Threshold Chloride Concentration Research Articles

Effect of early CO2 curing on the chloride induced steel bars corrosion in cement mortars

Early CO2 curing acting as a new curing technique is an effective way to cut down on carbon footprint in cement industry. It also improves the strength development and long-term durability of concrete without reinforcement. Given the fact that reinforcement concrete is most widely used in construction and building fields, the carbon sequestration potential will be enhanced remarkably if this curing technology can be used in reinforced concrete. However, the impacts of early CO2 curing on the corrosion behavior of steels, which is the major concern for reinforced concrete exposure to chloride, is unclear yet. In this work, the permeability of chloride, threshold chloride value and corrosion behaviors of steels in cement mortar were determined to comprehensively study the effect of early CO2 curing on the chloride induced corrosion of steels. Early CO2 curing lowers the chloride ions permeability of mortars, in particular for the mortars with 28 day curing ages. Steels take longer time to form stable passive films in early CO2-cured mortar than in conventional moist-cured ones. Early CO2 curing reduces the chloride threshold concentration of mortar, in particular for the mortars that the whole cross section is carbonated, the reduction reaches ∼50%. However, it exerts few effects on corrosion induction phase of steel rebar. Moreover, early CO2 curing decreases the corrosion rate of steel bars in corrosion propagation phase and thus the overall corrosion rate of steels during the whole testing process due to slower chloride ingress rate. Thus, early CO2 curing can act as an alternative curing technique in the production of pre-cast reinforced concrete exposed to chloride.

Effect of high-energy shot peening on the corrosion behavior and chloride threshold concentration of AISI 316L stainless steel rebar in simulated concrete pore solution

The corrosion behavior and chloride threshold concentration of AISI 316L stainless steel reinforcement bar (rebar), subjected to high-energy shot peening (HESPed), are compared with its solution-annealed (SAed) counterpart in simulated concrete pore solution (CPS). X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that HESP significantly reduces the surface grain size to the ultrafine/nanometric range and increases the density of various dislocation structures, including dislocation walls and cells. This augmented presence of grain boundaries and dislocations, acting as high diffusivity paths, contributed to the formation of a thicker surface film with higher Cr content and lower donor/acceptor density in the HESPed sample compared to the SAed. Moreover, corrosion behavior analyses in CPSs with NaCl concentrations ranging from 0.0 to 2.0 M demonstrated that HESPed samples exhibit broader passivation ranges, more positive corrosion potentials, and higher polarization resistance under equivalent chloride concentrations. Additionally, threshold chloride concentrations obtained from the potentiodynamic polarization tests show values between 0.5 and 1.0 M for the SAed and between 1.0 and 1.5 M for the HESPed samples. Based on diffusion equations, a physical model is proposed to elucidate the improved behavior resulting from the HESP treatment.

6-Methoxyquinoline immobilized nylon sensor for monitoring chloride concentration in reinforced concrete

An excess in the threshold concentration of chloride in concrete leads to the initiation of reinforcement corrosion, which in turn results in a reduction in the load-bearing capacity of reinforced concrete (RC) structures. Accordingly, the chloride concentration of RC is periodically investigated via a destructive method, but this process is time-consuming, labor-intensive, and costly. In order to overcome the limitations, this study sought to develop an optical chloride sensor that can be used in structural health monitoring. To this end, nylon was functionalized using formaldehyde and epichlorohydrine, after which 6-methoxy quinoline was covalently immobilized to manufacture a sensor. The developed sensor was found to be capable of measuring chloride concentrations (0–5,000 mM) and exhibited rapid responsiveness and reversibility. In addition, the developed sensor also showed a stable response even when exposed to the pH 13.4 environment for three months, and it could detect changes in chloride concentration even inside the mortar.

