Depassivation Process Research Articles

Corrosion protection performance of stretched graphene-modified cold-sprayed zinc coating for steel rebar

The cold-sprayed zinc technology exhibits unique advantages in the field of metal corrosion prevention due to its environmental friendliness and convenient construction. However, compared to hot-dip galvanizing, there is still a necessity to improve its corrosion resistance. Graphene as a filler material, effectively strengthens the anticorrosive properties of the coating. Nevertheless, the resistance of graphene-modified cold-sprayed zinc coating to chloride ion induced metal corrosion remains unknown. In this study, cold-sprayed zinc coating and graphene-modified zinc coating were employed to steel rebar that was subjected to different levels of tensile stress. The passivation and de-passivation process of the stretched zinc-coated rebar in the simulated concrete pore solution were investigated with electrochemical tests and microscopic analyses. The research findings reveal that compared with ordinary cold-sprayed zinc coating, graphene-modified coating can significantly improve the coating resistance and reduce the corrosion rate of rebar under different levels of tensile stress attributed to the high surface area of graphene. However, due to the discontinuity between zinc powders induced by graphene, a complete passivation film cannot form on the surface of graphene-modified zinc coating, which resulted in a lower resistance against chloride ion induced de-passivation. This study indicates that graphene-modified cold-sprayed zinc-coating is not fit for marine environment.

Computational chemistry analysis of passive layer formation and breakdown mechanisms in ferritic stainless steels

Despite extensive research and proposed theories on formation and breakdown of passive layers, several questions remain unanswered. These include the reasons behind the bi-layer nature of the passive layer, the decrease in hydrogen and oxygen diffusivity upon entering the passive layer, and the influence of microstructure on passive layer formation and function in stainless steels. In this study, we employed ReaxFF molecular dynamics simulations to investigate passivation and depassivation of ferritic stainless steels in a polycrystalline structure. Through static and dynamic calculations, we elucidated the underlying mechanisms of passive layer formation, which were primarily governed by clustering. Our analysis also highlighted the significant role of hydrogen diffusion and its reaction with metallic compounds in the depassivation process. We have identified several physical phenomena involved in the processes of passivation and depassivation, which can provide explanations for the questions posed above.

Nanoscale paraffin layer fabricated using spin coating technique for on-demand removable passivation

Passivation is an essential process in the research and application of reactive and air-sensitive materials, and there has been an increasing demand to develop materials for stable but readily removable temporary passivation layers. Paraffin coating has been suggested as a possible candidate for such applications, which, however, further requires the demonstration of thin paraffin coating with complete surface coverage as well as effective depassivation performance. In this study, we demonstrate on-demand removable passivation using nanoscale paraffin layers fabricated using a spin-coating technique. Based on the Emslie Bonner and Peck's and Mark-Houwink-Sakurada (MHS) equations, eicosane and hexane were selected as the solute and solvent, respectively, to ensure low viscosity for the fabrication of nanoscale paraffin layers, facilitating depassivation process under a vacuum. Optimizing the solution concentration and spin speed generates nanoscale paraffin layers with complete surface coverage, which is confirmed through optical microscopic images and subsequent image processing using Python. The simple removal of the nanoscale paraffin layer through vacuum treatment allows us to revisit a passivated surface underneath, and subsequent contact resistance and x-ray photoelectron spectroscopy measurements evince the suppressed surface oxidation of paraffin-coated silicon samples. These results further attest to the applicability of paraffin coating as an on-demand removable passivation technique.

Accelerated test device and method for critical chloride concentration initiating steel depassivation in cement-based materials

Abstract In order to overcome the disadvantages of traditional test devices and methods, new accelerated test device and efficient test method for critical chloride concentration (Ccrit) initiating steel depassivation in cement-based materials were developed. Based on the theory of electromigration, a new accelerated test device for Ccrit was invented first by combining with the chloride electromigration system and electrochemical test system. Then the influences of applied voltage and continuous power-on time on electrochemical parameters including open-circuit potential, polarization resistance, and corrosion current density during the steel depassivation process were investigated. Moreover, an efficient test method for Ccrit was proposed based on the quantitative criterion in terms of corrosion current density. Analysis results show that Ccrit can be measured quickly and accurately based on the accelerated test device and method by selecting appropriate applied voltage (e.g., 3–6 V) and continuous power-on time (e.g., 48–72 h).

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.

