Wet-dry Cyclic Condition Research Articles

Enhancing concrete self-healing capabilities of Bacillus sphaericus spores through the encapsulation in biopolymeric microcapsules

This study examined the encapsulation of Bacillus sphaericus LMG 22257 spores in biopolymeric microcapsules (MCs) for use in cement mortar, with a focus on enhancing self-healing properties. Biopolymers, including alginate (ALG), chitosan (CTS), carboxymethyl cellulose (CMC), and ALG/CMC blends at various ratios, were employed to fabricate the MCs through ionotropic gelation and freeze–drying. The physicochemical properties of MCs, including size, morphology, and swelling behavior under simulated concrete conditions, were assessed. Among these, ALG/CMC-MCs exhibited superior characteristics and demonstrated the highest urea hydrolysis activity when incorporated into the mortar, indicating optimal spore protection. Despite an initial decrease in compressive strength, the ALG/CMC blend with an ALG:CMC mass ratio of 6:4 achieved a crack healing efficiency of 96.7% over 28 days under cyclic wet-dry conditions. These findings highlight the potential of biopolymer encapsulation for embedding functional microorganisms in construction materials, contributing to a more durable and sustainable infrastructure.

Assessing the Settlement and Deformation of Pile-Supported Embankments Undergoing Groundwater-Level Fluctuations: An Experimental and Simulation Study

The intensification of extreme weather phenomena, ranging from torrential downpours to protracted dry spells, which trigger fluctuations at the groundwater level, poses a grave threat to the stability of embankments, giving rise to an array of concerns including cracking and differential settlement. Consequently, it is crucial to embark on research targeted at uncovering the settlement and deformation behaviors of pile-supported embankments amidst changes in water levels. In tackling this dilemma, a series of direct shear tests were carried out across a range of wet–dry cyclic conditions. The results confirmed that the occurrence of wet–dry cycles significantly impacted the resilience of silty clay. Additionally, it was observed that the erosion of cohesion and the angle of internal friction initially diminished sharply, subsequently leveling off, with the first wet–dry cycle exerting the most substantial influence on soil strength. Employing a holistic pile-supported embankment model, simulations revealed that variations in the groundwater level, fluctuations therein, varying descent rates, and periodic shifts in the groundwater level could all prompt alterations in soil settlement between embankment piles and could augment the peak tensile stress applied to geogrids. In summary, the orthogonal experimental method was utilized, indicating that, in terms of impacting embankment settlement under periodic water-level changes, the factors ranked in descending order were the following: pile spacing, pile length, embankment height, and the height of the groundwater table.

Multi-physical analysis of saturation-dependent creep behavior for RC beam under sustained load and wet-dry cycles

A multi-physical and multi-scale analytical method based on thermo-hydro-mechanical models is adopted to evaluate the long-term deflection of RC beams with cracking under sustained loads and cyclic wet-dry conditions. The major physical-mechanical coupling mechanisms considered in this study includes: 1) water transport and phase equilibrium in the microstructure of gel and capillary pores; 2) accelerated moisture transport affected by cracking in concrete; 3) saturation-dependent creep coefficient considering gel and capillary pores; 4) combination of creep deformation and non-uniform shrinkage. The numerical approach is firstly verified by experiments where the thermo-hydro-mechanical interactions are found to be well analyzed. Then, parametric cases of cyclic wet-dry conditions are investigated to achieve deeper and wider conclusions. It has been found that under the cyclic wet-dry ambient conditions, there is an extreme drying state for concrete members depending on the drying humidity and its period. For the long-term mid-span deflection of beams, generally it will grow faster if the drying humidity is lower, which owes to the saturation-dependent creep coefficient, which has been called as Pickett effect. Meanwhile, beam's bottom-side cracking caused by sustained loads may also accelerate the moisture transport, which leads to an uneven drying state between upper and lower part of beams and viaducts, and eventually causes up-warping of them. For some cases, this up-warping can be significant enough to alter the macroscale structural creep deformation. At last, the difference between wet-dry cycle and dry-wet one has also been discussed. It can be proved that for the long-term deformation under wet-dry cycles, the moisture state will greatly affect the time-dependency from many aspects, for which the thermo-hydro-mechanical numerical approach is beneficial and needed in practice.

