Chloride Threshold Research Articles

Exploring machine learning to study and predict the chloride threshold level for carbon steel reinforcement

Chloride-induced corrosion of steel reinforcing bar (rebar) is the primary cause of deterioration in reinforced concrete structures, posing a significant infrastructure challenge. The chloride threshold level (CTL) of rebar, which represents the critical amount of chloride needed to initiate active corrosion, is crucial in corrosion and service life prediction models. However, substantial uncertainties and a multitude of influencing factors, along with the absence of a universally accepted testing framework, hinder the achievement of a consistent CTL range for service life models and complicate comparisons of published values. This study addresses these challenges by developing multiple machine learning models to predict CTL, considering 21 carefully selected features. A comprehensive database of 423 data points was compiled from an exhaustive literature review. Seven machine learning models—linear regression, decision tree, random forest, K-nearest neighbors, support vector machine, artificial neural network, and an ensemble model—were developed and optimized. The ensemble model achieved superior prediction performance, with a mean absolute error of 0.218 % by weight of binder, root mean square error of 0.321 %, and a coefficient of determination of 0.751 on unseen CTL data. Partial dependence plots generated using the support vector machine model quantified the effect of each feature on CTL. The random forest model identified SiO₂ binder content and exposed rebar area to chlorides as the most influential factors. The study also examined the impact of supplementary cementitious materials (SCMs), finding that only blast furnace slag positively affected CTL.

On the Impact of Cathodic Polarization on the Chloride Threshold of Carbon Steel in Alkaline Solutions

On the Impact of Cathodic Polarization on the Chloride Threshold of Carbon Steel in Alkaline Solutions

Performance of cementitious systems containing calcined clay in a chloride-rich environment: a review by TC-282 CCL

In this review by TC- 282 CCL, a comprehensive examination of various facets of chloride ingress in calcined clay-based concrete in aggressive chloride-rich environments is presented due to its significance in making reinforced concrete structures susceptible to chloride-induced corrosion damages. The review presents a summary of available literature focusing on materials characteristics influencing the chloride resistance of calcined clay-based concrete, such as different clay purity, kaolinite content and other clay minerals, underscoring the significance of pore refinement, pore solution composition, and chloride binding mechanisms. Further, the studies dealing with the performance at the concrete scale, with a particular emphasis on transport properties, curing methods, and mix design, are highlighted. Benchmarking calcined clay mixes with fly ash or slag-based concrete mixes that are widely used in aggressive chloride conditions instead of OPC is recommended. Such comparison could extend the usage of calcined clay as a performance-enhancing mineral admixture in the form of calcined clay or LC2 (limestone-calcined clay). The chloride diffusion coefficient in calcined clay concrete is reported to be significantly lower (about 5–10 times in most literature available so far) compared to OPC, and even lower compared to fly ash and slag-based concrete at early curing ages reported across recent literature made with different types of cements and concrete mixes. Limited studies dealing with reinforcement corrosion point out that calcined clay delays corrosion initiation and reduces corrosion rates despite the reduction in critical chloride threshold. Most of these results on corrosion performance are mainly from laboratory studies and warrant field evaluation in future. Finally, two case studies demonstrating the application of calcined clay-based concrete in real-world marine exposure conditions are discussed to showcase the promising potential of employing low-purity calcined clay-based concrete for reducing carbon footprint and improving durability performance in chloride exposure.

Mitigation of Steel Corrosion Threshold Utilizing Plant-Based Green Corrosion Inhibitors through Electrochemical Techniques

Corrosion of steel reinforcement is a major cause of deterioration in reinforced concrete structures. The corrosion process is influenced by the concrete-steel interface, with the alkaline concrete pore solution initially providing passivation. However, ingress of aggressive substances like chlorides can disrupt the passive layer, initiating active pitting corrosion above a threshold level often taken as 0.4% chloride by cement weight. Once the chloride threshold is exceeded, corrosion propagation depends on oxygen and moisture availability. The resulting rust formation causes expansive cracking and spalling of the concrete cover. Corrosion damage can be mitigated through use of inhibitors like calcium nitrite though high dosages impair concrete strength. Recently, plant-based organic compounds have shown promise as green corrosion inhibitors. The corrosion behavior of steel in concrete can be evaluated through impressed accelerated corrosion testing along with electrochemical techniques like half-cell potential mapping and resistivity measurements. These allow assessment of the probability of corrosion and corrosion rate. Techniques like linear polarization resistance and electrochemical impedance spectroscopy can also quantify instantaneous corrosion rate. Proper structural condition assessment and repair using both conventional and green inhibitors is crucial to control steel corrosion, maintain service life and ensure safety. Further research is needed on green corrosion mitigation methods and advanced non-destructive testing techniques.

