be analyzed. The only known disadvantage of it is the calibration of material to be monitored [44].Figure 2.10 Gamma radiography technique for corrosion inspection, (a) tangential radiography technique and (b) double wall radiography technique.Pulsed Eddy CurrentsThis technique applies for knowing wall thickness of the structure during corrosion monitoring generally in refinery units with severe corrosion problems. The principle employs production of pulsed magnetic field to generate eddy currents in the metallic structures. If the metal is specifically steel, hence ferromagnetic, so only topmost exposed layer of structure can be magnetized [45, 46]. The schematic representation is shown in Figure 2.11a and b.It is showing stages of measurement for eddy currents on the metallic (steel) surface, which is near to the pulse eddy current (PEC) probe. As the time passes, the current passes into the specimen structures showing in stages 2 and 3. At last they reach to farthest surface, which is stage 4 in provided figure. The produced eddy currents induce a voltage signal in the receiver coils of the pulse eddy current probes. The pulse eddy current signals are then displayed in the form of plot, PEC signals vs. time. The free expansion of the metal (steel) as experienced by the eddy currents in different stages exposes the strength, which decreases in relatively slow manner. Hence, on reaching to farthest position in structure, the strength dropped suddenly, which can be clearly seen from sharp fall in the PEC signal. At early outset of this, acute decay of the monitored pulse eddy current signal shows wall damage or loss of the structure. The readings of the wall thickness are nearly circular in shape named as “footprints” where eddy currents can flow. The size of these footprints mostly depends on the length in between the probe and the metallic surface along with the dimensions of the probe itself. In all these way, pulse eddy current method is best suited to get wall deterioration or loss in metallic dimensions to get knowledge of corrosion [47].Figure 2.11 Pictorial presentation of (a) generation of pulse eddy current and (b) graph for signal.Infrared Thermographic DetectionOut of other routinely detection of deterioration in nondestructive testing methods, one is infrared thermographic detection. This technique seems to be impractical due to many safety reasons and prerequisite to have a two‐sided admittance to objects under study [48–50]. The principle associates thermal stimulation of the studied object by channeling with an optical heat source (convective/inductive heat sources) and analyzing the matter surface with an Infrared imager. The obtained data can be sequenced as “IR thermograms” and further recorded on computer for data processing methods like Fourier transform and principal component analysis. The area to study at once can be of dimension 0.2–1.0 m2. The results obtained can be in terms of binary maps of defects. For bigger areas, the study can be done by applying area‐by‐area flashing and bringing together multiple infrared thermograms in a panned image. The schematic presentation of infrared thermographic monitoring is shown in Figure 2.12 [51, 52].
Its application has shown versatile inspection of composite materials used in the aerospace, boilers, pipeline jacketing, aluminum airframes, and at many more places [53–59]. It can be seen that this technique can be able to detect material debt up to 10% [60–61].
It has to note also that the screening of metals by using present technique is more burdensome compared to nonmetals. It is due to the fact that metals are examined by lower absorption toward optical radiation and higher thermal diffusivity. This leads to the low number of temperature signals with short period. Other known disadvantage of having halogen lamp is the formation of reflected noisy radiation, which can be appeared in both heating and cooling phases. To eliminate these noises, special kind of algorithms are used in infrared thermographic detection [62]. Recently detailed study of this technique was done in 2019 by Doshvarpassand et al., where they showed its application in around 400 literature resources [63].
Figure 2.12 Schematic diagram for generating infrared thermogram.
2.3 Conclusion
Corrosion is the most general obstacle detected in the metallic structure, petrochemical industry, and at oil and gas refineries. This phenomenon of corrosion is occurring due to the metal deterioration and different types of chemical reactions with the pipes. All these problem causes an economic loss at extremely high in multiple industries. The different types of corrosion occur at different position in the same structure. So monitoring and disclosure is the must‐to‐do places where metal is one of the components. Out of destructive and nondestructive methods of monitoring, coupons and probes like electrical resistance and linear polarized resistance are best to monitor corrosion in pipelines. Electrochemical methods like potentiodynamic polarization and impedance spectroscopy are competent and prudent for corrosion analysis. Specifically electrochemical polarization method has a great potential for corrosion monitoring. It has benefit of being more conscious to and not destroying the assessed metallic surface.
So this cause is the only possible way to monitor deterioration based on the extracted data so that further action of replacement of the pipelines can be done in industries. Corrosion monitoring also offers multiple answers to the problems of whether further corrosion is happening now compared to yesterday. Also by knowing this data, it is important to eliminate the cause of corrosion along with its effects. This can be considered as important asset to altercate corrosion and supposing considerable economic prosperity to the country.
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