Chloride Threshold Research Articles

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

Modified hydrotalcites for improved corrosion protection of reinforcing steel in concrete – preparation, characterization, and assessment in alkaline chloride solution

Six MgAl hydrotalcites (Mg/Al = 2 and 3) were synthesized through the modification of two carbonate hydrotalcites (i.e., Mg(2)AlCO3 and Mg(3)AlCO3) by two amino acids (11aminoundecanoic acid and paminobenzoic acid) and sodium nitrite. They were characterized by means of XRD, FTIR, TG/DSC, and elemental analysis. The ion exchange of the modified hydrotalcites (MHTs) were investigated in alkaline chloride solution and the results showed that ion exchange occurred between free chloride ions and the intercalated inhibitive anions in MHT. The corrosion inhibition of three MHTs (Mg/Al = 2) was evaluated based on the open circuit potential and linear polarization resistance evolution of steel coupons in 0.1 M NaOH solution stepwise enriched with chlorides and the results showed that MHT, in particular, Mg(2)AlpAB exhibited an improved corrosion inhibiting effect in terms of a higher chloride threshold compared to its constituent free inhibitor ions (i.e., pAB). This study confirms the dual protection that MHT, in particular, Mg(2)AlpAB, offers to the steel: capturing chlorides and simultaneously releasing the intercalated inhibitors to further protect the steel from corrosion.

Influence of flexural fatigue on chloride threshold value for the corrosion of steels in Ca(OH)2 solutions

The flexural fatigue was enforced on reinforced concrete beam with stress level of 0.6 and different fatigue life cycles. Steels removed from the beams were soaked in the saturated Ca(OH)2 solution, which was used as a simulated concrete pore solution. The NaCl solution was chosen as the source of chloride ions. The Chloride Threshold Values (CTV) were detected by combining the open-circuit potentials (Ecorr) with the corrosion current densities (icorr), which were obtained by electrochemical impedance spectroscopy (EIS). The changes of microstructure caused by the flexural fatigue were observed by scanning electron microscopy (SEM). The results showed that as the fatigue cycle times increased, the CTV decreased under a certain stress level and range of fatigue life cycles. The grains became finer and cracks appeared on the surface of the steels. While the capacitive arcs under no flexural fatigue decreased gradually with the addition of chloride ions, the ones under flexural fatigue presented no regularity. Cracks at the surface were expanded because of sustaining flexural fatigue, which degenerated the later resistance to chloride ions of the steels.

Effectiveness of inhibitors in increasing chloride threshold value for steel corrosion

This investigation was aimed at evaluating the effectiveness of corrosion inhibitors in increasing the chloride threshold value for steel corrosion. Three types of corrosion inhibitors, calcium nitrite (Ca(NO2)2), zinc oxide (ZnO), and N,N′-dimethylaminoethanol (DMEA), which respectively represented the anodic inhibitor, cathodic inhibitor, and mixed inhibitor, were chosen. The experiment was carried out in a saturated calcium hydroxide (Ca(OH)2) solution to simulate the electrolytic environment of concrete. The inhibitors were initially mixed at different levels, and then chloride ions were gradually added into the solution in several steps. The open-circuit potential (Ecorr) and corrosion current density (Icorr) determined by electrochemical impedance spectra (EIS) were used to identify the initiation of active corrosion, thereby determining the chloride threshold value. It was found that although all the inhibitors were effective in decreasing the corrosion rate of steel reinforcement, they had a marginal effect on increasing the chloride threshold value.