Corrosion resistance behavior of enhanced passivation Cr-modified rebars and their service life prediction based on Monte Carlo simulation

This work investigates enhanced passivation and corrosion resistance of various Cr-modified rebars compared to carbon rebar. It also evaluates their service life through time-varying probability analysis based on Monte Carlo simulation, combined with the parameters obtained from sampling and testing reinforced concrete mortar samples exposed to various marine conditions over 5 years. The results show that the free energy required for oxide thickening of rebar decreases from 20.12 kJ/mol to 9.25 kJ/mol as the Cr content increases from 5 % to 11 % (wt.), which facilitates the developed Cr-modified rebars rapidly form a passive film, exhibiting a chloride threshold level 6–14 times higher than that of carbon rebar. In the harshest marine splash zone, sea sand concrete with Cr11 rebar ensures a theoretical 50-year service life, contrasting sharply with the much shorter lifespan estimated at 5–6 years for carbon rebar. The corrosion rate of Cr-modified rebars was notably lower than that of the carbon rebar, and the Cr oxides/hydroxides facilitate the transformation of FeO(OH) with larger volume expansion to Fe3O4 with smaller volume expansion, reducing from 130 μm in HRB to 70 μm in the case of the thinnest Cr11 rebar. The thin and dense rust layer of the developed rebars is advantageous in blocking Cl- and guarantees a smaller volume expansion of structure even if the rebars are corroded. This study underscores the suitability of Cr-modified rebars for sea sand concrete structures and sheds new light on rebar-reinforced concrete structure developments toward eco-efficient and sustainable infrastructures.

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.

Quantification of critical chloride threshold of reinforcing steel embedded in portland limestone cement systems containing supplementary cementitious materials

Quantification of critical chloride threshold of reinforcing steel embedded in portland limestone cement systems containing supplementary cementitious materials

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.

Function of Cu-Sb or Al microalloying on the corrosion resistance of 9Cr steel exposed to simulated concrete pore solution

The role of Cu-Sb or Al on the passivation and Cl-induced depassivation of 9Cr steel in simulated concrete pore solution was systematically obtained via electrochemical tests and microstructure characterization. Both microalloying enhanced electrochemical passivation even if the solution pH dropped. Cu and Al participated in the formation of natural passive film, and the properties of passive film were optimized in terms of resistance, composition, and inner film thickness. However, the chloride threshold level (CTL) was not increased with Cu-Sb microalloying and even decreased with Al microalloying, which is understood from the stability of Al-containing products and the hazards of inclusions.

Passivation behaviour of aluminium alloys in limestone calcined clay cement (LC3)

Limestone calcined clay cement (LC3) has emerged as a promising alternative to traditional Portland cement (PC), offering reduced carbon dioxide emissions and improved mechanical and durability-related properties. However, concerns persist regarding the durability of reinforced LC3 concrete, particularly its lower critical chloride threshold and carbonation resistance. To address this issue, aluminium alloys were suggested to be used as an alternative reinforcement. This study investigates the passivation behaviour of aluminium alloys in LC3 and compares it with that in PC. Electrochemical measurements indicate that LC3 paste exhibits higher resistance and forms a more compact film compared to PC paste, suggesting its more protective property. XRD, Raman spectroscopy, and SEM analysis provide insights into the chemical composition and thickness of the films formed on the surfaces of aluminium alloys. This study enhances our understanding of the passivation mechanisms of aluminium alloys in LC3, facilitating their potential application in LC3-based structures.

Computation of Particulate Deposition on Dry Storage Canisters

This paper provides an overview of ongoing work aimed at developing spent nuclear fuel (SNF) canister deposition models. Currently, it is known that stainless steel canisters are susceptible to chloride-induced stress corrosion cracking (CISCC). However, the rate of CISCC degradation and the likelihood that it could lead to a through-wall crack is unknown. While it is currently unknown if there is a threshold chloride surface concentration for CISCC initiation, it can be assumed that the onset and progress of material degradation will depend on the local contaminant concentration, the properties of the contaminant species, and the synergistic effects when multiple contaminants are present. This study uses well-developed computational fluid dynamics and particle tracking tools and applies them to SNF storage to determine the rate of deposition on canisters. Understanding the rate of deposition on SNF canisters could be important for making canister aging management predictions. This study is a part of an ongoing effort funded by the U.S. Department of Energy, Office of Nuclear Energy, Office of Spent Fuel and Waste Science and Technology, which is tasked with doing research relevant to enhancing the technical basis for ensuring the safe extended storage and subsequent transport of SNF. This work is being presented to demonstrate a potentially useful technique for SNF canister vendors, utilities, regulators, and stakeholders to utilize and further develop for their own designs and site-specific studies.

