Effects of cathodic prevention on the chloride threshold for steel corrosion in concrete

This paper presents the results of long-term experimental tests aimed at studying the effects of cathodic prevention on the critical chloride threshold for the onset of pitting corrosion on steel bars in concrete. Cathodic prevention is applied when the structure is new and rebars are still passive being in contact with alkaline and chloride-free concrete. The results obtained showed that even very low values of current densities can increase the critical chloride threshold and, therefore, contribute to increase the service life of reinforced concrete structures in chloride-bearing environments.     

Investigations of pitting corrosion of magnesium by means of DEIS and acoustic emission

The paper presents methodology and results of electrochemical examination of magnesium in 0.1 M NaCl solution with different pH. The measurements were conducted under potentiodynamic conditions using Dynamic Electrochemical Impedance Spectroscopy (DEIS). Analysis of DEIS results was performed based on electrical equivalent circuit. As the result of analysis changes of separate parameters of the equivalent circuit vs. potential for different pH values of the environment were obtained. Simultaneously with DEIS measurements an investigation using acoustic emission was conducted. The results showed that for pH higher than 12.50 the properties of layers on the surface of magnesium change significantly. One can observe a stable passive state that has properties considerably different from those of the layers that form in solutions with lower alkalinity. In such conditions the beginning of pitting corrosion process can be unequivocally determined by the level of acoustic emission and the values of electrochemical parameters.     

Dimensional analysis applied to pitting corrosion measurements

In this work, two new indices, S_E and S_G are proposed to evaluate pitting corrosion by dimensional analysis of three parameters of PSD, the slope of high frequency linear region (k), the critical frequency (f_c) and the low-frequency plateau level (W). And then they are employed to analyze the potential electrochemical noise obtained during the corrosion processes of stainless steel 1Cr18Ni9Ti and pure Al, two materials most susceptible to pitting corrosion in NaCl solutions. Meanwhile another two mathematical methods, statistics and wavelet transform techniques, are also carried on the same potential oscillation data to estimate the function (meaning) of these two indices (S_E and S_G) in monitoring corrosion processes. The results suggest that, the index S_E is directly proportional to the rapider potential oscillating and higher fluctuation amplitude and can be used to depict the distribution behavior of electrochemical noise, while the index S_G can be used to depict the slower corrosion processes.     

AC and DC studies of the pitting corrosion of Al in perchlorate solutions

The pitting corrosion behaviour of Al in aerated neutral sodium perchlorate solutions was investigated by potentiodynamic, cyclic voltammetry, galvanostatic, potentiostatic and electrochemical impedance spectroscopy (EIS) techniques, complemented by ex situ scanning electron microscopy (SEM) examinations of the electrode surface. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of Al₂O₃ on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film by ClO₄⁻ ions. SEM images confirmed the existence of pits on the electrode surface. Cyclic voltammetry and galvanostatic measurements allow the pitting potential (E_pit) and the repassivation potential (E_rp) to be determined. E_pit decreases with increase in ClO₄⁻ concentration, but increases with increase in potential scan rate. Potentiostatic measurements showed that the overall anodic processes can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involve pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time (t_i). The rate of pit nucleation (t_i⁻¹) increases with increase in ClO₄⁻ concentration and applied step anodic potential (E_s,a). EIS measurements showed that at E_s,a < E_pit, a charge-transfer semicircle is obtained. This semicircle is followed by a Warburg diffusion tail at E_s,a > E_pit. An attempt is made to compare the values of E_pit and E_rp obtained through different methods and to determine the factors influencing these values in each particular method.     

