Corrosion Meter Research Articles

Оценка локальной коррозии по данным, получаемым с датчиков электрического сопротивления (часть 2)

Corrosion monitoring of oilfield equipment condition is a necessary requirement for ensuring the reliability of technological processes of oil production and transportation. The «electrical resistance» (ER) method is one of the corrosion monitoring methods used in the oil and gas industry. The disadvantage of the ER method is that the devices currently used do not allow to assess the local corrosion losses on the surface of the material, that is, they cannot identify the process of local corrosion development, while it is local corrosion that leads to failures of oilfield equipment. In the article, based on the consideration and interpretation of experimental data (laboratory and field), it is shown that the analysis of the dependencies «electrical resistance of the sensor sensitive element – time» makes it possible to detect the manifestation of local corrosion. Laboratory experiments were carried out in an oxygen-free model of the aqueous phase of production from producing wells in Western Siberia and in a medium providing intense local corrosion of carbon steel at a partial pressure of carbon dioxide of 1 atm. The experimental curves were approximated using the least squares method using the MS Excel «Solver» application. Further development of the proposed approach will make it possible to improve corrosion meters implementing the ER method to register local corrosion.

Enhancement of hardness and corrosion resistance of Al-Si-N multilayer color coating via SiN/AlSiN/AlN compositional gradient interlayer

The color of AlN/Si/Al coating can be controlled by the thickness of the AlN layer according to the interference effect. However, the loading capacity is affected by the large hardness difference between the Si and Al layers. Corrosion resistance is relatively weak due to penetration defects in the AlN surface layer. In this work, therefore, a SiN/AlSiN/AlN interlayer is sputtered in between the Si/Al layer in the AlN/Si/Al coating as the transition layer. The chemical state, structure, morphology, color, hardness, and corrosion resistance of the as-deposited Al-Si-N coating are carefully characterized using x-ray photoelectron spectrometry, grazing incident x-ray diffraction, atomic force microscopy, scanning electron microscope, colorimeter, nanoindentation, and electrochemical corrosion meter, respectively. To evaluate the long-term corrosion resistance, the uncoated, AlN/Si/Al-coated, and AlN/Si/SiN/AlSiN/AlN/Al-coated AZ31B Mg alloys are immersed in salt solution for different durations, followed by characterization of morphology and composition. The results show that the SiN/AlSiN/AlN interlayer is of a gradient structure in both composition and hardness. The AlN crystals grow continuously from the Al bonding layer into the AlSiN layer, resulting in internal longitudinal grain boundaries. The coating surface becomes smoother with a roughness (Rq) of 12.6 nm. The color of the coating is controlled by the AlN surface layer thickness. The coating hardness increases from 6.5 to 20.6 GPa. The corrosion current density of the coating decreases from 2.02 × 10−6 to 1.99 × 10−8 A/cm2. The coating could withstand corrosion in salt solution for at least 192h. The gradient structure of the interlayer effectively alleviates the hardness difference between the Si layer and the Al layer and inhibits the penetration of the corrosive medium from the surface. The mechanism for the enhanced corrosion resistance is explained through a model.

Corrosion resistance of MoS2-modified titanium alloy micro-arc oxidation coating

To improve the corrosion resistance of the Ti-6Al-4V alloy treated by micro-arc oxidation (MAO), molybdenum disulfide (MoS2) particles were added into the electrolyte and a composite oxide coating was formed with different MoS2 contents on the surface of Ti-6Al-4V alloy. A scanning electron microscope and a handheld roughness meter were used to analyze the surface morphology and roughness of the coating, and the corrosion performance of the MAO-treated Ti-6Al-4V alloy was analyzed by galvanic corrosion meter and electrochemical workstation. The results show that MoS2 can be successfully incorperated into the coating and block the micropores during the process of micro-arc oxidation. Electrochemical experiments showed that the coating has a higher resistance and a lower corrosion rate, while in the galvanic experiment, the galvanic potential and galvanic current of the coating also decreased. Under the experimental conditions, the addition of MoS2 can significantly improve the corrosion resistance of the MAO coating, and the best corrosion resistance is obtained when the addition amount is 4g/L, this will expand the marine applications of Ti-6Al-4V alloy.