Modeling of chloride-induced corrosion in concrete bridge using the simplified and full probabilistic methods

Concrete and reinforced concrete structures are subjected to chloride corrosion, which can shorten their service life and safety of use. Reliability and safety prediction of concrete structures is a crucial task for optimizing their life cycle design and maintenance and for minimizing their life cycle costs. In the paper a probabilistic analysis of concrete durability in structural members is presented. Two methods: simplified-probabilistic and full-probabilistic have been applied for modelling chloride-induced corrosion in concrete structures. The uncertainty of the key parameters including surface chloride concentration, chloride threshold, cover depth and diffusion coefficient, which govern the chloride ingress into concrete and corrosion of reinforcing steel have been analyzed. A case study of a reinforced concrete bridge has been used to illustrate the capability and efficiency of these probabilistic methods in modeling the uncertainty and predicting the time-dependent probability of corrosion. FREeT-D and ProCAAT software have been used for the analysis.

Macrocell Effect on Chloride Threshold Value and Corrosion Rate of Steel Bar in Simulated Concrete Pore Solution

Macrocell Effect on Chloride Threshold Value and Corrosion Rate of Steel Bar in Simulated Concrete Pore Solution

Chloride absorption capacity of calcium silicate hydrate (C-S-H) and its effect on steel corrosion in simulated concrete pore solution

The effect of calcium silicate hydrate (C-S-H) on chloride (Cl−) binding and steel corrosion in simulated concrete pore solution (SCPS) was evaluated. The results showed that Cl− absorbed by C-S-H conformed to Freundlich isotherm for 0.1–1.0 M of Cl−. When C-S-H reached equilibrium in saturated Ca(OH)2 solution, its degree of polymerization increased. The high pH is unfavorable for the absorption of Cl− by C-S-H, but favorable for the passivation of steel. The synthetic C-S-H powder contains NO3 −, which prolongs the passivation time of steel, but the presence of silicate is favorable for the formation of a passivation film with good corrosion resistance. The addition of C-S-H promoted the chloride threshold value of steel corrosion and delayed the corrosion development process.

Input parameters and nomograms for service life-based design of reinforced concrete structures exposed to chlorides

Civil engineers are trying to promote the service-life-based design of reinforced concrete (RC) structures to ensure long corrosion-free service life with minimal maintenance, repair, rehabilitation, and life cycle costs. This paper highlights the difficulties in adopting the existing tools as a general practice, especially for high-performance concrete with novel materials. To overcome these difficulties, “SL-Chlor,” a MATLAB® computer program is developed to estimate service life, which uses Fick’s 2nd Law of Diffusion to model chloride ingress, a major factor causing corrosion. Based on SL-Chlor, user-friendly nomograms are then developed to estimate the service life of RC structures located at various distances from the seashore. With the development of nomograms, this paper attempts to bring a new perspective to promote service life-based design with the help of user-friendly tools. The use of nomograms is demonstrated using case studies with multiple input parameters. The effect of different input parameters on service life is found to be synergistic, in the following descending order of influence: cover depth (d) > chloride diffusion coefficient (DCl) > ageing coefficient (m) [maturity constant ofDCl)] > chloride threshold of the steel-cementitious system (Clth). These findings encourage not only the use of blended cements but also the implementation of 3C’s (adequate cover depth, compaction, and curing, which inherently influences d, DCl, and m) to help structures achieve the desired service life.

Service life prediction and environmental impact evaluation of recycled aggregate concrete with supplementary cementitious materials

The application of recycled aggregate concrete (RAC) can promote the harmonious development of resources and environment. However, the mechanical properties and chloride permeability resistance of RAC are worse than those of natural aggregate concrete (NAC), which limits RAC application in chloride-attack environment. In order to solve this problem, supplementary cementitious material (SCM) is added into RAC to improve its performance. In this research, the changes in durability-based service life of RAC due to the addition of unconventional SCMs composed of red mud (RM), ground granulated blast furnace slag (GGBFS) and/or glass power (GP) have been studied. Based on prepared reinforced concrete specimens with 4 kinds of recycled coarse aggregate (RA) substitution level and 5 kinds of binder composition, the accelerated corrosion tests were conducted for determining corrosion rate. Using the corrosion rate data detected in this paper, combined with the chloride rapid migration coefficient and chloride threshold etc. data, the service lives of reinforced concrete structures with SCMs concrete were found to be much longer than those with Ordinary Portland cement (OPC) concrete. Furthermore, life cycle assessment integrated with strength and service life was performed for concrete with specific mix design to study the effect of unconventional SCMs addition on the environmental behaviors of RAC with different RA content. The results showed that RA substitution level had little effect on material environmental benefits, but the improvement effect of SCMs was obvious. Finally, the multi-criteria analysis method was used to evaluate the overall performance of concrete with different binder composition and RA content in terms of strength, service life and environmental impact. For any substitution level of RA, the SCMs composition with optimal effect in improving overall performance of concrete was 10% RM and 15% GGBFS (percentage by weight of binder).

