Influence of metal properties on corrosion, forms of corrosion
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Structural heterogeneity of the metal affects the corrosion process; local inclusions and segregation of non-metallic inclusions are places of enhanced corrosion. In a steel ingot, the central part, especially of boiling steel, often has a zone of sulfur inclusions in a sharp form; when rolling sheets and bars from ingots, their central parts, especially from the ends, and sometimes from the sides, are often contaminated with these inclusions, and practice has noted increased corrosion of the ends of bars and sheets.
Types of metal corrosion
Defects in the form of shells, porosity and looseness on metals are corrosion intensifiers, since these defects naturally increase the contact surface of the metal with the solution; also due to the Evans principle and uneven aeration, these places, as less accessible to the action of oxygen, will be more accessible in relation to neighboring places, — anode.
The smoother the surface, the more resistant it is to corrosion. According to Kroenig, on polished steel with a carbon content of 0.8%, corrosion appeared after 28 days, and on the surface of the same steel, processed by a file or on a lathe, — in 10 days.
Metal riveted, hard-worked or cold-worked tends to be more corrosive than annealed metal. If the product has cold-worked parts, they are always anodic with respect to parts of this product that have not been subjected to such work hardening. Material that has been work-hardened by cambering, stamping or other cold processes is recommended to be annealed whenever possible to reduce corrosion damage. The work-hardened material has internal stresses and these stresses in some metals and products cause cracks in the presence of even slight corrosion on the surface of the work-hardened material. Corrosion, for example, causes cracks in pipes and sometimes in boiler seams.
Metal corrosion can manifest itself in various forms: uniform, when the thickness of the sample or product is uniformly reduced over the entire surface; local, when they appear on the surface of the shell (pitting); intercrystalline, when corrosion occurs along grain boundaries, and selective, when any one structural component of a multiphase alloy is destroyed.
Uniform corrosion weakens the metal over the entire section; the loss of strength in this case is proportional to the loss of weight. It takes place in pure and homogeneous metals or in solid solutions.
Local corrosion in varying degrees of manifestation is often observed in parts of the iron hull of the ship. The loss in weight of local corrosion is not an indicator of the loss of strength and suitability of the product for further service. The weight loss may not be great, but the local sinks are so deep that you have to consider replacing the product with a new one.
Intercrystalline corrosion is usually accompanied by little or no weight loss, but the strength of the product is significantly reduced, and the toughness is also reduced. This form of corrosion is susceptible to duralumin in sea water if it is poorly protected. In steel, this form of corrosion in water is not observed; it is sometimes observed on the tubes of the refrigerator.
Selective corrosion is essentially a process that changes the state of the structure, and, of course, this is a serious destruction of the metal, making it often unsuitable for further service if the destruction process has gone far. For example, when gray cast iron is left in water for a long time, corrosion captures the ferrite without damaging other components; an iron casting will retain its appearance, but its strength will be completely lost, since the casting will consist of oxidized iron and graphite inclusions.
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