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.     

Improved corrosion resistance in simulated concrete pore solution of surface nanocrystallized rebar fabricated by wire-brushing

Continuous surface nanocrystallization (SNC) of rebar was achieved through wire-brushing process. A uniform NC layer with thickness of 25 μm and average grain size of 50 nm was formed on the rebar surface. Due to the enhanced passivation performance of the NC layer, corrosion resistance of the SNC rebar was significantly improved in Cl⁻-containing saturated Ca(OH)₂ solution. High-energetic crystal defects of the nano-grains leads to the faster passivation and enhanced stability of the passive film of the SNC rebar.     

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.     

Polarisation behaviour of steel bar samples in concrete in seawater. Part 1: Experimental measurement of polarisation curves of steel in concrete

The most widely used technique for the investigation of corrosion of reinforcing steel in concrete is the linear polarisation resistance (LPR) method, which however needs to assume the Tafel slopes or the B constant for calculation of the corrosion rate. This paper aims to explore the use of a polarisation curve technique to study the polarisation behaviour and to evaluate simultaneously the corrosion rate and Tafel slopes of steel samples in concrete. As the first part of the study, this paper reviews the relevant literature, and examines the effect of the experimental technique on the measured polarisation curves of steel in concrete in seawater. In particular, differences in the results obtained by two different test procedures are examined. A comparison of the experimental polarisation curves with the theoretical curves based on charge transfer reactions is made.     

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.     

Application of harmonic analysis in measuring the corrosion rate of rebar in concrete

The corrosion rate (CR) of rebar embedded in cement mortar, concrete and cement extract is determined using harmonic analysis technique (HA). Simultaneously using other electrochemical techniques such as impedance spectroscopy (EIS) and Tafel extrapolation (TET), the CR was determined and compared with the weight loss method. CR obtained from HA is comparable to that of EIS provided that the Stern–Geary constant (B value) obtained from HA is used in the calculation. In concrete, comparable corrosion rates are obtained between TET and HA only under active condition of the rebar whereas under passive state, the corrosion current (i_corr) by TET is 10 times lower than that of HA. A good agreement is obtained between the HA and weight loss method. The outcome of the result suggests that HA is capable of providing a higher degree of accuracy than that of EIS and TET in the determination of i_corr in the medium like rebar in concrete having very low rate of corrosion.     

Corrosion of steel reinforcement in structural concrete with carbon material addition

Different types of carbonaceous materials have been added to concrete mixes and their effect on the corrosion of embedded steel has been studied. Using a constant water/cement ratio of 0.42 and different amounts of carbonaceous materials and different curing periods the evolution of the corrosion process in the embedded reinforced bars has been determined. The addition of small quantities of carbonaceous materials to the mixture produces a reduction of the concrete permeability. Tests demonstrate that a decrease of the corrosion level occurs when the content ratio of carbon material addition is increased.     

Corrosion behaviour of corrugated lean duplex stainless steels in simulated concrete pore solutions

This work studies the corrosion behaviour of two corrugated lean duplex stainless steels (SAF 2001 and 2304 grades) in eight alkaline solutions (carbonated and non-carbonated, saturated Ca(OH)₂ solutions with different chloride contents). 2001 stainless steel is a new grade in market because of its composition. 2304 is a grade previously studied under different conditions. However, its use as reinforcement in concrete is new. Studies are carried out by polarization curves following scanning electronic microscopy (SEM) and optical observations. Results are compared to those of carbon steel and austenitic AISI 304 and duplex SAF 2205 under similar conditions. After corrosion tests in alkaline media with chloride, ferrite tends to corrode selectively in 2304 duplex, while austenite corrodes selectively in 2001 under the same conditions. The influence of the duplex microstructure on attack development and morphology is analyzed. The electrochemical parameters obtained from the polarization curves suggest 2001 could replace 304 keeping the structure its corrosion performance (and with clear economical advantages). 2304 shows better corrosion behaviour than the more expensive 304, but somewhat lower than the excellent behaviour shown by 2205.     

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.     

The effect of silane-based hydrophobic admixture on corrosion of galvanized reinforcing steel in concrete

Sound or deliberately pre-cracked concrete specimens, with 0.5 or 1 mm wide crack, were manufactured with water to cement ratios (w/c) of 0.45 and 0.75, both in the presence and in the absence of a silane admixture. The specimens were exposed to wet-dry cycles in a 10% NaCl aqueous solution. The results, in terms of electrochemical measurements, and visual and metallographic observations carried out on the galvanized steel reinforcement removed from the specimens, showed that the hydrophobic concrete is able to protect galvanized steel reinforcement from corrosion even in the presence of cracks in the concrete cover, especially when a high w/c is used.     

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.     

Damage analysis and cracking model of reinforced concrete structures with rebar corrosion

The damage of concrete cover in reinforced concrete structures induced by reinforcing steel corrosion is investigated in this study. The damage process of the concrete cover can be divided into two distinct stages: the non-cracking stage and the partial cracking stage. An analytical model based on damage mechanics and elastic mechanics is developed to predict the concrete cracking due to steel corrosion. Based on this model, the expansive pressure and the radial loss of steel bar are discussed. Parametric studies are carried out to examine the effects of the correlative factors on the expansive pressure and the steel loss.     

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.     