Durability in Design of Light Rail Reinforced Concrete Structures

Recently, a number of light rail systems have been built or extended in North America. Typical design lives of metallic and concrete transit structures are required to exceed 75 y, with exposure to de-icing salts, freeze/thaw, and the potential for stray currents. Measures to mitigate the risks of reinforcement corrosion to rail structures have progressed over the last century, with some diversion between the preferred practices in Europe and North America. One significant difference with large cost impacts on projects is the means and methods to achieve continuity of the reinforcing steel within rail structures to control stray currents to negligible levels. Herein we review the available standards and literature as it relates to the risks of stray current corrosion of reinforcing steel, specifically with respect to the requirement for welding reinforcing. Field measurements of steel resistivity taken during the construction of rail structures are presented to clarify the as-built condition. Taking into account the effect of stray currents on the chloride threshold for corrosion initiation, methods are recommended to achieve durability requirements for the least lifecycle cost to asset owners.

Chloride threshold determination of hybrid inhibitor immersed in simulated concrete pore solution

The hybrid inhibitor performs excellently compared to either organic or inorganic inhibitor individually. In the present studies, a fixed amount i.e. 2 wt% l-arginine (LA) and 0.25 wt% trisodium phosphate dodecahydrate (TSPDH) is considered as hybrid inhibitor and its chloride threshold is determined by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) in simulated concrete pore solution and the formed passive films are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy after 240 h of immersion. The chloride threshold of this hybrid inhibitor is 6 wt% NaCl, i.e., C3 until 216 h of immersion, whereas if the amount is increased, the steel rebar leads to corrode severely. 5 wt% NaCl containing sample, i.e., C2 exhibits the noble OCP, highest impedance, and lowest passive current density attributed to the consumption of all zwitterion and phosphate ions in the formation of P-zwitterion-(Cl)-Fe, zwitterion-(Cl)-Fe and FePO4 complexes with NaCl, whereas lower or higher than this amount of NaCl, the Cl- ions either lesser or greater to be formed these complexes.

Influence of tensile stress on chloride-induced depassivation of carbon steel in simulated concrete pore solution

To study the influence of tensile stress on the depassivation behaviour of carbon steel, the critical chloride threshold CCl,crit, chemical composition, surface morphology, thickness, and shedding rate of carbon steel passive films were investigated using electrochemical tests and XPS and SEM analyses. The results showed that CCl,crit decreased gradually, and the shedding rate of the passive film increased remarkably with the applied tensile stress. Slight change of Fe0 content can be found under less tensile stress. As the tensile stress increased to 200 MPa, the content of Fe0 increased significant by about 57%. A correlation model between CCl,crit and the characteristic evolution of the passive film is proposed to describe the depassivation process of carbon steel under tensile stress. The decrease in CCl,crit could be attributed to the degradation of the passive films. Additionally, a relationship between CCl,crit and tensile stress is proposed, and a method for determining the depassivation of carbon steel under stress based on electrochemical parameters is established.

Determination of the Chloride Threshold of Cr-Based Steel Rebars in a Synthetic Concrete Pore Solution Based on Electrochemical Methods

Steel rebars are widely applied in reinforced concrete structures due to their contribution to significant improvements in mechanical properties. However, exposure to corrosive environments, such as solution-containing chloride, may induce an accelerated corrosion and jeopardize the sustainability and durability of reinforced concrete. The current work evaluated the chloride thresholds (in both wt% and [Cl−]/[OH−]) of steel rebar with different chromium contents (0 wt%, 4 wt%, and 23 wt%) in synthetic concrete pore solution based on AC and DC electrochemical measurements, and chemical and structural characterizations. The study found that the chloride threshold (CT) values varied slightly based on the different measurement methods and the values were compared with values obtained from the literature. The values are 0.01 wt% to 0.1 wt% (3.76), 0.1 wt% to 1 wt% (>23.8), and 2.9 wt% to 3.5 wt% (>23.8) for 615, 4% Cr, and SS23, respectively, where the values in parentheses are [Cl−]/[OH−]. Therefore, it is demonstrated that multiple measurements are necessary to determine a reliable CT value. Corrosion mechanisms giving rise to CT are included to illustrate the processes involved in establishing CT.