Experimental assessment of salinization effects on freshwater zooplankton communities and their trophic interactions under eutrophic conditions

Freshwater ecosystems are becoming saltier due to human activities. The effects of increased salinity can lead to cascading trophic interactions, affecting ecosystem functioning and energy transfer, through changes in community and size structure. These effects can be modulated by other environmental factors, such as nutrients. For example, communities developed under eutrophic conditions could be less sensitive to salinization due to cross-tolerance mechanisms. In this study, we used a mesocosm approach to assess the effects of a salinization gradient on the zooplankton community composition and size structure under eutrophic conditions and the cascading effects on algal communities. Our results showed that zooplankton biomass, size diversity and mean body size decreased with increased chloride concentration induced by salt addition. This change in the zooplankton community did not have cascading effects on phytoplankton. The phytoplankton biomass decreased after the chloride concentration threshold of 500 mg L−1 was reached, most likely due to direct toxic effects on the osmotic regulation and nutrient uptake processes of certain algae rather than as a response to community turnover or top-down control. Our study can help to put in place mitigation strategies for salinization and eutrophication, which often co-occur in freshwater ecosystems.

Localized Corrosion Susceptibility of 434 Ferritic Stainless Steel in Acid Chloride Media

The pitting corrosion resilience and passivation behaviour of 434 ferritic stainless steel was studied in 1 M H2SO4 solution at 0% to 2% NaCl concentration by potentiostatic technique and optical illustration characterization. Data showed 434 steel underwent metastable pitting activity at the lowest metastable pitting potential and current by reason of the effect of SO42- anions within the electrolyte. Addition and increase in Cl- anion concentration decreased the potential while simultaneously increasing the current value at which metastable pitting occurs till 1% NaCl concentration. The presence of chlorides decreased the passivation range of the steel with regards to Cl- anion concentration compared to the electrolyte without chlorides till 0.5% NaCl concentration. Beyond this concentration no further decrease in the steel’s passivation range was observed despite increase in chloride concentration. Beyond 1% NaCl concentration passivation of the steel completely collapsed signifying threshold chloride concentration for optimal steel utilization on astringent conditions. Potentiostatic data showed variation in chloride concentration from 0.25% to 1% had strong effect on the pitting resistance of 434 steel. Optical images of 434 steel morphology at 0%, 0.25% and 2% NaCl concentration in 1 M H2SO4 solution showed important details. At 0% and 0.25% NaCl concentration, corrosion pits were clearly visible though the dimension and depth of the pits from the electrolyte with 0.25% NaCl concentration were more defined and deeper. At 2% NaCl concentration, corrosion pits were larger and the surface morphology showed severe morphological deterioration by reason of the synergistic action of SO42- and excess Cl- anions.

Ca-LDH-modified cementitious coating to enhance corrosion resistance of steel bars

Layer double hydroxides (LDHs) are potential candidates for the application in reinforced concrete. The main perspective of this paper is to prepare cementitious coatings modified with Ca–Al–NO3 layered double hydroxides (Ca-LDH) and investigate its corrosion inhibition performance on steel bars by means of the isothermal adsorption experiment, open-circuit potential (OCP) and linear polarization resistance (LPR). The effect of Ca-LDH dosage on pore structure and chloride binding ability of cement pastes were studied. The primary aim was to design a novel composite coating applied to reinforced concrete. The results show that the incorporation of Ca-LDH increased the chloride consolidation ability of cement paste. Compared with the uncoated steel bars, the chloride concentration threshold (CCT) of the cement paste coatings with 2% and 4% Ca-LDH increased by 0.07 M and 0.06 M respectively, which effectively inhibited corrosion of steel bars in the simulated concrete pore (SCP) solution with chloride. The incorporation of Ca-LDH into cement paste reduced its porosity and strengthened the physical barrier effect. Moreover, Ca-LDH had the dual ability of consolidating chlorides and releasing corrosion-inhibiting anions.