On-site corrosion monitoring experience in concrete structures: potential improvements on the current-controlled polarization resistance method

The need for proactive maintenance of reinforced concrete structures with non-destructive testing (NDT) is less disputable today than ever. One of the most promising strategies in this regard is the in-situ measurement of the reinforcement corrosion rate. This study explored the reliability of modulated current confinement method (hereafter MCC) based on a review of in-situ measurements made with that technique in real-life structures over a 13-year period. The most prominent problems detected included defective confinement of the polarization current in low-resistivity environments and over-polarization of passive reinforcement. The findings, which showed enhancement of MCC reliability to depend on improving the electrochemical current regulation and control methodologies presently in place, are being applied to improve the design of the next generation of corrosion meters.

Development of a mathematical model of a transformer-type corrosion meter for pipelines

The article is devoted to the substantiation of a technical solution to the problem of monitoring corrosion changes in the walls of oil and gas pipelines using the electromagnetic method of non-destructive testing. To conduct a machine experiment on the qualitative and quantitative assessment of the characteristics of the magnetic field of solenoids with different geometric characteristics, a model has been developed that allows determining the magnetic induction and vector potential of the magnetic field of the solenoid and to visualize individual projections of vectors. The dependence of the vector magnetic potential of the field induced in the transformer and the role of the magnetic circuit in which the control object performs on the geometric and electromagnetic properties of the control object is determined.

Investigation of corrosion interaction between white tin cans and model media simulating tomato products

When choosing metal packaging for canned tomatoes and in tomato pouring, it is advisable to take into account their corrosiveness, since corrosion of the inner surface is one of the factors affecting the quality of products during storage. To rationalize corrosion testing of metal packaging and materials, it is advisable to use model media instead of food products. It should be borne in mind that, due to the presence in the composition of products of substances that affect the corrosion rate, the corrosiveness of the model environment and the product may vary. The corrosion kinetics of canned tinplate (ELC) in tomato juice and in an aqueous solution containing oxalic and citric acid was studied. The composition of the model medium (0.4% citric acid + 0.3% oxalic acid) was selected based on the results of previous studies. The uniform corrosion rate of ELC was measured by the polarization resistance method, the pitting corrosion rate was measured by zero resistance amperometry. The measurements were carried out using an Expert-004 corrosion meter in automatic mode. The mass of the tin coating on the contact surface of the ELC is 5.5–5.7 g/m2. According to the results of the studies, it was found that the kinetics of the rate of uniform and pitting corrosion of the ELC when interacting with tomato juice is similar to the kinetics of the process when interacting with a model medium. For both the model environment and tomato juice, the corrosion process is uniform - the average stationary pitting rates are 7.5-7.6 times lower than the corresponding values for uniform corrosion (for the model environment – 2.73 and 20.46 ?m/year; for tomato juice - 1.12 and 8.54 ?m/year). At the same time, the corrosiveness with respect to ELC for tomato juice is 2.4 times less than for a model medium. Thus, it is advisable to use a two-component model medium containing 0.4% citric and 0.3% oxalic acid for corrosion testing of metal packaging and materials, taking into account the correction factor.

Peculiarities of Corrosion of a Zinc Coating in Neutral Media in the Presence of Inhibitors Based on Benzotriazole, Cyclohexylamine, and Morpholine

