In pipelines, the early detection of structural or material defects helps to avoid severe collapses that lead to environmental damage and high costs. Therefore, an appropriate and reliable corrosion monitoring tool is necessary in order to guarantee the structural integrity of materials.
In this article, we’ll look at electrochemical noise measurements, the acoustic emission technique, and fiber optics. These form a set of highly complementary non-destructive testing (NDT) methods for monitoring ongoing corrosion in chemical production units. When some of these methods are combined, active corrosion processes can be revealed, and a reliable quantification of corrosion damage can be obtained.
Electrochemical Noise Measurements
A potential difference arises from the oxidation and reduction reactions of the corrosion processes at the metal–electrolyte interface. Accordingly, the short- and long-term evolution of this potential reveals information about the active corrosion processes.
One can use a nanovoltmeter with an input resistance > 10 GΩ and a resolution of 100nV for the potential noise measurements. A General Purpose Interface Bus (GPIB) connects the nanovoltmeter to a computer. The computer controls the nanovoltmeter and then stores the measured data on the hard disk. The potential differences are measured between the sensor body and an Ag/AgCl reference electrode.
General corrosion of carbon steel, general corrosion of stainless steel, pitting of stainless steel, and stress corrosion cracking (SSC) of stainless steel can be distinguished by pattern recognition techniques in combination with statistical analysis in the time domain. Electrochemical noise (EN) measurements have a higher accuracy than more conventional NDT methods (dye penetration, magnetic interference, or ultrasonic testing).
During the scaling-up stage, the potential noise measurements were found to be very reliable; no additional difficulties arose. In fact, no further filtering of the measured data was necessary. The developed measurement concept for the potential noise measurements can also be used in an industrial environment.
Acoustic Emission Technique
Corrosion processes cause microstructural changes at the metal–electrolyte interface. The energy released by these microstructural changes results in transient elastic waves. Sounds of higher frequency and lower intensity than audible sounds can be measured by the acoustic emission technique. One can use a Fracture Wave Detector for this technique.
Using pattern recognition techniques and statistical analysis, different corrosion types can be distinguished. These include pitting of stainless steel, SSC of carbon steel and stainless steel, etc.
Pit or crack initiation and growth can be monitored by the acoustic emission technique. This provides important additional information alongside the EN measurements. The accuracy of the acoustic emission technique is also higher than that of conventional non-destructive testing methods like dye penetration, magnetic interference, or ultrasonic testing.
Optical Fibers
First, an optical fiber (OF) is attached to the surface of interest. Then, a laser diode forces a light pulse through the fiber. Changes in the stress conditions on the surface due to the corrosion activity also change the stress conditions of the OF. The changing stress conditions of the OF modulate the light pulse. Analysis of this modulation is very useful in quantifying corrosion damage.
The level of corrosion is detected and quantified by the variation in intensity of the light pulse. The fiber is fixed on the surface of interest under a certain pre-stress. During ongoing corrosion, the pre-stress reduces, and the intensity of the laser pulse through the fiber will rise. The intensity rise curve is then plotted over time to depict the corrosion rate of the materials.
Optical fibers are very useful in monitoring general corrosion with relatively low corrosion rates. Therefore, the OFs are complementary to acousto-ultrasonics and have proven very reliable.
A technician welding a fiber optic cable at a job site. (Source: Koonsiri Boonnak / iStock)
Corrosion Detection Using Non-Destructive Testing Optical Inspection Systems
Internal pipe inspection is usually carried out using closed-circuit television (CCTV) cameras and offline human surveys through raw image assessment for failure identification. However, it has some limitations: a lack of visibility in the pipe interiors, poor-quality images obtained due to difficult lighting conditions, etc.
An intensity-based optical system for internal pipe inspection can be used as a new alternative. The system consists of a laser diode, an optical ring pattern generator, and a charged-couple device (CCD) camera. A surface map of the inside pipe wall is then generated through extracting the intensity information that exists in the pipe images.
The intensity of images is considerably changed at discontinuities, such as holes. For example, small surface defects result in an increase of intensity. These defects can be identified by dark or bright levels of intensity. Once the ring profiles have been accurately extracted, one can detect defects on each profile instead of searching the whole acquired image for possible defects.
Summary of Non-Destructive Testing
A new laser-based inspection system for inspecting the inner surface wall of a pipe is another excellent technique. The method is based on the projection of a laser-generated ring onto the inner wall of a pipe. The location of holes and defects on the internal surface of pipes is detected by analyzing the intensity of the projected light rings.
The combination of non-destructive testing methods discussed in this article has a higher accuracy than that of more conventional NDT methods, such as magnetic interference or ultrasonic testing.
Electrochemical noise measurements, the acoustic emission technique, and optical fibers are complementary NDT methods for monitoring the general corrosion of carbon steel and stainless steel, pitting of stainless steel, and SSC of carbon steel and stainless steel.
By combining the EN measurements and the acoustic emission results, the active corrosion processes are revealed. The electrochemical part of the corrosion processes is visualized by the EN measurements, while the acoustic emission technique monitors the mechanical part of the corrosion processes, such as the breakdown of passive films, crack growth, and pit growth.
In conclusion, advanced non-destructive testing is a highly effective approach for detecting and monitoring corrosion in pipelines. NDT techniques not only enhance the accuracy of corrosion detection but also offer valuable insights into the electrochemical and mechanical aspects of the degradation process. These methods can help ensure the structural integrity of pipelines, reduce maintenance costs, and prevent catastrophic failures. Ultimately, this contributes to safer and more sustainable pipeline operations.