In situ 3D monitoring of corrosion on carbon steel and ferritic stainless steel embedded in cement paste

In a X-ray microcomputed tomography study, active corrosion was induced by galvanostatically corroding steel embedded in cement paste. The results give insight into corrosion product build up, crack formation, leaching of products into the cracks and voids, and differences in morphology of corrosion attack in the case of carbon steel or stainless steel reinforcement. Carbon steel was homogeneously etched away with a homogeneous layer of corrosion products forming at the steel/cement paste interface. For ferritic stainless steel, pits were forming, concentrating the corrosion products locally, which led to more extensive damage on the cement paste cover.     

Chloride-induced corrosion of steel embedded in mortars containing fly ash and spent cracking catalyst

Stationary and non-stationary chloride diffusion coefficients of mortars prepared with ordinary Portland cement partially substituted by spent cracking catalyst (FC3R) and fly ash (FA) have been determined. An accelerated test of chloride migration through the mortar has been performed in order to determine these coefficients. The results show that the presence of FC3R improves chloride binding properties of mortars and the addition of FA reduces the total porosity, both factors leading to a higher resistance to chloride ingress. Corrosion rate of steels embedded in these ternary-binder mortars do not show significant differences compared to reference mortars (with plain cement), so a combined cement substitution as high as 45% by FC3R and FA do not lead to a higher risk of chloride-induced corrosion of steel when tested at an early age.     

Environmental factors affecting the corrosion behavior of reinforcing steel III. Measurement of pitting corrosion currents of steel in Ca(OH)2 solutions under natural corrosion conditions

Using a simple electrolytic cell, the pitting corrosion current of reinforcing steel is measured in Ca(OH)₂ solutions in presence of chloride and sulfate as aggressive ions. Pitting corrosion current starts to flow after an induction period which depends on the concentration of both the aggressive and the passivating anions. The pitting corrosion current densities reach steady-state values which depend also on the type and concentration of the corrosive and passivating anions. The corrosive action of the aggressive species decreased in the order: SO₄²⁻ > Cl⁻. Corrosion of the steel is found to be governed by a single electron transfer reaction. Raising the temperature decreases the induction period associated with pit initiation and increases the corrosion current associated with pit propagation. From Arrhenius plots, the activation energies for both pit initiation and pit propagation in presence of chloride and sulfate ions are calculated.     

Steel corrosion behaviour in carbonated alkali-activated slag concrete

Steel bars embedded in an alkali-activated slag (AAS) concrete were exposed (after curing for 28 days) to an accelerated carbonation test (3% CO₂, 65% relative humidity (RH), and 25 °C temperature) and a laboratory environment (0.03% CO₂, 65% RH, and 25 °C). Ordinary Portland cement (OPC) was also tested for comparative purposes and exposed to identical experimental conditions. The corrosion behaviour of uncarbonated and carbonated AAS and OPC concretes was tested for different times, performing corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy measurements. Corrosion products were analysed using the Mössbauer technique. The main corrosion products found were magnetite (Fe₃O₄), wüstite (FeO), and goethite (α-FeOOH).     

Quantitative characterisation of steel/cement paste interface microstructure and corrosion phenomena in mortars suffering from chloride attack

Chloride ions constitute one of the deleterious agents that may cause or promote corrosion of steel reinforcement in concrete. The influence of chloride ingress on mortar microstructure (including microstructural alterations of hydration products and of pore structure) has been studied by the authors on the basis of cross-section image analysis of reinforced mortar specimens [D.A. Koleva, J. Hu, A.L.A. Fraaij, N. Boshkov, Influences of chloride ions on plain and reinforced mortars, investigated by combined microstructure and electrochemical approaches, Paper 315, Eurocorr 2005, September 4–8 ’05, Lisbon, Portugal]. This paper specifically pursues exploring the morphological aspects and chemical compositions of the corrosion products deposited on steel surface. For this purpose, scanning electron images (SEM) were taken on the cylindrical surface of steel reinforcement and also on the corresponding positions on cement paste surface for visualisation and microstructural investigations of corrosion products. In addition, energy dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD) are employed for quantitative characterisation of the corrosion products at the steel–paste interface. Electrochemical impedance spectroscopy (EIS) is used to estimate the corrosion current and corrosion rate for the reinforced mortars. The EIS measurements are in good agreement with the microstructural observations and quantitative analysis of various corrosion products. The combination of electrochemical measurements with quantitative microstructure analysis of the steel–paste interface constitutes a reliable and useful tool for quantitative characterisation of the interface microstructure and thereby provides better insight into the electrochemical processes during corrosion of the steel reinforcement in concrete.     

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.     

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.     

