Eddy Current Inspection Technique Research Articles

Oxygen corrosion of reboiler tube served in production of dilution steam from heat exchanger of petrochemical refinery

This paper reports the oxygen corrosion of heat exchanger tubes serviced in dilute steam generation system. The floating head heat exchanger employed in a petrochemical industry underwent remnant corrosion failures particularly at lower passes of tube bundle. Initially remote field eddy current inspection technique was employed for tube bundle conditional assessment and further, root cause of degradation of tube had been analyzed. Comprehensive experimental techniques including visual examination, tube chemistry analysis, external tube surface morphological analysis, elemental mapping, scanning electron microscopy, light microscopy and x-ray diffraction studies were employed. The tube surface at the degraded position with its cross sectional examination was rigorously evaluated as well. Moreover, actual steam parameters such as pH, conductivity and iron & chloride concentration have been monitored in the petrochemical facility. The detailed analysis concluded that dissolved oxygen in boiler feed water (BFW) was the root cause for corrosion failure. Several other foreign elements were detected and source of each element was discussed in the context of actual heat exchanger operating conditions. Simulated conditions of BFW service have electrochemically been evaluated in two different dissolved oxygen concentrations at 7.5 ppm and 12.7 ppm, respectively. Even small increase of dissolved oxygen in BFW could lead to promotion of oxygen reduction reaction. Theoretically SA179 tubes operated at pH of 9.75 could lead to formation of bicarbonate and it could protect the tube in absence of dissolved oxygen as evaluated by Molecular Dynamics simulation. However, according to obtained results, tube has been degraded due to presence of dissolved oxygen in BFW. Various contributing factors such as external paint/coat removal, absence of de-aerator system in BFW treatment and improper corrosion inhibitor programme were discussed. Several practical countermeasures have been suggested to mitigate oxygen corrosion failure in petrochemical refineries.

Natural Rail Surface Defect Inspection and Analysis Using 16-Channel Eddy Current System

Trains are used as the fastest mode of transportation for both people and cargo. The train moves along a special path called “rail”, where fatigue can be accumulated due to wheel-rail contact load as a result of continuous train operation. Consistent and regularly scheduled safety management is required since corrosion rate of the rails located on outside environment is very high. Researchers have actively investigated and developed rail defect inspection systems employing non-destructive techniques to address these problems. In particular, the eddy current inspection technique does not involve contact with the surface of the test specimen and offers the advantage of excellent rail defect detection sensitivity. Therefore, a 16 Channel array eddy current inspection device was developed to inspect the surface defects of the rail. An equation was derived to predict the correlation between the depth and phase of an artificial defect using the eddy current inspection device, and the derived equation was applied to the natural defect specimen.

Detection of the Fatigue Cracks Initiated near the Rivet Holes by Eddy Current Inspection Techniques

Abstract Eddy current (EC) method is considered as most applicable for in-service detection of fatigue subsurface cracks initiated in aircraft multilayer structures near the rivet holes. At the same time, the successful solution of this problem is obstructed by additional noise created by defect-free rivets. All EC inspection techniques for the detection of subsurface cracks around the rivets can be classified into three main groups: 1) static mode – carried out by placing the EC probe concentrically on the rivet head; 2) rotational mode – when the EC probe is rotated around the rivet axle and 2) sliding mode – performed by the movement of EC probe along the rivet line or near it. All these approaches have some advantages and limitations. In this study, known EC techniques for the detection of cracks in multilayer aircraft structures are analyzed. New advanced EC techniques for the detection of fatigue cracks in internal layers of the riveted structures based on different types (ring, sliding, and rotational) probes are presented. The static EC method with developed low-height ring-type probe creates the possibility to detect cracks in the difficult of access areas. The possibility to estimate the length of detected cracks by a ring-type probe is shown. The proposed rotational remote field EC probe can detect as small as 1.0 mm long cracks under the button-head rivet and 2 mm thick upper skin with a high signal-to-noise ratio. Therefore, in many aircraft structures, fatigue cracks will be detected before a critical threshold achieved. New EC sliding techniques based on remote field and double differential probes were proposed for the rapid detection of cracks in internal layers of riveted aircraft structures. Remote-field EC probe for reliable detection of fatigue cracks in third and fourth layers of five-layer units was proposed. Another sliding technique based on a double differential EC probe gives the possibility to detect transverse cracks in the second layer without the rivet row area access. The main advantage of developed techniques is high inspection reliability due to the possibility to discriminate the signals created by cracks and defect-free rivets. Presented inspection procedures include the selective signal analysis in the complex plane diagram. Proposed EC inspection techniques were successfully implemented into the aircraft maintenance practice.

