Threshold Chloride Concentration Research Articles

Corrosion Mechanisms of Austenitic Steel in Salt Rock Repositories of High-Level Radioactive Waste

Salt rock is presently pondered as a host for a deep repository of high-level radioactive waste in Germany [1]. Its plasticity and impermeability reduce the probability of a direct contact of containments with aqueous solutions. In a worse scenario, however, the steel containment would meet a saturated brine. In Germany, repository concepts for salt rock envisage the use of Cr-Ni steels for containers of high-level radioactive waste [2]. Therefore, metallic corrosion in this environment defines the overall performance and longevity of the steel-based containment. After ca. 30 year of the closure, surface temperatures of 100-120°C can be reached due to the heat released by radioactive decay. The consumption of initially trapped oxygen by corrosion of the copper shell barrier (partial electrochemical reduction reaction), by microbiological processes and/or by mineral reactions leads to an anaerobic environment. Under these conditions the alloy corrodes with the production of hydrogen [3]. Apart from its extremely aggressive environment, the repository is also characterized by high hydrostatic and ground pressures (p>100 bar). The nature of corrosion products is also relevant, because they may retard the migration of radioactive contaminants that have been mobilized from the waste matrix [4]. Austenitic steels are susceptible to pitting corrosion over a certain chloride concentration threshold [5,6]. The passivity strength depends on concentration of Cr and Mo incorporated in the oxide film [7]. According to the empiric the pitting resistance equivalent number (PREN) = %wt Cr + 3.3 %wt Mo + 16 %wt N [8], it decreases in the order (AISI): 304L > 316L≈309S> 904L. The corrosion of AISI 309S was investigated in representative NaCl- and MgCl2-based saturated brines by a special designed high-pressure and high-temperature electrochemical reactor. Classical low potentiodynamic polarizations and electrochemical impedance (EIS) experiments were complemented by morphological and surface chemical analysis by SEM-EDX, XPS and Synchrotron-PES. Anodic polarizations show that stainless steel AISI 309S start to dissolve by a localized mechanism beyond a threshold potential. The breakdown potential decreases almost linearly with the temperature with a larger slope than the re-passivation potential does. Thus, the localized process governs the free corrosion of the alloy under contact with hot concentrated brines (t>100 °C). At moderate pressures (6 bar or lower), the breakdown of passivity is characterized by the formation of shallow pits which grow initially in the radial direction and then advance vertically engraving stripes on the surface. Under pressures of 50 bars, on the other hand, the dissolution front advances radially. The grain structure is revealed at the center of the spreading dissolution patches, characterized by distinct crystalline textures and the formation of strings assigned to FeCr-carbides released by dissolution of the alloy matrix (see Fig.1). The dissolution of chromium seems to play a decisive role for the advance of the corrosion front due to the related strong local acidification caused by its hydrolysis. Cr(OH)3, Ni(OH)2 and MoO3 are the main surface compounds identified by XPS. Fe 2p and O 1s signals suggest the presence of Fe2O3 and Fe3O4 outside the corrosion front, but not inside it. The formation of a chloride salt film is postulated to maintain the active state. The formation of sulfate salts, on the other hand tends to neutralize the active dissolution. The evolution of the chemical environment during corrosion was modelled with the help of thermodynamic calculations performed with the Geochemist´s Workbench® tool, which applies the Pitzer´s formulation for high ionic strength media. Figure 1: SEM micrograph showing the corroding front on AISI 309S in a cyclic current-voltage experiment performed at 80°C and 50 bar of pressure. T.von Berlepsch. B. Haverkamp, Elements, 12 (2016) 257.D.G. Bennett. R. Gens, J. Nucl. Mat., 379 (2008) 1.F. King. M. Kolář. P.G. Keech, Corr. Eng. Sci. Tech., 49 (2014) 455.J. Farrell. W.D. Bostick. R.J. Jarabek. J.N. Fiedor, Ground Water, 37 (1999) 618.H.P. Leckie. H.H. Uhlig, J. Electrochem. Soc., 113 (1966) 1262.E. Smailos. M.Á. Cuñado. I. Azkarate. B. Kursten. G. Marx, “Long-Term Performance of Candidate Materials for HLW/Spent Fuel Disposal Containers”, Forschungszentrum Karlruhe, FZKA 6809, Karlsruhe (2003).C.-O.A. Olsson. D. Landolt, Electrochim. Acta, 48 (2003) 1093.A.J. Sedriks, “Corrosion of Stainless Steels”, 2nd ed., Wiley Interscience, New York (1996). Figure 1

