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.     

Potentiostatic study of reinforcing steel in chloride contaminated concrete powder solution extracts

This paper presents the findings of an experimental investigation of potentiostatic study on reinforcing steel in chloride contaminated concrete powder solution extracts. Various zones of corrosion for the steel reinforcement at various chloride levels have been identified. In addition the chloride concentration and pH value of these solutions were also measured. The major test variables include steel type, w/c ratio, cement content, and admixed chloride content.     

Corrosion zones of rebar in chloride contaminated concrete through potentiostatic study in concrete powder solution extracts

In this paper an experimental investigation has been carried out to identify semi-immune, active, passive and pitting zones of corrosion of the steel reinforcement in concrete powder solution extracts at different levels of admixed chloride. Potentiostatic linear sweeping test was carried out on three different types of steel reinforcement in the solution extracts made from concrete with three different types of cement at four levels of admixed chloride content with three w/c ratios for two test conditions, i.e., with and without deaeration. From the results obtained it was concluded that chloride content is the major factor in defining the zones of corrosion and the same was further confirmed through ANOVA.     

Finite element modelling of chloride removal from concrete by an electrochemical method

This paper presents a finite element model for simulating the transport behaviour of ions in a porous medium when a constant DC current density is applied through electrode pairs emplaced in the medium. The model is then applied to predict the mass transport associated with electrochemical chloride extraction from hydrated cement paste. Numerical results are compared with those obtained in experiments of various duration. Effects of externally applied current density, treatment period, diffusion coefficients, ionic binding, boundary conditions and medium porosity on the efficiency of chloride extraction are discussed.     

A study of steel rebars embedded in concrete during 65 years

Several experimental techniques were used to study the corrosion products formed on steel rebars embedded in concrete during 65 years. The research has the objective of understanding the stages of rusting of steel in concrete. Such understanding would provide information concerning the environmental conditions that produce rusting. It was found that rust was composed of two layers: an inner one composed mainly of non-stoichiometric magnetite and probably Ca and Al substituted magnetite firmly adhered to the substrate, and an outer one composed of α- and γ-iron oxyhydroxides, final steel corrosion products. The chemical and mechanical properties of steel rebars and the surrounding concrete were investigated in order to establish the causes of corrosion initiation which were the drop of the concrete pH due to a carbonation process and the loss of steel passivity.     

Detection and identification of concrete cracking during corrosion of reinforced concrete by acoustic emission coupled to the electrochemical techniques

The tenacious oxide passive film, which is formed on the surface of embedded reinforcing steel under high alkaline condition of concrete, protects the steel against corrosion. However, the condition of passivity may be destroyed, due to processes such as leaking out of fluids from concrete, atmospheric carbonation or through the uptake of chloride ions. Passive steel reinforcing corrosion induced by chloride is a well-known problem, especially where chloride-containing admixtures or chloride contaminated aggregate are incorporated into the concrete. The objective of this work is on one hand to study the effect of chloride ions on passivity breakdown of steel, respectively, in simulated concrete pore solution (SCP) and in concrete reinforcement, and on the other hand to reproduce the carbonation phenomena by applying to the concrete samples a heating–cooling cycles. In this context, the acoustic emission coupled to the electrochemical techniques (potentiodynamic and electrochemical impedance spectroscopy (EIS)) are used.

The results show clearly that [Cl⁻]/[OH⁻] ratio of 0.6 is the critical threshold where the depassivation set-up can be initiated. In addition, the carbonation process is very aggressive with chloride ions and shows a perfect correlation with acoustic emission evolution.

A physical model of the reinforcement/electrolyte interface is proposed to describe the behavior of the reinforcement against corrosion in chloride solution.     

Evaluation of chloride contamination in concrete using electromagnetic non-destructive testing methods

We present the results of the sensitivity of some electromagnetic non-destructive testing (NDT) methods to chloride contamination. The NDT methods are resistivity, using a quadripole probe, capacitive technique, with few sets of electrodes, and radar technique, using different bistatic configurations. A laboratory study was carried out involving three different concretes with different water to cement ratios. The concretes were conditioned with different degrees of NaCl saturation by means of three solutions containing 0 g/L, 30 g/l or 120 g/l. The solution was homogenized in the concrete by using a specific procedure. Results show that the EM techniques are very sensitive to the chloride content and saturation rate and, on a second level, to the porosity. Multi-linear regression processing was performed to estimate the level of sensitivity of the NDT measurements to the three indicators. Values of ten ND observables are presented and discussed. At last, the uncertainties of the regression models are studied on a real structure in a tidal zone.     