Role of hygroscopic chloride salts on corrosion performance and hydrogen absorption of steel under cyclic wet-dry conditions

The corrosion morphology of steel under aqueous NaCl, MgCl2, and ZnCl2 droplets and the corresponding hydrogen permeation behaviour were investigated using surface characterization techniques and electrochemical methods. Under cyclic wet-dry conditions, Mg2+ and Zn2+-containing surface layers were formed on steel and the latter presented a high corrosion resistance. The most active hydrogen entry occurred under ZnCl2 during drying, while it happened under MgCl2 at low humidity. The effects of chloride salts on the formation of rust layers and hydrogen entry into steel during wet-dry cycles were discussed based on the hardness of metal cations and the deliquescence properties of salts.

Enhancement of Prebiotic Peptide Formation in Cyclic Environments.

The dynamic behaviors of prebiotic reaction networks may be critically important to understanding how larger biopolymers could emerge, despite being unfavorable to form in water. We focus on understanding the dynamics of simple systems, prior to the emergence of replication mechanisms, and what role they may have played in biopolymer formation. We specifically consider the dynamics in cyclic environments using both model and experimental data.Cyclic environmental conditions prevent a system from reaching thermodynamic equilibrium, improving the chance of observing interesting kinetic behaviors. We used an approximate kinetic model to simulate the dynamics of trimetaphosphate (TP)-activated peptide formation from glycine in cyclic wet-dry conditions. The model predicts that environmental cycling allows trimer and tetramer peptides to sustain concentrations above the predicted fixed points of the model due to overshoot, a dynamic phenomenon. Our experiments demonstrate that oscillatory environments can shift product distributions in favor of longer peptides. However, experimental validation of certain behaviors in the kinetic model is challenging, considering that open systems with cyclic environmental conditions break many of the common assumptions in classical chemical kinetics. Overall, our results suggest that the dynamics of simple peptide reaction networks in cyclic environments may have been important for the formation of longer polymers on the early Earth. Similar phenomena may have also contributed to the emergence of reaction networks with product distributions determined not by thermodynamics, but rather by kinetics.

Galvanic Interactions on a Coupled Microelectrode Array of AA7050-T7451 and 316 Stainless Steel in Chromate-Containing NaCl Solutions Under Thick Electrolyte Film and Cyclic Wet-Dry Conditions

The electrochemical behavior of an AA7050-316SS galvanic couple in chromate-containing NaCl environments under relatively constant thick electrolyte films and wet-dry cycling was investigated utilizing the coupled microelectrode array (CMEA) approach. The CMEA approach provided a means to analyze the in situ electrochemical kinetics as a function of spatial location and time. In inhibitor-free environments, the total net anodic charge associated with galvanic current increased with increasing conductivity and aggressiveness of the environment. The AA7050 electrodes supplied more than half of the total net cathodic charge in relation to the 316SS (316 stainless steel) electrodes in the more aggressive environments. Under thick electrolyte films, chromate became less effective at suppressing cathodic kinetics on the 316SS and AA7050 net cathodes as the chloride concentration increased. Under wet-dry cycling conditions, the effectiveness of chromate was diminished when compared to thick electrolyte film conditions, due to the cycling of the equilibrium chloride concentration as electrolyte thickness changed upon onset of drying and wetting while the loading density of the salt remained constant. Furthermore, chromate exhibited a diminished ability to suppress cathodic currents on the AA7050 net cathodes in comparison to the 316SS electrodes. This study highlighted the importance of Cu-rich intermetallic particles and replated Cu on precipitation-strengthened Al alloys when considering the driving force of cathodes in sustaining anodic dissolution in typical Al alloy macro-galvanic systems exposed to atmospheric conditions.

