Increase In Corrosion Rate Research Articles

Test and Theoretical Investigations on Flexural Behavior of High‐Performance H‐Shaped Steel Beam with Local Corrosion in Pure Bending Zone

This article undertakes the tested and theoretical investigations on the flexural behaviors of high‐performance steel (HPS) beams subjected to local corrosion in pure bending zone. Four H‐type HPS beams are designed, then the flexural bearing capacity test is carried out. Mechanical behaviors, including load‐deflection response, strain development, and failure modes are investigated in detail. A simple theoretical model of flexural strength is proposed. The tested and theoretical results show that: with the increase of corrosion rate, the random distribution of the residual section of HPS beam tends to be stable, the corrosion position also plays an important role in the random distribution of the residual area. The corrosion position has a greater influence on the residual flexural strength of corroded HPS beam than the corrosion degree. The theoretical flexural strength model can slight accurately predict the flexural strength of corroded H‐type HPS beam, the error is less than 5%. The corrosion of compressive flange has the most serious effect on the flexural strength degradation of HPS beams, followed by the corrosion of tensile flange and web corrosion. The influence of corrosion in the web height direction on the flexural strength of corroded HPS beam is extremely limited.

Performance of Corrosion Behavior of Commercial Magnesium Alloy Anode Electrode in Seawater-Powered Lamps

In modern times, alternative technologies that are efficient and environmentally friendly continue to be developed, such as lamps fueled by seawater. By reacting between a magnesium alloy anode electrode and an electrolyte solution of salt, a difference in electrical potential is generated, which can power electronic equipment. In order to achieve optimal performance with the magnesium alloy anode electrode, an initial assessment of this component is necessary. This research discusses the corrosion behavior of the magnesium alloy anode electrode caused by variations in salinity in seawater batteries. In this study, five samples of commercial product anode electrodes were prepared. Each sample was immersed in a salt solution with different salt concentrations. The salt solution was prepared by mixing 380 ml of distilled water with salt masses of 14g, 16g, 18g, 20g, and 22g respectively. Potentiodynamic testing was conducted in this research. The results showed that the corrosion behavior produced from the five samples yielded the highest corrosion potential, electrical current, and corrosion rate values of (Ecorr = 1.5419 Volts), (Icorr = 0.0010 Amps/cm2), and 12.5850 mm/a, respectively. These results indicate an increase in corrosion rate with increasing salinity. It is hoped that in future research, additional treatments will be applied to the electrode to achieve better performance.

Sub-minimum inhibitory concentration of tetrakis(hydroxymethyl)phosphonium sulfate enhances biocorrosion of carbon steel by Pseudomonas aeruginosa

Biocide treatments are commonly employed to mitigate unwanted microbial activities in industrial water systems. This study illuminates the intriguing phenomenon wherein sub-minimum inhibitory concentration (sub-MIC) of tetrakis(hydroxymethyl)phosphonium sulfate (THPS), a frequently used biocide, stimulates the formation of biofilms by Pseudomonas aeruginosa, consequently intensifying the corrosion of carbon steel. Introducing 160 µg/ml THPS, constituting a sub-MIC level, into the culture medium resulted in a notable increase in biofilm thickness and corrosion rate, elevating them from 82 µm and 10 mpy to 97 µm and 18.7 mpy, respectively. Electrochemical impedance spectroscopy, Tafel polarization and linear polarization resistance measurements substantiated the extent of corrosion. Furthermore, the treated biofilm exhibited a heightened presence of extracellular polymeric substances, improved adherence to the metal surface, enhanced structural integrity, and an extended dispersal phase. Confocal laser scanning microscopy (CLSM) images revealed a greater abundance of viable sessile cells within the inner layers of the treated biofilm. These findings underscore the role of sub-MIC levels of biocides as a potential driving force for developing more corrosive biofilms on industrial materials, emphasizing the critical importance of precise biocide dosing.

