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[Dongfeng He](https://orcid.org/0000-0001-5670-4419)

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[Review of Corrosion Evaluation Methods for Steel Reinforcement in Concrete](https://mdr.nims.go.jp/datasets/07fea84d-d756-4435-8c3a-53b82d5e3c2b)

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Academic Editor: María Criado SanzReceived: 20 June 2025Revised: 30 July 2025Accepted: 5 August 2025Published: 7 August 2025Citation: He, D. Review of CorrosionEvaluation Methods for SteelReinforcement in Concrete. Corros.Mater. Degrad. 2025, 6, 37. https://doi.org/10.3390/cmd6030037Copyright: © 2025 by the author.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).ReviewReview of Corrosion Evaluation Methods for SteelReinforcement in ConcreteDongfeng HeResearch Center for Structural Materials, National Institute for Materials Science, 1-2-1 Sengen,Tsukuba 305-0047, Ibaraki, Japan; he.dongfeng@nims.go.jp; Tel.: +81-29-859-2533AbstractCorrosion of steel reinforcement is one of the primary causes of deterioration in reinforcedconcrete structures, significantly impacting their durability and structural performance.This review comprehensively examines various techniques used to evaluate rebar corrosion,categorizing them into electrochemical, physical, and advanced non-destructive methods.Each method is discussed with respect to its operational principles, advantages, limitations,and field applicability. This comparative overview aims to support the selection of suitableevaluation strategies tailored to diverse structural conditions.Keywords: steel reinforcement; corrosion evaluation; non-destructive testing; reinforcedconcrete; structural health monitoring1. IntroductionSteel reinforcement, commonly referred to as rebar, plays a vital role in cement-basedstructures by providing tensile strength and enhancing the overall structural integrity ofconcrete [1–3]. Concrete alone is strong in compression but weak in tension; embeddingsteel reinforcement compensates for this weakness, allowing for the construction of durablebridges, buildings, and other infrastructure.However, the corrosion of embedded steel reinforcement poses a major threat to thedurability and safety of concrete structures, especially in aggressive environments such asmarine atmospheres or regions exposed to deicing salts [4,5]. Corrosion of reinforcing steelis an electrochemical process that progresses through two primary stages: the initiationperiod and the propagation period.During the initiation period, a passive layer—primarily composed of iron oxides—naturally forms on the steel surface in the highly alkaline pore solution of concrete(pH > 12.5). This passive film provides temporary protection against corrosion. However,its stability is compromised when chloride ions from external sources (e.g., seawater orde-icing salts) or carbonation reduce the local alkalinity. Once the chloride concentrationat the rebar surface exceeds a critical threshold or the pH drops sufficiently, the passivelayer breaks down, marking the onset of active corrosion [6,7].The breakdown of the passive layer involves complex physicochemical mechanisms.Chloride ions penetrate the concrete matrix and accumulate at the steel–concrete interface,leading to localized depassivation. This results in the formation of anodic sites whereiron dissolves into Fe2+ ions, and cathodic sites where oxygen reduction occurs. Thedifferential aeration around these sites accelerates corrosion through the formation ofmicrocells [8]. Furthermore, factors such as moisture content, oxygen availability, andtemperature fluctuations can influence the corrosion kinetics significantly.Corros. Mater. Degrad. 2025, 6, 37 https://doi.org/10.3390/cmd6030037https://doi.org/10.3390/cmd6030037https://doi.org/10.3390/cmd6030037https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/cmdhttps://www.mdpi.comhttps://orcid.org/0000-0001-5670-4419https://doi.org/10.3390/cmd6030037https://www.mdpi.com/article/10.3390/cmd6030037?type=check_update&version=1Corros. Mater. Degrad. 2025, 6, 37 2 of 24Once active corrosion begins, the propagation period is characterized by the formationof expansive corrosion products (e.g., Fe2O3, Fe3O4), which exert internal pressure onthe surrounding concrete, causing cracking, delamination, and eventually spalling. Thesedeterioration processes can severely impair the structural capacity and service life ofreinforced concrete components [9]. Figure 1 shows the corrosion mechanism of steelrebar [10].Figure 1. Corrosion mechanism of steel rebar [10].Given the complexity and critical importance of these degradation mechanisms, theearly detection and quantification of rebar corrosion are essential. Non-destructive testing(NDT) techniques offer valuable tools for assessing corrosion without damaging the struc-tural integrity of concrete. This paper provides a comprehensive review of current NDTmethods for evaluating corrosion in reinforced concrete, with a focus on their principles,capabilities, limitations, and potential for field deployment.2. Steel Rebar Corrosion Evaluation and MethodsNumerous techniques have been developed to detect the corrosion of steel rebar inconcrete. Commonly used methods include electrochemical techniques, ultrasonic methods,X-ray tomography, infrared thermography method, ground penetration radar method, mag-netic flux leakage method, and eddy current testing. This paper focuses on summarizingthese non-destructive evaluation approaches, while methods involving embedded sensorswithin the concrete will not be covered.2.1. Electrochemical TechniquesElectrochemical techniques are widely used for assessing the corrosion behavior ofsteel rebar in concrete due to their sensitivity, quantitative capabilities, and suitability forboth laboratory and field applications. These methods are based on the electrochemicalnature of corrosion and provide valuable information about the corrosion potential, rate,and mechanisms. The most common electrochemical techniques include half-cell potentialmeasurement, concrete resistivity measurement, linear polarization resistance (LPR), andelectrochemical impedance spectroscopy (EIS).2.1.1. Half-Cell Potential MeasurementHalf-cell potential (HCP) measurement is also called open-circuit potential (OCP) mea-surement, which is a widely used non-destructive technique to assess the corrosion activityCorros. Mater. Degrad. 