Failure mechanisms of high strength steels in bicarbonate solutions under anodic polarization

High strength steels used in prestressing concrete structures are not exempt from the effects induced by corrosion on the normal concrete reinforcement. Carbonation of surrounding concrete or mortar is not unlikely for prestressing tendons and strands. Moreover, these steels undergo to brittle fracture as a consequence of stress corrosion cracking phenomena. To evaluate if concrete carbonation can promote this kind of failure, constant load tests in bicarbonate aqueous solutions under anodic polarization were carried out on high strength steel wires. Microscopic examination pointed out that the wires exhibited a brittle fracture mode, while its natural feature is ductile, as indicated by air testing. Failure mechanism was evaluated by a fracture mechanic approach. Cracks initiation was attributed to an anodic dissolution mechanism, while its propagation, interpreted by means of the surface mobility theory, was related to interaction between hydrogen atoms and magnetite at a crack tip.     

Influence of steel-concrete interface condition on galvanic corrosion currents in carbonated concrete

This paper presents specific experiments which were developed in order to assess galvanic currents in macrocell corrosion specimens involving active steel in carbonated concrete and passive steel in sound concrete. The influence of the steel-concrete interface condition on the galvanic current was also experimentally investigated. To focus on macrocell corrosion rate assessment, the initiation time of the corrosion process (concrete carbonation) was accelerated. FEM simulations were carried out in order to enhance the physical comprehension of these corrosion experiments. It was found that, in realistic condition, the electrical coupling of active and passive steel areas leads to high galvanic currents and consequently high corrosion levels according to RILEM recommendation. Moreover, steel-concrete interfacial defaults significantly increase the macrocell driving potential and, therefore, the galvanic corrosion current.     

Effects of Tafel slope,exchange current density and electrode potential on the corrosion of steel in concrete

A parametric study is carried out to investigate the effect of variations in anodic and cathodic Tafel slopes, exchange current densities and electrode potentials on the rate of steel corrosion in concrete. The main goal of this investigation is to identify the parameters that have significant influence on steel corrosion rate. Since there is a degree of uncertainty associated with the selection of these parameters, particularly during modelling exercises, it is intended that the results of this study will provide valuable information to engineers and researchers who simulate steel corrosion in concrete. To achieve this goal, the effect of a parameter on the corrosion rate of steel is studied while all other parameters are kept constant at a predefined base case. For each parameter, two extreme cases of anode‐to‐cathode ratio are studied. The investigation revealed that the variations in the anodic electrode potential have the greatest impact on the corrosion rate, followed by the variations in the cathodic Tafel slope.     

Polarisation behaviour of steel bar samples in concrete in seawater. Part 2: A polarisation model for corrosion evaluation of steel in concrete

In the companion paper [Z.T. Chang, B. Cherry, M. Marosszeky, Polarisation behaviour of steel bar samples in concrete in seawater, Part 1: Experimental measurement of polarisation curves of steel in concrete, Corrosion Science 50(2) (2008) 357-364], influences of the experimental procedure on measured polarisation curves of steel in concrete in seawater were investigated. It was found that an undistorted full polarisation curve of a steel sample in concrete can be obtained by the two-test procedure to conduct separate anodic and cathodic polarisation tests and combine the two partial curves into one curve. However, polarisation curves of steel samples in concrete in seawater were found not to fit with the theoretical curves based on the kinetics of charge transfer reactions. This was considered to be due in the main to the influence of a passive film on the steel surface in concrete. This paper proposes an empirical model for the polarisation behaviour of steel in concrete based on the assumption of two major electrochemical processes taking place at the interfaces of steel/passive-film/concrete: one is the active corrosion process and the other is the passive film growth or dissolution process. Typical curve-fit results are presented using the proposed model to simulate the polarisation behaviour and to evaluate the corrosion rate and Tafel parameters of three types of steel corrosion in seawater: steel bars in concrete, new steel bars and corroded steel bars.     