Phase evolution and steam oxidation behavior of nickel-based composite coatings fabricated by laser cladding

In this study, a variety of synthesized carbides are used as reinforcing phases for Inconel 625 alloy to enhance the performance of surface coatings. The effects of scanning rate and laser power on the microstructure and mechanical properties of coatings were studied, and specific energy was introduced to comprehensively evaluate the mechanisms of their influence. The results show that the percentage of macroscopic defects in the nickel-based composite coatings decreases and the dilution rate increases gradually with increasing specific energy density and the mechanical properties of the composite coating deteriorate gradually. It is worth noting that when the specific energy density is 153.33 J/mm2, the comprehensive performance of the composite coating is the best, which is mainly due to the uniform distribution of the contained (Ti, W)C and Cr carbides. The coating exhibits excellent hardness, corrosion resistance, and high-temperature oxidation resistance. In addition, after determining the optimal process parameter combination, the high-temperature steam oxidation performance of the nickel-based composite coating is analyzed in depth. At 1200°C, the corrosion degree of the composite coating gradually increased with increasing oxidation time. Research shows that the ceramic phase structure of (Ti, W)C and Cr plays a significant inhibitory role in high-temperature environment, effectively decelerating the increase of corrosion rate. Interestingly, before the first 48 h of the oxidation process, the thickness of the oxidation layer on coating surface increases slowly. However, when the oxidation time reaches 96 h, large amounts of WO3 and NiCr2O4 phases are formed, leading to the generation of pores and cracks in the coating. This causes the oxide film to fall off, resulting in a sharp increase in the corrosion layer thickness. The research in this paper is of great significance for optimizing coating materials and preparation processes, as well as improving the long-term high-temperature corrosion resistance of coatings.

Influences of heat flow on corrosion behavior and interfacial reaction kinetics

Heat transfer surface of heat exchange devices often suffers from severe corrosion, and their corrosion behavior is dramatically influenced by both surface temperature and heat flow. In this work, a novel heat transfer interface corrosion testing system was designed and built based on a simulation calculation analysis (COMSOL Multiphysics 6.0 software). By the aid of it, effects of different heat flux on corrosion performance of steel Q235 was investigated through mass loss and electrochemical methods in 0.5 mol/L H2SO4 solution at controlled interface temperatures. Results indicate that variations in concentration and temperature impose a great contribution to corrosion rate but not corrosion mechanism. However, heat flow not only changed the concentration of reactants near the interfacial surface but also affected the rate and kinetic mechanism of corrosion. Positive heat flow could reduce corrosion rate of heat transfer surface, while negative heat flow increased corrosion rate. Heat flow significantly altered parameters of the corrosion reaction rate constant (such as pre-exponential factor, activation energy, entropy change, and enthalpy change), and an approximate exponential relationship between corrosion rate and heat flux was proposed.

Corrosion of Steel Pipelines in Supercritical CO2 Environments: Effects of Injection Pressure and Temperature

Combining carbon capture and sequestration with enhanced oil recovery (EOR) could potentially reduce the carbon footprint through permanent storage of carbon dioxide (CO2) at the end of the EOR operations. However, severe corrosion risks to surface facilities and pipelines come along with the benefits of CO2 injection. The present study, therefore, investigates the corrosion resulting from contact between supercritical carbon dioxide (sCO2)-saturated water and carbon steels typical of Alaska pipelines. Carbon steel was allowed to contact the corrosive environment for 72 h, while the corrosion rate was monitored using linear polarization resistance. It was found that injecting sCO2 into the test brine (synthetic Ugnu field salinity) increased the general corrosion by twofold compared to liquid CO2 owing to the solubility and the kinetics of the corrosion byproducts. The increase in sCO2 injection pressure at a fixed temperature (40°C) was proportional to an increase in corrosion rate (up to 8.83 mm/y). At a fixed pressure (12.7 MPa), increasing the temperature decreased the corrosion rate. As part of the effort to mitigate corrosion of CO2 in an sCO2 environment and to validate inhibitor performance outside of common operating conditions, the inhibiting potential of an imidazolium-based ionic liquid was also evaluated. The findings revealed inhibition efficiency up to 65% at low concentrations (up to 51 ppm) of the inhibitor. The addition of ionic liquid (IL) causes the corrosion to shift from a general type to pitting owing to a partial surface coverage. Results revealed further that ILs work better on carbon steel with low manganese concentration in the coupon steel.