2025, 6, 37 3 of 24of steel reinforcement in concrete structures [11,12]. Figure 2 shows a schematic diagram ofthe technique. It measures the electrical potential difference between the embedded steeland a standard reference electrode placed on the concrete surface. The technique is basedon electrochemical principles. When steel corrodes in concrete, electrochemical cells areformed, creating anodic (corroding) and cathodic (non-corroding) regions.Figure 2. Schematic diagram of half-cell potential measurement.A saturated calomel electrode (SCE), copper/copper sulfate electrode (CSE), or sil-ver/silver chloride electrode is usually used as the standard reference electrode. As perASTM C 876 [13] standards, the probability of reinforcement corrosion is as shown inTable 1.Table 1. Corrosion conditions related to HCP measurements.Potential Value(mV vs. SCE)Potential Value(mV vs. CSE) Corrosion Condition≥−125 ≥−200 Low (10% risk of corrosion)−126 to −275 −201 to −350 Intermediate corrosion risk−276 to −425 −351 to −500 High (<90% risk of corrosion)≤−426 ≤−501 Severe corrosionTo ensure accurate and reliable measurements, certain technical specifications mustbe met. The voltmeter used should have an input impedance of at least 10 MΩ to mini-mize current draw and avoid polarization of the reference electrode. Lower impedancedevices can cause current flow, leading to distorted readings and misinterpretation of thecorrosion state.Moisture content in the concrete plays a critical role in potential measurement. Ade-quate pore water conductivity is necessary to establish a stable electrical path between therebar and reference electrode. In dry concrete, measurements may be erratic or yield artifi-cially high potentials due to insufficient ionic conduction. To address this, pre-wetting ofthe concrete surface or the use of wet sponges is commonly practiced in field applications.Corros. Mater. Degrad. 2025, 6, 37 4 of 24Additionally, the stabilization time of the potential signal is essential. After connectingthe measurement circuit, the potential may fluctuate initially due to capacitive effects andelectrode settling. It is recommended to allow the signal to stabilize—typically for 30–60 s—before recording the reading. Automated systems may incorporate real-time stabilizationdetection to improve reliability.While HCP provides a qualitative indication of corrosion probability, it does not quan-tify the corrosion rate. Therefore, it is often used in combination with other electrochemicaltechniques such as linear polarization resistance (LPR) or electrochemical impedance spec-troscopy (EIS) for more comprehensive corrosion assessment.2.1.2. Concrete Resistivity MeasurementIn addition to corrosion potential (Ecorr) measurements, concrete resistivity is a keyelectrochemical parameter that provides valuable complementary information regardingthe likelihood and rate of corrosion [14]. While Ecorr reflects the thermodynamic tendencyfor corrosion, resistivity relates to the ionic conductivity of the concrete, which governs theavailability of electrolyte paths for electrochemical reactions.Concrete resistivity is typically measured using the Wenner four-point method, whichinvolves placing four equally spaced electrodes on the concrete surface and applying aknown current across the outer probes while measuring the potential drop across the innerprobes [15]. Figure 3 shows the setup of the measurement. The measurement is influencedby surface moisture, temperature, aggregate content, and the presence of cracks. Fieldprotocols often recommend performing resistivity measurements in tandem with half-cellpotential mapping, especially during condition surveys of bridge decks, parking structures,and marine platforms.Figure 3. Circuit for electrical resistance measurements.Low resistivity (<10 kΩ·cm) typically indicates high ionic mobility, which facilitatesthe corrosion process, especially under conditions where oxygen and moisture are present.Conversely, high resistivity (>100 kΩ·cm) suggests limited ion transport and may inhibitcorrosion, even if Ecorr values indicate a depassivated state. As such, the combined in-terpretation of Ecorr and resistivity provides a more robust and reliable assessment ofcorrosion risk [16].The importance of this complementary approach has been emphasized in severalinternational guidelines and research publications. In particular, Elsener et al. [17] highlightCorros. Mater. Degrad. 2025, 6, 37 5 of 24that half-cell potential mapping alone may be misleading when concrete resistivity is eitherextremely high (dry concrete) or extremely low (due to carbonated zones or high humidity).In such cases, resistivity measurements help contextualize Ecorr results and avoid falsepositives or negatives in structural assessments.2.1.3. Linear Polarization ResistanceThe linear polarization resistance (LPR) method is used to measure the corrosion rateof metal in an electrolyte [18–20]. Figure 4 shows the schematic diagram of the LPR method.Figure 4. Setup of linear polarization resistance measurement.If we apply small potential perturbation (typically ±10–20 mV) to the metal’s open-circuit potential (Ecorr), the electrochemical reaction at the metal surface will respond bydeveloping a small current. Within this small polarization range, the current (∆I) is directlyproportional to the applied potential (∆E). This linear relation obeys Ohm’s law:Rp =∆E∆I(1)Here Rp is the polarization resistance. Then, the corrosion current icorr can be calculatedby the Stern–Geary equation:icorr =BRp(2)The constant B (Stern–Geary constant) typically falls within 13–52 mV for steel inconcrete, depending on conditions. Once icorr is known, we can compute the corrosion rate(v) in millimeters per year by applying Faradays’ law:v = k·icorr (3)k is a conversion constant that depends on the atomic weight of iron, the valence of themetal dissolution reaction (typically 2 for iron), the density of steel, and Faradays’ constant.The LPR method is a powerful tool to quickly and non-destructively assess the cor-rosion rate of steel rebars in concrete structures. In order to overcome the problem ofconfining the current to a predetermined area, the use of a second auxiliary guard ringelectrode surrounding the inner auxiliary electrode has been developed [21]. Table 2 showsthe relationship between the condition of steel rebar and the corrosion current [22].Corros. Mater. Degrad. 