PSI - Issue 17

Garan Martin et al. / Procedia Structural Integrity 17 (2019) 514–519

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Garan, M.,Šulko, M.: / Structural Integrity Procedia 00 (2019) 000 – 000

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system can also change i.e. the geometry of the pipe. The mechanism which causes the change of geometry is e. g. the corrosion. The corrosion causes decreasing of the wall thickness and so the pipe becomes less rigid and losing the strength. Material properties can be also affected due to the embrittlement and ageing of the pipeline material. The effect of ageing and brittleness may be reflected in a change in ductility or notch toughness. These mechanisms, like corrosion, ageing and embrittlement, which can occur in operation of pressured pipeline systems, are generally called as degradation mechanisms. The occurrence of such degradation mechanisms can cause critical situations and may have a significant impact on reducing the safety of piping systems. The causes and impacts on such pipeline safety are briefly analyzed below. Corrosion as a physicochemical interaction between metal and environment begins with hydrogen or oxygen depolarization and the formation of an electrode potential, which then directly determines the rate of corrosion. In the case of complete homogeneity of the metal phase and the corrosive environment, the metals should be completely uniformly rusted from the original surface to the depth. However, such uniform corrosion is always influenced by the crystal structure of the metal in the sense that certain spots corrode faster and form an uneven surface. Therefore, the term uniform corrosion refers to the corrosion at which the entire surface of the corroding metal is attacked. If heterogeneity (on the environment or material side) exists or develops (during corrosion) in the corrosion system, the corrosion attack concentrates only on a certain limited area (often related to the metal structure); we are talking about uneven (localized, local) corrosion. While uniform corrosion progress is relatively easily predictable and during operation the operation controllable by non-destructive methods, prediction and corrosion detection are more difficult. Some demonstrations of uneven corrosion are (Philip 2009, Marcus 2017): Point corrosion on a flat limited area but deep pits - points where the metal surface can be completely unbroken. Point corrosion occurs mainly in passive metals (e.g. stainless steels). Hole, mottled corrosion in which the depth of attack is not as great as in point corrosion (e.g. pipe steel). Selective corrosion is a process in which only some chemical or structural parts of a metal material are corrosive. In this way, intergranular corrosion takes place in which the corrosion-attacked area is at the crystal boundaries. Trans crystalline corrosion is manifested by the presence of transcrystalline cracks. Slot corrosion occurs in semi-enclosed narrow spaces, arising e. g. at the point of contact of the two metal parts or in imperfect contact with the non-metallic material (e. g. seal). In addition to structural slots, solid particles can be used as shading material which are wafted by the environment and which sediment on the surface at the flow disturbance sites as well as deposits and some types of bacteria can also occur there. Galvanic corrosion occurs when two different conductively bonded metals are immersed in the electrolyte. Due to the different noble metals, corrosion of less noble material can occur. Corrosion with stress interaction is the process caused by interaction of corrosion and tensile stress i.e. residual stress. The effect of both factors multiplies and is the result of formation of cracks or corrosion cracking that means suddenly breaking of metal by refraction. The surface of the metal outside the crack does not have to be corrosively attacked at all. The resulting crack propagates either grain boundary (inter-granular) or penetrates the grain (trans-crystalline). Certain metal-environment combinations, e. g. non-alloyed steels in an environment that contains nitrate anion (industrial fertilizers) or in strongly alkaline environments, so-called luster brittleness. Anti corrosive steels are subjected to the corrosion cracking in chloride anion-containing environments. The presence of ammonium cations is a stimulating factor for cracking copper alloys. Corrosion fatigue is a process that leads to cracks and cracking of metal, interacting with varying mechanical tension with the tensile component and the corrosive environment. In the corrosion nomenclature we come across definitions of other types of corrosion that are related either to their specific mechanism (microbial, cavitation, erosion) or to the environment in which it occurs (atmospheric, soil corrosion, corrosion in concrete). 2. Corrosion