Performance test to evaluate the corrosion resistance of stainless steel in concrete

Abstract Stainless steel is a possible measure for increasing the service life of reinforced concrete structures exposed to chlorides. For the prediction of service life it is crucial to quantify the corrosion resistance of the material used by determining the chloride threshold at the steel concrete interface inducing corrosion at the rebar. However, currently the corrosion resistance of stainless steel is expressed by its pitting resistance equivalent number (PREN) which considers only the content of chromium, molybdenum and nickel along with certain factors. The higher the PREN-value, the more corrosion resistant the steel. But this PREN-value cannot be transferred into a chloride threshold concentration. Therefore, this paper presents a test method, the so called “ECISS”-test (ECISS = European Commission for International Steel Standardization), which enables the evaluation of the chloride threshold of stainless steel. The focus is on the performance of this test with respect to its limits and opportunities. Test results of different types of stainless steels are included and discussed. The benefit of stainless steel with respect to the service life of reinforced concrete structures is highlighted.

Pitting corrosion behaviour of 2101 duplex stainless steel in chloride solutions

ABSTRACTThe corrosion behaviour of 2101 duplex stainless steel (DSS) in NaCl solution was studied and compared with that of 2205 DSS. The effects of chloride concentration and solution temperature on pitting corrosion behaviour were focused. The relative sensitivity to pitting corrosion of the constituent phases in both 2101 and 2205 DSSs was also explored. Pitting corrosion susceptibility was evaluated by conducting cyclic polarisation curve measurement. The corrosion morphology was examined by a scanning electron microscope equipped with energy dispersive spectrometer. The experimental results showed that the pitting corrosion resistance of 2101 DSS was inferior to that of 2205 DSS by exhibiting a lower threshold chloride concentration and a lower critical pitting temperature. For both 2101 and 2205 DSSs, the ferrite phase was more susceptible than austenite phase to pitting corrosion.This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).

Threshold Chloride Content for Corrosion of Steel in Concrete

Threshold Chloride Content for Corrosion of Steel in Concrete

Numerical Prediction of Chloride Penetration in Concrete Exposed to a Marine Environment at Tide

Reinforced concrete structures under cyclic exposure to the corrosive environment such as the tidal zone as a part of marine structure entail the higher risk of steel corrosion. In this paper, chloride penetration in concrete exposed to the tidal zone was predicted using a combined moisture and chloride transport model. For the analysis of moisture transport, pore size distribution in concrete was determined from the experimental observation and used to determine the moisture permeability and degree of saturation. Then, the chloride profile through nonsaturated concrete cover was calculated by applying the moisture distribution along the penetration depth to the chloride convection and diffusion model. To assess sensitivity of the service life to the environment and concrete mix conditions, this study used three types of tide levels and water to cement ratios in the simulation. Under 10 years of tidal exposure condition, the minimum required cover depth at which the threshold chloride concentration reaches on the steel embedment increases only about 1.13 times as the tide level increases from the minimum to the highest while that for the w/c ratio increases about 1.69 times.