2025, 6, 37 6 of 24Table 2. The relationship between the condition of steel rebar and the corrosion current.Corrosion Current (Icorr) Condition of Steel Rebar<0.2 µA/cm2 No corrosion expected0.2~1.0 µA/cm2 Corrosion possible in 10–15 years1.0~10 µA/cm2 Corrosion expected in 2–10 years>10 µA/cm2 Corrosion expected in 2 years or lessFigure 4 presents a typical laboratory setup for electrochemical testing where theenvironmental parameters are controlled. However, on-site applications of electrochemicalmethods require additional considerations due to variable conditions and unknown elec-trode geometries. In field settings, the exact surface area of the rebar is generally unknown,and corrosion products or concrete cover may introduce contact resistance. To addressthese issues, protocols such as those proposed by Polder [16] recommend using a referenceelectrode in combination with a guard ring or signal confinement method to estimatepolarization area and reduce edge effects [16]. Portable systems have been developed toapply short current pulses to induce polarization and analyze the electrochemical response,which is then correlated to the corrosion rate through calibration models.Moreover, recent advances in electrochemical testing have led to the development ofnovel techniques that enhance sensitivity and adaptability for both laboratory and field use.For instance, a new method utilizing a modified three-electrode configuration embeddedinto concrete has been shown to improve accuracy and reduce noise in LPR readings, evenunder highly resistive conditions [23]. Another innovative approach employs embeddedmicrosensors for continuous electrochemical monitoring in concrete structures, whichenables real-time corrosion diagnostics without surface access [24].These new developments are particularly important for in situ monitoring of in-frastructures like bridges, tunnels, and marine platforms, where continuous or periodiccorrosion assessment is needed without damaging the structure. Such techniques contributesignificantly to the implementation of predictive maintenance strategies and lifecycle-basedasset management.2.1.4. Electrochemical Impedance SpectroscopyElectrochemical impedance spectroscopy (EIS) is a powerful, non-destructive electro-chemical technique used to investigate the corrosion mechanisms and kinetics of metal in anelectrolyte [25,26]. In standard laboratory practice, EIS is performed using a three-electrodeconfiguration. Figure 5 shows the setup [27]. The reinforcing steel acts as the workingelectrode (WE). A reference electrode (RE)—typically a saturated calomel electrode (SCE)or Ag/AgCl—is placed near the rebar. A counter electrode (CE) is also used, which is oftenmade of stainless steel or graphite.Instead of applying small DC polarization (like in LPR), EIS involves applying a smallalternating voltage (AC) of low amplitude (typically 5–10 mV) to the rebar’s surface andmeasuring the resulting AC current. The frequency is varied over a wide range (e.g., 0.01 Hzto 100 kHz) to probe different electrochemical processes. The system’s impedance (Z) ismeasured as a complex quantity (real Z′ and imaginary Z′′ components). The responsedepends on the electrochemical processes at the rebar–concrete interface (e.g., chargetransfer, diffusion, oxide layer behavior).Corros. Mater. Degrad. 2025, 6, 37 7 of 24Figure 5. Setup for EIS measurements.Electrochemical impedance spectroscopy (EIS) has been applied to investigate thecorrosion of steel rebars within concrete structures. Dawson et al. [28] performed impedancemeasurements on mild steel rebars embedded in concrete using two-electrode or three-electrode configurations and subsequently interpreted these spectra with the Randlesequivalent circuit. Multiple equivalent circuits have been put forward to describe thedissolution process of steel rebars in concrete or mortar, based on impedance spectracollected with two- or three-electrode setups [29–31].To avoid having to connect directly to the steel rebar, some researchers have in-vestigated an indirect approach employing a four-electrode or two-electrode configura-tion [32,33], as shown in Figure 6.Figure 6. Schematic of (a) four-electrode setup and (b) two-electrode setup of electrochemicalimpedance spectroscopy. Based on ref. [33].It should be noted that EIS is predominantly a laboratory technique due to severallimitations in field environments: long measurement times at low frequencies; the needfor stable environmental and moisture conditions; difficulty in establishing proper con-tact with embedded reinforcement; and the complexity of interpreting multi-componentimpedance spectra.Corros. Mater. Degrad. 2025, 6, 37 8 of 242.2. Ultrasonic MethodUltrasonic testing is a powerful non-destructive method for evaluating the conditionof steel rebars in reinforced concrete structures. Figure 7 shows the approach of ultrasonictesting. It operates by sending high-frequency stress waves into the material and analyzingtheir propagation, reflection, and attenuation to assess internal conditions.Figure 7. Approach of ultrasonic testing.As ultrasound propagates through the concrete and reaches the steel, it reflects towardthe transducer, allowing for measurement of the travel time and reflection amplitude. Areduction in the cross-sectional area due to corrosion, the presence of voids, or poor bondconditions can affect these signals, making it possible to identify and quantify damagewithout physically removing material. Ultrasonic testing is frequently used on-site becauseit is lightweight, portable, and can provide localized information about a small area ofa structure.The pulse velocity (UPV) method has been broadly applied to assess the condition ofconcrete structures. This approach, which involves measuring the speed of acoustic wavestraveling through the material, is sensitive to the bond between