Impact of service life and system boundaries on life cycle assessment of sustainable concrete mixes

Partial substitution of cement/clinker in concrete mix helps reduce its environmental impact. The supplementary cementitious materials such as fly ash and ground granulated blast furnace slag (GGBS), and limestone calcined clay concrete (LC3) are thus been exploited without affecting the short-term performance of concrete. The long-term performance of concrete largely affects the service life of a structure and consequently the environmental impact during the service period. Hence, a comparative life cycle assessment study of four concrete mixes (controlled, fly ash blended (FA30), GGBS blended (GGBS50) and LC3) considering cradle-to-gate and cradle-to-grave system boundaries is conducted for the scenario in Singapore. The chloride resistance of these concrete mixtures is considered for service life estimation as it is the major cause for corrosion of the reinforcement. The uncertainty study reflects the dependence of service life on threshold chloride content. The service life is in the order of LC3 > FA30 > GGBS50 > controlled mix. LC3 and controlled concrete exhibit minimum and maximum environmental impact, respectively irrespective of system boundary. In cradle-to-gate system boundary, GGBS50 exhibits lesser environmental impact than FA30 excluding abiotic depletion, human toxicity, freshwater aquatic ecotoxicity and marine aquatic ecotoxicity impact categories owing to higher cement substitution level. However, the consideration of service life in cradle-to-grave system boundary resulted in lesser environmental impact for FA30 than GGBS50 in all impact categories (apart from acidification, GWP and terrestrial ecotoxicity), owing to the fewer repair cycles and subsequent utilization of concrete.

Quantitative Non-Linear Effect of High Ambient Temperature on Chloride Threshold Value for Steel Reinforcement Corrosion in Concrete under Extreme Boundary Conditions.

This paper investigates the effect of high ambient temperatures on the chloride threshold value for reinforced concrete (RC) structures. Two commonly available carbon steel rebars were investigated under four different exposure temperatures (20 °C (68 °F), 35 °C (95 °F), 50 °C (122 °F), and 65 °C (149 °C)) using environmental chambers at a constant relative humidity of 80%. For each temperature, six different levels of added chloride ions (0.00%, 0.15%, 0.30%, 0.60%, 0.90%, and 1.20% by weight of cement) were used to study the chloride threshold value. Corrosion initiation was detected by monitoring the corrosion potential and corrosion rate using electrochemical techniques. The water-soluble (free) and acid-soluble (total) chlorides were determined using potentiometric titration according to the relevant ASTM standards. The threshold chloride content for each exposure temperature was determined by analyzing the corrosion potential, corrosion rate, and chloride content of each specimen. The results showed that the chloride threshold values were significantly temperature-dependent. At temperatures of 20 °C (68 °F) and 35 °C (95 °F), the chloride threshold value (expressed as free chlorides) was approximately 0.95% by weight of cement. However, as the temperature increased to 50 °C (122 °F), the chloride threshold decreased significantly to approximately 0.70% by weight of cement. The reduction in the chloride threshold value became more dramatic at an exposure temperature of 65 °C (149 °F), decreasing to approximately 0.25% by weight of cement. The trends were similar for the rebars from the two sources, indicating that the rebar source had little influence on the chloride threshold value.

Chloride-related steel corrosion initiation in cement paste prepared with the incorporation of blast-furnace slag

Steel corrosion caused by chloride penetration is an important problem in construction practices threatening the long-term durability of reinforced concrete structures. In this study, the effects of blast-furnace slag on the passivation and corrosion processes of carbon steel embedded in cement pastes were investigated by electrochemical measurements, and equivalent circuit models were proposed for analysis of the passivation and corrosion processes. The spatial chloride distributions in the cement paste after corrosion initiation were determined distinguishing between amounts of total, free and bound chloride. The values of the open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements indicated that the carbon steel embedded in cement pastes was passivated due to the presence of an alkaline pore solution. However, the formation of passive layers on the surface of carbon steel in the slag-blended cement paste was weakened by the more negative OCP value due to a lower pH value and higher sulphide concentration in the pore solution. Upon initiation of corrosion during the electrical acceleration test, the OCP value dropped to a significantly more negative value whereas the Warburg impedance in the equivalent circuit model sharply decreased. Moreover, the addition of slag decreased the threshold chloride concentration at the steel surface to initiate steel corrosion. On the other hand, the enhanced resistance against chloride penetration and the higher chloride binding capacity resulted in an overall increase of the corrosion resistance of carbon steel in slag-blended cement paste.