Influence of pH on the nitrite corrosion inhibition of reinforcing steel in simulated concrete pore solution

Measurements of corrosion rate of reinforcing steels have been carried out in solutions simulating electrolytic chloride environments in corrosion pits in the propagation period with sodium nitrite. A significant corrosion inhibition has been observed at relatively low chloride contents of 0.045 M due to the presence of nitrite in these systems, but its efficiency decreases when the pH is reduced. The corrosion intensity seems to be related to the [Cl⁻]/[OH⁻] ratio. Three different pH regions from acid to alkaline have been observed in terms of corrosion activity. An explanation on the behaviour of nitrite at different pH is given.     

Corrosion inhibition of carbon steel in hydrochloric acid solution by fruit peel aqueous extracts

The inhibitive action of the aqueous extracts of fruit peels against corrosion of carbon steel in a 1 M HCl solution was investigated using electrochemical impedance spectroscopy, potentiodynamic polarization curves, weight loss measurements and surface analysis. We analyzed aqueous extracts of mango, orange, passion fruit and cashew peels in different concentrations and found that the extracts act as good corrosion inhibitors for the tested system. The inhibition efficiency increases with increasing extract concentration and decreases with temperature. The adsorption of components of the fruit peel extracts on the surface of the carbon steel follows the Langmuir adsorption isotherm.     

On the corrosion behaviour of phosphoric irons in simulated concrete pore solution

The corrosion behaviour of three phosphoric irons P₁ (Fe-0.11P-0.028C), P₂ (Fe-0.32P-0.026C) and P₃ (Fe-0.49P-0.022C) has been studied in simulated concrete pore solution (saturated Ca(OH)₂ solution) containing different chloride concentration. This has been compared with that of two commercial concrete reinforcement steels, a low carbon steel TN (Fe-0.148C-0.542Mn-0.128Si) and a microalloyed corrosion resistant steel CS (Fe-0.151C-0.088P-0.197Si-0.149Cr-0.417Cu). The beneficial aspect of phosphoric irons was revealed from potentiodynamic polarization experiments. The pitting potentials and pitting nucleation resistances for phosphoric irons and CS were higher than that for TN. Electrochemical impedance spectroscopy (EIS) studies revealed thickening and growth of passive film as a function of time in case of phosphoric irons and CS in saturated Ca(OH)₂ pore solutions without chloride and in the same solution with 0.05% Cl⁻ and 0.1% Cl⁻. In case of TN, breakdown of passive film resulted in active corrosion in simulated pore solution containing 0.1% Cl⁻. Linear polarization resistance measurements complemented EIS results. Visual observations indicated that phosphoric iron P₃ was immune to corrosion even after 125 days of immersion in saturated Ca(OH)₂ solution containing 5% NaCl. The good corrosion resistance of phosphoric irons in simulated concrete pore solution containing chloride ions has been related to the formation of phosphate, based on ultraviolet spectrophotometric analysis and Pourbaix diagram of phosphorus-water system.     

The effect of temperature on the corrosion of steel in concrete. Part 2: Model verification and parametric study

A comprehensive model for predicting the corrosion rate of steel in concrete has been developed using the concept of simulated polarization resistance experiments. This model is developed by carrying out a nonlinear regression analysis on data obtained from numerical experiments that are based on the solution of Laplace’s equation in a domain determined by the polarized length of the rebar. This part of the paper provides a comprehensive verification of the developed model and illustrates the application of the model to investigate the coupled effects of parameters affecting corrosion of steel in concrete. The results of the verification study show that the model predictions are in good agreement with the experimental data.     

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.     

Corrosion behaviour of steel in concrete in the presence of stray current

Effects of stray current on the corrosion behaviour of steel in concrete have been studied, with regards to both corrosion initiation and propagation. Results showed that DC current can induce corrosion initiation on the reinforcement in the anodic zone only after it has circulated for a certain time, which depends on the anodic current density, the presence of chloride in the concrete and interruptions in the current. AC current proved to be much less dangerous than DC, although it can influence the corrosion rate of steel in chloride-contaminated concrete and stimulate macrocouples.     

The initial stage of pitting corrosion on coated steels investigated by photon rupture in chloride containing solutions

A photon rupture method, film removal by a focused pulse of pulsed Nd-YAG laser beam irradiation, has been developed to enable oxide film stripping at extremely high rates without contamination from the film removal tools. In the present study, Zn-55mass%Al alloy and Al-9mass%Si alloy-coated steel specimens covered with protective nitrocellulose film were irradiated with a focused pulse of a pulsed Nd-YAG laser beam at a constant potential in 0.5 kmol m⁻³ H₃BO₃-0.05 kmol m⁻³ Na₂B₄O₇ (pH = 7.4) with 0.01 kmol m⁻³ of chloride ions to investigate the initial stage of localized corrosion. At low potentials, oxide films on both coated alloys were reformed after the nitrocellulose films were removed by this method. The oxide film formation kinetics follows an inverse logarithmic law, in agreement with Cabrera-Mott theory. However, at high potentials, localized corrosion producing corrosion products occurs at the area where nitrocellulose film was removed. Nevertheless, when the applied potential is less noble, the dissolution current of the Zn-55mass%Al-coated steel samples is higher than that of Al-9mass%Si-coated samples.