Transitions between pitting and intergranular corrosion in AA2024

The pitting and intergranular corrosion (IGC) behavior of various tempers of AA2024 was investigated in 1 M NaCl. The breakdown potentials associated with pitting or IGC were determined. The breakdown potentials were found to be almost independent of sample orientation for any given temper. Artificial aging had a strong effect on polarization behavior and localized corrosion morphology. The anodic polarization curves of AA2024 in the solution heat treated and water-quenched condition, T3, and T3+ tempers exhibited two breakdown potentials, whereas overaged AA2024-T8, T8+, and solutionized and furnace cooled AA2024 exhibited only one breakdown potential. When two breakdown potentials were observed, the more active one was found to be related to the transient dissolution of S phase Al₂CuMg particles leading to pitting while the noble one was thought to result primarily from initiation and growth of IGC. The breakdown potentials decreased with increasing aging time at 190 °C, and only one breakdown potential was measured for T8 and T8+ tempers. Unlike the T3 temper, no sharp IGC was found for these tempers. Selected granular attack from breakdown of the copper-depleted matrix was believed to be the cause for localized corrosion in the T8 and T8+ tempers. The effect of nitrate and sulfate ions on the localized corrosion behavior was also studied.     

Chloride induced corrosion of reinforcing steel evaluated by concrete resistivity measurements

The chloride threshold (Cl_TH) concentration for rebar corrosion initiation has received extensive attention over the last years. The chloride threshold concentration depends on several factors involving concrete composition and quality, exposure conditions and rebar surface characteristics. As a consequence, many researchers have proposed Cl_TH ranges that take into account the relative influence of each of these many factors. On the other hand, the electrical resistivity of concrete has proven to be an effective parameter that can be used to estimate the risk of reinforcing steel corrosion, particularly when corrosion is induced by chloride attack. The present study is based on a correlation of electrochemical parameters such as corrosion potential (E_corr) and current density (i_corr) together with concrete resistivity (ρ) and chloride concentration data. A relationship between chloride threshold values for rebar corrosion initiation and resistivity values (indicative of concrete quality) is proposed. According to this correlation, when the electrical resistivity of concrete increases from 2 to 100 kΩ cm, the value of Cl_TH increases from 0.44 to 2.32% relative to the weight of cement.     

Uniform and pitting corrosion events induced by SCN anions on Al alloys surfaces and the effect of UV light

The influence of the alloying elements on the uniform and pitting corrosion processes of Al-6061, Al–4.5%Cu, Al–7.5%Cu, Al–6%Si and Al–12%Si alloys was studied in 0.50 M KSCN solution at 25 °C. Open-circuit potential, Tafel polarization, linear polarization resistance (LPR) and ICP-AES measurements were used to study the uniform corrosion process on the surfaces of the tested alloys. Cyclic polarization, potentiostatic current-time transients and impedance techniques were employed for pitting corrosion studies. Obtained results were compared with pure Al. Passivation kinetics of the tested Al samples were also studied as a function of applied potential, [SCN⁻] and sample composition by means of potentiostatic current transients. The induction time, after which the growth of stable pits occurs, decreased with increasing applied potential and [SCN⁻]. Regarding to uniform corrosion, alloyed Cu was found to enhance the corrosion rate, while alloyed Si suppressed it. Alloying elements of the tested samples diminished pitting attack to an extent depending on the percentage of the alloying element in the sample. Among the investigated materials, Al–Si alloys exhibited the highest corrosion resistance towards uniform and pitting corrosion processes in KSCN solutions. The passive and dissolution behaviour of Al was also studied under the conditions of continuous illumination (300–450 nm) based on cyclic polarization and potentiostatic techniques. The incident photons had a little influence on pit initiation and a marked effect on pit growth. These explained in terms of a photo-induced modification of the passive film formed on the anode surface, which render it more resistant to pitting. The effects of UV photons energy and period of illumination on the morphology of the pitted surfaces were also studied.     