Physical Manifestation of a90/95 in Remnant Life Revision Studies of Aero-engine Components

Damage Tolerance (DT) lifing methodology for aero-engines require the reliability of Non-Destructive Testing (NDT) techniques used. Probability of Detection (POD) for measuring NDT reliability yields the a90/95 (flaw detection with 90% probability and 95% confidence) value. This a90/95 or the largest crack size missed by an NDT technique is in general, incorporated into the DT calculations for estimating the remaining fatigue cycles the component can withstand before failure. Hence, it is essential to estimate the a90/95 value to the closest accuracy. However, the NDT inspection data at a site containing multiple cracks results in ambiguity of HIT/MISS approaches to be adopted for the estimation of POD or a90/95 values. Several approaches were attempted by the researchers to minimize the ambiguity but with limited success due to the restrictions in implementing them. Moreover, to the best of the author’s knowledge, the physical significance of the a90/95 value obtained from different HIT/MISS approaches on the remnant life calculations of aero-engine components was not available in the literature. Therefore, in the current study, physical manifestation of a90/95 in remnant life calculations obtained from the maximum flaw size and sum of flaw sizes approaches for inspection of natural fatigue cracks in a nickel based superalloy using fluorescent penetrant (FPI) and eddy current inspection (ECI) techniques was attempted. It was observed that ECI technique provides the higher number of remnant cycles than the FPI technique due to its higher sensitivity. In addition, it was also observed that regardless of the NDT techniques used, maximum flaw size approach results in higher number of fatigue cycles. However, the actual number of remnant cycles of the component can be exactly known provided the capability of the current NDT techniques in resolving a group of flaws in a particular location is enhanced.

Development of an Eddy Current Inspection Technique with Surface Magnetization to Evaluate the Carburization Thickness of Ethylene Pyrolysis Furnace Tubes

An eddy current inspection system that incorporates a C-core probe and a direct current magnetizer for HP-grade ethylene pyrolysis tubes was developed to estimate the degree of carburization of the tubes. Through the development process, the characteristics of the B–H curves and the electric conductivities of service-aged tubes were clarified, and a method of counteracting the external magnetism of the tube, which disturbs the accurate detection of the degree of tube carburization, by installing a direct current magnetizer was validated. The influence of tube external magnetism in the oxide/nitride layer was properly canceled by installing a magnetizer with a magnetizing current of 0.5 A on the external surface of the tube. In addition, a calibration curve correlating the carburization depth from the external surface of the tube with the normalized inductance was obtained by finite element analysis simulation. The calibration curve revealed that the degree of carburization could be quantitatively evaluated from the detected normalized inductance of service-aged tubes using the developed inspection system.

