Chloride Threshold Research Articles

Modeling the corrosion initiation time of slag concrete using the artificial neural network

This paper focuses on the Artificial Neural Network (ANN) as an alternative approach to simulate the corrosion initiation time of slag concrete obtained from the error function solution to Fick’s second law of diffusion. The adopted network architecture consists of four neurons in the input layer, which represents the values of concrete cover depth, apparent chloride diffusion coefficient, chloride threshold value and surface chloride concentration, and one neuron in the output layer, which represents the value of the corresponding corrosion initiation time. Back Propagation (BP) algorithm was employed for the ANN training in which a Tansig function was used as the nonlinear transfer function. The research results obtained from both ANN model and the error function solution to Fick’s second law of diffusion demonstrate that the corrosion initiation time of slag concrete increases with increasing both the concrete cover and the chloride threshold value and decreases with increasing both the surface chloride concentration and the chloride diffusion coefficient. Through the comparison of the estimated results from ANN model and the error function solution to Fick’s second law of diffusion, it was clear that there was a high correlation between the corrosion initiation time obtained from the error function solution to Fick’s second law of diffusion and the corresponding corrosion initiation time predicted by the ANN model.

Experiences on corrosion inhibitors for reinforced concrete

Corrosion of carbon steel reinforcement is the most important cause of premature failure on reinforced concrete structures. Prevention of corrosion is primarily achieved in the design phase by using high quality concrete and adequate cover. Additional prevention methods are adopted when severe environmental conditions occur or on structures requiring very long service life. Among these methods, corrosion inhibitors seem to offer a simple and cost effective prevention technique. They may be used both as a preventative techniques, if added to fresh concrete, and as a repair system, if applied on hardened concrete. The performance of corrosion inhibitors for reinforced concrete structures affected by chloride induced and carbonation corrosion has been studied at PoliLaPP, Laboratory of Corrosion of materials “P. Pedeferri” of the Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, in the last 15 years. Organic commercial admixed corrosion inhibitors delayed the occurrence of chloride induced corrosion. This result is related to two effects: reduction of the rate of chloride transport into concrete and increase of the critical chloride threshold. Commercial migrating corrosion inhibitors (MCI) are able to delay time-to-corrosion of passive rebars in concrete subjected to chlorides ponding; this effect is mainly related to the reduction of chlorides diffusion coefficient. MCI can reduce the corrosion rate after corrosion initiation only in carbonated concrete, nevertheless efficiency is low and residual corrosion rate is not negligible. Commercial MCI can penetrate into concrete mainly through capillary sorption but penetration depth is limited to 20 mm. The results obtained with nitrite-based inhibitor confirm literature data: the inhibitor is effective if the molar ratio 2 [NO ] /[Cl ]  is higher than 0.5–0.6; in carbonated concrete, due to insufficient amount of inhibitor, no significant effect has been observed on corrosion rate. Among the tested organic substances, compounds containing carboxylic group showed the best results solution tests: pitting potential, time-to-corrosion and critical chloride content are similar to those obtained with sodium nitrite. In concrete tests, only one amine and one amino acid showed good performance increasing the critical chlorides

Effect of temperature on the corrosion behaviour of low-nickel duplex stainless steel bars in concrete

Stainless steel reinforcing bars can be a means for prolonging the service life of reinforced concrete structures exposed to tropical climates. To select a suitable grade of stainless steel according to exposure conditions and design service life, the definition of the chloride threshold for pitting corrosion initiation is required. This paper investigates the effect of temperature in the range 20–60°C on the resistance to chloride-induced corrosion of low-nickel duplex stainless steel rebars and, for comparison, of traditional austenitic stainless steel rebars. Tests in concrete and in solutions simulating the concrete pore liquid were performed and an attempt to evaluate the chloride threshold levels for corrosion initiation was carried out. Results showed lower corrosion resistance and higher sensitivity to increase in temperature for low-nickel duplex stainless steel bars compared to traditional austenitic stainless steels.