Enzyme-induced ennoblement of AISI 316L stainless steel: Focus on pitting corrosion behavior

The use of purified enzymes in microbial influenced corrosion (MIC) studies is increasingly recognized as a powerful tool to understand electrochemical interfacial processes, especially the ennoblement of stainless steels (SS) in natural waters. The ennoblement has attracted the interest of many researchers as the consequences in terms of pitting corrosion are still not well understood.In the present study, the ennoblement of AISI 316L SS was induced by glucose oxidase (Gox) catalyzed reaction or by adding hydrogen peroxide (H₂O₂) in synthetic fresh water, on the basis of previous work. The corrosion behavior of the sample was studied using potentiodynamic and galvanostatic polarization tests. When the ennoblement occurs, the pitting potential (E_p) becomes nobler as well. The involvement of H₂O₂ to enhance pits repassivation seems to be a key consideration in this respect.Results obtained using this enzymatic system enable us to reappraise the commonly acknowledged hypothesis that the ennoblement increases the risk of localized attacks.     

Passivation and pitting corrosion of nanostructured Sn-Ni alloy in NaCl solutions

The passivation and pitting corrosion of tin-nickel alloy (34% Ni-66% Sn) in NaCl solution was studied using potentiodynamic, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques complemented by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of concentration of the chloride ion, the switching potential, scans rate and pH on the electrochemical behavior of Sn-Ni alloy is discussed. The data indicate that the corrosion rate and the pitting corrosion of Sn-Ni alloy increases by the increasing of chloride ion concentration. The observed corrosion resistance of electrodeposited Sn-Ni alloy is due to the formation of a thin passive film from tin and nickel oxides.     

Initiation and propagation of pitting and crevice corrosion of hydrogen-containing passive films on X70 micro-alloyed steel

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential E_p of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO₃ solution. Pre-charged hydrogen does not have a significant effect on the pitting potential E_p and open circuit potential E_corr of the steel in 0.5 M NaHCO₃ solution. However, a synergistic effect of hydrogen and Cl⁻ on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO₃ solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed.     

Chloride threshold dependence of pitting potential of reinforcements

Chloride threshold is one of the key parameters needed for service life prediction of concrete structures. The main difficulty that the determination of this parameter encounters is that the chloride threshold inducing active corrosion is not a unique value and depends on several variables. Among them the electrical potential exhibited by the rebar surface is the one less experimentally studied, even though it was identified some decades ago as the most comprehensive controlling parameter for the disruption of passivity. In the present paper the influence of the potential of reinforcements embedded in mortar on the chloride threshold has been studied for the first time in a region from +250 to −650 mV with respect to SCE. Identical samples have been tested in order to perform a statistical treatment and so mean values are given. The chloride thresholds found are expressed as total and free chloride as well as chloride/hydroxide (Cl⁻/OH⁻) ratio. The results show that chloride threshold is independent of the potential for values more anodic than −200±50 mV (SCE) being this region the typical potentials exhibited by passive reinforcements in atmospheric conditions. For potentials more cathodic than −200±50 mV (SCE), the threshold progressively increases and a linear fitting is possible. The dependence of chloride threshold with the potential approaches to the understanding why in previous works researchers have found significant differences.     

An investigation on general corrosion and pitting of iron with the in-line digital holography

In the anodic processes of the iron electrode in both neutral and acidic solutions, general corrosion and pitting dynamical processes in the presence of chloride ions were observed in situ with the in-line digital holography. The dynamic processes such as formation and dissolution of the passive film, initiation and propagation of pitting have been studied. During the general corrosion, not only is the temporal structure in order, evidenced by the regular change of current with time, but also the spatial structure, indicated by the uniform fringes in the holograms at the electrode/electrolyte interface. However, during the pitting processes, the order of the spatial structure is destroyed, which is indicated by the wrinkled fringes in the holograms; the temporal order is broken as well evidenced by the irregular change of current with time. The experimental result shows that there is synergistic effect between H⁺ and Cl⁻ on both the initiation stage and the propagation stage. With the presence of chloride ions, pitting occurs much more easily in acidic solution than in neutral solution.     