Advances in Superalloys

This topic concerns advances in superalloys, and is sponsored by the TMS High Temperature Alloys Committee. Near-term developments on nickel-base superalloys are presented, as well as longer-term considerations for new superalloys. The High Temperature Alloys Committee sponsored a very successful International Symposium on Superalloy 718 and Derivatives on October 10–13, 2010. A wide range of processing, properties, and applications of these alloys was considered in depth. The entire published proceedings are available at http://www.wiley.com/WileyCDA/WileyTitle/ productCd-0470943165.html. The High Temperature Alloys Committee also recently sponsored a symposium entitled ‘‘Advances in Processing’’ at the TMS 2011 Annual Meeting that was very well attended. Several interesting papers are presented from these two symposia. Superalloy 718 has a very versatile microstructure which can include c, c¢, c¢¢, and d phases. Rick Kearsey and coworkers describe how varied grain size, precipitate size, morphology, and phase fraction influence mechanical properties at high temperatures in one improved derivative alloy, ATI 718Plus . This alloy is shown to offer promising combinations of monotonic and cyclic mechanical properties in comparison with a superalloy employed in many turbine disk applications, Waspaloy. Some aerospace applications require small, complex-shaped components. A near-net shape metal injection molding process is investigated for use with Superalloy 718 by Eric Ott and coworkers. This process requires injection of a powder–binder blend into complex tooling, removal of tooling, debinding, densification, heat treatment, and final machining. With sufficient control of process and materials through appropriate industry specifications, this process is shown to have promising potential for aerospace components. Processing of superalloys for aerospace applications often includes shot peening after machining, to introduce compressive residual stresses near the surface that can prevent surface cracking. Verification of the stress magnitude is often required. R. Chandrasekar and co-authors describe the use of an advanced eddy current inspection technique to measure these stresses in Superalloy 718, and the effects of different microstructures on response. As the oil and gas industry has advanced to drill deeper wells with higher pressure, temperature, and corrosive media, conventional steel components have been selectively replaced with superalloys. John ‘‘Jack’’ deBarbadillo describes how Superalloy 718 and derivatives have been adapted to these demanding requirements. More advanced cast and wrought superalloys are being optimized to withstand extreme conditions in deep-water sour gas and oil drilling and production applications. Advanced ultrasupercritical power generation plants have the potential to attain significantly higher efficiency, reducing fuel consumption and gas emissions. Here also, many steel components will need to be replaced with superalloys for the associated increases in temperature. Paul Jablonski and coworkers describe the potential for cast versions of selected wrought superalloys to attain the temperature–stress requirements, in large cast components. Several candidates show promising results in screening tests at relevant conditions. Dispersion strengthening can be used to provide exceptional strength in superalloys at high temperatures. However, this can make the superalloys very difficult to fabricate, due to poor hot working and joining capability. Mike Fahrmann and co-author describe an innovative new Co-base superalloy that uses fine-scale stable nitrides for dispersion strengthening, introduced by a gas nitriding process after fabrication processes are completed. Thermodynamic and kinetic properties are shown to strongly influence the nitridation process. The mission of the TMS High Temperature Alloys Committee is to provide a means of communication among those interested in superalloys and other high-temperature alloys. Emphasis is placed on JOM, Vol. 64, No. 2, 2012

Nonlinear Eddy Current Technique for Characterizing Case Hardening Profiles

Industrial components are often case hardened to improve their strength and wear characteristics. Traditionally, component samples are collected from the production line at specific intervals and destructively tested for case depth profile assessment. This process is time-consuming, laborious, and can potentially allow an improperly treated component to escape detection. This paper presents a novel nonlinear eddy current technique for assessing the case hardening profile based on the premise that the magnetic characteristic of the case hardened region is different from that of the host material. A custom electromagnetic excitation-sensor array is used to both apply sinusoidal excitations to the component and measure the nonlinear response at multiple excitation frequencies and spatial locations, taking advantage of the different penetration regions due to the skin depth phenomenon. Each response signal obtained from the component under test is compared with that from a reference component subjected to the same excitation. Two pattern recognition algorithms (an artificial neural network and the Iterative Dichotomiser 3 (ID3) algorithm) are then used to process selected characteristics of the difference signal to determine the case depth profile of the component. The nonlinear eddy current technique has been applied to evaluate the case hardening profile of automotive bearing assemblies. This problem is challenging due to the variations in geometry across assemblies as well as the limited accessibility to the case hardened surface. In the neural network test, the system achieved a 95.77% accuracy. For the ID3 algorithm, the system achieved a 95.65% accuracy. These results demonstrate that the nonlinear eddy current inspection technique is highly promising in characterizing the case profile of induction hardened parts.