The influence of domestic nitrogen-containing corrosion inhibitors of the VNKh-L type on corrosion regularities of a zinc coating on steel in neutral media is investigated. This work is aimed at studying the surface structure of a corroding zinc coating, as well as the influence of conditions simulating the degradation of inhibitors during real operation on their protective properties. Mechanical activation in a ball planetary mill is used to simulate the deformation and thermal operational conditions of inhibitors. Corrosion of a zinc coating on steel is carried out in a sulfate–chloride medium simulating atmospheric corrosion and in a borate buffer solution. The concentration of inhibitors in the corrosion media was 0.2 wt %. The morphology of a corroded surface of a zinc coating is studied using a Philips SEM-515 scanning electron microscope (at an accelerating voltage of 10 kV) with an X-ray microprobe. Studies of the corrosion rate of zinc coating on St 08 are carried out by the indirect measurement of corrosion resistance with the help of a MONIKOR-1 corrosion meter. A borate buffer solution (Na2B4O7 + H3BO3, pH 6.6) and a solution simulating atmospheric corrosion (NaCl + Na2SO4, pH 6.0) are used as corrosion media. The corrosion rate of samples in corrosive media without an inhibitor is accepted 1. The exposure time of each sample in corrosive media is 3 h. The chemical composition of corrosion products is studied by the mirror reflection in the IR range. The IR spectra of the metal plate surface are recorded using an FSM-1202 IR Fourier spectrometer in a wavelength range of 450–4000 cm–1 with a resolution of 2 cm–1 and an accumulation of 100 scans. To record the reflection spectra, a mirror-reflection attachment with a 10° angle of incidence is used. The corrosion rate of zinc coating in sulfate–chloride and solvent media in the presence of inhibitors based on benzotriazole and cyclohexylamine is practically not decreased when compared with the corrosion rate in the same media without inhibitors. The addition of both initial and mechanically activated inhibitors based on morpholine and benzotriazole to the corrosion medium decreases the corrosion rate of iron when compared with the corrosion in the same media without inhibitors. The pitting corrosion of a zinc coating is observed in the presence of initial and mechanically activated inhibitors of both types in studied corrosion media. Herewith, the pitting depth is smaller than the zinc-coating thickness under these conditions.

Peculiarities of zinc coating corrosion in neutral environments with inhibitors based on benzotriazole, cyclohexylamine and morpholine

The influence of domestic VNKh-L type nitrogen-containing corrosion inhibitors on the corrosion patterns of zinc coating on steel in a neutral environment was investigated. The paper aims to study the structure of the corroding zinc coating surface, as well as the influence of conditions simulating the degradation of inhibitors under actual application conditions on their protective properties. Mechanical activation in a ball planetary mill was used to simulate the thermal and deformation conditions of inhibitors. Zinc coating corrosion on steel was carried out in a sulfate-chloride environment simulating atmospheric corrosion and in borate buffer solution. The concentration of inhibitors in corrosion environments was 0,2 wt.%. The corroded surface morphology of the zinc coating was studied using the Philips SEM-515 scanning electron microscope (at an accelerating voltage of 10 kV) with an X-ray micro probe. Studies of the zinc coating corrosion rate on St 08 were carried out by the indirect measurement of corrosion resistance using the MONIKOR-1 corrosion meter. Borate buffer solution (Na2B4O7 + H3BO3, pH = 6,6) and the solution simulating atmospheric corrosion (NaCl + + Na2SO4, pH = 6,0) were used as corrosive environments. The corrosion rate of samples in corrosive environments without inhibitors was taken as 1. Exposure time of each sample in corrosive environments was 3 h. The chemical composition of corrosion products was studied by mirror reflection in the IR range. The IR spectra of metal plate surfaces were recorded on the FSM-1202 IR Fourier spectrometer in a wavelength range of 450–4000 cm–1 with a resolution of 2 cm–1 and an accumulation of 100 scans. A mirror reflection attachment with a 10° angle of incidence was used to obtain reflection spectra. The zinc coating corrosion rate in sulfate-chloride and borate environments in the presence of inhibitors based on benzotriazole and cyclohexylamine was virtually not reduced compared to the corrosion rate in the same environments without inhibitors. When both initial and mechanically activated inhibitors based on morpholine and benzotriazole are added to the corrosion environment, the iron corrosion rate decreases compared to the corrosion rate in the same environments without inhibitors. In the presence of initial and mechanically activated inhibitors of both groups, pitting corrosion of the zinc coating in the studied corrosion environments is observed. At the same time, the pitting depth under corrosion conditions is less than the zinc coating thickness.

Effect of corrosive media on galvanic corrosion of complicated tri-metallic couples of 2024 Al alloy/Q235 mild steel/304 stainless steel

Galvanic corrosion of tri-metallic couples is more complicated than that of bi-metallic couples. In this study, the effect of the pH of corrosive media on the galvanic corrosion of 2024 Al alloy/Q235 mild steel/304 stainless steel tri-metallic couples was investigated using potentiodynamic polarization, scanning electron microscopy, scanning vibrating electrode technique and a multi-channel galvanic corrosion meter. The results show that 2024 always acts as the only anode in 3.5 wt% NaCl at pH 5.56, 9.72 and 12.0, while both Q235 and 2024 act as anodes at pH 2.39 in the initial stage and then the role of Q235 changes at longer coupling time, which can be attributed to the effect of pH on the surface film of 2024. It is also found that the galvanic current density of a tri-metallic couple is the superposition of two bi-metallic couples when cathodic reactions are controlled by the diffusion of oxygen, otherwise it is smaller than that of the sum of two bi-metallic couples. The localized corrosion instead of uniform corrosion of anodic metal is accelerated by galvanic corrosion.

