Chloride-induced Depassivation Research Articles

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.

Chloride-induced depassivation and corrosion of mild steel in magnesium potassium phosphate cement

This study investigated the chloride-induced depassivation and corrosion of mild steel in the magnesium potassium phosphate cement (MKPC) pore solutions at different magnesia-to-phosphate (M/P) ratios via several electrochemical methods and surface characterisations. Results showed that although the pH of MKPC is much lower than that of PC, the corrosion resistance of mild steel is significantly higher, with critical chloride values in MKPC being several orders higher than that in PC. Chloride-induced corrosion resistance increases with increasing M/P ratios. Whilst both pH and HPO42-/PO43- ions influence the formation of corrosion products, the pH plays a dominant role in the corrosion resistance.

Passivation and Chloride-Induced Depassivation of Additively Manufactured Duplex Stainless Steel Clads in Simulated Concrete Pore Solution

AbstractThe metallurgical and electrochemical behavior of additively manufactured super duplex stainless steel (SDSS) clads on carbon steel [low-carbon steel (LCS)] substrates are investigated in a...

Role of red mud in natural passivation and chloride-induced depassivation of reinforcing steels in alkaline concrete pore solutions

The influence of red mud (RM) on electrochemical behaviour, chemical composition, surface morphology and thickness of passive films naturally formed on low-carbon steel and 2304 stainless steel in alkaline concrete pore solutions was investigated using electrochemical measurements, XPS, AFM and SEM/EDS. Furthermore, chloride-induced pitting corrosion of passivated steels was characterized with and without RM. Results show that RM was detrimental to the passivity of steels by forming defective passive films with reduced thickness. Despite the formation of a less protective passive film on low-carbon steel, beneficial role of RM was highlighted in preventing chloride-induced depassivation and corrosion propagation.

Thermodynamic feasibility of the four-stage chloride-induced depassivation mechanism of iron

The atomistic mechanism of chloride-induced depassivation of iron is still debated. A recent study suggests a four-stage depassivation mechanism, in general agreement with the point defect model. The proposed four-stage mechanism is based on reactive force field molecular dynamics simulations and is rather complex but here we use density functional theory to confirm the thermodynamic feasibility of the proposed mechanism. We find that the four surface species, formed in the four stages, have decreasing surface stability, which is consistent with the order of species formed in the depassivation process proposed in the reactive force field molecular dynamics study. The Fe vacancy formation energy, that is the energy needed to form a surface Fe vacancy by removing different surface species, indicates that surface species with more chlorides dissolve more easily from the surface, suggesting that chloride acts as catalyst in the iron dissolution process. The results are consistent with the suggested four-stage reaction mechanism and the point defect model.

The Effect of Surface Defects on Chloride-Induced Depassivation of Iron—A Density Functional Theory Study

Chloride-induced depassivation is a large contributor to the degradation of metals, but defects are likely to play a key role in that process. Here density functional theory calculations are used to investigate the mechanism of the initial stages of chloride-induced depassivation of iron by studying the Cl interactions with stepped α-Fe2O3 (0001) surfaces and how that can lead to degradation of the passive oxide film. The low coordinated Fe sites near the step edge and O vacancies facilitate high local coverages of adsorbed Cl, which enhance surface Fe vacancy formation significantly. The step edge also lowers the Cl insertion energy, relative to the flat surface, but insertion by exchange with O is still endothermic. This study illustrates the importance of surface defects, step edges, and O vacancies in the depassivation mechanism, but the findings generally support the point defect model as a description of the depassivation mechanism.

Investigation of chloride-induced depassivation of iron in alkaline media by reactive force field molecular dynamics

The passivity of iron in alkaline media enables the use of carbon steel as reinforcement in concrete, which makes up the majority of modern infrastructure. However, chlorides, mainly from deicing chemicals or marine salts, can break down the iron passive film and cause active corrosion. Despite recent advances in nanoscale characterization of iron passivity, significant gaps exist in our understanding of the dynamic processes that lead to the chloride-induced breakdown of passive films. In this study, chloride-induced depassivation of iron in pH 13.5 NaOH solution is studied using reactive force field molecular dynamics. The depassivation process initiates by local acidification of the electrolyte near the film surface, followed by iron dissolution into the electrolyte, and iron vacancy formation in the passive film. Chlorides do not penetrate the passive film, but mainly act as a catalyst for the formation of iron vacancies, which diffuse toward the metal/oxide interface, suggesting a depassivation mechanism consistent with the point-defect model.