Effect of passivation on chloride concentration threshold of steel reinforcement corrosion

PurposeWhen concrete is manufactured, it can be instantaneously contaminated by chloride (Cl−) ions or later by their intrusion from the environment. This work aims to study the electrochemical behavior of the passive layer formed on the reinforcing steel surface in the presence of the same Cl− ion concentration, with and without passivation time. This will, undoubtedly, affect the corrosion threshold values thereafter.Design/methodology/approachElectrochemical polarizations were carried out in two concrete pore solutions. The surfaces of samples immersed for 255 days in saturated Ca(OH)2 solution were examined with optical and scanning electron microscopy and Raman microspectroscopy.FindingsCl− ion origins in reinforced concrete lead to different values of corrosion thresholds. The passive layer behaves like a physicochemical barrier, and corrosion occurs at higher NaCl concentration thresholds. The formed passive film on the steel surface shows differences in the chemical composition and the morphology. The results show a rich presence of hematite. Maghemite, lepidocrocite, akaganeite and goethite are also present in much lower concentrations. The Cl− ion presence in fresh concrete at the beginning of the manufacture harms the good formation and the good stability of these oxides, leading to corrosion initiation.Originality/valueThis study contributes to a better understanding of the passive layer role, not only in reducing the corrosion rate value but also in reconsidering new Cl− ion corrosion threshold values.

The Influence of Coating Thickness and Composition on the Corrosion Propagation Rates of Galvanized Rebar in Cracked Concrete

A study has been conducted of the chloride-induced corrosion behavior of four different batches of galvanized steel reinforcement embedded in sound and in cracked concrete. One batch of bars was of conventionally produced hot-dipped galvanized (HDG) steel, two were prototypes of continuously galvanized rebar, and the fourth was a hot-dipped bar with an experimental Zn-Al alloy coating. Carbon (black) steel bars were also tested for comparison purposes. The continuously galvanized process is aimed at producing a thinner, but more ductile coating than that formed by conventionally hot-dipped galvanizing process. Metallographic examination of the as-received galvanized bars showed a wide variation of the coating thickness around and along the bars, and the continuously galvanized coatings were consistently thinner than specified. All bars were cast in concrete which was subsequently cracked either parallel to or perpendicular to the embedded bars. Additional specimens were tested in the sound (non-cracked) concrete. All specimens were constantly exposed to a chloride brine for 64 weeks, and were electrochemically assessed bi-weekly during the exposure period. The electrochemical results and visual examination after autopsy showed that no active corrosion was initiated in either the galvanized or black rebar reinforced non-cracked concrete specimens. Therefore, the data in this project give no indication of initiation time or chloride threshold concentration for corrosion of these bars. On the other hand, in all cracked concrete specimens, corrosion initiated at the base of the crack and extended along or around the bars. In the cracked specimens, all galvanized bars exhibited lower current densities than the black bars, with the HDG being the lowest. Recommendations are given for appropriate interpretation of half-cell potentials of the galvanized bars investigated in terms of high or low probability of active corrosion.

The Effect of Chloride-Hydrogen Sulfide Synergism on the Stress Corrosion Cracking Susceptibility of Type 321 Stainless Steel

The paper presents the results of a study conducted on the effects of chloride-hydrogen sulfide (H2S) synergism on the electrochemical behavior and stress corrosion cracking (SCC) susceptibility of Type 321 stainless steel (SS). Thiosulfate was used as a replacement for H2S. The electrochemical work included double-loop potentiokinetic reactivation to assess the degree of sensitization and cyclic polarization to study the susceptibility toward pitting. Slow strain rate test (SSRT) was used to assess the susceptibility of Type 321 SS to SCC. The tests were performed on the solution annealed and aged Type 321 SS. Aging was performed at 500°C for 100 h and 1,000 h. The results showed that the presence of H2S in NaCl solution increased the susceptibility of the alloy to pitting and SCC. The fracture surface of specimens failed by SCC was covered with sulfide scale, which was enriched in nickel and depleted in iron. A threshold chloride concentration was found necessary to induce SCC in the chloride-H2S enviro...