Electrochemical behavior of corroded and protected construction steel in cement extract

The corrosion behavior of corroded, cathodically protected and control (reference) construction steel, previously embedded in concrete, was studied in cement extract (pH 12.6, considered as concrete pore water), using cyclic voltammetry (CVA) and potentiodynamic polarization (PDP). The necessity for this investigation occurred from the previously observed and commented discrepancies in the recorded corrosion parameters for corroded and protected steel in embedded conditions 1 . Therefore this study aimed to evaluate how the “naturally” formed in concrete product layers (after 460 days) will influence the electrochemical behavior of the steel in cement extract. The PDP measurements reveal the lowest corrosion resistance to be for the previously corroded steel samples, for which the most active corrosion potential (?0.7 V SCE) and the highest anodic current in the potential region 0 to 0.6 V (SCE) were recorded. The CVA tests support the results from PDP and correlate the properties of the naturally formed layers with the recorded peak current densities and peak potentials with cycling. For all specimens, except the corroded ones, the peak potential initially shifts anodically, which denotes for a high corrosion resistance in the former and low corrosion resistance in the latter case. For the control and protected specimens, the passive current in the potential region of 0 to 0.6 V (SCE) remains almost unchanged with cycling, i.e. the protective properties of the initial layers remain unchanged. Thickening of the film with cycling does not influence the corrosion resistance of the previously formed layers. For the protected specimens, however, a tendency to reach a steady state and change of peak currents' height only were observed, without a pronounced shift to more anodic values. An increase in the peak current only, not accompanied by anodic shift of the peak potential, suggests that the layers in the cathodically protected specimens are more homogeneous and compact. Overall it can be stated that in cement extract, the product layer with lowest corrosion resistance is the one on the surface of the corroded steel reinforcement. The product layers in the protected specimens (although similar to control conditions) are with the highest corrosion resistance.     

Untersuchung über die Auswirkung der Elementbildung zwischen Stahl in Beton und Stahl in Erdboden

Investigation of the effect of galvanic corrosion between steel in concrete and steel in soil The investigation of cathodic polarization of steel in concrete on samples of portland cement and blast furnace cement revealed that oxygen reduction on steel in portland cement is less hindered than on steel in blast furnace cement. As to the magnitude of the cathodic current the aeration of the samples is important. Dry stored samples before measuring case a higher current density than wet stored ones. A significant relationship of the cathodic current depending on the water/cement value and the storage time of concrete could not be ascertained. The polarization resistances for portland cement samples lie within 86 and 3000 kΩ · cm² and for blast furnace cement samples between 430 and 5100 kΩ · cm². The measurements of the anodic current densities on corrosion cells of steel in concrete/steel in solution and steel in wet sand respectively rendered, by varying the cathode/anode aspect ratio from 1000: 1 to 1: 1, values for portland cement samples between 560 and 1,2 μAJcm² (corresponding to an annual corrosion of 6,5 and 0,014 mm respectively), and for blast furnace cement samples values between 730 and 0,28 μA/cm² (corresponding to an annual corrosion rate of 8,5 and 0,003 mm respectively). A decrease of the anodic current density variable with time could not be observed during the 28 days of measurements. On account of the investigations the following conclusions can be made: A formation of a corrosion cell with steel in concrete/steel in soil is particularly then to be expected if the concrete is dry on one side, e.g. the foundation of a building where oxygen diffuses by way of the gas pores in the concrete from the inner side to the cathode. This is less serious if the concrete is fully embedded in soil and thoroughly moistened. In this case the transport of oxygen has to be carried via waterfilled pores.     