steel and concrete and tothe presence of cracks [34]. Table 3 shows the relationship between ultrasonic pulse velocityand the quality of concrete [15,35].Table 3. Longitudinal pulse velocity vs. quality of concrete.Longitudinal PulseVelocity (km/sec.)Approximate CompressiveStrength (N/mm2) Quality of ConcreteBelow 2.0 --- Very poor2.0 to 3.0 4.0 Poor3.0 to 3.5 Up to 10 Fairly good3.5 to 4.0 Up to 25 Good4.0 to 4.5 Up to 40 Very goodAbove 4.5 Up to 40 ExcellentThe impact echo (IE) method, which relies on striking the surface mechanically andthen recording the signals captured by nearby sensors, has been used less frequently to iden-tify damage stemming from the corrosion of embedded steel [36]. Some researchers havepointed out that the reliability of IE decreases as the thickness of the concrete increases [37].Nonlinear ultrasonic (NLU) techniques have demonstrated potential for evaluatingmaterial deterioration [38,39]. Specifically, these methods are well-suited for detectingCorros. Mater. Degrad. 2025, 6, 37 9 of 24early-stage cracking [40]. NLU is based on the principle that the interaction of ultrasoundwith a flawed material produces higher-frequency components in the acoustic signals [41].Nevertheless, factors such as surface roughness, material heterogeneity, and moisturecontent can influence the signals and sometimes make their interpretation challenging.Nevertheless, when applied by skilled inspectors, ultrasonic testing is a valuable tool forcondition assessment and maintenance planning of reinforced concrete structures.2.3. X-Ray Tomography MethodX-ray tomography, particularly X-ray computed tomography(X-CT), has becomea powerful non-destructive tool for evaluating the internal condition of steel rebars inreinforced concrete structures [42–45]. It utilizes X-rays to generate a series of radiographicimages from different angles. These two-dimensional projections are then computationallyreconstructed into a high-resolution, three-dimensional view of the material’s internalstructure. X-ray tomography enables a direct visualization of the rebar’s geometry, surfacemorphology, and any internal defects or abnormalities that may be present due to corrosion.This method operates on the principle of X-rays being differentially absorbed bymaterials of different densities and atomic numbers. Steel has a higher density than thesurrounding concrete, which results in greater absorption of X-rays. Furthermore, areaswhere corrosion has removed metal or filled voids with rust products appear as regions oflower density. The result is a detailed 3D map of the rebar’s condition, allowing engineersto quantify the depth of pitting, reduction in cross-sectional area, and the distribution ofcorrosion products.Figure 8 shows the schematic diagram of the X-ray tomography method to evaluate thecorrosion of steel rebar. The X-ray source emits a beam of X-rays that penetrates the concretesample containing the steel rebar. The intensity of X-rays is attenuated differently by variousmaterials (concrete, steel, corrosion products), enabling contrast in the captured images.The concrete sample with embedded steel rebar is mounted on a rotating foundation. TheX-ray detector is positioned on the opposite side of the X-ray source; the detector collects thetransmitted X-rays after they pass through the sample. A reconstructed 2D cross-sectionalimage of the sample shows the internal structure. By combining multiple cross-sectionalslices, a 3D volume image is reconstructed. This 3D view enables the comprehensivevisualization of corrosion morphology, such as pit depth, distribution, and volume loss.Figure 9 shows the progression of corrosion of steel rebar observed by 3D X-ray micro-computed tomography [46]. After 14 days of chloride-accelerated corrosion testing, clearsigns of corrosion appeared on the rebar, primarily in the portions exposed outside theconcrete cube. By day 42, corrosion on the exposed rebar had significantly intensified, whilechanges within the concrete remained relatively minor. Similar patterns were observedafter 70 days, with external corrosion continuing to advance and internal corrosion showingonly slight progression.One key advantage of X-ray tomography is its ability to produce high-resolution,non-destructive, and volumetric data. Traditionally, assessing internal corrosion requiredcutting or coring, which is invasive and destroys the specimen. X-CT avoids this by allowinga complete view of the interior without physical intervention. Furthermore, it can resolvefine details, often at a micron scale, which makes it suitable for identifying small defectsand initiation sites of corrosion that might not be detectable by surface methods.Despite its capabilities, X-ray tomography has some limitations. The method typicallyrequires specialized equipment, a radiation-safe environment, and extensive computingresources for data reconstruction and analysis. Furthermore, the size of the specimenthat can be scanned at high resolution is limited by the dimensions of the X-ray imagingapparatus. Dense materials, large dimensions, or thick protective covers can diminishCorros. Mater. Degrad. 2025, 6, 37 10 of 24resolution and contrast. Nonetheless, for small cores or segments of reinforced concrete,X-CT provides unparalleled insight into internal conditions and damage mechanisms.Figure 8. Schematic diagram of X-ray tomography method.Figure 9. The 3D X-ray micro-CT visualization of steel rebar, based on ref. [46]: (a) before corrosionacceleration experiment, (b) 14th day of the experiment, noticeable first corrosion changes within therebar, (c) 42nd day of the experiment, and (d) 70th day of the experiment.2.4. Infrared Thermography MethodInfrared thermography (IRT) is a non-destructive testing (NDT) technique that enablessurface and subsurface defect detection based on thermal imaging. In the field of civilengineering, IRT has emerged as a promising tool for evaluating the corrosion of steelreinforcement (rebar) in concrete structures [47–50]. All materials emit infrared radiationCorros. Mater. Degrad. 