Threshold Chloride Concentrations and Passivity Breakdown of Rebar Steel in Real Concrete Solution at Different pH Conditions with the Addition of Glycerol

Addition of glycerol as a viscosity modifier in concrete is proposed to decrease the permeability of corrosion-inducing ions such as chloride and sulfate. In addition to controlling the permeability of concrete, glycerol could perform as an inhibitor of corrosion of rebar steel. Cyclic polarization studies were carried out on metallographically polished rebar steel specimens in actual concrete solutions at two different pH conditions (pH 12.5 and 9.0) and different chloride concentrations. The threshold concentration of chloride for passivity breakdown at pH 12.5 was greater than 50 × 10−3 mol/L in the absence of glycerol addition. The threshold increased to 81 × 10−3 mol/L upon addition of 2 wt% glycerol. The threshold chloride concentration for passivity breakdown in pH 9.0 cement solution was 0.2 × 10−3 mol/L without glycerol addition. No beneficial effect of glycerol was observed in the low pH condition. However, glycerol enhanced the passivation kinetics of the rebar steel in saturated cement solution, but did not affect the electronic properties of the passive layer. The passive layers exhibited n-type semiconductivity with a charge carrier density in the range of 2–7.5 × 1020 cm−3. Polarization of the specimens to potentials is higher than oxygen evolution potential, resulted in transition top-type semiconducting character due to an accumulation of holes. This phenomenon could be related to the passivity breakdown.

Threshold Chloride Concentration for Passivity Breakdown of Mg–Zn–Gd–Nd–Zr Alloy (EV31A) in Basic Solution

Mg–Zn–Gd–Nd–Zr alloy (EV31A) is a heat-treatable magnesium (Mg) cast alloy that can be used up to 200 °C for automobile and aerospace applications. This alloy has excellent mechanical properties (ultimate tensile strength: 280 MPa at room temperature, and ~230 MPa at 200 °C) and improved corrosion resistance. Electrochemical corrosion studies were conducted on this alloy under different heat treatment conditions in 0.1 M NaOH solution with the addition of 0–1000 ppm of chloride. The alloy showed excellent passivity in the 0.1 M NaOH solution. The passive potential range typically extended to more than 1.2 VAg/AgCl. The transpassive potential was observed to be dependent on heat treatment condition of the alloy. More than 80 ppm of chloride was required to induce passivity breakdown in any heat treatment condition. Peak aging at 200 °C for 16 h imparted better resistance for localized corrosion than other heat-treated conditions. The alloy in the as-received condition showed the highest passivation kinetics due to its smaller grain size that possibly increased the diffusion of reactive elements to form protective oxide. The passive film of the EV31A alloy showed n-type semiconductivity with a charge carrier density of ~2 × 1021 cm−3 with no chloride addition. The charge carrier density increased with chloride addition in the electrolyte which could be correlated with the susceptibility to localized corrosion.

Effect of supplementary cementitious materials (binder type) on the pore solution chemistry and the corrosion of steel in alkaline environments

The pore solution compositions of paste samples produced with Ordinary Portland Cement (PC), slag 25%, 50% and 75%, fly ash 30%, condensed silica fume (SF) 7%, and a ternary blend of 50% PC, 43% slag and 7% SF were determined. Not only are there significant variations in the concentration of the major cations and anions but also, and equally important from the perspective of development of the passivity of steel in solution, in the level of dissolved oxygen and redox potential.Further, the impact of changes in the pore solution chemistry of cement pastes with SCMs on the passivation and corrosion of steel was investigated with mild steel in simulated pore solutions (SPS). Sulphides and thiosulphates, typically found in slag bearing pastes, appeared to reduce the chloride threshold concentration and increase the rate of corrosion in SPS, which has potential implication for the long term performance of reinforced concrete structures.