Theoretical considerations on the supposed linear relationship between concrete resistivity and corrosion rate of steel reinforcement

Traditionally, the assessment of service life of steel reinforced concrete structures has been focused on the prediction of the time required to achieve a transition from passive to active corrosion rather than to accurately estimate the subsequent corrosion rates. However, the propagation period, i.e. the time during which the reinforcing steel is actively corroding, may add significantly to the service life. Consequently, ignoring the propagation period may prove to be a conservative approach. On the other hand the prediction of the corrosion rate may result in a very complex task in view of the electrochemical nature of corrosion and the numerous parameters involved. In order to account for the various influences an essentially empirical model has been introduced in which the electrolytic resistivity of the concrete environment serves as the major parameter. This model will be discussed for carbonation‐induced corrosion based on the commonly accepted theory of aqueous corrosion. An alternative model for microcell corrosion is proposed which is based on the commonly accepted view that anodic and cathodic sites are microscopic and their locations change randomly with time. In line with this view electrolytic resistivity can be incorporated and thus may play a significant role in the kinetics of the corrosion process. For a wide range of corrosion current densities the relationship between corrosion current density, log(i_corr), and concrete resistance, log(R_con), can then be approximated by an almost ideal linear relationship. Assuming a fixed geometrical arrangement of anodic and cathodic sites on the steel surface, this linear relationship is also valid for concrete resistivity, ρ_con. However, from the theoretical treatment of the electrochemical processes underlying reinforcement corrosion it becomes evident that a linear relationship between corrosion current density and concrete resistivity does not necessarily imply that concrete resistance is dominating the overall corrosion cell resistance. In most cases a significant portion of the driving voltage of the corrosion cell will be consumed by the transfer of electrical charge involved in cathodic reactions, i.e. cathodic activation control will dominate.     

Effect of inclusions on initiation of stress corrosion cracks in X70 pipeline steel in an acidic soil environment

The effect of inclusions on the initiation of stress corrosion cracking (SCC) X70 pipeline steel was investigated in an acidic soil solution using slow strain rate test, scanning electron microscopy and energy-dispersive X-ray techniques. The results demonstrated that stress corrosion cracks are not initiated in X70 steel when it is under anodic polarization. At cathodic polarization, hydrogen evolution is enhanced, and hydrogen is actively involved in SCC processes. Two types of inclusions exist in the steel and play different role in crack initiation. The inclusions enriching in Al are brittle and incoherent to the metal matrix. Microcracks and interstices are quite easily to be resulted in at the boundary between inclusions and metal. There is no crack initiating at inclusions containing mainly Si.     

Passivity and Pitting Corrosion of X80 Pipeline Steel in Carbonate/Bicarbonate Solution Studied by Electrochemical Measurements

This work investigated the effects of chloride ions and hydrogen-charging on the passivity and pitting corrosion behavior of X80 pipeline steel in a bicarbonate-carbonate solution by electrochemical and photo-electrochemical techniques. It was found that a stable passivity can be established on the steel in the absence and presence of chloride ions. The hydrogen-charging does not alter the transpassive potential, but increases the passive current density. When chloride ions are contained in the solution, pitting corrosion will be initiated. The pitting potential is independent of the hydrogen-charging. Hydrogen-charging would enhance the anodic dissolution and electrochemical activity of the steel, but does not affect the pitting potential, which indicates that the charged hydrogen is not involved in the pitting initiation. However, hydrogen may accelerate the pit growth. Photo illumination could enhance the activity of the steel electrode, resulting in an increase of photo-induced anodic current density.     

A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H2SO4

Phosphorous as a minor element (0.03%) in AISI 304 austenitic stainless steel greatly affects the polarization and corrosion behavior in sulfuric acid solution. The presence of P in stainless steel created a second current maximum in the anodic polarization curve and the current increased with increasing aging in the solution. An adhesive corrosion surface layer, rich with phosphate, formed on the surface of a P-containing steel during active dissolution. The layer lowered the cathodic Tafel slope at low current densities, and is likely, due to a change in hydrogen evolution mechanism. Phosphorous increases the H-adsorbed (and/or absorbed) atoms on the surface, leading to the appearance of a second anodic current peak that is interpreted as re-oxidation of hydrogen atoms. Also, P shifted the corrosion potential to the noble side, decreased effectively the active anodic dissolution, and lowered the corrosion rate.     