Reducing stress corrosion cracking susceptibility of high-strength aluminum alloy and its fastener by a novel electromagnetic shocking treatment

A novel electromagnetic shocking treatment (EST) is put forward to reduce the stress corrosion cracking (SCC) susceptibility of high-strength aluminum alloys and its fasteners by selectively modifying grain boundary (GB) microstructure. The effect of EST on the microstructure, mechanical properties and SCC susceptibility of T73-Al7075 alloy and its high-lock nuts has been investigated. Mechanical properties, electrochemical performance and constant strain SCC test have been carried out for T73-Al7075 alloy, microhardness test and SCC test under wet-dry cyclic condition have been carried out for its high-lock nuts, as well as microstructure characterizations via SEM, EBSD and TEM have been carried out for T73-Al7075 alloy. Comparing to the received sample, the strength of EST sample basically keep consistent while the elongation increases slightly, besides, the electrical conductivity and electrochemical corrosion performance improves. The SCC susceptibility of T73-Al7075 alloy and its high-lock nuts have been reduced after EST. Microstructure characterization shows that the grain morphology and matrix intragranular precipitate distribution of the EST sample keep almost the same as those of the received sample, which is consistent with the almost unchanged mechanical properties of alloy sample. However, EST induces the transformation of grain boundary precipitates (GBPs) and the formation of stripy/wavy GB as observed experimentally, which indicates that EST promotes nonlinear GB wetting or pre-melting and even grain bridging. The phase transformation and GB complexion transition induced by EST are main factors attributing to the obvious improvement of SCC resistance of T73-Al7075 alloy and its high-lock nuts. It can be deduced that anodic dissolution cracking will be relieved because of the dissolution and coarsening of GBPs. Hydrogen induced cracking and crack propagation can be relieved with the occurrence of grain bridging. They are both beneficial to improve SCC resistance of Al7075 alloys. This work provides a novel EST method to targetedly embellish GB microstructure, optimize GB complexion and connectivity, and then enhance the service performance of solid alloys and even finished parts.

Comparative experiment of steel bar corrosion at concrete construction joints

Introduction: Construction joint is common and even inevitable in most of the reinforcement concrete structures. This study was to assess the effect of construction joints on chloride-induced corrosion of reinforcing steel in concrete.Methods: Test parameters included two environmental conditions (salt solution immersion condition and cyclic wet-dry condition), two forms of construction joint (direct wet joint and roughened wet joint) and four types of steel bar (mild steel bar, ferritic stainless-steel bar, austenitic-ferritic stainless-steel bar and epoxy-coated steel bar). The corrosion test of 90 specimens was carried out by electrochemical accelerated corrosion method. The weight loss of each steel bar and steel bar section in specimens was measured. An influence coefficient (k_j) of construction joint on local weigh loss of steel bars was defined.Results: Except for epoxy-coated steel bars, the most severe corrosion of experimental steel bars in concrete specimens all occurred at the joints, while the corrosion in non-joint sections of steel bars was relatively uniform and less. The weight loss rate of specimens has the range of 1.18% to 15.73% with an average value of 6.22%. The average k_j of mild steel bars, S11203 stainless steel bars, and S23043 stainless steel bars are 1.38, 1.92, and 1.97, respectively. The average k_j of specimens in immersion condition and cyclic wet-dry condition are 1.44 and 2.07. The corrosion of epoxy-coated steel bars mainly occurred at the damage locations of epoxy coating, not mainly at the joints.Conclusion: Chloride-induced corrosion of steel bars at construction joints was always more severe than at non-joints, especially in cyclic wet-dry environments, even for stainless-steel bar, but epoxy-coated steel bars were excluded.

(Digital Presentation) The Role of Mg in Cut-Edge Corrosion Protection of 55 Mass% Al-Zn-Mg Coated Steel Under a Wet-Dry Cyclic Condition