An Ensemble Kalman Filter approach for service life prediction of reinforced concrete structures subject to chloride-induced corrosion

Chloride induced reinforcement corrosion is one of the most common causes of deterioration of reinforced concrete structures. Condition assessment of such structures is typically based on comparing the chloride content at steel level to the critical threshold chloride concentration. This study aims at developing a robust non-destructive structural health monitoring (SHM) methodology for assessing the state of the structure and predicting its remaining corrosion free service life. The presented methodology relies on embedded sensors that measure the chloride concentration at certain locations, and a predictive model that governs the dynamics of chloride ingress. The system is propagated forward in time using a finite element/finite difference scheme, and the Ensemble Kalman Filter (EnKF) is employed to calibrate the predictive model parameters. The calibrated parameters are then used to assess the remaining corrosion free service life of the structure. The robustness of the presented methodology is demonstrated on 1-D and 2-D numerical examples yielding accurate estimation of the spatial and temporal variability of the chloride content and consequently enhancing maintenance efficiency and structural sustainability.

Correlation between chloride-induced corrosion initiation and time to cover cracking in RC Structures

Numerical value of correlation between effective parameters in the strength of a structure is as important as its stochastic properties in determining the safety of the structure. In this article investigation is made about the variation of coefficient of correlation between effective parameters in corrosion initiation time of reinforcement and the time of concrete cover cracking in reinforced concrete (RC) structures. Presence of many parameters and also error in measurement of these parameters results in uncertainty in determination of corrosion initiation and the time to crack initiation. In this paper, assuming diffusion process as chloride ingress mechanism in RC structures and considering random properties of effective parameters in this model, correlation between input parameters and predicted time to corrosion is calculated using the Monte Carlo (MC) random sampling. Results show the linear correlation between corrosion initiation time and effective input parameters increases with increasing uncertainty in the input parameters. Diffusion coefficient, concrete cover, surface chloride concentration and threshold chloride concentration have the highest correlation coefficient respectively. Also the uncertainty in the concrete cover has the greatest impact on the coefficient of correlation of corrosion initiation time and the time of crack initiation due to the corrosion phenomenon

The Influence of the Cations in Anti-Icing Brines on the Corrosion of Reinforcing Steel in Synthetic Concrete Pore Solution

Concentrated salt solutions are currently being used as antiicing and deicing agents on the roads and highways in Ontario, Canada. The four commonly-used brines are predominantly CaCl2, NaCl, MgCl2, or a multi-chloride (CaCl2/NaCl) mix. The purpose of this investigation was to determine if the different cation(s) of the brine have an influence on the chloride threshold concentration for corrosion initiation of reinforcing steel and on the subsequent active corrosion rates. The tests were carried out in synthetic pore solution in order to allow for control of the chloride content and observation of any corrosion. The results show that the MgCl2 results in a significant drop in pH of the solution and a corresponding high level of corrosion, while the NaCl solution has the lowest chloride threshold value and most anodic corrosion potential.

The inhibitive effect of bicarbonate and carbonate ions on carbon steel in simulated concrete pore solution

The critical chloride threshold concentration and the polarizability of the barrier layer/outer layer (as defined by the Point Defect Model) have been determined for AISI 1020 carbon steel in simulated concrete pore solutions of pH 9, 10, 11 and 12.5 with varying chloride and bicarbonate/carbonate concentrations. It was found that bicarbonate/carbonate ions have a beneficial effect on the charge transfer resistance, the critical chloride threshold concentration and the pitting potential. When the critical chloride threshold concentration for pitting is exceeded, the inhibitive effectiveness of bicarbonate/carbonate against pitting corrosion is around one order of magnitude lower compared to hydroxide.

Service life prediction of cracked RC pipe piles exposed to marine environments

Cracks due to shrinkage, chemical reactions and weathering process occur unavoidably during construction and application of RC pipe piles. These cracks may be the main pathways through which chloride diffuses into the concrete and also can be one of the major reasons of steel corrosion in RC pipe piles exposed to marine environments. In the present paper, an analytical method for predicting the service life of cracked RC pipe pile with considering the effect of cracks on the chloride diffusion is presented. The governing equation of chloride diffusion into RC pipe pile is described and solved analytically by using the Bessel functions. The equivalent chloride diffusion coefficient is derived based on the partition of chloride diffusion through cracked RC pipe pile. The threshold chloride content is estimated from a wide review of previous experimental studies. Comparisons of analytical results with experimental data are conducted to establish the validity of the proposed analytical method. Furthermore, an application of the analytical method is demonstrated by predicting service life of cracked RC pipe piles exposed to marine environments. The analysis results demonstrate the importance of considering the effect of cracks for accurate prediction of service life in RC pipe piles exposed to marine environments.