Corrosion resistance of carbon steel in simulated concrete pore solution in presence of 1-dihydroxyethylamino-3-dipropylamino-2-propanol as corrosion inhibitor

The inhibition behaviour of 1-dihydroxyethylamino-3-dipropylamino-2-propanol (HPP) as an environment friendly corrosion inhibitor for reinforcing steel was investigated in simulated concrete pore solution contaminated by 0·1 mol L^?1 Cl^? by means of linear polarisation resistance, electrochemical impedance spectroscopy and cyclic voltammetry (CV). The surface morphology and corrosion products were also examined by scanning electron microscopy and X-ray diffraction (XRD). The results show that HPP can effectively inhibit the corrosion of reinforcing steel. The CV interpreted the corrosion inhibitor by restraining the reaction of cathodic reduction and anodic oxide. The XRD shows that the corrosion inhibitor reduces Cl^? adsorption on oxide film and reduces the production of corrosion products. In the conditions of this investigation, HPP behaves as an anodic corrosion inhibitor, protecting steel against corrosion in chloride contaminated environments.     

Electrochemical studies of steel in cement mortar containing chloride and micro-silica

Steel electrodes have been exposed for 3.5 years in cement mortar with and without chloride and ELKEM micro-silica. Corrosion potentials in mortar without Cl⁻ varied between −0.1 and +0.06 V(SCE), indicating passivity. One percentage of cement weight of Cl⁻ reduced the corrosion potentials to about −0.2 V(SCE). Micro-silica in chloride containing mortar increased the severity of corrosion the first year. However, after 2 years exposure reduction of interconnected porosity and increased resistivity caused by 15% micro-silica, raised the corrosion potentials and counteracted chloride-attack.The polarisation resistance was undefined, the apparent values varying with the technique and parameters used.     

Macrocell sensor systems for monitoring of the corrosion risk of the reinforcement in concrete structures

Since 1990, a special macrocell system, the so-called anode-ladder-system, has been used world-wide to monitor the corrosion risk of new concrete structures, besides other systems. This sensor-system indicates the depth of the critical chloride content initiating corrosion, i.e. the critical depth of the reinforcement with respect to corrosion. Subsequently, the time-to-corrosion can be determined, enabling the owners of buildings to initiate preventive protection measures before cracks and spalling occur. By measurement of the potentials and the electrical resistance of the concrete around the sensors, an estimation of the humidity, the availability of oxygen and the corrosion behaviour after depassivation is possible.To be able to monitor the corrosion risk for existing structures as well, a new sensor-system has been developed called expansion-ring-system. It consists of six measuring rings (‘anodes’) separated by sealing rings as parts of the main sensor and a ‘cathode-bar’, which are installed in small holes, to be drilled into the concrete.     

Application of EIS to cathodically protected steel: Tests in sodium chloride solution and in chloride contaminated concrete

The efficiency of cathodic protection is usually checked by switching off the current and either by measuring the polarized potential or by recording the evolution of the potential decay. These methods although based on a long experience are not indicating directly on the state of the metal, and the interpretation of the results is not always evident. It remains as an interesting goal for the research on alternative methods to verify the efficiency of the cathodic protection. In this paper, a study made by using linear polarization and EIS is presented based on the behaviour of the two cathodically protected systems. The results indicate that the impedance diagrams show important and consistent changes in shape and associated parameters when the steel is protected or unprotected. Although changes have been noticed before by other authors in the EIS diagrams, this study assumes that the system is a galvanic couple.     

Fluoride induced corrosion of steel rebars in contact with alkaline solutions, cement slurry and concrete mortars

Corrosion behaviour of mild steel rebars has been studied in contact with 0.01 N NaOH, saturated lime water, cement slurry and embedded mortars having different concentrations of fluoride ions. Weight-loss, electrochemical DC cyclic polarisation and polarisation resistance, surface topographic and X-ray diffraction (XRD) techniques have been used to investigate the effects of fluoride ion on the corrosion behaviour of the material. Under all the conditions of the exposure it is observed that a low content of fluoride (?25 ppm) in the corrodent has deleterious action on the performance of the steel, where as at its higher content (?100 ppm) the ion has rather an inhibiting effect on corrosion rate. XRD studies of the corrosion product accumulated on the steel surface in the presence of fluoride ion indicate the predominance of magnetite oxide phase. Accelerating effect of the ion is observed to be due to depolarisation of cathodic reaction of the corrosion process. Observations show that in accelerating range of fluoride (?25 ppm) it has almost double corrosive effect than noted for equal concentration of chloride ion.     