Pre-painted hot-dip 55 mass% Al-Zn coated steel sheets are widely used as a building material due to their superior corrosion resistance. In consideration of its long-term service, a pre-painted hot-dip 55 mass% Al-Zn-2 mass% Mg coated steel sheet with enhanced corrosion resistance has been developed in recent years.1 It has been shown that the addition of 2 mass% Mg not only reduced the corrosion rate by 1/3 but also inhibited the delamination of organic paints from the cut-edge. As yet, however, the role of Mg in the cut-edge corrosion protection of the 55 mass% Al-Zn-2 mass% Mg coated steel has not been completely clear. In this study, we focused on the influence of Mg-containing corrosion products on the electrochemical properties of the coating layer and the steel substrate. Hot-dip 55 mass% Al-Zn-2 mass% Mg-1.6 mass% Si and 55 mass% Al-Zn-1.6 mass% Si coated steel sheets were used in this study. The Al-Zn-Mg coating layer is composed of Al-rich, Zn-rich, and Mg-Si phases. Our previous study has reported that for the Al-Zn-Mg coating layer, the Zn-rich phase preferentially dissolves during the initial process of cut-edge corrosion.2 In addition, Mg also preferentially dissolves from the Mg-Si phase in the Al-Zn-Mg coating layer. The wet and dry cyclic corrosion test showed that the formation of the Zn-based corrosion products at the cut-edge for the Al-Zn-Mg coated steel was more inhibited than that for the Al-Zn coated steel. In addition, the sacrificial corrosion protection ability of the Al-Zn-Mg coated steel was higher than the Al-Zn coated steel. It was found that the dense corrosion products including Mg were formed on the steel substrate at the cut-edge for the Al-Zn-Mg coated steel 1 hour after the onset of the corrosion test. The Mg-containing corrosion products slightly reduced the cathodic current for the oxygen reduction reaction, and this is thought to be one of the factors for the improved cut-edge corrosion resistance.1) N. Shimoda, Y. Morimoto, Y. Kubo, S. Fujii, and S. Shiragaki, Galvatech 2015 Proceedings, 802 (2015).2) S. Tokuda, I. Muto, Y. Sugawara, M. Takahashi, M Matsumoto, and N. Hara, Corros. Sci, 129, 126 (2017).

Polyurea reinforcement of disintegrated mudstone embankments and the underlying mechanism

Disintegrated mudstone (DM) is often discarded during highway constructions because of its potential continuous disintegration and large wet deformation. The purpose of this study is to improve DM so that it can be used to fill highway embankments. The chemical reinforcement method based on polyurea (PU) was selected to reinforce disintegrated carbonaceous mudstone (DCM) and disintegrated red-bed mudstone (DRM). Tensile strength (TS) tests under various temperature-humidity conditions, and then contact angle tests, unconfined compressive strength (UCS) tests under cyclic wet-dry conditions were carried out on PU modified disintegrated mudstone (PU-MDM). The hydrophobicity and mechanical characteristics of PU-MDM were studied. In addition, a series of microscopic tests including scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and fourier transform infrared spectroscopy (FTIR) were conducted to examine the curing mechanism of PU from a microscopic perspective. The results show that PU modified disintegrated carbonaceous mudstone (PU-MCM) is insensitive to temperature and high humidity when it is modified into a hydrophobic material. The TS and contact angle increase gradually with increasing PU content and component B, and they tend to be stable after the PU content and dosage ratio reach 5 wt% and 1:4, respectively. The stabilized TS is 2.42 MPa, and the corresponding contact angle is 94.84°, indicating that the material is modified to be hydrophobic. At natural temperature (30 °C), the specimen with a PU content of 5 wt% and a dosage ratio of 1: 4, the TS value can reach 77% of the maximum after curing for 1 d. PU-MDM with 5 wt% PU shows a good resistance to wet-dry cycles. After 10 wet-dry cycles, the UCS of PU-MDM is still significantly higher than that of DM specimens without wet-dry cycles. The microscopic results indicate that PU can effectively enhance the cementation of soil particles, which leads the soil structure to be denser and exhibit a certain hydrophobic effect. It is found that PU is not only environmentally friendly but also effective and fast in modifying DM.

Effect of Environmental Factors on Hydrogen Absorption into Steel Sheet under a Wet-dry Cyclic Corrosion Condition

Effect of Environmental Factors on Hydrogen Absorption into Steel Sheet under a Wet-dry Cyclic Corrosion Condition

Experimental Investigation on Tensile Strength of Jurassic Red-Bed Sandstone under the Conditions of Water Pressures and Wet-Dry Cycles

Jurassic Red-Bed sandstone is one of the main types of rock composing unstable slopes on both banks of the Three Gorges Reservoir (TGR) in China. It is vulnerable to tensile strength degradation when exposed to cyclic wet-dry conditions under reservoir operation. Therefore, the law behind this needs to be clarified in order to accurately evaluate the stability of a sandstone slope located in the drawdown area of the TGR. In this paper, Jurassic Red-Bed sandstone specimens from the Majiagou landslide in the TGR area were collected for experiments with cyclic wet-dry procedures. After each wet-dry cycle, efforts were made to implement splitting tests, measure clay mineral contents and ion contents in the soak solution, and observe changes in microstructures. The tensile strength is experimentally demonstrated to be a process of damage accumulation under the integrated actions of water pressure, calcite dissolution, and clay mineral hydration, among which hydration of illite contributes the most to the damage. The loss of the cement material and the cyclic expansion and shrinkage of clay minerals weaken the cementation between grains, leading to the tensile strength degradation.