Breakdown Voltage Instability Mechanism on the Field Limiting Rings Edge Termination of Buried Layer Rectifier

In this article, an innovated buried layer rectifier (BLR) breakdown voltage (BV) instability has been tested and analyzed. Under the reverse current stress (RCS) condition, the BV read-out profiles show walk-in and walk-out phenomena. Based on the experimental results and surface traps theory, a sort of time-dependent degraded simulation is developed by TCAD tools, which is considered with hydrogen depassivation and diffusion process. In order to explore the inner mechanism, the dynamic electric parameters are analyzed during the degraded period. Due to the hole interface traps, the termination Si-SiO2 surface leakage current is deteriorated and the depletion boundary is shrunk, which reveal the BV values instability. Thus, the proposed physical model could clearly explain the whole process of degradation, while it fits BV and junction temperature curves to the experimental results identically. Furthermore, the investigated mechanism will be helpful to future device design and failure situations analysis.

Atomic scale insight into the mechanisms of chloride induced steel corrosion in concrete

Chloride (Cl) induced steel corrosion is a major cause of durability issues of reinforced concrete (RC) structures and causes a large economic loss every year. Understanding the mechanisms of Cl induced steel corrosion in concrete is of great importance to develop anti-corrosion methods and further lowers down the repair and strengthening frequency and cost. Hence, the nature of steel depassivation and corrosion induced by Cl at atomic scale was revealed through the density functional theory (DFT) calculations to better understand the Cl induced corrosion mechanism. The results indicated that Cl species weakened the bonding of oxygen (O) with outer iron (Fe) atoms while enhanced the hybridization of O with inner Fe atoms, thus induced the breakdown of passive films. Besides, Cl facilitated the charge loss of outer Fe atoms and improved the interactions of water molecular (H2O) with these Fe atoms. When co-adsorption with O atom, hydroxyl (OH) generated by dissociative adsorption of H2O enhanced the charge loss of outer Fe atoms. Then OH strongly bonded with the neighboring Fe atoms to form the initial corrosion products, iron hydroxides (Fe-OH). The above DFT calculations are consistent with the experimental results about the steel depassivation and corrosion processes in concrete and hopefully these results can guide the anti-corrosion design of RC structures.

SERVICE LIFE PREDICTION’S ALGORITHM: LOADING, CARBONIZATION, CHLORIDE AGRESSION

Corrosion reinforcement marine hydraulic structures due to chloride aggression and carbonization of concrete leads to a sharp decrease in thesafety of the structure. The steel reinforcement will be subjected to a so-called depassivation process, once the chloride concentration on surface exceedsa certain threshold concentration, or the pH value in the protective layer of concrete decreases to a threshold value due to carbonation. Electrochemicalreactions begin to occur with the formation of corrosion products with the penetration of oxygen on the steel reinforcement surface. This leadsto cracking of the protective layer of concrete. It should also be taken into account that, due to corrosion mechanisms, the cross-sectional area of thereinforcement also decreases. The article suggests a method for predicting the complex degradation of reinforced concrete structures, taking into accountvarious mechanisms of corrosion wear, which will allow developing effective ways to improve the durability and maintainability of structures operatedin the marine environment.

Correlation between depassivation and repassivation processes determined by single particle impingement: Its crucial role in the phenomenon of critical flow velocity for erosion-corrosion

The correlation between depassivation and repassivation processes, which is significant in erosion-corrosion, was quantitatively investigated by single particle impingement tests at various flow velocities and impact angles. The results show that both repassivation and depassivation processes are associated with the kinetic energy of solid particle, and demonstrate that the repassivation is retarded by depassivation. This phenomenon probably results from the depassivation-induced microstructure evolution. On this basis, the dependence of critical flow velocity (CFV) for erosion-corrosion on the solid particle concentration and diameter is further theoretically predicted and experimentally verified. Accordingly, the crucial role of depassivation-repassivation in CFV phenomenon is further highlighted.

Removal of heavy metal ions and polybrominated biphenyl ethers by sulfurized nanoscale zerovalent iron: Compound effects and removal mechanism