Determination of diffusion coefficient of chloride in concrete using Warburg diffusion coefficient

Assessment of diffusion coefficient of chloride (D) using chloride profile method based on Fick’s law (D_FL) is either overestimates or underestimates the time to initiation of corrosion (T_i). An alternate method for predicting ‘D’ using Warburg diffusion coefficient (D_WI) which is determined from Electrochemical Impedance Spectroscopy technique (EIS) is established. The results reveal that EIS being non-destructive appears a promising technique to arrive at time-dependent characteristics of D_WIin situ in concrete structures. D_WI is an intrinsic effective diffusivity measures the diffusion of free chloride through the pore solution present in the interconnected pores. Pore constriction by pozzolanic reaction and higher chloride binding capacity reduces the D_WI in PPC and PSC concrete by a factor of 1.65 and 4 times that of OPC concrete in 20 MPa concrete; 1.83 and 2.52 in 30 MPa concrete; 24 and 16 times in 40 MPa concrete respectively.     

Cathodic protection of steel in concrete using magnesium alloy anode

Corrosion of steel embedded in concrete structures and bridges is prevented using cathodic protection. Majority of the structures protected employ impressed current system. Use of sacrificial system for the protection of steel in concrete is not as widely employed. The use of magnesium anodes for the above purpose is very limited. This study has been carried out with a view to analyse the use of magnesium alloy anode for the cathodic protection of steel embedded in concrete.Magnesium alloy anode, designed for three years life, was installed at the center of reinforced concrete slab, containing 3.5% sodium chloride with respect to weight of cement, for cathodic protection. Potential of the embedded steel and the current flowing between the anode and the steel were monitored, plotted and analyzed. Chloride concentration of concrete at different locations, for different timings, were also determined and analyzed.The magnesium anode was found to shift the potential of the steel to more negative potentials initially, at all distances and later towards less negative potentials. The chloride concentration was found to decrease at all the locations with increase in time. The mechanism of cathodic protection with the sacrificial anode could be correlated to the removal of corrosive ions such as chloride from the vicinity of steel.     

On the pitting corrosion currents of zinc by chloride anions

The changes in the pitting corrosion current density with time on zinc electrode concerning the concentration of both the passivating borate and the aggressive chloride anions were followed using a simple electrolytic cell. The pitting corrosion currents started to flow after an induction period, τ. This period is found to be a function of the concentration of Cl⁻ anion, according to the relation logτ=β−γlogC_Cl⁻. The pitting corrosion currents finally reached a steady-state value, which depended on the concentration of both B₄O₇²⁻ and Cl⁻ anions. At a constant B₄O₇²⁻ anion concentration, the pitting corrosion current varied with the concentration of Cl⁻ anion according to the relation logi_pit=a₁+b₁logC_Cl⁻. It also varies at constant Cl⁻ anion concentration and various B₄O₇²⁻ anion concentration according to the relation logi_pit=a₂−b₂logC_B4O7²⁻. The susceptibility of the passivating zinc to pitting corrosion was found to be increasing as the temperature and pH of the solution increases. Results are discussed on the basis of adsorption of the aggressive anion on the passivating film, followed by penetration through the film and incorporation in it. This undermines the oxide film and causes pitting corrosion.     

Electrochemical study on the effect of Schiff base and its cobalt complex on the acid corrosion of steel

The effect of the Schiff base N,N′-bis (salicylaldehyde)-1,3-diaminopropane (Salpr) and its corresponding cobalt complex on the corrosion behaviour of steel in 1 M sulphuric acid solution were studied by electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization techniques. Spectrophotometry measurements were employed to investigate the stability of the complex in acid media. The inhibitive effect of Salpr and its stable octahedral cobalt complex is argued to their adsorption over the steel surface. Theoretical fitting of different isotherms, Langmuir, Flory–Huggins and the kinetic–thermodynamic model were tested to clarify the nature of adsorption. The data revealed that there might be non-ideal behaviour in the adsorption processes of Co(Salpr) complex on the steel surface. The Co(Salpr) could displace more water molecules from the steel surface than the corresponding Salpr. The bulky Co(Salpr) molecule could cover more than one active site.     