Effects of NH4+, Na+, and Mg2+ ions on the corrosion behavior of galvanized steel in wet–dry cyclic conditions

AbstractThe effect of cations on the corrosion of galvanized steel (GS) is scarcely reported. In this study, a wet–dry cyclic test was conducted to study NH4 +, Na+, and Mg2+ cation effect on the corrosion behavior of GS available in Nepal. Fourteen wet–dry cycles (18 h wet and 6 h dry) were performed by exposing samples at 298 K with a relative humidity of 90% in a wet cycle and 50% in a dry cycle for 14 days. The cations strongly affect the corrosion rate of the GS sample estimated by weight loss and potentiodynamic polarization. The potentiodynamic polarization curves showed the inhibition of cathodic and anodic reactions by Mg2+ ion, while the NH4 + ion only changed the cathodic reaction. Mg2+ ion was found to shift the corrosion potential to noble values compared with NH4 + and Na+ ions. A compact and thin corrosion products layer was developed in Mg2+ salt solution in contrast to a thick and porous corrosion products layer in NH4 + and Na+ salt solutions. Red rust due to corrosion of underlying steel appeared in the presence of NH4 + and Na+ salt solutions. Finally, the weight loss data revealed that the corrosivity of cations for GS decreased in the order Na+ > NH4 + > Mg2+.

(Keynote) EIS Study of Galvanic Corrosion of Al Alloy Coupled with High Strength Steel in Simulated Atmospheres

Introduction Weight reduction of automobiles is required to improve fuel efficiency. Therefore, the uses of lightweight materials such as Al alloys and CFRP is expected to increase. When an automobile is made of multi-materials, there is a concern that galvanic corrosion and crevice corrosion may occur at the joint parts.We successfully applied EIS to study corrosion of metals covered with thin solution films [1–4]. A two-electrode cell, which was composed of two identical metal plates embedded in epoxy resin with a small gap distance, was used to measure the EIS. Under very thin solution films, uneven current distribution must be considered in the EIS measurements because the current lines may concentrate on the nearest edge of the two electrodes when AC voltage is applied. The EIS characteristics of a metal/solution film interface was successfully described by an equivalent circuit of transmission line (TML) [2,3]. We reported that accurate charge transfer resistance can be obtained by curve fitting the measured EIS with a TML circuit, even if the current lines are not evenly distributed on the electrode surface [1].In this study, EIS was applied to the study of galvanic corrosion of Al alloy coupled with high strength steel under thin NaCl solution films with different thicknesses. The monitoring of the galvanic corrosion of All alloy was also carried out under a wet-dry cyclic condition by EIS. Experimental Al alloy (6016–T4, 10 mm×10 mm) and high-strength steel (980 MPa, 10 mm×10 mm) were used as specimens. They were embedded in the epoxy resin at a gap distance of 0.1 mm. A solution film of 2 M NaCl was placed on the cell surface. Three different films thickness, 0.025, 0.050 and 0.1 mm were used. The EIS was measured under the condition that the potential difference of Al alloy and high strength steel was set to zero by potentiostat. Amplitude of AC voltage applied between the two electrodes was 10 mV and the frequency range was 10 kHz to 10 mHz. Using the same cell, the galvanic corrosion under wet-dry cyclic conditions was also monitored by continuously measuring two impedance at high and low frequency. To investigate the effect of crevice, the cells with and without a crevice were employed. Result and Discussion All Nyquist plots of the galvanic couple measured under thin solution films showed two capacitive semi-circles. One semicircle with a smaller time constant can be expressed by a parallel combination of double layer capacitance Cdl and charge transfer resistance Rct for the All alloy. Another semicircle with a larger time constant can be given by Cdl / Rct for the high strength steel. The corrosion rate (CR) of the Al alloy estimated from the Rct increases with decreasing the solution film thickness. This tendency is consistent with that of the galvanic current between the two electrodes.The galvanic corrosion behavior under the wet-dry cycles in the presence of chlorides was monitored. In above experiment, the optimum frequency for monitoring Rct of the Al alloy was determined to be 0.3 Hz. Using this frequency, the corrosion rate of the Al alloy coupled with the high strength steel was monitored under the wet-dry cycles. The corrosion rate of Al alloy changed in response to the wet-dry cycles. The galvanic corrosion behavior will be discussed in the conference on the basis of the EIS monitoring data. Acknowledgements This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References 1) Y. Shi, E. Tada, A. Nishikata, J. Electrochem. Soc. 162[4], C135-C139 (2015).2) A. Nishikata, Y. Ichihara, T. Tsuru, Corros. Sci. 37, 897-911(1995)3) A. Nishikata, Y. Ichihara, Y. Hayashi, T. Tsuru, J. Electrochem. Soc.144, 1244-1252 (1997)4) A. Nishikata, Q. Zhu, E. Tada, Corros. Sci. 87, 80-87 (2014)