A time-dependent chloride diffusion model for predicting initial corrosion time of reinforced concrete with slag addition

The effect of granulated blast furnace slag (GBFS) addition on the threshold chloride concentration (TCC) for rebar corrosion was investigated. A modified diffusion model, coupled with a time-dependent effective diffusion coefficient and surface chloride concentration, was proposed to predict the chloride profile. The corrosion initiation time was estimated based on the model predictions and the measured TCC. The results indicate that adding GBFS decreases the TCC by lowering the pH value of the pore solution. The evolution of corrosion potential and current density is found to obey a 3-parameter Weibull distribution. MIP tests show that adding GBFS contributes to refinement of pore structure by decreasing the fraction of large capillary pores. The time-dependent model exhibits good predictive strength and helps understand how GBFS addition delays the corrosion initiation by retarding the chloride diffusion, though a lower TCC is obtained.

Influence of Chloride Concentration on Stress Corrosion Cracking and Crevice Corrosion of Austenitic Stainless Steel in Saline Environments.

Stainless steels are used as canister materials for interim storage of spent fuel. Crevice corrosion has proved to be a safety concern of 304L stainless steel spent fuel canisters, when exposed to the saline environments of coastal sites. To study the effects of chloride concentration and test duration on the crevice corrosion behavior, and the effect of relative humidity on the initiation of discrete SCC cracks, a test program was conducted on the 304L steel specimens sprayed with synthetic sea water of 3.5 wt.%. The salt-deposited specimens, wrapped up with a crevice former to form a crevice configuration, were then exposed to an environment at 45 °C with a pre-set 45%, 55%, and 70% relative humidity (RH), for 400 h and 10,000 h, respectively. The surface features and crack morphology of the tested 304L stainless-steel specimens were examined by energy-dispersive spectrometry (EDS) and electron back scatter diffraction (EBSD). For the specimens deposited with a chloride concentration of 1 g/m2, no cracks were found in the corroded regions after 400-h exposure, whereas SCC cracks were observed with the specimens tested for 10,000 h at all three pre-set relative humidity. The specimens tested at the pre-set relative humidity 45% are characterized with discrete SCC cracks, but, on the other hand, those exposed to the environments of 55% and 70% relative humidity show SCC cracks of distinct features. From the results of 10,000-h tests, it is inferred that the chloride concentration threshold for SCC initiation of 304L stainless steel at 45 °C is between 0.1 g/m2 and 1 g/m2.

Chloride binding in cement paste with calcined Mg-Al-CO3 LDH (CLDH) under different conditions

The chloride binding capacity of cement paste with Mg-Al CLDH, obtained through calcining a commercial LDH, is investigated under different chloride concentration, environmental temperature, and concrete pore solution alkalinity conditions. The transformation mechanism of Mg-Al CLDH to Mg-Al-Cl LDH is mainly explored by means of X-ray diffraction (XRD) , Fourier-transform infrared spectroscopy (FT-IR) and Differential thermogravimetric analysis (DTG). There appears to be a threshold chloride concentration of 1.0 mol/L for Mg-Al CLDH (6% incorporation) effectively enhancing the chloride binding capacity of cement paste. Both lowering and raising (>38 °C) the environmental temperature is disadvantage to the transformation of Mg-Al CLDH to Mg-Al-Cl LDH in cement paste, and the amount of bound chloride at different curing temperatures follows as 38 °C > 50 °C ≈ 20 °C > 3 °C. Higher pore solution alkalinity is also adverse to both of the formation of Friedel’s salt and the rehydration of CLDH. With increasing pH from 12.5 to 13.5, the improvement of Mg-Al CLDH on the chloride binding capacity of cement paste became weaker. Mg-Al CLDH improves the chloride binding capacity of cement paste is not only related to the excellent anions exchanging ability of it , but also related to the promotion of Friedel’s salt formation of it.