DETERMINATION OF CORROSION INITIATION OF REBAR IN CONCRETE BY MONITORING HALF-CELL POTENTIAL

Corrosion of rebars in concrete is a key mechanism which determines the durability and service life of concrete structure. Half-cell potential of rebars in concrete subjected to a NaCl solution of 10% in the concentration has been monitored using embedded reference electrode in the concrete and computer operated automatic data acquisition system. A portable corrosion meter was also employed to measure half-cell potential and AC impedance. Initiation of corrosion in rebars were estimated by the former system and verified by the later. Corrosion initiation period was found to be from as early as 119 days and up to 360 days of chloride application depending on the characteristics of test specimens. Upon successful detection of corrosion initiation threshold chloride concentration for corrosion initiation was estimated. The estimated chloride threshold level for deformed rebar was found almost half of that with ideal plain steel rebar. Corrosion was spread over rebars depending on its surface characteristics.

Corrosion Meters of New Generation Based on the Improved Method of Polarization Resistance

We use the method of polarization resistance for the purposes of corrosion monitoring in aqueous technogenic media. The influence of the electrode capacitance and electrochemical transformations in the layer of corrosion products on the accuracy of determination of the polarization resistance is thoroughly investigated. The results of theoretical investigations were used as basic in the development of domestic means of corrosion monitoring. We describe Ukrainian corrosion meters: an R5126 laboratory eight-channel corrosion meter, an ІK-4p portable corrosion meter, and an ІK-4s stationary corrosion meter. The application of corrosion meters makes it possible to perform continuous monitoring of the corrosion state of pipelines, rapidly take measures of corrosion protection, determine the service life of equipment, and perform the repair works in due time.

DEVELOPMENT OF EMBEDDED MINI-SENSOR FOR DIAGNOSIS OF CORROSION OF STEEL REINFORCEMENT IN CONCRETE NONDESTRUCTIVELY

Corrosion of steel reinforcement embedded in concrete is one of the main causes of degradation of reinforced concrete structures. Degradation occurs in reinforced concrete structures from corrosion caused by the Chloride ingress into concrete. That degradation has a severe impact on the structure in terms of maintenance and rehabilitation costs. Therefore, early detection of reinforcement corrosion is important for efficient maintenance, repair and planning. Meanwhile, the evaluation of the corrosion of reinforcement by non-destructive measurements have been used a lot. In particular CM-II (corrosion meter) is used to measure the polarization resistance, but has some disadvantages. Embedded mini-sensor has been developed in order to overcome these disadvantages. In this study, measurement of corrosion by using the mini-sensor is compared with the measured results by CM-II to verify the validity of the newly developed mini senor. Results show that there are agreement in trends of the parameters measured and as such the developed mini sensor has a promising start to be used.

Closure to Discussion by Jae Chung and Henry Bollmann on “Development of a Substructure Instrumentation System at the New I-10 Twin Span Bridge and Its Use to Investigate the Lateral Behavior of Batter Piles” [Geotechnical Testing Journal, GTJ103109, Vol. 34, No. 4

Abstract An instrumentation system was installed at M19 eastbound pier of the new I-10 Twin Span Bridge for short-term and long-term monitoring of the bridge, and was used to monitor the lateral behavior of the pier during a unique lateral load test. The system includes strain gauges, In-Place Inclinometers, triaxial accelerometers, water pressure cells, tiltmeters, and corrosion meters. The strains and deformations within the foundation piles were measured during the lateral load test using the strain gauges and IPI inclinometers. The horizontal movement of the pier was also monitored using an automated survey station. The results of lateral load test were compared with the FB-MultiPier predicted values. This article presents the closure to the discussion on the Geotechnical Testing Journal, GTJ103109, Vol. 34, No. 4.