Galvanic currents and corrosion rates of reinforcements measured in cells simulating different pitting areas caused by chloride attack in sodium hydroxide

Chlorides induce localized corrosion in the reinforcement. The pits formed a substantial reduction in the cross-sectional area. The smaller is the ratio between the size of the corrosion spot and the surrounding passive metal area acting as cathode, the higher is the corrosion rate or the progression in the pit depth. This, however, is not always the case. To understand well the macro or microcell behavior of the corrosion of reinforcements, this paper reports a study on the effect of decreasing sizes of anodic zones until relatively small, maintaining the size of the cathode constant. In addition, to enhance the pure anodic behavior of the corroding zone, an acidic solution was used as an anolyte. Measuring the galvanic current (I_g) and the corrosion rate (I_corr) by using zero resistance ammeter (ZRA) and polarization resistance (R_p) techniques, respectively, was crucial to differentiate the microcell contribution from that of the macrocell. The results indicate that while I_g increased as the anodic area decreased, I_corr presented a maximum for the area ratios, S_A/S_C = 1/3.     

Characterization of steel corrosion in mortar by various electrochemical and physical techniques

In the study characterization of steel corrosion in concrete at the macro- and micro-level was performed. Physical (electrical-resistance probes) and electrochemical techniques (coupled multi-electrode arrays) were implemented in order to upgrade the general information that conventional electrochemical techniques can provide. Measurements were performed in mortar exposed to periodic wetting and drying. Steel corrosion damage was assessed by micro X-ray computer tomography (CT) and SEM. The results were compared and interpreted. By combined use of micro-CT and electrochemical methods, new insights into the corrosion mechanisms of steel in concrete were obtained.     

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.     

Comparison of the corrosion of bulk amorphous steel with conventional steel

The corrosion behaviour of one prominent bulk amorphous steel (BAS) alloy, (Fe_44.3Cr₅Co₅Mo_12.8Mn_11.2C_15.8B_5.9)_98.5Y_1.5, is compared to that of its crystalline counterpart and of the conventional steel X210Cr12. In acidic solutions, due to its single-phase nature, the BAS exhibits a much higher stability than the multiphase crystalline counterpart and the conventional steel. With increasing pH value, the overall elemental composition plays a more important role and the reactive elements Mo, Mn and Co, are assumed to be responsible for the observed lower stability of the BAS and its crystalline counterpart as compared to the conventional steel.     

Environmental factors affecting the corrosion behaviour of reinforcing steel. V. Role of chloride and sulphate ions in the corrosion of reinforcing steel in saturated Ca(OH)2 solutions

The corrosion behaviour of reinforcing steel in saturated naturally aerated Ca(OH)₂ solutions in absence and presence of different concentrations of NaCl, NH₄Cl, Na₂SO₄ and (NH₄)₂SO₄ is followed by measuring of the open circuit potential complemented with SEM and EDS investigation. These salts cause breakdown of passivity and initiation of pitting corrosion. The rates of oxide film thickening by OH⁻ ions and oxide film destruction by the aggressive ions follow a direct logarithm law and depend on the concentration and type of aggressive salts anions and cations. The values of the activation energies for oxide film thickening are calculated and discussed.     

Chloride induced reinforcement corrosion: Electrochemical monitoring of initiation stage and chloride threshold values

Reinforcement steel embedded in six different concrete mixes was exposed to chloride by wetting/drying cycles. Various parameters were continuously monitored during more than 1 year. Cement replacement with fly ash had beneficial long-term effects regarding chloride penetration resistance. Concerning corrosion performance, by far the most dominant influencing parameter was the steel/concrete interface since corrosion initiated on the lower side of the rebar (with respect to casting direction) regardless of binder type and w/b ratio. In many cases, after the first signs of depassivation, a marked increase in chloride content was required to prevent repassivation and to enable stable pit growth.     

Zr-based bulk glassy alloy with improved resistance to stress corrosion cracking in sodium chloride solutions

In order to evaluate stress corrosion cracking (SCC) susceptibility of Zr-based bulk glassy alloys and develop the BGAs with low susceptibility to SCC, the SCC behaviour of Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₀Al₁₀Ni₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ BGAs in various environments including sodium chloride solution has been investigated using a slow strain rate technique at an initial strain rate of 5 × 10⁻⁶ s⁻¹. It is found, for the first time, that the Zr₇₀Cu₆Al₈Ni₁₆ BGA has no susceptibility to SCC in a 0.5 M NaCl solution. On the other hand, Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₀Al₁₀Ni₁₀ BGAs are highly susceptible to SCC in the NaCl solution, although they are not susceptible to SCC in de-ionized water, phosphate buffer, 0.5 M Na₂SO₄ and 0.5 M NaNO₃ solutions. The possible cause of the high susceptibility to SCC in the NaCl solution for the Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₀Al₁₀Ni₁₀ BGAs is discussed.     