Correlation of fatigue life with potential NDE corrosion metrics

Attempts are being made to move away from the 'Find-and-Fix' approach and to manage corrosion damage in aircraft so that it can be rectified during scheduled maintenance periods. This 'Identify-and-Manage' approach will only be possible if effective non-destructive evaluation (NDE) methods are available to detect, characterise and monitor the corrosion that is present within the air frame and suitable life-prediction models exist to predict the interaction of corrosion and fatigue from the characterised corrosion damage. In order to investigate potential NDE corrosion metrics, a set of dog-bone fatigue specimens was fabricated from unclad 2024-T3 aluminium alloy. The specimens were corroded in a salt-fog chamber for a range of time intervals to induce both pitting and crevice corrosion, which was then characterised using ultrasonic and eddy current inspection techniques. Ultrasonic inspection was carried out at three frequencies (10, 22 and 50 MHz) using focused immersion transducers. Amplitude and depth scans at each frequency were obtained. Eddy current inspection was carried out at a single frequency of 10 kHz. A number of parameters were calculated from the scans and correlated with the fatigue life.

Signal processing for remote field eddy current inspection technique : Udpa, S.S.; Ou-Yang, T.H.; Lord, W. Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22–26 Jun. 1987. Vol. 7A, pp. 831–838. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988

Signal processing for remote field eddy current inspection technique : Udpa, S.S.; Ou-Yang, T.H.; Lord, W. Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22–26 Jun. 1987. Vol. 7A, pp. 831–838. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988

Eddy current inspection of graphite fiber components : National Aeronautics and Space Administration, Huntsville, Alabama (United States), N91-10294/7/GAR, 52 pp. (Oct. 1990)

The recognition of defects in materials properties still presents a number of problems for nondestructive testing in aerospace systems. This project attempts to utilize current capabilities in eddy current instrumentation, artificial intelligence, and robotics in order to provide insight into defining geometrical aspects of flaws in composite materials which are capable of being evaluated using eddy current inspection techniques. The unique capabilities of E-probes and horseshoe probes for inspecting probes for inspecting graphite fiber materials were evaluated and appear to hold great promise once the technology development matures. The initial results are described of modeling eddy current interactions with certain flaws in graphite fiber samples.

Signal processing for remote field eddy current inspection technique Review of progress in quantitative nondestructive evaluation, Williamsburg, Virginia (United States), 22–26 Jun. 1987. Vol. 7A, pp. 831–838. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Signal processing for remote field eddy current inspection technique Review of progress in quantitative nondestructive evaluation, Williamsburg, Virginia (United States), 22–26 Jun. 1987. Vol. 7A, pp. 831–838. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Signal processing for remote field eddy current inspection technique : Review of progress in quantitative nondestructive evaluation, Williamsburg, Virginia (United States), 22–26 Jun. 1987. Vol. 7A, pp. 831–838. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

Signal processing for remote field eddy current inspection technique : Review of progress in quantitative nondestructive evaluation, Williamsburg, Virginia (United States), 22–26 Jun. 1987. Vol. 7A, pp. 831–838. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

History of the remote-field eddy current inspection technique : Materials evaluation, Vol. 47, No. 1, pp. 14–22 (Jan. 1989)

History of the remote-field eddy current inspection technique : Materials evaluation, Vol. 47, No. 1, pp. 14–22 (Jan. 1989)

The reliability of rotating probe eddy-current inspection techniques for fastener holes in aircraft Reliability in non-destructive testing, Proceedings of the 27th annual british conference on non-destructive testing, Portsmouth (United Kingdom), 12–15 Sep. 1988. pp. 45–64. Edited by C. Brook and P.D. Hanstead. Pergamon Press, 1989

The reliability of rotating probe eddy-current inspection techniques for fastener holes in aircraft Reliability in non-destructive testing, Proceedings of the 27th annual british conference on non-destructive testing, Portsmouth (United Kingdom), 12–15 Sep. 1988. pp. 45–64. Edited by C. Brook and P.D. Hanstead. Pergamon Press, 1989