A SEM/XPS/SKP study on the distribution of chlorides in contaminated rusty steel

In most laboratory studies carried out to obtain metallic substrates contaminated with chlorides, the specimens are usually prepared by dosing the saline contaminant in a uniform or homogeneous way on the clean substrate. The result, in terms of chloride distribution, is considerably different to that found in practice, where chlorides are heterogeneously distributed among the corrosion products, mostly in their inner layers and frequently inside corrosion pits. An alternative to this conventional contamination method consists of exposing fresh clean steel in the salt fog cabinet, with the aim of obtaining a more similar salt distribution to that which occurs in real situations. The SEM/XPS/SKP comparative study performed confirms that the chloride distribution achieved with climate cabinets is much closer to the real case of natural contamination than that achieved with the conventional homogeneous dosing method.     

Ability of the direct wave of radar ground-coupled antenna for NDT of concrete structures

The research work presented in this paper aims at evaluating the potential of the direct wave of radar ground-coupled antennas for the on site physical characterization of concrete. According to studies highlighting the ability of radar reflected waves to characterize concrete, the potential of the direct wave is studied by systematic comparison of direct and reflected wave attenuations. Among the various features of electromagnetic waves propagating through concrete, attenuation is the most sensitive to moisture and chloride contents. In order to study the sensitivity of the direct wave to the physical condition of the concrete, laboratory experiments were conducted on concrete slabs with various porosities, water contents and chloride contamination levels.The results of this study show very satisfactory correlations between attenuations of direct and reflected waves, indicating that both waves provide similar information regarding the physical state of concrete. This correlation was confirmed on two bridges by comparison of direct wave attenuation maps with reflection attenuation maps. Therefore, radar interpretation method based on the direct wave attenuation should be able to detect, in a very fast way, physical contrasts in concrete structures and, consequently, identify probable pathological areas.     

Investigation of effect of fineness dependent volume of fine aggregate on passive layer formation and corresponding corrosion of steel reinforced concrete using electrochemical approach

This paper aims to determine the effect of the fineness modulus of fine aggregate on the quality and development of passive film and its subsequent effect on the corrosion of reinforced concrete structures. The study uses both electrochemical laboratory controlled experimentation and finite element modelling approach. Corrosion is an electrochemical process that requires an electrolyte for the occurrence of a corrosion reaction. Therefore, it is necessary to calculate the effective corrosion rate with reference to the saturated area of the concrete only when there is corrosion of reinforced concrete structures. Theoretically and numerically, the saturated area depends on the capillary zone porosity, gel zone porosity and degree of saturation in the aggregate free volume of concrete, which is a heterogeneous material. It is thus important to understand the effect of aggregate fineness on the corrosion of steel rebars. Therefore, detailed investigations were conducted using a variety of fine aggregates and combinations thereof. The initiative for this research came from the observation that the interfacial transition zone (ITZ) around the steel bar in concrete does not contain coarse aggregate, but could be surrounded by mortar and not only paste. Hence, the fine aggregate volume may also influence the corrosion rate. Some fine aggregate, which is finer than coarse aggregate, could be present in the vicinity of the steel bar surface, depending on the fineness modulus, which may affect both the development of the passive layer and the corrosion rate. The previous research data in this area were found to be limited. Hence, these factors have been investigated in the present study. Both mortar and concrete prismatic specimens were cast, and the quality of passive layer development and the corrosion severity were compared between specimens to determine the effect of fine aggregate volume with respect to the respective fineness modulus values. The corrosion potential, current and rate values obtained through experimentation showed significant differences in their magnitudes with respect to the variation in the fineness modulus. A directly proportional relation was observed between the fineness modulus of fine aggregate and the corrosion of rebars. Therefore, in the light of the experimental results obtained in this study, a finite element based numerical model incorporating the effect of the fineness modulus of fine aggregate on corrosion of steel rebars in concrete has been developed and successfully verified in this study.     

Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41

The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)₂ in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.