The Role of Mo Addition in the Improvement of Corrosion Resistance of Weathering Steel Under a Wet-Dry Cyclic Condition

It is known that Mo improves corrosion resistance of weathering steels in atmospheric corrosion environments.1 Mo dissolves as MoO4 2- in neutral solutions. If MoO4 2- is adsorbed to the surface of rust particles, it is expected that Mo-containing rusts are selectively permeable to cations and inhibit the permeation of chloride ions into the steel matrix.2 However, there is little report on the role of Mo addition in the improvement of the corrosion resistance of weathering steels. In this study, solution chemistry of electrolytes permeating through rust layers of a Mo-added weathering steel was investigated. In addition, a model interface between Mo-containing rusts and the steel matrix was fabricated, and its corrosion behavior under a wet-dry cyclic corrosion test was observed. From the results obtained in this study, the beneficial effect of Mo on the corrosion resistance of weathering steels was discussed.Commercial-type weathering steel (WS: Fe-0.5Cr-0.3Cu-0.2Ni) and 0.5 mass% Mo-added weathering steel (Mo-WS: Fe-0.3Cu-0.5Mo) sheets were used in this study. All the steel sheets were polished up to 100–150 µm in thickness with SiC papers. We conducted a wet-dry cyclic corrosion test based on ISO 16539 Method A. The dew point was kept at 301 K. An electrolyte layer of 1.0 mm in thickness was formed every cycles using 0.18 M NaCl. In order to examine the solution chemistry of the electrolyte permeating through rust layers, a model specimen simulating a rust/steel interface was fabricated. The steel sheets were placed on a silica glass. A pH test paper or a filter paper was also placed between the steel sheet and the silica glass. Those were fixed by a masking tape and an epoxy resin. Using the specimen, we conducted the cyclic corrosion test to form rust layers. When a penetration hole through the steel sheet was formed by rust growth, the permeating electrolytes were trapped by the pH test paper or the filter paper. The pH of the electrolytes was evaluated by the color change of the pH test paper. The electrolytes absorbed in the filter paper was extracted to pure water, and Cl- concentration was measured by ion chromatography. In addition, we observed the corrosion behavior of the steel sheets in the model specimen simulating the rust/steel interface without pH test and filter papers. When a penetration hole through the steel sheet was formed under the wet-dry cyclic corrosion test, the steel sheet in the back side corroded by the permeating electrolytes. The corrosion behavior of the steel sheet in the back side can be regarded as that in the rust/steel interface.After 10–15cycles of the corrosion test, the penetration holes were observed from the back side of the model specimens with both WS and Mo-WS. In both cases, the pH of the electrolytes was about 1. The Cl- concentration for Mo-WS was approximately 1.0 M, one-third of that for WS. From the EDS analysis for the cross section of the rust/steel interface of Mo-WS, the signal of Mo was distributed in the whole rust layers. It is confirmed that Mo-containing rust layers suppressed the permeation of Cl- into the rust/steel interface. Black and brown rusts were observed from the WS sheet in the model specimen immediately after the penetration by rust growth. Raman spectroscopic analysis showed that the black rust was Fe3O4, and the brown rusts were β-FeOOH, which is often detected from rust layers formed in high chloride environments. In the case of Mo-WS, only black rusts (Fe3O4) were observed. This result also supported the inhibition of the Cl- permeation through Mo-containing rust layers. It is thought that the Mo addition suppressed the enrichment of Cl- and the formation of highly reducible β-FeOOH at the rust/steel interface, leading to the decrease in the corrosion rate. Reference: 1) H. E. Townsend, Corrosion, 57, 497(2001).2) N. Sato, in Passivity of Metals and Semiconductors, Proceedings of the Eighth International Symposium, M. B. Ives, J. L. Luo, and J. R. Rodda, Editors, P. 283, The Electrochemical Society, Pennington, NJ (2001).