Sulfurized nanoscale zerovalent iron (S-nZVI) has been widely reported to be able to quickly remove heavy metals/persistent organic pollutants, but the limited understanding of the complicated removal process of heavy metals-organic combined pollutants restricts the application of S-nZVI. Here, we demonstrate that there is significant difference in the effectiveness of S-nZVI for removing single pollutant and complex pollutants. The removal kinetic constant (kobs) of heavy metals by S-nZVI followed a sequence of Cr(VI)>Pb(II)>Ni(II)>Cd(II) with or without polybrominated diphenyl ethers (PBDEs). While the capacity of co-existing cations increasing the kobs of PBDEs followed the order: Ni(II)>Pb(II)>Cd(II), and the co-existence of Cr(VI) anion inhibited the reduction of PBDE by S-nZVI because the generated Cr-Fe precipitate hindered the electron transfer. The de-passivation process on S-nZVI surface by Cd(II) ions slightly accelerated the transformation rate of electron. Nevertheless, the co-existing Pb(II) significantly accelerated the transformation of BDE-209 via the galvanic effect from the generated Pb0/Fe0 bimetal. Interestingly, the kobs of BDE-47 in Ni(II)/S-nZVI system was 5.51 times higher than that of Pb(II)/S-nZVI system, implying that an atomic hydrogen mechanism dominated the reduction of BDE-47 by Ni(II)/S-nZVI. In conclusion, the results provided a deep comprehending of removal mechanism of heavy metal-organic complex pollutants by S-nZVI.

Corrosion prediction models for steel bars in chloride-contaminated concrete: a review

Concern has been expressed in the recent decades about the corrosion prediction of concrete structures in a chloride environment. This paper summarises the corrosion prediction models developed for chloride-contaminated concrete, including both the empirical and electrochemical corrosion models. The definition methods of the steel de-passivation process for electrochemical models, such as anode–cathode ratio (Aa/Ac), critical chloride concentration (CCl,crit), modification of anodic Tafel slope (βa) or passive film resistance (Rf), are considered essential and discussed in detail. Furthermore, electrochemical parameters regarding the definition and selection in electrochemical models and experimental measurements are discussed. This study confirms that Aa/Acand CCl,crithave been generally used in recent years, and that the modifications of βaand Rfrelated to free chloride concentration are technically feasible. In addition, the discreteness of electrochemical parameters in electrochemical models and experimental measurements are observed, and time-dependent characteristics of electrochemical parameters are scarcely presented in the existing literature.

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.

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.

Corrosion Resistance Evaluation of Some Stainless Steels Used in Manufacture of Hydraulic Turbine Runner Blades

The paper presents corrosion resistance testing results of three stainless steels that may be used in hydropower turbine blades manufacture. Two of these have a chemical composition close to that of some other stainless steels previously employed in producing these parts, being updated steel grades of the former ones. The third one is of a new conception, having a chemical composition close to that of a maraging steel. The three materials were produced in an induction furnace with cold copper crucible under vacuum and argon atmosphere in order to obtain improved mechanical and corrosion resistance characteristics as well as an inclusion � free structure. Quenching and tempering heat treatments were subsequently applied. Tests were carried out at room temperature in normally aerated 1N Na2SO4 and 3% NaCl solutions. Corrosion rates were calculated using the Tafel slope method. All steels have a passivation tendency in a chlorine-free aqueous medium. The newly conceived steel has a more pronounced anodic field as a result of a chromium content below 12%. However, the general corrosion behavior of this material is rebalanced by the content of about 10% Ni which leads to a mainly martensitic structure in quenched state. The corrosion rate values obtained for all samples enframe the three materials in highly and very highly corrosion resistant steels. Nevertheless it must be specified that in chlorine environments the overall corrosion rate is not a sensitive indicator of corrosion resistance performance due to the local depassivation process followed by corrosion pits.

Reinforced Concrete under the Action of Carbonization and Chloride Aggression: a Probabilistic Model for Service Life Prediction

Reinforcement corrosion of marine and coastal hydraulic structures due to chloride aggression and concrete carbonization leads to a sharp decrease in structure safety. The reinforcement is subjected to a depassivation process as soon as a chloride concentration on its surface exceeds a certain threshold concentration, or the pH value in a concrete protective layer is decreased to a threshold value due to carbonation. Electrochemical reactions are realized with formation of corrosion products due to penetration of oxygen up to reinforcement surface. This leads to cracking of the concrete protective layer and decrease in reinforcement cross-section. The paper proposes a method for predicting a complex degradation of reinforced concrete structures with due account of various mechanisms of corrosion wear that allows to develop efficient methods for improvement of structure durability and maintainability which are operated in the marine environment. A methodology for forecasting of reinforced concrete service life prediction has been developed under a combined effect of carbonization and chloride aggression while using finite-difference and probability models. The paper takes into account initiation periods of reinforcement corrosion and propagation periods for conditions of Sakhalin shelf zone. Field surveys of Kholmsk and Korsakov port facilities are presented in the paper. Carbonization front and chloride content have been estimated according to depth of the concrete protective layer. The paper proposes a model that allows to determine an average period prior to repair while taking into account rate of concrete protective layer degradation caused by simultaneous action of two corrosion processes: carbonization and chloride aggression.