单氟磷酸钠在混凝土模拟孔隙液中的缓蚀特性

采用动电位极化曲线和电化学阻抗谱研究了Na2PO3F在模拟混凝土孔隙液中对钢筋的保护作用。Na2PO3F的加入导致混凝土钢筋的腐蚀电位负移,对钢筋的阴极和阳极电化学过程均有抑制作用,是一种混合型缓蚀剂。Na2PO3F能够在钢筋表面形成沉淀型保护膜,阻挡侵蚀性离子对钢筋的侵蚀。     

Corrosion properties of HVOF-coated steel in simulated concrete pore electrolyte and concentrated chloride environments

The corrosion susceptibility of steel and HVOF-coated steel in solutions simulating the alkaline concrete pore environment and with the addition of chloride was investigated using potentiodynamic polarization and potential step techniques. The surface characterization was performed using SEM and the surface elemental analysis was determined by EDS. The concentration of chloride was 2.8 M to simulate the concentration of chloride spread in many local regions of Saudi Arabia and called Sabkha. It was found that, in the case of the simulated concrete pore electrolytes, the HVOF coating resulted in an anodic shift of the corrosion potential with marginal effect on the corrosion current. However, upon addition of 2.8 M chloride solution, the corrosion rate of the HVOF-coated steel was found to increase by a factor of two. SEM showed a network of pores within the coating which provides a path for the electrolyte. This would result in preferential corrosion around splat boundaries and confirmed by EDS which showed that the corroded splats have higher oxide contents. Potential step experiments at 400 mV above open circuit potential showed a suppressed current of the HVOF-coated steel compared to the steel substrate alone. The corrosion potential versus time experiments resulted in a more anodic Ecorr which decreased with time and became equal to the Ecorr of the bare steel after 34 h. After that, the corrosion potential of the HVOF-coated steel decreased due to the increase in galvanic coupling between the steel and the HVOF coating.     

Electrochemical corrosion of X65 pipe steel in oil/water emulsion

The electrochemical corrosion behavior of X65 pipeline steel in the simulated oil/water emulsion was investigated under controlled hydrodynamic and electrochemical conditions by rotating disk electrode technique. Results demonstrated that mass-transfer of oxygen plays a significant role in the cathodic process of steel in both oil-free and oil-containing solutions. Electrode rotation accelerates the oxygen diffusion and thus the cathodic reduction. The higher limiting diffusive current density measured in oil-containing solution is due to the elevated solubility of oxygen in oil/water emulsion. The anodic current density decreases with the increase of electrode rotating speed, which is attributed to the accelerated oxygen diffusion and reduction, enhancing the steel oxidation. Addition of oil decreases the anodic dissolution of steel due to the formation of a layer of oily phase on steel surface, increasing the reaction activation energy. The steel electrode becomes more active at the elevated temperature, indicating that the enhanced formation of oxide scale is not sufficiently enough to offset the effect resulting from the enhanced anodic dissolution reaction kinetics. The corrosion reaction mechanism is changed upon oil addition, and the interfacial reaction is activation-controlled, rather than mass-transfer controlled. When sand particles are added in oil/water emulsion, there is a significant increase of corrosion of the steel. The presence of sands in the flowing slurry would impact and damage the oxide film and oily film formed on the steel surface, exposing the bare steel to the corrosive solution.     

Environmentally assisted cracking mechanism of pipeline steel in near-neutral pH groundwater

A modified Devanathan dual cell was used in environmentally assisted cracking (EAC) tests to independently control the concentration of dissolved hydrogen and anodic current density. The experimental evidence indicates that the corrosion may reduce the micro-hardness of steel, suggesting a dissolution-induced degradation of the mechanical properties in the surface layer. An EAC model is described in which surface micro-plastic deformation can be enhanced by the dissolved hydrogen and anodic dissolution, and plays a crucial role in cracking processes. The model can provide a reasonable prediction for the dependence of the cracking resistance of pipeline steel on the applied potential.     

