect cooling of the material can lead to complete deterioration of the material structure.
3) If the Alloy Of Stainless Steel Is Inadequate For That Environment:
Although the headings in the 1st and 2nd articles are the factors that cause the rusting of stainless steel, it is the wrong quality selection that causes the most rusting of stainless steel. Stainless steel can be corroded due to incorrect quality selection and due to poor stainless steel quality choice for that environment. The idea that all stainless steels will not corrode in every environment because of the name stainless steel is quite wrong. Stainless steels should be chosen correctly and it is very important to choose the most suitable stainless steel for every environment. For example, if 316L quality material is used in an environment where 904L quality stainless steel will be sufficient, 316L quality material may be corroded or dissolved due to acid. You can check our “STAINLESS STEEL QUALITY SELECTION” page for the right material selection in the right place.
4) If there was a metal transfer:
Environments where stainless steel materials are processed are very important. During the processing of stainless steel, it should be noted that a non-stainless material has not been previously processed in the workbench where stainless is processed. For example, if a stainless steel is processed with the same pencil tip right after carbon steel on a workbench where carbon steel is processed, dust particles remaining from the carbon steel will be transferred on the surface of the processed stainless steel in an invisible way. In the meantime, the stainless steel surface can be covered with the powders of the previous material and an invisible outer layer can be formed. Since this layer is much more corrosive then the material may easily appear to rust, but this is not a true rust. This is metal transfer, and in order to prevent this metal transfer, care must be taken not to process non-stainless steel material in environments where stainless steels are processed. Another way to prevent this situation is to wash the parts that come out of production in a metal washing machine with chemicals.
5) The mixture of stainless steel could not be made homogeneously:
Homogeneous Mixture is one of the indispensable topics in stainless steel. In order to obtain a homogeneous mixture, it is necessary to dissolve the material in the melting furnaces for a longer time, to spend electricity for a longer period, and all this increases the production costs considerably. Avoiding such production cost may cause the end product produced to have a more heterogeneous, inhomogeneous structure. For this reason, one of the two stainless materials of the same content, which cannot be noticed even during the analysis, can withstand corrosion for a long time, while the other can rust in a much shorter time in the same environment. You can review more detailed information from the section below.
Could the mixture not be homogeneous cause rust?
Microstructure homogeneity is very important in stainless steel materials. As shown in the example below, in one of 2 different stainless steel materials with the same chemical analysis, the elements are homogeneously mixed, while in another, they are not homogeneously mixed. Over time, the material of the non-homogeneous manufacturer will begin to rust from the regions that do not contain “NICKEL” and “MOLIBEN”. Since the materials produced by French UGITECH sold in Birçelik Stainless are much more homogeneous, their corrosion resistance is better than that of other producers who cannot produce homogeneously even if they have the same chemical content.