Study on corrosion behavior and first-principle analysis of M50 steel in lubricating oil contaminated by salt water

The corrosive wear of M50 steel in lubricating oil contaminated with saline is a form of wear that often occurs within the main shaft bearings of aviation engines carried by aircraft working at sea. In the present paper, the corrosion behavior of M50 steel in lubricating oil contaminated with saline was studied. The open-circuit potential, polarization curve and impedance spectrum were analyzed by rotating electrochemical corrosion wear test. A three-layer interface microscopic molecular model was established with Materials Studio to simulate the dynamic corrosion evolution process of M50 steel self-matching pairs and the adsorption energy was calculated. The results show that the corrosion type is pitting corrosion. With the increase of saline concentration, the corrosion area becomes wider and shallower, and the size of the pitting core gradually decreases. The weight loss of M50 steel increases over time, but the trend slows down, which is also shown in the MS simulation results. Electrochemical tests show that the tribocorrosion of M50 steel is related to load, speed. As the load increases, the corrosion rate decreases. And, as the speed increases, the corrosion rate increases. This has practical significance for extending the understanding of tribocorrosion of M50 steel in marine atmospheric environments.

Developing high elongation of Ca-containing Zn alloys with superior osteogenic and antibacterial properties

The development of high-strength zinc (Zn) alloys significantly contributes to the field of orthopedics by enhancing their osteogenic properties. Calcium (Ca) is pivotal for bone composition; however, its incorporation into the Zn matrix often hampers elongation due to the formation of large-sized CaZn13 phases. In this study, two Zn alloys with different Ca contents were prepared, and their microstructures, mechanical properties, corrosion behavior and biocompatibility were investigated. Results indicate that the Zn alloy with high Ca content exhibits large-sized and high-volume-fraction CaZn13 phases, which do not significantly change during equal channel angular pressing (ECAP). The grain sizes of ECAPed Zn alloys decreased from 36.75 μm and 35.81 μm of as-cast state to 4.32 μm and 1.71 μm, respectively. Refined microstructures contributed to the improvement of mechanical properties of Zn alloys, which have elongation over 15 % higher than that of all Zn-Mg-Ca alloys reported so far. The high elongation of Zn alloys originates from the inhibition of the propagation of microcracks which generated from the Zn/CaZn13 interphase. Moreover, the formation of large-sized CaZn13 phases leads to the severe local corrosion through enhancing galvanic effect, finally increasing corrosion rate. During degradation, the release of metallic ions, such as Zn2+, Mg2+, and Ca2+, are beneficial to the cell viability and osteogenic properties. And the accelerated release of Zn2+ plays a role in antibacterial performance. These results are very helpful for promoting the application of biodegradable Zn alloys in the field of orthopedics.

Integrating Flow Field Dynamics and Chemical Atmosphere Predictions for Enhanced Sulfur Corrosion Risk Assessment in Power Boilers.

In this work, we attempt to explain the phenomenon of sulfur corrosion of power boiler water walls under the conditions of large fluctuations in carbon monoxide concentrations. To assess the conditions required for corrosion formation, a criterion based on the chemical and flow field parameters of the flue gas is proposed. The formulated sulfur corrosion criterion is based on the mixture fraction variance and the turbulence time scale. Numerical modeling of coal combustion in a 250 MW power boiler is performed using ANSYS. Two cases of combustion in a boiler are analyzed, with the first simulating the boiler operated using classic high-swirl burners and the second one accounting for boiler operation with modified low-swirl burners. Calculations of pulverized coal combustion are performed using the standard k-ε turbulence model and the combustion described by the mixture fraction. The simulation results reveal that the low-swirl burner is characterized by higher values of the mixture fraction variance and a higher frequency of fluctuation of the velocity field, which is strongly related to an increased corrosion rate. The study outcomes show the validity of using the criterion of the mixture fraction variance and velocity field fluctuations to determine the areas at risk of sulfur corrosion.

Inhibitive Effect of Mangifera Indica Extract on Mild Steel in Hydrochloric Acid Solution

This research investigated the corrosion inhibition potential of Mangifera Indica Peel Extract (MIPE) for mild steel in a 1 M HCl solution. The study explored the effects of extract concentration, solution temperature, and immersion time on the inhibition potential of MIPE using weight loss measurements at extract concentrations of 0, 1.0, 1.5, and 2.0 g/L, temperatures of 303 K and 323 K, and immersion times of 1, 2, 4, 6, and 8 h. Experimental results showed that MIPE significantly reduced the corrosion rate of mild steel, with maximum inhibition efficiency reaching 97.26% and 94.83% at 2.0 g/L MIPE concentration and solution temperatures of 303 K and 323 K, respectively. The uninhibited mild steel experienced increased corrosion rates with rising temperatures and longer immersion times. The inhibition efficiency of MIPE improved with higher extract concentrations and immersion periods. These findings underscore the potential of MIPE as an effective and environmentally friendly corrosion inhibitor in acidic environments.