Effects of alternating current on corrosion of a coated pipeline steel in a chloride-containing carbonate/bicarbonate solution

In this work, the alternating current (AC)-induced corrosion of a coated pipeline steel was studied in a chloride-containing, concentrated carbonate/bicarbonate solution, which simulated the trapped high pH electrolyte under coating, by potentiodynamic polarization measurements, immersion tests and surface characterization technique. It was found that an application of AC resulted in a negative shift of corrosion potential of the steel, caused an oscillation of anodic current density, and degraded the steel passivity developed in the solution. With the increase of AC current density, the corrosion rate of the steel increased. At a low AC current density, a uniform corrosion occurred, while at a high AC current density, pitting corrosion occurred extensively on the steel electrode surface. At individual applied AC, there was a higher electrochemical dissolution activity of the coated steel electrode containing a 1 mm defect than that of the electrode containing a 10 mm defect.     

The effect of temperature on the galvanic corrosion of the copper/AISI 304 pair in LiBr solutions under hydrodynamic conditions

The corrosion behaviour of copper and AISI 304 stainless steel and the galvanic corrosion generated by the copper/AISI 304 pair, have been studied by electrochemical methods. These materials have been tested in an 850 g/L LiBr solution at different temperatures (25-75 °C) and at different Reynolds numbers (1456-5066) in order to study their performance in absorption machines. Results show that copper was always the anodic element of the pair and its corrosion resistance decreases due to the AISI 304 stainless steel galvanic effect. Galvanic corrosion increases with temperature and Reynolds number. However, it was proved that the effect of temperature on galvanic corrosion is more influential than the Reynolds number effect. This fact is also certain for corrosion of uncoupled copper and for corrosion of AISI 304 stainless steel. Experimental values of the corrosion current densities fit well the Arrhenius plot at all the Reynolds numbers analysed and a potential relation between the corrosion current densities and the Reynolds number has been found.     

Embedded sensor system to detect chloride permeation in concrete: an overview

Exposure of concrete structures to chloride-bearing solutions such as sea water or de-icing salts brine can affect their durability to stand against deterioration caused by corrosion of steel reinforcement. In order to monitor the chloride ingress into concrete, embedded chloride sensors can be used in concrete structures. Through the sensors, they monitor the initiation of corrosion of steel reinforcement with time, and measure the open circuit potential and polarisation of steel reinforcement, humidity inside the concrete, chloride content and many more. This paper provides an overview of the latest development of chloride permeation measurement in concrete by evaluating the current techniques being used in industry and highlighting a new approach to monitor chloride ion progress in concrete structures wirelessly.     

Chloride threshold for rebar corrosion in concrete with addition of silica fume

The effect of silica fume on the chloride threshold for the initiation of pitting corrosion of steel in concrete was investigated. Laboratory tests were carried out in concrete specimens made with ordinary Portland cement and with 10% of silica fume. Chloride contents up to 2% by mass of cement were added to the mixes, in order to investigate the corrosion rate of embedded bars made of both strengthened and mild steel. A lower chloride threshold was observed in the bars which were embedded in concrete with silica fume compared to those embedded in concrete made of Portland cement.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

Study of localized corrosion of 304 stainless steel under chloride solution droplets using the wire beam electrode

Localized corrosion of 304 stainless steel under droplets of 1 M sodium chloride solution was investigated by the wire beam electrode (WBE) method. It was found that the current distributions were heterogeneous with isolated anodic current peaks mostly located near the edge of the droplet. During the corrosion process, the stainless steel WBE exhibited the stochastic characteristics with the disappearance of some anodic sites. In addition, stainless steel suffered more serious localized corrosion with the increase of the droplet size. The increase of the cathodic area and the three-phase boundary (TPB) length was believed to be the reason.