2025, 6, 37 11 of 24in proportion to their temperature; by employing a thermal camera, this radiation can becaptured and processed into a thermal image. The principle of infrared thermography forcorrosion evaluation lies in the thermal anomalies that arise from differences in materialproperties, defects, or damage beneath the surface.When corrosion occurs in a steel rebar, the products of corrosion (such as rust) typicallyhave different thermal properties, including thermal conductivity and emissivity, thanthe base metal. Furthermore, the formation of corrosion products can affect the bondbetween steel and the surrounding concrete, creating small voids or detachments. Theseabnormalities manifest as localized temperature variations on the surface of the structure.Two main approaches are used in IRT for corrosion detection: passive thermographyand active thermography. Passive thermography relies on naturally occurring thermalgradients (e.g., solar heating) to reveal anomalies [51,52]. Active thermography involvesexternal stimulation (e.g., heat lamps, flash lamps, eddy current) to induce thermal waves,allowing for better subsurface defect detection [53–56].Figure 10 shows the schematic setup of the eddy current heating infrared thermog-raphy method to evaluate the corrosion of steel rebar [56]. A coil is positioned above theconcrete surface. When an alternating current is applied to the coils, eddy currents are in-duced within the steel rebar. The eddy current generates localized heating in the rebar dueto its electrical resistance (Joule heating). The heat generated in the steel rebar diffuses out-ward through the surrounding concrete. Any variations in the diffusion pattern—causedby corrosion or defects—affect the temperature distribution on the concrete surface. Aninfrared (IR) camera is used to monitor the surface temperature changes by capturing theemitted infrared radiation. The camera records a sequence of thermal images showing howthe heat propagates over time. A PC controls the system, including both the heating pulse(timing and intensity) and the infrared camera. The captured image sequence is processedon the PC to analyze thermal contrasts and detect anomalies associated with corrosion.Figure 10. Setup of eddy current heating infrared thermography method to evaluate corrosion ofsteel rebar.Infrared thermography is a quick, non-invasive, and non-contact technique. Largeareas of a structure can be scanned rapidly, making it well-suited for the preliminaryscreening of extensive reinforced concrete components, such as bridges, decks, and parkinggarages. The captured thermal images can be processed to highlight regions of potentialCorros. Mater. Degrad. 2025, 6, 37 12 of 24deterioration, guiding further investigations with more invasive or localized methodsif necessary.However, the IRT method has some limitations. The accuracy of infrared thermog-raphy depends on environmental conditions, such as solar radiation, wind, and surfacewetness, which can affect temperature signals. The depth of defects detectable by infraredis typically limited to shallow abnormalities—typically 2~3 cm below the surface—andthe method may be less effective for detecting deep or uniformly distributed corrosion.Furthermore, proper data interpretation requires specialized knowledge and experience toaccount for these influencing factors.2.5. Ground Penetrating Radar MethodGround penetrating radar (GPR) is a non-destructive testing method that utilizes elec-tromagnetic waves to image the interior of materials [57–59]. Figure 11 shows a schematicdiagram of the GPR system. The microwave source generates high-frequency electromag-netic pulses (usually in the range 10 MHz to 2.6 GHz), and the transmitter (Tx) antennadirects the microwave signal into the concrete structure. As the signal penetrates the con-crete, it interacts with embedded objects, such as steel rebar. The presence of steel rebarcauses part of the microwave signal to reflect due to its different dielectric properties. Thelevel and characteristics of this reflection can be affected by corrosion, which alters theelectrical and magnetic properties of the rebar [60–62]. The receiver (Rx) antenna capturesthe reflected microwave signals after interacting with the steel rebar. The time delay andamplitude of the received signal provide information about the depth and condition of therebar. The pulse extender enhances and shapes the received signal for better analysis andresolution. It helps distinguish between reflections from different layers or objects. TheA/D converter (analog-to-digital converter) converts the analog microwave signals intodigital data for computational processing. The processor analyzes the digitized signals toextract meaningful information about the location and condition of the steel rebar. It mayapply signal processing techniques like filtering, deconvolution, or time-domain analysis.The visual display presents the processed data as images or graphs. It typically shows rebarposition, potential corrosion zones, or signal amplitude vs. depth.Figure 11. Schematic diagram of ground penetrating radar for evaluation of steel rebar.The principle of GPR for evaluating corrosion lies in the contrast in electromagneticproperties—particularly the permittivity and conductive properties [63]. As rebar corrodes,rust products form and microcracks develop in the surrounding concrete, affecting itsability to reflect radar signals. Furthermore, thinning of the steel due to corrosion weakensCorros. Mater. Degrad. 2025, 6, 37 13 of 24the reflection from the rebar. The degree of reflection attenuation and the spreading of theradar pulse can be indicators of the condition of the rebar.Figure 12 shows the GPR scanning profile of steel rebar with different corrosion lev-els [64]. Since the ground penetrating radar (GPR) profile presents a simulated signal image,visual differences between corrosion states are not readily apparent. Therefore, corrosionassessment in this study relies on coded signal analysis. The reflected electromagneticwave voltage from steel bars embedded in concrete was examined at various stages ofaccelerated corrosion (ranging from 0 to 408 h). GPR results indicated that during theselected corrosion durations of 72, 192, and 288 h, the reflected voltage varied accordingto the extent of corrosion, as illustrated in Figure 13. These findings demonstrate that thedegree of corrosion increases with longer exposure to the accelerated corrosion process.Figure 12. GPR scanning profile of steel rebar corrosion (thickness of 4 cm), based on ref. [64].Figure 13. Reflected voltages of steel rebars with 4 cm protective layer, based on ref. [64].GPR is a high-resolution, non-invasive, and rapid method for scanning large areas ofreinforced concrete structures. It is frequently used to produce two-dimensional and, bycollecting data on a grid, even three-dimensional images of the internal configuration ofrebars. GPR can provide valuable information on the depth, spacing, and condition of thereinforcements without disturbing or damaging the structure [65,66].Corros. Mater. Degrad. 