Influence of Threshold Chloride Concentration on Corrosion Behavior of Low-Alloy Steel Rebar in Concrete Pore Solutions

Influence of Threshold Chloride Concentration on Corrosion Behavior of Low-Alloy Steel Rebar in Concrete Pore Solutions

CO2 Corrosion of Carbon Steel: The Synergy of Chloride Ion Concentration and Temperature on Metal Penetration

This paper investigates the synergy of chloride ion concentration and temperature on the general and pitting corrosion of carbon steel in CO2-saturated environments. Experiments were conducted over 168 h in different concentrations of NaCl brines (1 wt%, 3.5 wt%, and 10 wt%) and temperatures (30°C, 50°C, and 80°C) with the aim of elucidating the combined effect of changes in chloride ion concentration and temperature on overall metal degradation, taking into consideration general and pitting corrosion. This also includes a correlation with the formation and properties of FeCO3 corrosion products. Linear polarization resistance was implemented to monitor the electrochemical responses. Corrosion product characteristics and morphologies were studied through a combination of scanning electron microscopy and x-ray diffraction. Pitting corrosion evaluation was conducted through the application of 3D surface profilometry to study pit geometries such as the depth and diameter. The results show that general and pitting corrosion are strongly correlated to the synergistic effects of changing chloride ion concentration and temperature in carbon steel as a result of their combined influence on ferrite (Fe) dissolution and FeCO3 formation. This represents a paradigm shift from the already established mechanisms on chloride ion and temperature effects on passive alloys such as stainless steel. Increasing chloride ion concentration and temperature up to 10 wt% NaCl and 50°C to 80°C, respectively, is observed to increase the rate of Fe dissolution and formation of semiprotective FeCO3 corrosion products, leading to the increased manifestation and severity of pitting corrosion. The results also show that a “threshold chloride concentration” exists at 30°C, above which there is no significant increase in corrosion rate. However, such “threshold effects” were not observed at higher temperatures evaluated in the range of chloride concentration considered in this study.

Service life of reinforced concrete seawalls suffering from chloride attack: Theoretical modelling and analysis

This paper presents a theoretical model for predicting the service life of reinforced concrete (RC) seawalls under chloride attack. The developed theoretical model not only takes into account the effects of chloride-induced degradation, but also properly considers the external water and earth pressures acting on the RC seawalls. To theoretically model the service life of the RC seawall, the whole service life is divided into the initiation stage for chloride ions diffusion and the propagation stage for corrosion products development. The diffusion of chloride ions in the seawall is simulated by a two-sided chloride diffusion model on the basis of the Fick's second law, which is sufficiently validated by the corresponding ABAQUS numerical model. The propagation stage is modeled by a mechanical model that considers both the external pressures and the internal expansive pressure induced by the rust productions. Example analyses are conducted to illustrate the application of the model and investigate the effects of some key factors on the service life of RC seawall. The results show that the service life of reinforced concrete seawall increases with cover thickness, tensile strength of concrete, threshold chloride concentration, burial depth and surface load, while decreases with the increase of rebar diameter and water-cement ratio. The proposed theoretical model can serve as a theoretical base and benchmark for design and prediction of service life of coastal structures.

Effect of stray current coupled with chloride concentration and temperature on the corrosion resistance of a steel passivation film

The combined effects of stray current, chloride ion concentration and temperature on the corrosion resistance of a steel passivation film were studied by electrochemical methods. Potentiodynamic polarization tests and other auxiliary tests were carried out to reveal the mechanism of damage to a steel passivation film. The experimental results indicate that the film becomes unstable under the influence of high voltage stray currents. This leads to a decrease in the threshold chloride concentration and breakdown potential in the depassivation process. High temperature can obviously reduce the thickness of the passivation film, which aggravates damage by chloride ions. However, high voltage plays the decisive role in the destruction of the passive film, and the adverse effect of coupling is much greater than that of a single factor.