Influence of temperature on corrosion behavior of additively manufactured SS316L stainless steel in CO2-saturated brine solution

The present study investigates the influence of temperature on corrosion of AM SS316L in CO2 saturated brine solution. As the temperature increased from 25 °C to 70 °C, the corrosion rate rose nearly fivefold, from 1.78 to 10.06 mpy. The pitting potential decreased from 818 mV to 450 mV, with the gap between |Epit – Erep| widening from 669 mV to 853 mV. EIS results corroborated these results, revealing a decline in Rct from 4755 to 858 Ωcm2 and an increase in Yct from 38 to 119 snΩ-1cm−2. Subsequent surface morphology analysis post-corrosion testing revealed increased corrosion severity with rising temperature. In summary, elevated temperature led to increased corrosion rates, more susceptibility to pitting corrosion, and hindered repassivation.

The role of climate change and corrosion modeling strategy in dynamic response of pile-supported wharves

This paper explores the fragility of pile-supported wharves to environmental hazards, notably climate change and corrosion, and underscores the critical need to understand the interplay between these factors when assessing structural safety. The research advocates for comprehensive methodologies that encompass climate change effects, aging, and time-dependent deterioration in evaluating the seismic fragility functions of pile-supported wharves. An examination of aging and seismic effects is performed on a representative pile-supported wharf at designated time intervals. This study highlights the pronounced impacts of climate change and corrosion on the structural integrity of concrete and steel in marine environments. Specifically, it considers effects such as sea level rise, increased temperatures, and heightened relative humidity on pile-supported wharves. Additionally, three corrosion pitting configurations in prestressed strands with and without climate change considerations are analyzed to determine their influence on the strength and ductility of materials, limit states, and ultimately, on the fragility curves. The findings indicate that climate change significantly exacerbates the corrosion of materials in pile-supported wharves, and increases failure probability. The relative increase in corrosion rate after 50 years due to climate change is found to be 94%.

The impact of graphene on the mechanical properties, corrosion behavior, and biocompatibility of an Mg–Ca alloy

AbstractThis investigation explores the impact of incorporating graphene (Gr) reinforcement on the microstructure, mechanical properties, corrosion behavior, and biocompatibility of a composite derived from a magnesium–calcium (Mg–Ca) alloy. Two concentrations of Gr (0.1% and 0.2%) were introduced to an Mg–Ca alloy. The addition of 0.1% Gr resulted in a refined grain structure, enhancing both tensile and compression strength. However, electrochemical analysis and immersion testing revealed an increase in corrosion rate () with the incorporation of Gr. Although the corrosion rate of the Mg–Ca‐0.1%Gr composite was comparable to that of Mg–Ca, the Mg–Ca–0.2%Gr exhibited higher corrosion rates attributed to the enhancement of micro‐galvanic corrosion. Interestingly, cell survival rate tests demonstrated improved biocompatibility for the Mg–Ca–0.1%Gr sample, emphasizing its potential for applications in the biomedical domain, which requires enhanced mechanical strength, corrosion resistance, and biocompatibility.

Managing Green and Sustainable Technologies: Climate-Informed Corrosion Prediction for Steel Structures

The unpredictability of atmospheric circumstances is one of the major elements that contribute to the capability to anticipate the corrosion growth in metal structures over time accurately. Climate shifts can potentially modify the long-term attributes of these factors throughout the operational life of metal structures, both those currently in existence and those newly developed. The impact of climate irregularity on the probabilistic nature of atmospheric variables, which significantly impact corrosion situations, can add intricacy to corrosion predictions in these constructions. This project presents an incorporated framework to quantify the impact of climate alteration on the corrosion rates of steel structures in Jordan. It considers the changes in environmental conditions, specifically temperature, relative humidity, and wind speed, and their impacts on atmospheric corrosion. Global Climate Models are employed to assess the long-term effects of climate transformation on these environmental circumstances. An analytical model for anticipating corrosion rate is integrated with climate transformation models to predict modifications in the corrosion rates of steel parts relative to historical situations. This project also examines the impact of climate transformation on the fluctuations of these climatic parameters and offers a contrast between historical data and projected conditions across the country. The findings indicate a significant increase in corrosion rates across Jordan, which calls for localized green building codes and standards to ensure that future infrastructure is sustainable and capable of withstanding the new climatic norms. This approach addresses the immediate challenges posed by climate change and contributes to the broader goals of sustainable urban development and managing green technology adoption in Jordan. Doi: 10.28991/CEJ-2024-010-08-016 Full Text: PDF