Potential measurement to determine the chloride threshold concentration that initiates corrosion of reinforcing steel bar in slag concretes

The chloride content at which reinforcement corrosion occurs (Ccrit) is an important parameter for the determination of the service life of a concrete structure. Many studies have been conducted on Portland cement concrete but few data are available concerning slag concrete. In this study, a method for determining the Ccrit value is proposed. It uses capillary suction and a diffusion phase with steel-potential monitoring during the progressive contamination of the concrete by chloride. The same experimental process, capillary suction then diffusion, was performed simultaneously with tap water and chloride solution to verify the specific behaviour of specimens due to the presence of chloride. During the phase of diffusion and potential monitoring, samples were kept immersed in saturated conditions. The criterion for detecting corrosion initiation was a variation of about –250 mV (Ghods et al., Corrosion Science 52:1649–1659, 2010), which corresponds to the difference usually noted between steel in the passive and active state. Two concrete compositions were investigated: one with only Portland cement as the binder and one incorporating slag with a substitution level of 60 %. As expected, in the Portland cement concrete, a drop in potential occurred when corrosion started. For slag concrete, no potential drop was recorded because the initial potential of the steel before the introduction of chlorides was lower. The method for determining Ccrit, based on the fall in potential recorded when corrosion started, would not be suitable to compare the different compositions of binder in saturated conditions. The presence of sulphides in the pore water solution of the slag concrete is a possible explanation of this result. Tests in a solution with added sulphides tended to confirm this hypothesis.

Chloride Resistance of 9% Cr Steel in a Simulated Pore Solution

The resistance to chloride-induced corrosion of a 9% chromium-ribbed reinforcing steel is compared to plain carbon steel-ribbed and round bar. The results from this investigation show the considerable variability in chloride threshold concentration and corrosion rate of the three steel types exposed to simulated pore solution. Incremental chloride additions were made over a period of 46 weeks after allowing 24 weeks for passivation of the exposed steel.

Quantitative estimation on delaying of onset of corrosion of rebar in surface treated concrete using sealers

Surface treatment on the concrete surface using sealers reduces the rate of permeability of chloride and moisture through the concrete. The delaying of onset of corrosion is evaluated for surface treated and untreated concrete using electrochemical techniques. After conducting rapid chloride permeability test (RCPT), using Nernst–Plank equation, the diffusion coefficient of chloride (Deff) is calculated. Substituting threshold chloride concentration of rebar (Cth) from cyclic polarization test in the ficks second law, the time to initiation of corrosion (Ti) is arrived. From the results it is found that the treated concrete with alkyltrialkoxy silane sealer delays the onset of corrosion by four times than that of untreated concrete.

Probabilistic Model of Corrosion in Reinforced Concrete Structures Considering Coupling Effects of Influence Factors

A probabilistic model of the chloride-induced corrosion process is provided in this paper. Based on the Third-moment method, the uncertainty of various influence factors and the coupling effects of them are concerned. Considering the corrosion process as a statistic process, the probability of corrosion initiation at certain time can be obtained. Moreover, the effect of micro-crack in chloride ingress is taken into account, too. Due to the results of stochastic analysis, the surface chloride concentration is the most important factor that affects the corrosion probability, while the thickness of concrete cover, chloride diffusion coefficient, environmental relative humidity, critical threshold chloride concentration, micro-crack rate, water-to-cement ratio and temperature are also important factors. And the Third-moment method is proved to be reasonable in the durability assessment of corrosion-induced concrete structure. The results in this paper can be used to predict the rest life of corrosion-induced concrete structures.