Cyclic voltammetry evaluation of inhibitors for localised corrosion in alkaline solutions

The paper focuses on chloride corrosion of steel rebar in alkaline solutions. The effect of chloride content on corrosion initiation was assessed through cyclic voltammetry (CV) tests at pH values between 12.6 and 13.8. The potential value E1,1st was derived by CV tests to define the pitting initiation condition. The effect of inhibitor addition on the critical chloride content was also assessed. The performances of two innovative organic-acid-based inhibitors were compared to that of sodium nitrite, which is the most used for chloride corrosion prevention in concrete; the parameter f* was proposed for comparison purposes; this parameter defines the efficiency of the substance compared to alkalinity to prevent pitting initiation.The results confirm the role of the chloride-alkalinity ratio in corrosion initiation; when the value of the ratio exceeds 0.6, the probability increases, confirming the literature data. The addition of nitrites causes an increase in the critical chloride threshold, as expected. The efficiency of sodium nitrite is similar to the effect of alkalinity; the value of the coefficient f* is equal to 0.84. Sodium aspartate exhibited the highest effect at pH values approaching 13.7; lower pH values resulted in lower efficiencies, nearly reaching zero at pH 12.6. Sodium lactate showed the lowest efficiency (f*=0.01).

Effect of phosphate based inhibitor on the threshold chloride to initiate steel corrosion in saturated hydroxide solution

In this paper, a phosphate-based inhibitor was used to increase the well-known critical ratio [Cl−]/[OH−] (R), inducing the localized corrosion of steel reinforcements in saturated Ca(OH)2 solution. For this purpose, electrochemical measurements and characterization methods (SEM, EDS, XRD, etc.) were first employed to identify the phosphate layer formation, and then to evaluate the inhibition effectiveness for the localized corrosion of steel reinforcements. The process of pitting corrosion is evaluated by cyclic polarization and is followed by acoustic emission.The experimental results show that electrochemical techniques can evaluate the pitting corrosion and reveal that the introduction of phosphate-based inhibitor increases strongly the critical ratio [Cl−]/[OH−] from which the depassivation and pitting corrosion can be initiated. This clearly confirms its positive effect on the localized corrosion resistance of steel reinforcements in saturated Ca(OH)2 solution. A perfect correlation with the evolution of the acoustic activity is also demonstrated. Thus, it was significantly possible to highlight the acoustic signature of depassivation damage and to chloride concentration.

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.

A model for a long-term diffusion of multispecies in concrete based on ion–cement-hydrate interaction

A model for simulating a long-term diffusion in concrete under submerged conditions has been developed. The model focuses on the interaction between cement hydrates and electrolyte solution adopting the physical properties of concrete as alternative parameters for estimating long-term diffusion. The model was verified by the large variety of long-term experimental data involving verification of cement hydrates, porosity properties, pH value, element distribution, and chloride penetration for sulphate-resistant Portland cement (SRPC) concretes. Evaluating impacts of hundreds of years’ exposure on concrete durability, a simulation was also performed for an exposure time of 500 years. The results confirmed the importance of ion–cement-hydrate interaction in the evaluation of a long-term diffusion of harmful substances such as chlorides into concrete. The simulation results show also that the solid phases of the SRPC concrete mix that was used are stable in the long-term. The threshold concentration of chlorides which may initiate reinforcement corrosion defined as Cl−/OH− ratio could be exceeded in concrete after moderate exposure period under the conditions investigated with typical protective concrete layers. Using of case-specific values for a threshold chloride content and evaluating the method used for estimating the initiation time of reinforcement corrosion are recommended in the structures with a long-design service life.

An approach for measurement of chloride threshold values

Chloride induced corrosion is one of the major causes for degradation of reinforced concrete structures. One of the most important factors in this process is the critical chloride content (chloride threshold value) in the vicinity of the reinforcement that causes initiation of corrosion. In this work the development of a practice-related approach that includes specimen shape, preconditioning, corrosion measurement techniques, and calculation concepts, for determining the critical chloride content are presented. The results in this study showed that the presented approach for determining chloride threshold values functioned fairly well, and suggestions for further improvement are proposed.