Experimental and prediction model of axial compressive responses of corroded RC cylinders strengthened with CFRP

Currently, the capacity of corroded RC cylinders has been proven to significantly improve after being strengthened with carbon fiber reinforced polymer (CFRP). However, existing experimental data remains insufficient, resulting in few models available for predicting the post-strengthening capacity of corroded cylinder. The current understanding of the mechanism by which corrosion affects the mechanical properties of CFRP strengthened cylinders is also not yet clear. Against this background, this study conducted monotonic axial compression tests on CFRP-strengthened corroded RC cylinders. The experimental results indicate that while the CFRP layer predominantly controls the specimen's load-bearing capacity, the reduction in CFRP rupture strain due to increased corrosion rates should not be overlooked. When the corrosion rate reaches 35 %, the experimentally measured average rupture strain was only 65 % of the theoretical value, marking a 30 % decrease compared to non-corroded specimens. Subsequently, a predictive model for peak stress and strain of CFRP-confined corroded RC cylinders was proposed, which considers the confinement mechanisms of stirrups and CFRP as well as the degradation of CFRP rupture strain. Leveraging the existing test data, a dataset comprising 80 CFRP-strengthened corroded RC cylinder specimens was established to calibrate the key parameters in the proposed prediction model. Comparative analysis with existing models indicates that the peak stress and strain prediction model proposed in this study demonstrated the highest forecasting accuracy (the average errors for peak stress and strain were 14 % and 32.6 %, respectively), while existing other predictive models based on limited datasets suffer from a lack of universality. The experimental data and research conclusions of this article can provide reference for engineering practice and the establishment of models in subsequent research.

Investigation of buckling capacity behaviours of corroded tanks

The use of thin-walled steel shells is rapidly becoming widespread. Depending on their purpose, they can be susceptible to corrosion in the environments they are used in. This leads to metal loss and consequently a decrease in buckling strength. In this study, the effect of corrosion and CFRP reinforcement on thin-walled cylindrical steel tanks was experimentally investigated. Six samples with a length and diameter of 400 mm were tested; two samples were not subjected to corrosion, two samples were subjected to 2.5 % HCl corrosion, and two samples were subjected to 5 % HCl corrosion. The effect of CFRP usage was examined in both corroded and non-corroded samples. The results show that as the corrosion rate increases, the buckling strength decreases, and CFRP reinforcement recruitments the loss in buckling strength.

Effect of seawater salinity, pH, and temperature on external corrosion behavior and microhardness of offshore oil and gas pipeline: RSM modelling and optimization

This research aims to investigate the effects of seawater parameters like salinity, pH, and temperature on the external corrosion behaviour and microhardness of offshore oil and gas carbon steel pipes. The immersion tests were performed for 28 days following ASTM G-1 standards, simulating controlled artificial marine environments with varying pH levels, salinities, and temperatures. Besides, Field emission scanning electron microscopy (FESEM) analysis is performed to study the corrosion morphology. Additionally, a Vickers microhardness tester was used for microhardness analysis. The results revealed that an increase in salinity from 33.18 to 61.10 ppt can reduce the corrosion rate by 28%. In contrast, variations in seawater pH have a significant effect on corrosion rate, with a pH decrease from 8.50 to 7 causing a 42.54% increase in corrosion rate. However, the temperature of seawater was found to be the most prominent parameter, resulting in a 76.13% increase in corrosion rate and a 10.99% reduction in the microhardness of offshore pipelines. Moreover, the response surface methodology (RSM) modelling is used to determine the optimal seawater parameters for carbon steel pipes. Furthermore, the desirability factor for these parameters was 0.999, and the experimental validation displays a good agreement with predicted model values, with around 4.65% error for corrosion rate and 1.36% error for microhardness.