Retracted: Stress corrosion cracking of high strength stainless steels for use as strand in prestressed marine environment concrete construction

Stainless steel reinforcement has an elevated chloride threshold and is an attractive alternative for prestressed concrete applications. However, due to the high strength requirements for prestressing strands, concern exists about the possibility of stress corrosion cracking (SCC). A possible candidate high strength alloy for use as prestressed strands in a Florida marine environment is UNS# S24000. Previous testing methods used on other stainless steel alloys were investigated to determine the best way to evaluate the UNS# S24000 as compared to other alloys. The alloys tested were a common austenitic stainless steel with high nickel content UNS # S31603, UNS# S24000, and a duplex stainless steel with high chromium and an additional constituent, molybdenum UNS# S32205. Prior work on high strength stainless steels have tested various alloys of austenitic steels or austenitic and duplex steels, but did not include UNS# S24000. The alloys examined were evaluated at various temperatures in magnesium chloride solutions and in a simulated concrete pore water solution at 60 °C, followed by an anodic polarization regime as an alternative test acceleration method. The results suggest that duplex high‐strength stainless UNS# S32205 performed overall better than the other two alloys. The UNS# S24000 still performed well in simulated concrete pore water and chloride solution tests, and performed better that UNS#S32205 at high temperatures in the tests in the magnesium chloride solution.

The challenge of the performance-based approach for the design of reinforced concrete structures in chloride bearing environment

The performance-based approach, published by the International Federation for Structural Concrete (fib), was applied for the design of a RC element in a marine environment, with corrosion resistant reinforcement, to analyse the potentiality of the model as well as the possible reasons which limit its use. Results showed that the fib model allows to compare different solutions and to consider the benefits connected with the use of preventative measures. However the definition of reliable values for some input parameters, as the critical chloride threshold for corrosion resistant reinforcement, is demanded to the designer and this aspect clearly limits a widespread use.

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.

Evaluation of corrosion resistance of coated steel strips embedded in mortar under chloride exposure

Purpose – The purpose of this paper was mainly to select one of the three types of coatings for protection of steel used as reinforcement in composite pipes (thin steel shell covered by cement-mortar) subjected to chloride exposure. To achieve this target, an attempt was made to develop a simple methodology for evaluating the performance of corrosion protection measures in terms of chloride threshold level (CTL) and corrosion initiation time (TI). Design/methodology/approach – Bare, epoxy, red oxide and zinc primer-coated steel strips were embedded in cement mortar with sand/cement and water/cement ratios of 2 and 0.5 (by mass), respectively, to prepare the specimens which were exposed to chloride solution having a high concentration of 10 per cent NaCl. For determining the amounts of the water-soluble chloride diffused inside the specimens, powdered samples of mortar were collected from two different depths from the exposed surface of specimens on completion of each of the four different exposure times. The corrosion current densities were determined at two different stages. A step-by-step procedure for calculating CTL and TI using the measured chloride contents and corrosion current densities was established with the help of relevant information available in the literature. Findings – Based on the comparison of the values of CTL and TI calculated for bare steel and steel with all three types of coatings, utilizing the experimental data and the proposed calculation procedure, the epoxy-coated steel was found to have the best performance. Originality/value – This research has resulted into development of a simple methodology for evaluation of the performance of protective measures against corrosion of steel embedded in mortar or concrete exposed to chloride-bearing environment.

On the Durability of Sealable Circular Concrete Structures under Chloride Environment

Reinforcement corrosion resulting from chloride attack is one major mechanism that compromises concrete durability. Numerical methods were commonly used for tackling Fick’s diffusion equations. In this paper, we developed a Crank-Nicolson based finite difference scheme suitable for circular concrete structures. Both a time-dependent surface chloride model and diffusivity were considered. The impact of an ideal sealer on chloride redistribution was further investigated. Results suggested that the chloride threshold depth in a concrete structure is greatly affected by the radius of curvature, environment severity, and diffusivity. For sealable concrete structures, both the sealer application timing and location are of great importance.