Analysis of Tools to Evaluate Chloride Threshold for Corrosion Onset of Reinforced Concrete in Tropical Marine Environment of Yucatán, México

The chloride threshold for corrosion onset of reinforced concrete depends on several factors and is only valid for the specific condition of each factor. The main objective of this paper was to analyze different tools to obtain the chloride threshold for corrosion onset of reinforced concrete in tropical marine environment. Sixty Portland cement concrete cylinders were exposed in a tropical marine environment. One half of the samples had a reinforcing bar embedded at the center of the sample (corrosion measurements), and the other half was made with plain concrete (chloride measurements). Five water/cement ratios were tested representing the common practices of this region. The corrosion rate was monitored using the polarization resistance technique, and the chloride content was determined using an ion selective electrode. The chloride threshold was in the range of 0.3% to 1.1% by weight of cement for specimens at 50 m from the seashore. Main conclusion indicates that iaccum is a better tool to determine and/or predict chloride threshold in this study according to a conservative point of view in civil engineering. Using this tool, chloride threshold was 0.26% to 0.73% by weight of cement.

Initial corrosion presented by chloride threshold penetration of concrete up to 10 year-results under marine site

This research studied the initial corrosion of reinforced concrete presented by the chloride threshold penetration depth of concrete exposed to marine environment. The results of an on-going study of fly ash concrete exposed to a marine environment for up to 10years were reported. Water-soluble chloride testing was conducted to measure the free chloride penetration in fly ash concretes that had been subjected to wet and dry cycles in a marine environment for 2, 3, 4, 5, 7, and 10years. The free chloride penetration and chloride threshold levels obtained from this testing led to the determination of the chloride threshold penetration depth and time to initial corrosion of marine concrete. The study found that the chloride threshold penetration depth increased at a high rate during the first 5years, after which the rate of increase declined. It seems that the water-to-binder (W/B) ratio has a greater effect on the reduction of the chloride threshold penetration depth in Portland cement concrete than in fly ash concretes. The use of fly ash at least 35% replacement in concrete with W/B ratio of 0.65 provides an initial corrosion resistance as good as the cement concrete with W/B ratio of 0.45. For concretes with the same cover depth and the same W/B ratio, a high content of fly ash (over 25% fly ash replacement) greatly prolongs the time to initial corrosion of the reinforcing steel in the concretes.

Effect of Nitrite Concentration and pH on Steel Corrosion Induced by Chloride in Simulated Concrete Pore Environment

The influence of nitrite concentration on the corrosion of steel immersed in three simulated pH environments that really exist on the surface of steel in concrete has been investigated by comparing and analyzing the change of threshold level of Cl- and NO2-/Cl- mole ratio. The results indicate that with the presence of nitrite, the corrosion of steel can be inhibited effectively. Chloride threshold level increases with the increasing of nitrite concentration and highly alkaline environment plays an important role in assisting nitrite to inhibit corrosion. The use of NO2-/Cl- mole ratio as the parameter to guarantee the inhibition efficiency is suitable in highly alkaline environment, but not appropriate in weakly alkaline and neutral environments.

An electrochemical investigation on the effect of the chloride content on CO2 corrosion of API-X100 steel

This research argues that the corrosion rate for high strength steel in CO2-saturated solutions reaches a maximum at a threshold chloride content. At 20°C, the corrosion rates increased from 60μA/cm2 in 5g/L solutions to a maximum of 185μA/cm2 in 15g/L solutions, and then became less in the more concentrated solutions. The anodic dissolution accelerated at 90°C and the cathodic reactions decelerated with adding 10vol.% oil where the chloride content effect remained relatively the same. Some EIS experiments demonstrated an agreement with the polarization results without confirming that threshold effect of chloride on charge transfer.

Probabilistic modeling of chloride-induced corrosion in concrete structures using first- and second-order reliability methods

Abstract Concrete structures are subjected to chloride-induced corrosion that can lead to shortened service life. Reliable predictions of life cycle performance of concrete structures are critical to the optimization of their life cycle design and maintenance to minimize their life cycle costs. This paper presents two simplified semi-analytical probabilistic models based on the first- and second-order reliability methods to model 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. A case study of a reinforced concrete highway bridge deck is used to illustrate the capability and efficiency of these simplified probabilistic models in modeling the uncertainty and predicting the time-dependent probability of corrosion. The models enable to quantify the impact of the different governing parameters on probability of corrosion and service life, which can be used to develop cost-effective management strategies.