2025, 6, 37 14 of 24However, GPR has some limitations. The depth of penetration is influenced by concreteproperties, moisture content, and frequency of the antenna [67]. Higher-frequency antennasprovide greater resolution but less penetration depth. Additionally, GPR signals can becomeweak or distorted in materials with high clay content or heavy chloride contamination [68].The interpretation of GPR data also requires expertise and experience, as signals can beinfluenced by numerous factors unrelated to corrosion.2.6. Magnetic Flux Leakage MethodThe magnetic flux leakage (MFL) method is a well-established, non-destructive testing(NDT) technique used for the inspection of ferromagnetic materials [69–71]. It has beenincreasingly adopted for the evaluation of corrosion in steel reinforcement bars (rebars)embedded in concrete structures [72–74]. The principle of MFL relies on the distortion ofmagnetic fields in regions where metal loss or structural discontinuities, such as corrosionpits, are present.Figure 14 shows the setup of the MFL method for the corrosion evaluation of steel rebar.The excitation coils, on either side of a magnetic yoke, generate a controlled magnetic fieldthat passes through the concrete and steel rebar. The coils are driven by a function generatorand power amplifier connected to a PC, which allows for precise control of the excitationcurrent and waveform (typically sinusoidal or pulsed). The magnetic yoke provides a pathfor the magnetic field to flow through and concentrate over the steel rebar. It ensures thatmagnetic flux enters the steel bar and returns to the coil circuit, creating a closed magneticloop. When the rebar is magnetized, intact regions guide the flux smoothly, while corrodedregions cause magnetic flux leakage due to air gaps or material discontinuities from rustloss. The leakage field appears on the concrete surface and can be detected above therebar. The magnetic sensor is positioned in the center of the magnetic yoke and (e.g., Hall-effect sensor, giant magnetoresistance sensor) detects changes in magnetic flux density.Anomalies in the measured signal indicate the presence and severity of corrosion.Figure 14. Setup of magnetic flux leakage device for evaluation of steel rebar.Figure 15 shows the typical MFL signals for the steel rebars with no defect, saw cuttingdefect, and grinding defect [73]. The MFL signals display a sinusoidal waveform as thesensor passes over the ribs of the reinforcing steel, as shown in Figure 15a. When a defectCorros. Mater. Degrad. 2025, 6, 37 15 of 24is present in the specimen, the waveform changes in both amplitude and bandwidth,reflecting variations in defect width and depth, as shown in Figure 15b. In the case ofgrinding-induced defects (rib loss), the signal loses its oscillatory pattern and becomesstraighter, connecting directly to the next rib peak, as shown in Figure 15c.Figure 15. Typical waveforms of MFL signal over reinforcing steel: (a) new rebar, (b) rebar withcutting defects, and (c) rebar with grinding defects. Based on ref. [73].The MFL method is a powerful tool for detecting and quantifying rebar corrosion, offer-ing advantages in speed and accuracy. Recent advancements in sensor technology [75–77]and AI-driven signal processing have enhanced its reliability [78–80]. However, chal-lenges like concrete cover interference necessitate further research. Future developments inmulti-sensor fusion and automated inspection systems will broaden MFL’s applicability instructural health monitoring.2.7. Eddy Current Testing MethodEddy current testing (ECT) is a non-destructive testing technique widely used forinspecting conductive materials. Its ability to detect surface and near-surface flaws makes ita powerful tool in various industries, including aerospace, automotive, and civil infrastruc-ture [81–85]. The principle of this technique relies on alternating current passing througha coil, which generates an alternating magnetic field. When this coil is brought close toa conductive material, eddy currents are induced in the material’s surface. The flow ofthese eddy currents depends directly on the material’s properties, such as its electricalconductivity, permeability, dimensions, and its condition, including the presence of defectsor corrosion.Figure 16 shows the principle of the ECT method to evaluate the corrosion of steelrebar. The excitation coil generates a primary oscillating magnetic field when powered bythe AC source. The magnetic field penetrates the concrete and induces eddy currents inthe conductive steel rebar located beneath the surface. Corrosion in the rebar affects themagnitude and distribution of these eddy currents. The detection coil detects the secondarymagnetic field produced by the eddy currents in the steel rebar. The strength and phase ofthis induced field are influenced by changes in the rebar’s condition (e.g., due to corrosionor section loss). The lock-in amplifier is used to filter out noise and extract the desiredsignal that is synchronized with the reference signal from the AC source.Corros. Mater. Degrad. 