Fretting, erosion, cavitation, and galvanic reactions.
Corrosion is a natural process, which converts refined metal to their more stable oxide. It is the gradual destruction of materials (usually metals) by chemical reaction with their environment. Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. These can result from a number of causes including the method of application, the condition of the surface or any pre-treatment. Corrosion is an electro-chemical process which transforms steel influenced by oxygen and water, in iron hydroxyde (rust). Corrosion is most often caused when steel is exposed to atmospheric conditions, such as salt, moisture, and oxygen. A protective coating to the steel required to prevent corrosion. Corrosion of stainless steels can be categorised as one of: Stainless Steel and the passive layer Steel is an alloy of iron and carbon. Contrary to carbon steel, the presence of a minimum of 10.5 % chromium in the stainless steel gives it the property of corrosion resistance. Indeed, on contact with oxygen, a chromium oxide layer is formed on the surface of the material. Metal corrosion is the main cause of metal destruction, like steel rusts due to immersion in seawater. Similarly iron reacts with oxygen to form rust by exposure to moist air. Iron rust is iron oxide Fe2O3. XH2O where X is the amount of complexed water with ferric oxide, which can vary. It is also called flow-assisted corrosion. This is due to the movement of corrosive liquids on metal surface which damages the material. It can be seen in ship propellers which are constantly exposed to sea water or in soft alloys. The damage can be seen as waves or rounded holes etc. Reactions from other metals, acids, etc. cause corrosion as well. Friction can cause stress cracking, fretting, and intergranular corrosion and erosion. Two pieces of metal that rub on each other can cause fretting and lead to corrosion. Answer 1: oxygen and carbon in the air :-). Answer 2: Depends on what type of corrosion you are referring to. Rust and corrosion are typically used interchangeably. Rust is one type of corrosion. Corrosion is essentially the disintegration of a engineered material. Rust is a chemical reaction, but there are other chemical reactions that cause corrosion of steel (typically acids). Other types of corrosion include fretting, erosion, cavitation, and galvanic reactions. There are many methods to prevent The most common forms of corrosion in stainless steel are: 1  Pitting corrosion-The passive layer on stainless steel can be attacked by certain chemical species. 2  The chloride ion Cl-is the most common of these and is found in everyday materials such as salt and bleach. 1 Galvanic corrosion-If two dissimilar metals are in contact with each other and with an electrolyte e.g. water or other solution, it is possible for a galvanic cell to be set up. 2  This is rather like a battery and can accelerate corrosion of the less 'noble' metal. Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms. Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. When two different metals, such as aluminum and steel, are in contact within concrete, corrosion can occur because each metal has a unique electrochemical potential. A familiar type of dissimilar metal corrosion occurs in an ordinary flashlight battery. The zinc case and carbon rod are the two metals, and the moist paste acts as the electrolyte. When the carbon and zinc are connected by a wire, current flows. In reinforced concrete, dissimilar metal corrosion can occur in balconies where embedded aluminum railings are in contact with the reinforcing steel. From Steelconstruction.info. The corrosion of structural steel is an electrochemical process that requires the simultaneous presence of moisture and oxygen. Essentially, the iron in the steel is oxidised to produce rust, which occupies approximately six times the volume of the original material. [top] The corrosion process. The corrosion of steel can be considered as an electrochemical process that occurs in stages. Initial attack occurs at anodic areas on the surface, where ferrous ions go into solution.
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what causes steel corrosion