Reactive Molecular Dynamics Modeling of Chloride-Induced Depassivation of Iron Passive Film in Alkaline Media

Chloride-induced depassivation of iron passive film in 13.5 pH NaOH solution was studied using Reactive Force Field Molecular Dynamics (ReaxFF-MD). The study involved the simulation of the passive oxide film formed in chloride-free 13.5 pH NaOH solution and the effect of incremental chloride addition to the solution on the oxide film. The interactions of chloride with the oxide film and the changes in the oxide film structure were investigated by monitoring different species, charge distributions, and the pair distribution functions over time. The relationship between different surface termination of the passive film (i.e., O-terminated surface vs. Fe-terminated surface) and chloride interactions was also studied. The results of the investigation were linked to existing depassivation models (e.g, the ion exchange model and the point defect model) and to a parallel Density Functional Theory (DFT) investigation of iron oxide and chloride interactions in water.

Chloride-induced corrosion of steel rebars in simulated pore solutions of alkali-activated concretes

The passivation and chloride-induced depassivation of steel rebars immersed in varying alkaline environments (0.80M, 1.12M and 1.36M NaOH solutions), simulating the pore solutions of low-Ca alkali-activated concretes, were investigated using a range of electrochemical techniques. The passive film on the steel rebars was complex in chemical makeup, composed of Fe–hydroxides, oxy-hydroxides and oxides. An increased degree of passivation of the rebars was observed when exposed to solutions with higher hydroxide concentrations. The critical chloride level ([Cl−]/[OH−] ratio) required to induce depassivation of steel was strongly dependent on the alkalinity of the pore solution, and was found to be 0.90, 1.70 and 2.40 for 0.80M, 1.12M and 1.36M NaOH solutions, respectively. These values all correspond to a constant value of [Cl−]/[OH−]3=1.25, which is a novel relationship to predict the onset of pitting, interlinking chloride concentration and the solubility of the passive film.

Kinetics of Passivation and Chloride-Induced Depassivation of Iron in Simulated Concrete Pore Solutions Using Electrochemical Quartz Crystal Nanobalance

Kinetics of passivity and chloride-induced depassivation of iron exposed to simulated concrete pore solutions were studied using electrochemical quartz crystal nanobalance (EQCN), electrochemical impedance spectroscopy (EIS), and open circuit potential (OCP) monitoring. Passivation followed a two-stage logarithmic film formation process: protective film mostly formed within the first 10 min to 20 min of exposure to the passivating solutions as indicated by a sharp mass increase accompanied by impedance and phase angle data showing trends toward passivation. After this initial passivation period, mass continued to increase, albeit at a significantly slower rate. Electrochemical indicators during this period remained relatively constant and stable, suggesting that the iron remained passive. The mass increase during the post-passivation period was indicative of the formation of additional oxides, while relative stability of the OCP, impedance and phase angle measurements suggested that these oxides were like...

Semiconductive Properties of Passive Films on Carbon Steel Rebar in Highly Alkaline Environments

In the highly alkaline environment of concrete, carbon steel rebar is protected against corrosion by a passive oxide/oxyhydroxide film. Understanding the characteristics of this passive film, and how it depassivates, is the key to mitigate problems associated with steel corrosion in concrete. Although a large number of electrochemical and analytical studies have been conducted on the passivity of steel in concrete, current mechanistic models do not explain all experimental observations about the chloride-induced depassivation process. One area that is not studied extensively is the electronic properties of passive films which show semi-conductive properties; a better understanding of these properties will provide additional information to improve the mechanistic models for chloride-induced depassivation of carbon steel in concrete. The present study uses Mott-Schottky (M-S) analysis along with other electrochemical techniques to study the electronic properties of passive films that form on carbon steel exposed to simulated concrete pore solutions. Both passivation and chloride-induced depassivation processes are investigated, and changes in electronic properties during these processes are monitored. The main parameters of the study include the concrete pore solution composition, pH, and chloride amount. Challenges of using M-S analysis to study the electronic properties of passive films will also be discussed.