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.

The role of surface film on the critical flow velocity for erosion-corrosion of pure titanium

One of the key issues in clarifying the mechanism of critical flow velocity (CFV) for erosion-corrosion is whether the surface film is ruptured mechanically below the CFV. This paper aims to address such an issue for titanium by using ion-labelling and colour-labelling methods. The results show that the pre-labelled ion and colour cannot be detected on the titanium surface after impingement in liquid-solid two-phase fluid below the CFV, suggesting the surface film has been ruptured mechanically. Accordingly, the phenomenon of CFV is attributed to the competition between the depassivation process induced by solid particle impacting and the subsequent repassivation process. Furthermore, a critical criterion is proposed and the expressions of depassivation time and repassivation time are derived and discussed.

COMBINED EFFECT OF CARBONATION AND CHLORIDE AGGRESSION: DEG-RADATION OF REINFORCED CONCRETE STRUCTURES

Corrosion reinforcement marine hydraulic structures due to chloride aggression and carbonization of concrete leads to a sharp decrease in the safety of the structure. The steel reinforcement will be subjected to a so-called depassivation process, once the chloride concentration on surface exceeds a certain threshold concentration, or the pH value in the protective layer of concrete decreases to a threshold value due to carbonation. Electrochemical reactions begin to occur with the formation of corrosion products with the penetration of oxygen on the steel reinforcement surface. This leads to cracking of the protective layer of concrete. It should also be taken into account that, due to corrosion mechanisms, the cross-sectional area of the reinforcement also decreases. The article suggests a method for predicting the complex degradation of reinforced concrete structures, taking into account various mechanisms of corrosion wear, which will allow developing effective ways to improve the durability and maintainability of structures operated in the marine environment.

Density Functional Theory Study of the Interactions of Cl and α-Fe2O3 Surfaces: The Role of Cl in the Depassivation Process

Under the alkaline environment inside reinforced concrete, a protective passive film is formed on the iron surface. Chloride ions have been shown to be one of the aggressive ions that can cause depassivation of this passive film under the same condition but the atomistic mechanism of depassivation is not fully understood. Several hypotheses have been proposed for the role of Cl in the depassivation, such as ion exchange model and point defect model. Here we use density functional theory (DFT+U) calculations of Cl interactions with hematite (α-Fe2O3) to test these different hypotheses of Cl induced depassivation of iron passive film. The different hematite surfaces are used to represent the Fe (III) oxides which are the dominant structure in the outer layer of the passive film. On the flat (0001) surface, both pristine surfaces and surfaces with point defects are studied. Three point defects on the surface are considered, Fe vacancy, O vacancy and Fe-O pair vacancies. We found that the O vacancies enhanced the adsorption of Cl while the Fe vacancy weakens the adsorption. The insertion of the Cl into the sub surface was studied to investigate the stability of Cl staying in the bulk of oxide proposed in ion exchange model and was found to be endothermic for all four surfaces but surface defects have positive effects on the insertion of Cl, making it less endothermic with the smallest reaction energy of about 0.3 eV on the O vacancy surface. However, it is more difficult for Cl to stay in the sub surface lattice further away from the surface. The vacancy diffusion direction was studied by calculating the energy difference of vacancies in the different layers to investigate the point defect model. Fe vacancy is energetically more favorable in the deeper sub surface while O vacancy prefers to stay in the layers closer to the surface. The adsorbed Cl can affect the two diffusion process by making the two diffusion process more energetically favorable. These results indicate that the point defect model is the more promising model to explain the Cl-induced depassivation process.

Modelling of corrosion fatigue crack initiation on martensitic stainless steel in high cycle fatigue regime

This paper presents an analytical model for assessing the corrosion fatigue crack initiation life on a martensitic stainless steel X12CrNiMoV12-3 in high cycle fatigue regime (between 105 and 107 cycles). Based on in-situ electrochemical measurements during corrosion fatigue tests in NaCl aqueous solution, the corrosion fatigue crack initiation mechanism was identified. Two main stages were investigated: (i) the fracture of the passive film by slip bands and (ii) the free dissolution of the metal developing fatigue crack initiation from a critical corrosion defect. The depassivation stress threshold corresponds to the median fatigue strength at 107 cycles for fatigue corrosion tests. For an applied stress range less than this threshold, the depassivation phenomenon was not observed at 107 cycles and no crack initiation occurred. The proposed model takes into account the depassivation process induced by the slip bands emergence at the specimen surface and the corrosion rate under cyclic loading. The experimental results are compared to the proposed model taking into account mechanical and electrochemical material parameters.