Free and bound chloride relationships affecting reinforcement cover in alkali activated concrete

This paper investigates the free chloride profiles, diffusion parameters and chloride binding capacity of an alkali activated concrete (AACM) together with a control Portland cement (PC) concrete. Ggbs based AACM concrete specimens with different molarity of activator were exposed to a 5% NaCl solution for 540days to determine their free chloride diffusion properties. The relationships between the free and bound chloride concentration were determined by applying Freundlich and Langmuir isotherms. The required cover to steel reinforcement for corrosion prevention is derived to satisfy the limiting thresholds of free and bound chloride concentrations.The results show that Fick's second law of diffusion applies to the free chloride profiles of AACM concrete and provides higher values of diffusion coefficients than a similar grade of PC concrete. The relationship between the free and bound chlorides is defined by the Langmuir isotherm. PC concrete has higher chloride binding capacity than AACM concrete for both water and acid soluble chlorides. Less concrete cover to steel reinforcement is required in AACM than PC concrete when calculated by using the bound chloride concentration threshold limit. The values of cover based on the corresponding free chloride limit in AACM concrete are higher than its bound chloride values.

Study on the Chloride Threshold and Risk Assessment of Rebar Corrosion in Simulated Concrete Pore Solutions under Applied Potential

The pitting corrosion behavior of HRB400 steel in various simulated concrete environments was investigated by the combination of polarization curves and statistical method. The results indicated that the chloride concentration threshold ([Cl−]th) for pitting of the steel was greatly affected by pH when the applied potential exists, which is always caused by random stray current. The interaction of applied potential and chloride concentration on the pitting behavior was discussed. Finally, pitting-risk-evaluation diagrams were built up, which could be easily used to assess the pitting risk of reinforcing bars under the chloride-containing environment with stray current.

Effect of limestone powder addition on threshold chloride concentration for steel corrosion in reinforced concrete

The effects of limestone replacement fraction, limestone specific surface area and water-to-binder (W/B) ratio on the threshold chloride concentration (TCC) for steel corrosion were evaluated. XRD and SEM analyses were conducted to examine the crystalline phases and microstructure of limestone-blended cement paste, respectively. The influence of pH value of concrete pore solution on the chloride binding capability was also investigated. The results demonstrate that the chloride threshold decreases as the limestone content or the W/B ratio increases, while limestone fineness shows negligible effect. XRD analyses indicate that addition of limestone inhibits the transformation from ettringite (AFt) to monosulfate (AFm), which exhibits superior chemical binding capability for chloride ions. SEM analyses suggest that addition of 15 wt% limestone contribute to a denser microstructure. Reduction in the pH value of pore solution compromises the chloride binding capability of concrete.

Corrosion Resistance to Chloride of a Novel Stainless Steel: The Threshold Chloride Value and Effect of Surface State

To evaluate the corrosion resistance of a novel stainless steel intended for use within reinforced concrete (RC) structures exposed to aggressive environments, the threshold chloride concentration of three stainless steels (316, 2205, novel 2205) and two carbon steels (HRB400, HRB500) exposed to pore solutions of fresh concrete was experimentally studied by means of electrochemical methods. The effect of steel surface state on the corrosion resistance was also experimentally investigated. The results showed that the novel stainless steel has a much higher corrosion resistance than those of the carbon steels and stainless steels when subjected to chloride environments. The presence of surface damage leads to significant decrease of corrosion resistance for carbon steel, however the corrosion can be certainly inhibited with the accumulation of rust on the steel surface. Although the oxide layer was worn, the novel 2205 stainless steel still has a great corrosion resistance.