Nano-scale study of passive films and chloride-induced depassivation of carbon steel rebar in simulated concrete pore solutions using FIB/TEM

Analytical TEM techniques were used to characterize passive film on carbon steel and how it breaks down with chloride addition. Before chloride addition, the thicknesses of the oxide films on rebars were found to be uniform between 5 and 13nm. The films consisted mainly of iron oxides that included nano-crystalites of FeOOH with an epitaxial relationship to the metal surface. Elements from the passivating solutions were located in regions of the oxide closest to the free surface. With the addition of chloride at concentrations below depassivation thresholds, there was evidence for change in the oxide closest to the free surface, but generally the oxides were similar to those grown before chloride addition. After exposure to chloride concentrations above depassivation thresholds, the oxide films were no longer uniform, with parts of the surface bare, and the average oxide thickness reduced. Pitting and enhanced corrosion were seen to accompany cementite lamella.

Angle-resolved XPS study of carbon steel passivity and chloride-induced depassivation in simulated concrete pore solution

The passivity and chloride-induced depassivation of carbon steel in simulated concrete pore (CP) solution (pH=13.3) was investigated using angle-resolved X-ray photoelectron spectroscopy (XPS). After full passivation, the thickness of the oxide film on steel specimens was approximately 5nm. The oxide films close to the steel substrate were mainly composed of protective Fe(II) oxides while near the free surface the film consisted mostly of Fe(III) oxides. The addition of chloride to the CP solution decreased the thickness of the oxide film and changed its stoichiometry such that near the film/substrate interface the Fe(II)/Fe(III) decreased.

Microscopic investigation of mill scale and its proposed effect on the variability of chloride-induced depassivation of carbon steel rebar

Electrochemical experiments have shown that mill scale on carbon steel rebar affects corrosion initiation and variability associated with chloride-induced depassivation thresholds. In this study, scanning electron microscopy and focused ion beam microscopy were used to investigate the properties of mill scale and its effect on chloride thresholds of the underlying steel. The variability found in the literature for chloride thresholds is suggested to result, in part, from the variability in the physical and chemical properties of mill scale. A crevice corrosion mechanism is proposed to explain why as-received rebar is more susceptible to depassivation than rebar without mill scale.

Electrochemical investigation of chloride-induced depassivation of black steel rebar under simulated service conditions

Electrochemical measurements of chloride thresholds are reported in simulated concrete pore solution for as-received and surface-modified rebar in an experimental apparatus designed to simulate service conditions. Surface modification led to higher chloride thresholds and reduced variability. The variability in thresholds for as-received rebar was represented by a log-normal distribution; therefore, simple averages of chloride thresholds, without reference to underlying distribution, might not provide reliable indicators of depassivation. The relative constancy of electrochemical measurements below thresholds, and the dependence of the thresholds on surface conditions, suggests that rebar depassivation is likely caused by local critical chemical conditions at the steel surface.

A study on chloride-induced depassivation of mild steel in simulated concrete pore solution

The effect of chloride ions on passivity breakdown of steel in simulated concrete pore (SCP) solution was studied using electrochemical techniques. In this regards, the sensitivity of cyclic potentiodynamic parameters such as Δ E (difference between E pit and E rep), i peak and AC-impedance parameters like R ct, R f, C i, R ad and C ad to chloride ion concentration was investigated. Adsorption of OH − ions on the metal surface in free chloride SCP solution and also displacement of those ions by Cl − ions were demonstrated in high frequency part of Nyquist plots. In addition, a severe decrease in resistance of interfacial reaction ( R f+ R ct) was observed through breakdown of passive film in the [Cl −]/[OH −] ratio of 0.6. The interfacial capacitance, C i, was abruptly raised when localized corrosion changed to general one.