Some Open Issues in Forecasting Corrosion Risk of Steel in Concrete

Useful corrosion damage forecast and risk evaluation methods for steel in concrete were developed starting about three decades ago and are now being used by increasing numbers of engineers. Much room for improvement remains, however, and the following non-exhaustive selection of issues merit special attention in the next generation of predictive models: incorporation of chloride binding in predicting chloride ion penetration to avoid overly conservative design, resolution by additional mechanistic understanding, and a wider database of uncertainty in the long-term evolution of chloride diffusivity in aged concrete, consideration of geometric chloride transport obstructions at the rebar level, and integration of initiation and propagation stage modeling, in particular accounting for the dependence of the chloride threshold on steel potential. Each feature is discussed with attention to the conservative or non-conservative result of its eventual incorporation, and with indication of practical means for its ...

Influence of compression fatigue on chloride threshold value for the corrosion of steels in simulated concrete pore

Compression fatigue test was performed on stress level 0.8 for a group of fixed cycle numbers. The steels were soaked in the saturated Ca(OH)2 solution, which was used as simulated concrete pore solution. NaCl was chosen as the sources of chloride ion to corrode the steels. The chloride threshold value (CTV) was detected by combining the open-circuit potential (Ecorr) with corrosion current density (Icorr), which was obtained by electrochemical impedance spectroscopy (EIS). The changes of microstructure that caused by the compression fatigue were observed by optical microscopy (OM) and scanning electron microscopy (SEM). The results showed that the CTV first increased and then decreased with increasing the fatigue cycle times under a certain stress level and range of fatigue life cycles. The grains of the steels became finer and cracks appeared on the protective firms which formed quickly on the surface of the steels. The capacitive arcs under no compression fatigue decreased gradually, while the ones under compression fatigue presented no regularity due to the uneven distribution of the protective films. Cracks on the protective films propagated because of sustaining compression fatigue, which degenerated the later resistances of the steels to chloride ions. Meanwhile, the ranges of corrosion currents were declined.

Laboratory investigation to study the corrosion initiation of rebars in fly ash concrete

The objective of this study is to detect reliably the onset of corrosion in fly ash concrete using an embedded reference electrode. The half-cell potential of rebars in fly ash concrete subjected to 10% sodium chloride solution is monitored using an embedded reference electrode and an alternating current impedance spectroscope, along with other impedance parameters such as polarisation resistance and specific concrete resistance. At initiation of corrosion, the values of chloride threshold levels are obtained and the effect of long curing for fly ash concrete on chloride threshold values is analysed. The combination of two corrosion monitoring systems allows the estimation of corrosion initiation in fly ash concrete. It is found that chloride threshold values for fly ash concrete given a longer curing period approach the values for Portland cement concrete. Probabilistic analysis of the experimental data is carried out to find a best-fit probability distribution for time to initiate corrosion and chloride threshold values of fly ash concrete.

Service Life Forecasting for Reinforced Concrete Incorporating Potential-Dependent Chloride Threshold

The chloride corrosion threshold of steel in concrete depends not only on concrete and steel properties but also on the potential of the steel while it is still in the passive condition. An innovative model to predict corrosion in reinforced concrete structures is presented, incorporating potential dependence and corrosion macrocell effects. The novel approach integrates the initiation and the propagation stages of corrosion in a single predictive model. The model is applied to a generic, partially submerged, reinforced concrete marine pile. The purpose of this paper was to establish and illustrate the concept of a functional approach to implement potential-dependent chloride threshold in corrosion damage projections, plus articulation of the initiation and propagation stages, in one predictive model. The long-term damage projection results are markedly lower contrasted with those of an identical system with a time-invariant chloride threshold. The results show that implementing chloride threshold depende...

Corrosion resistance of galvanized steel reinforcements in carbonated concrete: effect of wet–dry cycles in tap water and in chloride solution on the passivating layer

The effect of wet–dry cycles in tap water and in 3.5% NaCl solution on the passivating products of galvanized steel after concrete carbonation was investigated. The results show that carbonation destroys the passivating layer of calcium hydroxyzincate with the formation of the amorphous products ZnCO3 and Zn5(CO3)2(OH)6, which are also protective. The exposure to wet–dry cycles in tap water after concrete carbonation does not significantly increase the corrosion rate of the galvanized steel with respect to the values measured in non-carbonated concrete. On the contrary, the exposure to chloride solution after concrete carbonation leads to a marked decrease in the chloride threshold for galvanized steel corrosion with respect to that found in non-carbonated concrete.