Development of a Substructure Instrumentation System at the New I-10 Twin Span Bridge and Its Use to Investigate the Lateral Behavior of Batter Piles

AbstractThis paper presents the development of the substructure instrumentation system that was installed at a selected pier (M19 Eastbound) of the new I-10 Twin Span Bridge for short-term and long-term health monitoring of the bridge, and its use to monitor the behavior of the pier during a lateral load test. The selected pier is supported by 24–36 in. diameter batter pile foundation. The substructure instrumentation for the M19 Eastbound pier includes sister bar strain gauges and MEMS In-Place Inclinometers (IPI) installed inside the foundation piles, triaxial accelerometers to measure lateral movements pile cap, water pressure cells to measure wave forces, tiltmeters, and corrosion meters on the pile cap rebar. A unique lateral load test was designed and conducted to investigate the lateral behavior of the pier and to assess the validity of the analysis method used to design the batter pile foundations using the FB-MultiPier software. Two high strength steel strand tendons were used to impose about 1900 kips of lateral load. The horizontal movements of the pier caps and bents were monitored using an automated survey station with prisms. The strains and deformations within the foundation piles were measured using the strain gauges and IPI inclinometers. This paper will discuss the substructure instrumentation plan and the novel approaches to preparation and design of lateral load testing of batter pile group foundation. The results of lateral load tests will be presented, discussed and compared with the predicted values using FB-MultiPier software. The soils’ p-y curves were also back-calculated and presented.

Monitoring and measuring electrochemical behavior of engineering alloys by optical interferometry

In a previous study, an optical corrosion meter was built based on a mathematical model relating to surface and bulk behaviors of metals in aqueous solution. The optical corrosion meter was established based on principles of holographic interferometry for measuring microsurface dissolution, i.e., mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e., corrosion current. The corrosion meter consists of an electrochemical cell in which the sample is immersed in aqueous solution. Furthermore, the corrosion meter has a holographic camera with a thermoplastic film for in situ processing holograms in order to obtain real-time holographic interferograms of the sample in the electrochemical cell. Results of the present work indicate that optical holography is a very useful technique for measuring the corrosion current density of different alloys in aqueous solutions. As a result, the corrosion current density of aluminium, stainless steel, low carbon steel, copper, and brass in 1M KCl, 1 M NaCl, 1 M NaOH, seawater and seawater solutions, respectively, were obtained. The obtained corrosion data from the optical corrosion meter (interferometric technique) were compared with corrosion data obtained on the same alloys in the specified solutions from the linear polarization method as well as from the weight-loss method. The comparison among the three techniques indicates that there is contrast in the results among the investigated alloys. In general, the new method of optical interferometry can be considered a more conservative method of corrosion measurement with respect to the linear polarization and weight-loss methods.

Localized corrosion of copper alloys by optical interferometry and other techniques

An optical corrosion meter has been developed for materials testing and evaluation of different corrosion phenomena. The idea of the optical corrosion meter was established based on principles of 3D-holographic interferometry for measuring microsurface dissolution, i.e., mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e., corrosion current of metallic samples in aqueous solutions. In the present work an early stage of crevice corrosion and pitting corrosion of an aluminium brass and ure copper samples in seawater and tap water, respectively, were monitored in situ by the optical corrosion meter during the cyclic polarization test. The observations of pitting corrosion were basically interferometric perturbations detected on the surface of both alloys. However, the observations of crevice corrosion were basically interferometric perturbations detected on the surface of both alloys underneath a crevice assembly, made of Teflon bolt, Teflon nut, and Teflon washer. The crevice assembly used on all tested samples to crevice corrosion to create a differential aeration cell between the surface of the sample and areas underneath the crevice assembly in seawater. Each Teflon washer contained radial grooves and had 20 plateaus which formed crevices (shield areas) when pressed against the surface of the sample. The interferometric perturbations interpreted as a localized corrosion in a form of an early crevice corrosion or pitting corrosion of a depth ranged between 0.3 μm to several micrometers. Consequently, results of the present work indicate that holographic interferometry is a very useful technique as a 3D-interferometric microscope for monitoring crevice corrosion and pitting corrosion at the initiation stage of the phenomena for different metallic samples in aqueous solutions.