A SECM assisted EQCM study of iron pitting

The local breakdown of the iron passive layer and the resulting pitting corrosion of iron were studied for massive material and for a thin film on an electrochemical quartz crystal microbalance (EQCM). The initiation and the propagation of a single pit on iron were controlled through the local production of chloride anion thanks to the silver chloride reduction at the tip of the scanning electrochemical microscope (SECM). Experiments were performed in borate buffer and in KOH solution, in order to investigate the influence of the solution pH. The influence of the iron potential and of the amount of chloride anion necessary for the breakdown of the passive layer were also investigated. The frequency response of the quartz was correlated to the pit evolution with respect to time. After the breakdown of the passive layer, the frequency changes were then directly linked to the generation of Fe(II) during the pit growth.     

Effects of Cr2N on the pitting corrosion of high nitrogen stainless steels

Effects of nitrogen (N) content on the precipitation behavior of Cr₂N with its relevant influences on the resistance to localized corrosion of high nitrogen stainless steels (HNSs) were examined using an electrochemical microcell. HNS of Fe-18Cr-20Mn-3Mo-1.07N was highly resistant to pitting corrosion. With aging the alloy at 900 °C, Cr₂N precipitated initially along grain boundaries, then within grain, and eventually formed a lamellar structure, accompanied with a drastic decrease in the resistance to pitting corrosion. It was found from the electrochemical microcell tests that the lamellar Cr₂N was the most susceptible site for pitting corrosion when compared with any other heterogeneity such as a (Mn, Cr)-oxide or a Cr₂N precipitated along the grain boundaries.     

Surface and electrochemical characterization of pitting corrosion behaviour of 304 stainless steel in ground water media

The effectiveness of aminotrimethylidene phosphonic acid (ATMP) as a corrosion inhibitor in association with a bivalent cation like Zn²⁺ and non-ionic surfactant like polyoxyethylene sorbitan monooleate (Tween 80) were investigated by measuring corrosion losses using electrochemical techniques. The corrosion of 304 stainless steel in the ground water medium was inhibited by complexation of the inhibitor. A combined inhibition effect was achieved by adding both ATMP and Zn²⁺ along with Tween 80. The formulation functioned as a mixed type inhibitor. The synergistic effect of the inhibitor compound is calculated. Luminescence spectra, FTIR spectra, XRD, XPS and scanning electron microscopic studies were carried out to understand the mode of corrosion inhibition and also the morphological changes on the metal surface.     

The role of inhibitors on the repassivation of pitting corrosion of carbon steel in synthetic carbonated concrete pore solution

The semiconductive properties and pitting susceptibility of passive films formed on carbon steel in synthetic carbonated concrete pore solution containing Cl⁻ ions were studied by Mott–Schottky (M–S) plots and electrochemical noise (EN). The results show that both nitrite and tetraethylenepentamine (TEPA) can reduce the donor density in passive film, leading to a more positive film breakdown potential. Moreover, nitrite can rehabilitate metastable pits and passivate the broken passive film more quickly than TEPA. Polarization curves also show that nitrite can increase the pitting potential more than TEPA at same concentration. Atomic force microscopy (AFM) imaging shows that Cl⁻ ions could induce large metastable pits on passive film, but the size of pits decreases and the density of pits increases with addition of nitrite. However, AFM imaging fails to detect the metastable pits under TEPA adsorbed film in spite of a smooth-like image on the steel surface is observed. The force curves of AFM indicate a stiffness (elasticity) order of films: nitrite oxidation film > original passive film > enamine adsorbed film.     

Investigation of hydrogen induced pitting active sites

Hydrogen-induced pitting active sites on iron were investigated by electrochemical noise (EN) and scanning reference electrode techniques (SERT). The significant increase in the number of current and potential fluctuations in EN measurements indicates that hydrogen increases the number of active sites for formation of metastable pits, which are precursors of stable pits. The potential distributions over the surfaces of hydrogen charged and uncharged iron polarized at 0.07 V (vs. SCE) for different polarization times also show clearly that hydrogen significantly increases the number of pitting active sites on the surface of charged iron. It was also observed that to reach equal numbers of pitting active sites, a higher applied anodic potential is required on uncharged iron than on hydrogen-charged iron. The increase in the number of pitting active sites due to hydrogen was discussed in terms of the effect of hydrogen on the nature of the passive film and the dissolution rate of iron.     