Monitoring of Carbon Steel Corrosion in Soil Under Wet-Dry Cyclic Conditions

Corrosion of carbon steel in soil is affected by various factors such as particle size, water content, soli depth, pH, temperature and microorganisms. Based on the results of exposure tests conducted in the past, it is common to design a steel material with a 1 mm "corrosion allowance" in 100 years. However, environmental factors of soil change depending on regions and seasons. Thus further improvement and rationalization of design are required to reduce material cost.Water content is one of important environmental factors that determines the ratio of the gas and liquid phases in soil, and it is considered to have an influence on corrosion. In addition, the corrosion rate near the soil surface may change significantly due to cyclic wet-dry such as rainfall and daily humidity changes. In this study the corrosion rate of carbon steel in simulated soil under cyclic wet-dry was monitored by electrochemical impedance method, and the effect of cyclic wet-dry on carbon steel corrosion was examined.A pair of identical carbon steel plates (10 × 5 mm) were embedded in an epoxy resin (40 mm ϕ) and used as a probe electrode for the corrosion monitoring in soil. SiO2 particles with 100 μm diameter were used as a simulated soil. An acrylic pipe with a flange was fixed vertically on the probe surface with a gasket sandwiched, and the silica powder was put into the acrylic pipe so that the height was 1 cm. Furthermore, 3% NaCl solution was poured until the soil surface was completely immersed (100%). The experimental cell was placed in a chamber set at 25 ℃ and 40% RHDuring drying, impedance at two frequencies, 10 kHz and 10 mHz was continuously measured. The solution resistance R sol and charge transfer resistance R ct were monitored by Z 10kHz and (Z 10mHz – Z 10kHz), respectively. The R sol increased with the drying time and reached a constant. The impedance measurement was terminated when the R sol became a constant. In this study, the reciprocal of R ct was used as an index of the corrosion rate.The corrosion rate reached a maximum after 1 ~ 3 hours from the start of the drying. It is considered that the increase in the corrosion rate was due to the decrease in the water content by drying. At the onset of the drying, the spaces among the particles were completely filled with 3% NaCl solution. In such a condition, the corrosion will be strongly suppressed because oxygen diffusion through the soil is very slow. As the drying progresses, the gas phase in soil increases and enhances the oxygen diffusion. Thus, the corrosion rate can increase. However, further drying decreases the solution phase and decreases the average corrosion rate because the anodic dissolution of Fe could be hindered. Finally, when the simulated soil was completely dried out, the 1/R ct became negligibly small.After the dry-up, ultrapure water was added to the soil, so that NaCl concentration became 3% and the cell was exposed to the 2nd drying. The tendency of the change in R ct -1 during the drying was the same as the first drying process. However, the maximum value of the R ct -1 seemed to increase by repeated drying. This may be attributed that the corrosion product formed in the drying process becomes an oxidant in the next drying process.