Detection of localized corrosion of stainless steels by optical interferrometry

An optical corrosion meter has been developed for materials testing and evaluation of different corrosion phenomena. The meter was based on principles of 3D holographic interferrometry for measuring microsurface dissolution, i.e. mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e. corrosion current of metallic samples in aqueous solutions. The early stage of pitting corrosion of UNS No. 304 stainless steel and crevice corrosion of UNS 316 stainless steel in natural seawater were monitored, in situ by the optical corrosion meter during the cyclic polarization tests. The observations of pitting corrosion were basically interferrometric perturbations detected on the surface of the UNS No. 304 stainless steel in seawater. In contrast, the observations of crevice corrosion were basically interferrometric perturbations detected on the surface of the UNS No. 316 stainless steel underneath a crevice assembly. Eventually, the interferrometric perturbations were interpreted as localized corrosion in the form of an early pitting or of an early crevice corrosion, of a depth ranging from 0.3 μm to several micrometers. Consequently, holographic interferrometry is shown to be a useful technique as a 3D-interferrometric microscope for monitoring pitting corrosion or crevice corrosion at the initiation stage of both phenomena for different metallic samples in aqueous solutions.

Monitoring Pitting Corrosion by Holographic Interferometry

Abstract The corrosion current density of aluminum in 5% sulfuric acid (H2SO4) and copper in seawater was measured using an optical corrosion meter developed to test and evaluate materials in relation to different corrosion phenomena. The optical corrosion meter is based upon principles of holographic interferometry for measuring microsurface dissolution (i.e., mass loss) and on principles of electrochemistry for measuring bulk electronic current (i.e., corrosion current) of metallic samples in aqueous solutions. In the early stage of corrosion monitoring of both metals, pitting phenomena were observed using holographic interferometry. Observations of pitting basically were interferometric perturbations detected on the surface of both metals. These perturbations were interpreted as localized corrosion in the form of early pitting. Depths ranged between 0.3 μm and several micrometers. Results indicated optical holography is a very useful technique for monitoring pitting at the initiation stage for differen...

Optical corrosion meter

In a previous study, a mathematical model relating surface and bulk behaviours of metals in aqueous solution has been developed. The model was established based on principles of holographic interferometry for measuring microsurface dissolution, i.e. mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e. corrosion current. In the present work, an optical corrosion meter was built based on the above model. The corrosion meter consists of an electrochemical cell in which the sample is immersed in aqueous solution. Furthermore, the corrosion meter has a holographic camera with a thermoplastic film for in situ processing of holograms in order to obtain real-time holographic interferoms of the sample in the electrochemical cell. Results of the present work indicate that optical holography is a very useful technique for measuring the corrosion current density of different alloys in aqueous solutions. As a result, the corrosion current density of aluminium, stainless steel, and low-carbon steel in 1 M KCl, 1 M NaCl, and 1 M NaOH solutions were obtained. A comparison between the corrosion data of samples showed that the corrosion current density of the stainless steel in 1 M NaCl is nearly three-folds higher than that of the aluminium in 1 M KCl and the low-carbon steel in 1 M NaOH.

Monitoring methodologies of the acid cleaning of steam generators

AbstractSome general considerations about the validity and usefulness of corrosion rate monitoring using the polarization resistance method are developed and the theory of Stern and Geary is shortly discussed. The basic aspects of the acid cleaning of steam generators, the role of corrosion inhibitors and the approach of the author with respect to the problem of monitoring this operation are also reviewed. The main aim of this applied research was to verify the reliability of a corrosion meter in predicting the correct order of magnitude of the corrosion rate of a plain carbon steel and two low alloy steels. Measurements under dynamic conditions, with the solution flow rates of 0.5, 1.0 and 1.5 ms−1, were performed in 5% by weight HCl solutions at 75°C containing a specific commercial inhibitor at two concentrations. Results indicate that the manufacturer calibration of the corrosion meter is not able to provide a realistic evaluation of the corrosion rate of the probe electrodes. Furthermore, it was experimentally demonstrated that probe and tubular specimens of the same material exhibit a different electrochemical behaviour. Therefore, a reliable evaluation of the true corrosion rate of tubular specimens should be made using appropriate calibration charts for the given environment and geometry of the electrochemical probe. At last, some alternating current measurements performed at 10 Hz showed that corrosion rate monitoring based on low‐frequency signals could prove very useful for controlling the aggressiveness of the acid‐cleaning solution.