Chloride threshold value for reinforcement corrosion in concrete with additions of silica fume or fly ash

The objective of this study is to evaluate the influences of silica fume and pulverised fly ash on the chloride threshold value for the corrosion of steel reinforcement in concrete. The chloride ions were introduced into concrete specimens made with ordinary Portland cement alone and a mixture of ordinary Portland cement and the partial replacements of cement with silica fume and pulverised fly ash at three levels by an immersing–drying cycle procedure. Half-cell potential and linear polarisation methods were applied to detect the initiation of pitting corrosion for the steel reinforcement. The threshold values were expressed as free chloride or total chloride by mass of binder, as well as [Cl−]/[OH−] and [Cl−]/[H+]. It has been found that the changes of the chloride threshold value with the additions of pulverised fly ash and silica fume intensively depend on the representations. Especially, the critical [Cl−]/[H+], regardless of the type and content of mineral admixtures, approximately keeps constant at 0·02. Accordingly, [Cl−]/[H+] is suggested as a more suitable form to represent the chloride threshold value.

Steel corrosion resistance in model solutions, containing waste materials

This work reports on the corrosion resistance of low-carbon steel in alkaline model solutions, i.e. “red mud” (RM) containing extracts of Ordinary Portland Cement (OPC) and Blast Furnace Slag (BFS) cement. The objective was to define the steel electrochemical behavior and the properties (morphology and composition) of the product layer that forms on the steel surface within treatment in chloride-free and chloride-containing model solutions. The motivation for this research is related to several aspects, mainly to exploring the possibility for achieving superior corrosion performance of steel in cement-based systems by using (combination of) waste materials. The study reveals that RM has a distinctive positive effect on the corrosion performance of steel, especially in chloride containing environment. The presence of RM in both OPC and BFS solutions (with and without NaCl) favors steel passivation and results in an increased corrosion resistance. The positive effect of RM is especially pronounced for steel, exposed to BFS solutions. The observed behavior is attributed to the presence of Fe 2O 3 and Al 2O 3 (from RM and BFS). The presence of Fe 3+ stabilizes the passive layer. Additionally, the adhered red mud particles act as adsorption sites for Ca 2+, resulting in a product layer with higher corrosion resistance. Further, the RM particles with regard to their Al 2O 3 content enhance steel passivity and increase the chloride threshold for corrosion initiation and/or propagation.

Stochastic Modeling of Service Life of Concrete Structures in Chloride-Laden Environments

Chloride-induced rebar corrosion is a common degradation process for concrete infrastructure, which is a practical concern for cold-climate states and coastal areas. In this work, numeric models based on the FEM are utilized to study service life of concrete structures subject to chloride ingress, where two models are utilized. The first one deals with multiple ionic species in the concrete pore solution, while the second one only accounts for the presence of chloride ions. The stochastic nature of model inputs is taken into consideration because each factor of interest is subject to random variabilities and inherent uncertainties. Specifically, the surface chloride concentrations and concrete cover depth follow the normal distribution; the diffusion coefficients obey the gamma distribution; the actual chloride threshold features the triangular distribution. The nonlinear partial differential equations (PDEs) to characterize the spatial and temporal evolution of ionic species are numerically solved, the results of which were utilized to elucidate the influence of various factors on concrete service life, such as mix design, surface chloride concentrations, cracking level, and coarse aggregate and concrete cover depth.

Influence of Mineral Admixtures on Chloride Threshold Value for the Corrosion of Steel Reinforcement

The influence of mineral admixtures on chloride threshold value was investigated by EIS method, MIP test and TG test, respectively. The experimental results showed that the chloride threshold value decreased with increasing the fly ash and slag content in composite cementitious. The increasing of fly ash and slag replacement resulted in the decreasing of porosity, average pore size and Ca(OH)2content. The effect of mineral admixtures on the chloride threshold value was the comprehensive results of concrete pore structure and Ca(OH)2content. The chloride threshold value decreased with decreasing the Ca(OH)2content in concrete, and there was an approximate linear relationship between the logarithm of chloride threshold value and the logarithm of CCa(OH)2content when the specimens exposed in tidal zone. The specimen exposed in tidal zone has a higher chloride threshold value than that exposed in splash zone was due to the lower dissolved oxygen around steels in concrete.