Durability of flexurally strengthened RC beams with prestressed CFRP sheet under wet–dry cycling in a chloride-containing environment

Using prestressed carbon fibre reinforced polymer (CFRP) to strengthen reinforced concrete (RC) members is currently considered a promising technology. This study aims to investigate the durability of RC beams externally strengthened with prestressed CFRP sheet in a chloride-containing environment. The prestressed load levels were set at 7.5%, and 15% of the ultimate tensile strength of the CFRP sheet at room temperature (~26 °C), and the test parameters included an inclined U-jacket (or not) and 90-day exposure to a wet–dry cyclic environment. The wet–dry cyclic condition was set as 8-h immersion in 3.5% NaCl solution at 40 °C and 16-h drying at 25 °C and 60% RH (relative humidity). After exposure to the wet–dry environment, the prestressed CFRP sheet strengthened RC beams were tested under four-point bending test. The evolution of time-dependent prestress losses in the strengthened beam was discussed, and then the flexural behaviour of the strengthened beams was analyzed. Finally, a theoretical model based on the classical beam theory was proposed to predict the bearing capacity of prestressed-CFRP strengthened RC beams. The results show that the prestressed CFRP sheet can significantly improve the flexural performance of RC beams, and the cracking and ultimate loads of the RC beam strengthened with a prestressing level of 15% could increase by approximately 50% and 40%, respectively. Moreover, the inclined U-jacket enhanced the flexural performance of the strengthened RC beams and changed the failure mode from CFRP debonding to CFRP fracture. Although the exposure in chloride-containing environments increased the prestress losses, a 90-day exposure in the simulated subtropical marine climate environment did not cause a significant detrimental influence on the flexural performance of the strengthened RC beams. The proposed model agrees well with the experimental results, indicating it could conservatively predict the bearing capacity of strengthened RC beams failed by CFRP fracture.

Effect of Environmental Factors on Hydrogen Absorption into Steel Sheet under a Wet-dry Cyclic Corrosion Condition

Effect of Environmental Factors on Hydrogen Absorption into Steel Sheet under a Wet-dry Cyclic Corrosion Condition

Insight into the corrosion evolution of Fe-based amorphous coatings under wet-dry cyclic conditions

Understanding the corrosion behavior of Fe-based amorphous coatings under wet-dry cyclic conditions is of pretty importance for coating applications in marine environments. In this paper, the corrosion evolutions of the coatings under wet-dry cyclic conditions were investigated by electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Accurate charge transfer resistance (Rt) was obtained through fitting EIS results using a modified transmission line equivalent circuit model and a landscape map of Rt was constructed for a panoramic investigation on coating corrosion. The results show that the corrosion rate of the coating changes cyclically under wet-dry cyclic conditions. Corrosion within a single wet-dry cycle can be presented as four stages: an initial decrease of corrosion rate due to passivation or repassivation when the coating is wetted, then a slow increase due to concentrated aggressive ions with solution evaporation, followed by a rapid increment associated with fast oxygen transport under ultrathin solution film and finally a stable stage as solution dries out. It is deduced that an oxygen concentration gradient can form between the inside and the outside of coating pores when the solution film over the coating is very thin, which further promotes the localized corrosion in the pores. According to the landscape map of Rt, the coating maintains its high corrosion resistance for the initial 33 wet-dry cycles. But its corrosion resistance decreases significantly in the 35th wet-dry cycle accompanied by the attack of localized corrosion. After 65 wet dry cycles, the coating fails due to the penetration of localized corrosion. The interparticle oxide layers are defective, which enhances the localized corrosion process.

Hydrogen Absorption Behavior of Pre-Rusted Steels under an NaCl Droplet

In this study, we investigated the hydrogen absorption behavior of pre-rusted steel in an NaCl droplet by measuring the hydrogen permeation current and corrosion potential. To prepare the pre-rusted steel, a 30-μL droplet of a 0.1-mol/dm3 NaCl solution was placed on the surface of steel and exposed to wet-dry cyclic conditions by changing the relative humidity (RH) repeatedly from 40% to 95% at a constant rate of 27.5%/h and 298 K. The hydrogen permeation current and corrosion potential of the pre-rusted steel were measured during the drying of an electrolyte droplet using an electrochemical hydrogen permeation test and the Kelvin probe (KP) technique, respectively. The corrosion potential of the pre-rusted steel shifted in the negative direction simultaneous with the evolution of the hydrogen permeation current during the drying of the droplet. The hydrogen permeation current increased with the number of wet-dry cycles in the early stages of the rusting process, which related to the negative shift in corrosion potential. As the whole surface of the steel was covered with iron rust in the later stages of the rusting process, the corrosion potential shifted in the positive direction and the hydrogen absorption current decreased gradually.