2025, 6, 37 16 of 24Figure 16. Principle of eddy current testing method for corrosion detection of steel rebar.ECT is used to detect pitting corrosion, loss of cross-sectional area, and surface cracksin steel rebars [86–92]. It is particularly effective in early-stage corrosion detection, whereminimal visual signs are present. The technique can be applied in laboratory settings andon-site inspections, using portable equipment [93,94].Figure 17 shows a portable compact ECT system for the corrosion evaluation of steelrebar. Figure 18 shows the block diagram of the system [94]. The frequency of 80 kHz isused. All the core components, including the excitation coil, detection coil, signal amplifier,lock-in amplifier, signal generator, and analog-to-digital (AD) converter, were integratedinto a compact plastic probe box with a size of 17 cm in length, 8.5 cm in width, and 6 cmin height. The probe was connected to a computer via a single USB cable, which served adual function by supplying both electrical power and enabling data communication. Thetotal power consumption of the system was under 1 watt, making it highly energy-efficient.A Helmholtz excitation coil with a diameter of 5 cm and a differential detection coil with adiameter of 1 cm were used. The corrosion of steel rebar with a covering depth of 75 mmcan be detected using this technique [94]. Figure 17. Photo of corrosion detection system with eddy current testing method.Corros. Mater. Degrad. 2025, 6, 37 17 of 24Figure 18. Block diagram of ECT system for corrosion detection of steel rebar.Figures 19 and 20 present the signals obtained using the same detection coil andexcitation coils with diameters of 3 cm, 5 cm, and 8 cm, corresponding to steel rebar depthsof 25 mm and 45 mm, respectively [94]. The phase differences between corroded anduncorroded rebars are clearly observed. Increased detection depth can be obtained using abigger excitation coil.Figure 19. ECT signals of steel rebars with depth of 25 mm. (a) Excitation coil diameter: 3 cm.(b) Excitation coil diameter: 5 cm. (c) Excitation coil diameter: 7 cm. Based on ref. [94].Figure 20. ECT signals of steel rebars with depth of 45 mm. (a) Excitation coil diameter: 3 cm.(b) Excitation coil diameter: 5 cm. (c) Excitation coil diameter: 7 cm. Based on ref. [94].With the advancement of sensor technologies and data processing algorithms, eddycurrent testing is expected to become more accurate and accessible. Developments inhigh-frequency probes, machine learning for signal analysis, and integration with otherCorros. Mater. Degrad. 2025, 6, 37 18 of 24NDT techniques may improve the reliability of corrosion assessment in complex structuralelements. ECT remains a valuable technique for ensuring the long-term performance ofreinforced concrete infrastructure.3. Summary, Discussion, and RecommendationsThe evaluation of corrosion in steel rebars is a crucial aspect of structural health moni-toring and maintenance. A range of methods—electrochemical, ultrasonic, radiographic,thermal, and electromagnetic—are available to assess the condition of rebars in a non-destructive or minimally invasive manner. Each method operates on different physicalprinciples and has unique capabilities, limitations, and applications. Table 4 summarizesthe principles, advantages, limitations, typical applications, corrosion stage to be detected,and relative cost of each technique.Table 4. Comparison of different steel rebar corrosion evaluation methods.Technique Principle Advantages Limitations Applications Stage and CostHalf-cell potential(HCP)Measurescorrosion potentialSimple,standardizedSensitive tomoisture,qualitativeBridge decks,piers; fieldInitiationcorrosion; low costConcrete resistivity Conductivity ofconcreteSupports EcorrinterpretationInfluenced bysurface moistureCombined withHCP; fieldInitiationcorrosion; low costLinear polarizationresistance (LPR)MeasurespolarizationresistanceCorrosion rate,quantitativelyRequire rebarexposure, stableenvironmentConcrete cores, labvalidation, fieldPropagationcorrosion,medium costElectrochemicalimpedancespectroscopy (EIS)Impedance overfrequencyspectrumDetailedmechanisminsightsComplex, notpractical on-siteLab testing,material studiesInitiation andpropagation;high costUltrasonicSends stress wavesand measuresreflectionPenetrates deepRequires couplingmedium; surfaceconditions affectsignalsLarge structures,internal defectmapping; laband fieldPropagation;low—medium costX-ray tomography Uses X-rays toproduce 3D viewDetailed internalview, highresolution,quantitativeRequiresspecializedequipment;limited size;radiation safetySmall components,primarily labInitiation andpropagation;high costInfraredthermographyMeasures surfacetemperature andabnormalitiesLarge areascanning,non-contact,fastShallow depth;surface conditionsand solarinfluence resultsBridge decks, largestructures; fieldPropagation;low–medium costGPRDetects reflectionof electromagneticsignalsMeasures depth,large area scanningAffected bymoisture, materialpropertiesBridge decks,highwaystructures; fieldCover loss,delamination;medium costMFL Magnetic fluxdisturbance signalsLocalized pitting,cross-sectionreductionNeed strongmagnet, limiteddepthPipelines, rebars,storage tanks; fieldPropagation;medium costEddy currentMeasures eddycurrent induced byAC magnetic fieldSmall defects,initial corrosion,non-contactShallow depth,influenced bynearby conductivematerialsBridge decks,aircraft parts, smallrebars; laband fieldInitiationcorrosion;medium costElectrochemical techniques, such as half-cell potential, polarization resistance, andelectrochemical impedance spectroscopy, directly measure electrochemical parameters atthe metal’s surface. They are well-suited for identifying active corrosion and estimating itsCorros. Mater. Degrad. 2025, 6, 37 19 of 24rate of progression. Nevertheless, these techniques typically require physical contact andcareful surface preparation, which can be challenging in some cases.Ultrasonic methods rely on the propagation of high-frequency stress waves throughmaterials. Disruptions in wave signals can reveal defects and thinning in rebars. Ultrasonictesting is useful for deep penetration and imaging internal defects, although it may beaffected by surface conditions and material heterogeneity.X-ray tomography provides a non-destructive, high-resolution, 3D view of internalstructures. It can visualize corrosion damage directly and accurately; however, it requiresspecialized equipment and radiation safety measures and is typically unsuitable for largestructures in situ.Infrared thermography detects abnormalities by measuring temperature distributionson a structure’s surface. It can quickly scan large areas to find regions of potential