Micro-electrochemical characterization of corrosion of pre-cracked X70 pipeline steel in a concentrated carbonate/bicarbonate solution

The electrochemical corrosion behavior of a stressed, pre-cracked X70 pipeline steel was studied in a bicarbonate/carbonate solution by electrochemical and micro-electrochemical measurements, numerical calculation and surface analysis technique. The effects of stress and potential on passivity, corrosion and electrochemical behavior of the steel at crack-tip were mechanistically determined. It was found that the passive film formed at crack-tip was less stable than that formed in the region ahead of the crack. Moreover, the crack-tip is more susceptible to pitting corrosion than other region of the specimen. The applied stress enhances the anodic dissolution of the steel. In particular, the stress concentration at crack-tip further increases the local anodic dissolution rate. The enhancement of the anodic dissolution of the steel at crack-tip is also resulted from the formation of a galvanic couple, i.e., the crack-tip as an anode and the surrounding region as a cathode.     

Probabilistic considerations on the effect of specimen size on the critical chloride content in reinforced concrete

A probabilistic model was used to predict the critical chloride content (C_crit) for reinforcement corrosion as a function of specimen size. The specimen size is likely to be a reason for the large scatter of C_crit values as well as for the high threshold levels often observed in laboratory setups. Regarding test methods, it is discussed that the common combination of small specimens and low numbers of parallel samples results in poor reproducibility. If experimental data is to be used for service life modelling, the geometrical dimensions of the specimens on which C_crit was measured have to be taken into account.     

The influence of cracks on chloride-induced corrosion of steel in ordinary Portland cement and high performance concretes subjected to different loading conditions

Many reinforced concrete structures are subjected to variable loads, but little information is available on the effect of such loading, and the accompanying opening and closing of cracks, on the initiation and propagation of chloride-induced rebar corrosion. The purpose of this paper is to provide that information for reinforced ordinary Portland cement concrete and high performance concrete exposed to static or cyclic three-point bending and exposed to salt solution.It was observed that: (i) corrosion occurred only at intersections of the rebar with cracks in the concrete; (ii) high performance concrete was more protective than ordinary Portland cement concrete and (iii) the type of loading had less impact on corrosion than the type of concrete and exposure conditions.     

Corrosion of reinforcing steel in simulated concrete pore solutions: Effect of carbonation and chloride content

The corrosion susceptibility of as-received reinforcing steel bars (rebars) in solutions simulating the pore liquid of alkaline and carbonated concrete has been studied by means of potentiodynamic polarisation tests and polarisation resistance measurements. The effect of different degrees of carbonation and the presence of several chloride contents in the simulated pore solutions was investigated. Results show the beneficial effect of high alkalinity on the localised corrosion of steel caused by chloride ions. From the results of the potentiodynamic tests a critical chloride concentration above which pitting could take place was evaluated for each solution. The chloride threshold values here found are of the same order than those previously reported in the literature for film-free steel. The results obtained in solutions simulating carbonated concrete showed that under weak carbonation conditions carbon steel does not passivate while in the presence of high levels of carbonate and bicarbonate the resistance to localised corrosion is improved.     

Use of a 3D optical measurement technique for stochastic corrosion pattern analysis of reinforcing bars subjected to accelerated corrosion

The 3D corrosion patterns of 23 reinforcing bars subjected to accelerated corrosion are characterised using an optical surface measurement technique. A stochastic signal processing methodology is employed for corrosion pattern analysis of the measured data. The statistical analysis of corrosion pattern data shows that a lognormal distribution model can represent the non-uniform distribution of pitted sections along the corroded bars. It was observed that the frequency of corrosion is independent from the mass loss ratio and the length of the bars. Finally, a set of probabilistic distribution models for the geometrical properties of corroded bars is developed.     

Analysis of the galvanostatic polarization method for determining reliable pitting potentials on stainless steels in crevice-free conditions

In this paper the potential of the galvanostatic polarization technique as accelerated method for determining the characteristic pit potentials on stainless steels in crevice-free conditions is examined. Measurement of the potential change as a function of time shows a maximum that agrees with the nucleation pit potential. Thereafter, a stationary potential is reached corresponding to the protection potential against pit. Possible limitations of this kind of measurements have been remedied by refinements in the test procedure and conditions. The state of the surface oxide film and the applied anodic current are two basic parameters that must be well defined because they govern the pitting susceptibility. It has been found that with applied anodic currents in the range 40–200 μA/cm² and with prior electrode exposures to solution between 30 and 60 min it is possible to obtain results in excellent agreement with the conventional potentiodynamic tests with the advantage of smaller data scattering and absence of crevice at electrode/holder interfaces. These effects are the result of the rapid pitting stimulated in the galvanostatic method. This implies a short duration of the experiment thus also favouring the elimination of the time-dependent crevice, which notoriously contributes to the poor reproducibility of pit potentiodynamic potentials. A detailed series of experiments have been conducted on several stainless steels and in different test conditions to validate the accuracy of the galvanostatic polarization method.