Effect of corrosion on bond slipping between steel and concrete in SRC structures

Corrosion critically impacts bond performance in steel-reinforced concrete (SRC) structures. In this study, we investigate the impact of corrosion on bond-slipping between steel components and concrete within SRC structures. Initially, SRC specimens were subjected to electrochemical corrosion tests to induce varying corrosion rates. Then, pull-out tests were conducted to study the specimens' failure modes, bond stress–slip curves, and displacement distribution. The experimental results indicate that lower corrosion rates can increase bond strength due to enhanced steel surface roughness. As the corrosion rate increased from 0% to 5%, the initial bond stress, peak bond stress, and residual bond stress increased by 133%, 101%, and 102%, respectively. However, when the corrosion rate increased from 5% to 15%, these stresses decreased by 57.1%, 49.6%, and 49.6%, respectively. Furthermore, with an increase in corrosion rate from 15% to 20%, the initial, peak, and residual bond stresses decreased by 100%, 67.6%, and 69.0%, respectively. Corrosion considerably affected the initial and peak slipping observed between the steel components and the concrete, yet its impact on residual slip was relatively minor. In addition, a nonlinear finite element model for characterising the corrosion and pull-out tests was developed and validated according to the experimental results. The numerical results showed that the four shear transfer mechanisms, including chemical bonding, micromechanical interlocking, macromechanical interlocking, and rust interface bonding, exhibited distinct behaviours at different loading stages. Finally, a parametric study was conducted using a finite element model. With variations in corrosion rates, four modes of cracking patterns on the concrete surface were observed.

Experience in Processing Alternative Crude Oils to Replace Design Oil in the Refinery

A comprehensive investigation of a highly complex petroleum refinery (Nelson complexity index of 10.7) during the processing of 11 crude oils and an imported atmospheric residue replacing the design Urals crude oil was performed. Various laboratory oil tests were carried out to characterize both crude oils, and their fractions. The results of oil laboratory assays along with intercriteria and regression analyses were employed to find quantitative relations between crude oil mixture quality and refining unit performance. It was found that the acidity of petroleum cannot be judged by its total acid number, and acid crudes with lower than 0.5 mg KOH/g and low sulphur content required repeated caustic treatment enhancement and provoked increased corrosion rate and sodium contamination of the hydrocracking catalyst. Increased fouling in the H-Oil hydrocracker was observed during the transfer of design Urals crude oil to other petroleum crudes. The vacuum residues with higher sulphur, lower nitrogen contents, and a lower colloidal instability index provide a higher conversion rate and lower fouling rate in the H-Oil unit. The regression equations developed in this work allow quantitative assessment of the performance of crucial refining units like the H-Oil, fluid catalytic cracker, naphtha reformer, and gas oil hydrotreatment based on laboratory oil test results.

Corrosion behaviors of Al2O3–20TiO2 and Cr2O3–3TiO2–5SiO2 coatings in both artificial seawater and high-pressure hydrogen sulfide seawater

In this study, Al2O3–20TiO2 (AT) and Cr2O3–3TiO2–5SiO2 (CT) coatings were prepared by atmospheric plasma spraying (APS), The microstructure, phase compositions and corrosion behavior were studied comparatively. The results indicated that both coatings had layered structure, with an average surface roughness RA around 6 μm, an adhesive strength around 35 MPa, and a porosity around 4 %. The AT coating mainly composed of γ-Al2O3 and α-Al2O3, and the CT coating composed Cr2O3 respectively. Moreover, the AT coating exhibited better corrosion resistance than the CT coating. In artificial seawater, the corrosion process was strongly related to the porosity. The corrosion rate (based on the corrosion current density) of the two coatings showed a same trend which decreased first and then increased. It was thought that the blocking of early corrosion products in holes was helpful to delaying the progress of corrosion. In contrast, the later corrosion products would promote the formation and propagation of cracks, accelerating the corrosion. In the simulated environment with high pressure and hydrogen sulfide seawater, the corrosion process was strongly related to the porosity as well as the brittleness of the coatings. The corrosion rate (based on the weight loss) of the AT coatings still remained the same trend as it in the artificial seawater. However, the CT coating showed a continuously increasing corrosion rate, due to higher porosity and brittleness resulted in a large number of cracks under external stress, which offered corrosion channels.