Electrochemical state conversion model for occurrence of pitting corrosion on a cathodically polarized carbon steel in a near-neutral pH solution

Pitting corrosion occurs frequently on oil/gas pipelines which are protected by both coating and cathodic protection (CP). Since the steel is in an active state, the classic theories attributing pitting to the passive film breakdown do not apply. This work is anticipated to advance the understanding of the fundamentals of pitting corrosion of carbon steel pipelines occurring in an active system. In this work, a square wave polarization method was used to simulate the CP fluctuation and its effect on pipeline steel pitting in a near-neutral pH solution. Moreover, an electrochemical state conversion model was developed to illustrate the pit initiation and growth on the cathodically polarized steel. According to ESCM, when a potential fluctuation is generated on the steel electrode, the local double-charge layer structure is disturbed. The defect area undergoes a temporary anodic potential field, resulting in the local anodic dissolution to nucleate pits. Thus, local anodic dissolution (pitting) of steel would occur under an unstable cathodic polarization. Furthermore, it is found that the potential shifting range plays an important role in pitting initiation and growth. Pits nucleate in high-density under square wave polarization if the polarization potential range is relative narrow and the upper potential is not so negative, i.e., close to OCP. When under a wide potential range and a relatively negative upper potential, pits tend to grow into big size with a low density. Therefore, although the pipeline is protected by CP, the CP fluctuation could occur during service, which would introduce the polarization fluctuation on the steel, resulting in pitting corrosion.     

Evaluation of localized corrosion in duplex stainless steel aged at 850 °C with critical pitting temperature measurement

Effect of aging at 850 °C on pitting corrosion of UNS S31803 duplex stainless steel was examined in chloride solution by potentiostatic critical pitting temperature (CPT) measurements. The quantitative metallography coupled with X-ray diffraction technique was employed to follow the microstructure evolution. Moreover, the initiation and propagation of pitting corrosion had been imaged in relation to microstructure variations using scanning electron microscopy (SEM). The results demonstrated that the corrosion behavior is strongly dependent on the microstructure, namely the presence of sigma phase. A deterioration of pitting corrosion resistance is found after aging 4 min, resulting in a drop in CPT. In particular, the metastable current transients during CPT test can clearly reflect the initiation of pitting process. Pitting nucleates preferentially in the austenite phase for the solution-annealed specimen, while the initiation of pitting corrosion takes place around sigma phase, in the newly formed secondary austenite for the aged specimen.     

Inhibition of Aluminium Alloy AA2024-T3 pitting corrosion by copper complexing compounds

A scanning vibrating electrode technique (SVET) is used to quantify the influence of a range of copper complexing/precipitating reagents on the kinetics of pitting corrosion affecting AA2024-T3 immersed in 5% aqueous sodium chloride (NaCl). It is shown that organic anions such as 2,5 dimercapto, 1,3,4 thiadiazolate (DMTD), benzotriazolate (BTA) and N,N- diethyl, dithiocarbamate (DEDTC), and inorganic ions such as ferrocyanide effectively inhibit pitting corrosion by film formation on Cu rich intermetallic particles (IMPs) and/or precipitation of aqueous copper cations. Inhibitor efficiency increases in the order ferrocyanide < BTA < DEDTC ≈ DMDT. In contrast, the presence of ethylenediamminetetraacetic acid (EDTA) is shown to prevent copper (Cu) replating, but does not lead to any decrease in localised corrosion currents. This supports the idea that that ample cathodic activity can be maintained on Cu-rich IMPs in the absence of any copper replating to sustain stable pitting on AA2024.