Threshold Chloride Levels for Localized Carbon Steel Corrosion in Simulated Concrete Pore Solutions Using Coupled Multielectrode Array Sensors

Threshold chloride levels for localized corrosion of carbon steel material have been studied in three types of simulated concrete pore solutions: sodium hydroxide-potassium hydroxide (NaOH-KOH) with pH 11.6, calcium hydroxide (Ca[OH]2) with pH 12.6, and sodium hydroxide-potassium hydroxide-calcium hydroxide (NaOH-KOH-Ca[OH]2) with pH 13.3. The nonuniform corrosion rates of carbon steel were measured with coupled multielectrode array sensors (CMAS) when the chloride concentration was changed from 0.0005 mol/L to 1 mol/L in each solution. Open-circuit potentials were also measured from the coupling joint of the CMAS probes and electrodes made of rebar specimens immersed in the simulated pore solutions to verify the results from the CMAS probes.

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.

The influence of reinforcement steel surface condition on initiation of chloride induced corrosion

This paper describes a part of the work in the development of a “standard” test method for determining chloride threshold values required to initiate corrosion on reinforcement in concrete. The prerequisites of the test set-up are that the test conditions should be reasonably comparable to those in service and the test method should be fairly reproducible and as rapid as possible concerning the slow diffusion nature of the investigated phenomenon. This paper presents the results from a study on the influence of steel bar surface condition on chloride induced corrosion. Various electrochemical techniques were employed in the study to monitor the corrosion behaviour of the embedded bars with three different surface conditions. It is shown that the steel surface condition has a strong effect on the corrosion initiation of reinforcement in concrete, and can likely be the most decisive parameter attributing to the variability in the reported chloride threshold values.

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.

Analysis of chloride threshold from laboratory and field experiments in marine atmosphere zone

Studies focusing on chloride threshold analysis based on field experiments are still scarce. Although taking longer than laboratory experiments, they can help on setting chloride thresholds closer to realistic conditions. The present work analyses results from reinforced concrete specimens exposed in a marine atmosphere zone of the northeast coast of Brazil and also from specimens subjected to wetting and drying cycles in sodium chloride solution. After the process of corrosion had just been initiated, the concrete specimens were broken and the total and free chloride contents at the rebar level were measured and assumed as the chloride thresholds for starting corrosion. Results show that average total chloride threshold varies in a range between 1.82 and 2.45 (% cement weight) and between 0.88 and 1.58 (% cement weight) for laboratory and field exposure experiments, respectively. Environmental interaction contributes for these differences. On the other hand, a good correlation could be obtained between both experimental conditions, helping on transporting laboratory results to field conditions.

Corrosion Resistance of Calcium Aluminate Cement Concrete Exposed to a Chloride Environment.

The present study concerns a development of calcium aluminate cement (CAC) concrete to enhance the durability against an externally chemically aggressive environment, in particular, chloride-induced corrosion. To evaluate the inhibition effect and concrete properties, CAC was partially mixed with ordinary Portland cement (OPC), ranging from 5% to 15%, as a binder. As a result, it was found that an increase in the CAC in binder resulted in a dramatic decrease in the setting time of fresh concrete. However, the compressive strength was lower, ranging about 20 MPa, while OPC indicated about 30–35 MPa at an equivalent age. When it comes to chloride transport, there was only marginal variation in the diffusivity of chloride ions. The corrosion resistance of CAC mixture was significantly enhanced: its chloride threshold level for corrosion initiation exceeded 3.0% by weight of binder, whilst OPC and CAC concrete indicated about 0.5%–1.0%.