Corrosion it is a physicochemical interaction between metal and environment. The result of this interaction is changes in metal properties (e.g. stainless steel) that may lead to significant deterioration of metal function, environment or technical system it is an element of. Corrosion phenomena are mostly classified according the mechanisms, types of corrosion damage, environment and occurrence within a given branch of an industry.
Corrosion is a natural enemy of metals. Regular carbon steel reacts with oxygen in air leading to iron oxide layer formation on the steel surface. Such surface is porous and allows for further oxidation of steel, causing corrosion leading to rust. In other words, corrosion is a gradual destruction of metal and non-metal materials as a result of chemical and electrochemical impact of an environment leading to change of condition and properties of a material being destroyed.
Bimetallic corrosion, also called galvanic corrosion, is a corrosion caused by an impact of a corrosive cell, where system elements, e.g. electrodes, are represented by different metals. If two different metals are placed in an electrolyte environment, it allows for electrolytic current to flow from less precious metal (anode) to more precious metal (cathode), and the anodic metal will corrode faster comparing to a case when the metals are not in contact. Stainless steels in electrochemical pairs usually represent cathodic material, thus in this combination, the other metal is exposed to corrosion.
Erosion corrosion – process, wherein corrosion and erosion phenomena occur – surface, mechanical material abrasion e.g. by flowing fluids or gases that include fragmented solids. Erosion corrosion in stainless steel systems is common within the area of cross-section and direction of medium flow change. Erosion corrosion is also closely related to the rate of medium flow within a system.
Local corrosion appears locally on a metal surface subjected to corrosive environment. Steels resistant to corrosion may undergo various types of local corrosion, such as pit, crevice, intergranular and stress corrosion.
Intergranular corrosion – takes place when a solution attacks boundaries of grains without affecting their interior. In other words, it is a selective dissolution of grain boundaries or adjoining areas as a result of a corrosive process. The factor initiating this process is the difference of potential between the Cr (chromium) depleted grain boundary, in case of chromium carbides – anode, and an inclusion, intermetallic phase or contamination on a grain boundary. It depends on chemical composition and heat treatment. This corrosion starts at the surface and penetrates metal. Strength and ductility drop rapidly. Sample of metal affected by such a corrosion does not make metallic sound, and during bending – it breaks. In extreme case, it can disintegrate to powder. This type of corrosion is very dangerous. Accurate determination of this type of corrosion extent is very difficult. Evaluation of this corrosion is made using microscopic methods and through the increase of electrical resistance.
Stress corrosion (corrosive cracking, ageing cracking) – local corrosion taking place in a material, in which permanent technological or operational stresses are present. Stress corrosion is a result of total impact of aggressive environment and mechanical stresses on materials susceptible to such corrosion type. Stress corrosion results in cracking of metal (e.g. steel, brass or aluminium) or polymer objects. Corrosion of metals is of electrochemical nature.
Stress corrosion takes place mostly in relatively low aggressive solutions, mostly chlorides (bromides and iodides).
Corrosive cracking of alloys can be caused by the following factors: e.g. brass – corrodes in ammonia or sulphur dioxide atmosphere; carbon steel – corrodes in alkaline environment and at presence of nitrates; aluminium alloys, magnesium alloys and chromium-nickel steels – corrode at presence of chlorides.
These can be stresses caused by external forces as well as internal stresses caused by e.g. cold compression (pull broaching) or welding. It is believed that material under stress when being subjected to corrosive environment, is less resistant to its impact comparing to a material that is not under stress. As a result of stresses, metal surface is not energetically uniform. There are places energetically richer, where the metal ions on the surface pass into a solution more easily than from non-defective surface. Defective surface becomes a negative pole and non-defective surface becomes a positive pole of a cell. Within a corrosive cell, the oxidization process takes place on an anode (electrochemical corrosion). Cracks run perpendicularly to main stresses and may run along grain boundaries (intergranular stress corrosion) or through grains (intragranular stress corrosion).
The phenomenon of corrosive cracking is explained with uneven location of alloying components, especially when they differ with chemical activity (e.g. zinc and copper in brass). In case of brass, corrosive cracks are formed at spots of locally greater concentration of zinc atoms. Therefore, brass containing up to 20% of zinc (e.g. tombac) is the most resistant to corrosion.
Uniform corrosion is a corrosion that takes place uniformly across the whole surface of metal present within a corrosive environment. This type of corrosion affects reduction of mechanical properties of a material through a uniform reduction of cross-section and loss of weight, thus reduction of strength properties of a corroded element. Uniform corrosion of steel resistant to corrosion is observed in case when steel is in an active state, when the passive layer is not stable.
Crevice corrosion – electrochemical local corrosion occurring in hardly accessible spots. Connection of elements without penetration of fused elements, e.g. screwed of riveted connections, is endangered to this type of corrosion. The process of crevice corrosion formation starts after oxygen depletion within hardly accessible surrounding of a crevice. This results in sedimentation of almost insoluble compounds at the crevice outlet and easily soluble salts inside it – acidifying the corrosive environment of a crevice.
Pit corrosion – is a local corrosion causing formation of pits – indentations penetrating a steel starting from its surface. Pit corrosion of steels resistant to corrosion consists in formation of pits initiated at spots of passive oxides layer damage, which may lead to complete perforation of thin-walled products. Major factors affecting pit corrosion resistance of stainless steels include the condition of material surface (low surface roughness is favourable) and main alloying additives, such a chromium, molybdenum and nickel.
Passive layer – it is a thin, non-porous, covering the whole element, stable and impermeable layer of oxides formed on a surface of steel resistant to corrosion (stainless) as a combination of air or water with chromium within an alloy. Passive layer in stainless steels can be characterized by susceptibility to self-acting reconstruction in an air-containing environment. The layer protects steel resistant to corrosion against naturally present corrosive factors.