damage.Nevertheless, it mainly reveals near-surface abnormalities and is influenced by surfaceconditions and environmental factors.GPR emits electromagnetic signals into a structure and detects reflection signals fromrebars and defects. GPR is effective for mapping rebars’ depth and spacing and identifyingareas of deterioration, although it has limited resolution at greater depth, and signals maybe affected by material properties and moisture.MFL is a powerful method for detecting localized metal thinning and pitting bymeasuring disturbances in a magnetic field. However, the method mainly responds tometal loss defects and is not sensitive to the initial corrosion of steel rebar. The device isnormally heavy and needs a big working space.Eddy current testing utilizes electromagnetic induction to generate eddy currents inthe metal. It is highly sensitive, contactless, and well-suited for the detection of early-stagecorrosion of steel rebar; a small, compact ECT device can be realized, although its depth ofpenetration is limited, and signals can be influenced by nearby conductive materials.Despite the extensive theoretical basis and demonstrated accuracy of each method,practical implementation challenges often arise during field applications. For instance,electrochemical techniques like half-cell potential or EIS require direct electrical connectionto the rebar, which may not be feasible without partial concrete removal [13,22]. X-raytomography, although offering unmatched internal resolution, is generally confined tolaboratory settings due to the need for bulky radiation-safe equipment and computationalresources [36,39]. Similarly, EIS and LPR may face signal interpretation difficulties undervarying concrete compositions, moisture levels, or electromagnetic interference [19,20,27].Infrared thermography, while fast and non-contact, depends heavily on ambient thermalconditions and is limited to shallow defects [41,44]. Field engineers must also contend withfactors such as equipment portability, accessibility of measurement locations, environmentalvariability, and the need for skilled personnel, which may restrict the real-world deploymentof otherwise promising methods.In addition to operational challenges, cost–performance trade-offs are also importantfor selecting appropriate techniques in practice. High-resolution methods such as X-ray CTand 3D ultrasonic imaging involve expensive and sophisticated equipment, which maybe justified only for critical structures or advanced diagnostic stages. In contrast, moreaffordable techniques such as half-cell potential, infrared thermography, or the ECT method,while limited in depth and resolution, provide valuable preliminary insights and are oftenmore suitable for large-scale or routine inspections. Understanding the balance betweenequipment cost and the actionable information provided is essential when developing anefficient and realistic inspection strategy.Corros. Mater. Degrad. 2025, 6, 37 20 of 24Given these practical constraints, it is increasingly evident that no single method offersa universal solution. Therefore, a combined or hybrid approach is often more effective,such as the following examples:- Pairing electrochemical techniques (for quantifying corrosion rates) with thermo-graphic imaging (for rapid scanning of large surface areas) allows for both quantitativeassessment and spatial screening [44,47].- Ground penetrating radar (GPR), which is effective in mapping rebar location andcover depth, can be complemented by magnetic flux leakage (MFL) or eddy currenttesting (ECT) for detecting localized corrosion or loss of cross-sectional area [53,63,88].- Ultrasonic pulse velocity measurements, although affected by heterogeneous mate-rials, can offer internal profiling when combined with X-ray or thermography data,improving defect localization [28,30,36].As a hypothetical case example, in bridge deck inspections, passive infrared ther-mography can be used for preliminary screening during daytime heating [45]. Suspectedcorrosion zones can then be examined using GPR to determine depth and rebar layout [52],followed by LPR measurements to quantify corrosion rates [15]. This multimodal frame-work improves diagnostic reliability and prioritization for maintenance.In practice, selecting appropriate techniques requires balancing cost, accessibility,required expertise, and structural characteristics. Future trends in this field are likelyto emphasize integrated sensor systems, automated scanning platforms, and AI-baseddata fusion algorithms, which can synthesize signals from multiple NDT methods into acomprehensive corrosion map [72,73]. The miniaturization of equipment and increasedportability will further enhance on-site applicability, while cloud-based diagnostic systemsmay allow for remote expert evaluation [88].In conclusion, while each technique offers distinct advantages, a multidisciplinary andintegrative strategy is recommended for robust, efficient, and practical corrosion evaluationin reinforced concrete structures. This approach ensures both early detection and reliablecondition assessment, contributing to extended service life and enhanced structural safety.Funding: This work was not supported by any funding agency.Data Availability Statement: Data are available upon request.Conflicts of Interest: The author declares no conflict of interest.References1. Amir, O. A topology optimization procedure for reinforced concrete structures. Comput. Struct. 2013, 114, 46–58. [CrossRef]2. Melchers, R.E. Long-Term Durability of Marine Reinforced Concrete Structures. J. Mar. Sci. Eng. 2020, 8, 290. [CrossRef]3. 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MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1007/s44251-023-00018-1https://doi.org/10.3390/standards4040014 Introduction  Steel Rebar Corrosion Evaluation and Methods  Electrochemical Techniques  Half-Cell Potential Measurement  Concrete Resistivity Measurement  Linear Polarization Resistance  Electrochemical Impedance Spectroscopy  Ultrasonic Method  X-Ray Tomography Method  Infrared Thermography Method  Ground Penetrating Radar Method  Magnetic Flux Leakage Method  Eddy Current Testing Method  Summary, Discussion, and Recommendations  References