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Chloride ion is known to accelerate corrosion in numerous metallic systems, although it is not well understood why. Using first-principles methods, such as density functional theory, it is possible to directly compute the energy of adsorption for chloride, and, using a thermodynamic framework based on statistical mechanics, to predict the environmental conditions under which it may be anticipated that chloride could displace other ions (such as hydroxide, incipient oxide, or other potentially corrosion inhibiting species) from the bare metal and/or oxide surface. By reviewing the literature, as well as performing some original density functional theory calculations, the authors present in this work the dependence of the surface coverage for H 2 O, OH, O and Cl on the electrochemical potential, pH and chloride concentration for a number of metallic systems, including nickel, iron, magnesium and aluminum, to predict the fundamental surface processes that may related to the role of chloride in potentially interfering with processes such as repassivation and metal dissolution. It is found that under some conditions certain metals can possess zones of mutual stability where both chloride and hydroxide may be coadsorbed, whereas other metals appear to have intrinsic resistance to chloride adsorption. Variations among the available first-principles adsorption energies obtained from different sources indicate that uncertainty analysis is necessary moving forwards. An analogous approach could be taken to consider the interaction of chloride with clean and defective oxide and/or hydroxylated surfaces. [ABSTRACT FROM AUTHOR]

This investigation focuses on sodium biphosphate (Na 2 HPO 4 ) as corrosion inhibitor for construction steel. All the tests are carried out in a solution that simulates the composition of the pores in chloride-contaminated carbonated concrete. The carbonated solution (CS) contained Na 2 CO 3 (0.0015 mol L −1 ), NaHCO 3 (0.03 mol L −1 ) and NaCl (0.1 mol L −1 ), resulting in [Cl − ]/[OH − ]=10000. Inhibited solutions (IS20, IS60 and IS100) incorporated 20, 60, and 100 mmol L −1 Na 2 HPO 4 respectively. These were labeled IS20, IS60 and IS100 respectively and result in [HPO 4 2− ]/[Cl − ] = 0.2, 0.6, and 1. Cyclic voltammograms and anodic polarization curves were complemented with micro-Raman spectroscopy and XPS, to evaluate the surface film composition. The results show that chloride contamination promotes active corrosion. When phosphate ions are incorporated, steel becomes passive with a more positive corrosion potential (Ecorr), and pitting presents as the predominant form of localized corrosion. Raman spectra show a broad band, centred in 982 cm −1 , suggesting that phosphates incorporate to the passive film. Phosphates are also present in the corrosion products. The surface film becomes more protective to pitting for the highest biphosphate content. However, after pitting no repassivation was detected. After over one month in immersion, steel remains passive in the condition IS100, with inhibition efficiency higher than 99%. In contrast, in the case of IS60 and IS20, pitting was detected. It can be concluded that phosphate ions are good candidates to be used as corrosion inhibitors for steel in chloride-contaminated concrete. [ABSTRACT FROM AUTHOR]

The purpose of this investigation is to analyze the effectiveness of phosphates as inhibiting agents for steel bars embedded in mortars. Three mix designs were selected for this study: mix A had no admixed chlorides and was used as reference; mix designs B and C were contaminated with 1% in weight of chlorides/weight of cement and mix C also incorporated 7% Na 3 PO 4 .12 H 2 O. After setting for 48 h, the samples were cured for 7 days immersed in water. Three specimens of each group were then immersed in aerated saline solutions 0.5 mol L −1 NaCl during 720 days. The corrosion potential ( Ecorr ), the polarization resistance ( Rp ) and the electrochemical impedance spectra were recorded regularly. After 720 days of immersion one set of mortars was anodically polarized and another set cathodically polarized. After that, Raman spectra of corrosion products were registered. Additional specimens were used to evaluate porosity and chloride profiles. The presence of phosphate ions as inhibitors has no effect on Ecorr values and suggests mixed-type inhibition. Mix C remains passive until 180 days of exposure, with Rt> 100 kΩ cm 2 . At longer times, Rt decreases in time but the inhibition percentage is always higher than 95% when evaluated by impedance spectra. Using Rp , inhibition stays above 70%. Anodic and cathodic polarization curves, together with corrosion potential values, suggest that phosphates behave as a mixed-type inhibitor. [ABSTRACT FROM AUTHOR]

Highlights: [•] Anticorrosive properties of the coating modified with Ce-enriched zeolite microparticles. [•] Adsorption of Cu2+ and Cl− on zeolite in response to release of Ce(III) by ion exchange process. [ABSTRACT FROM AUTHOR]

Abstract: Phosphate ions were evaluated as corrosion inhibitors in solutions that simulate the composition of the pores in concrete contaminated with chloride ions. Cyclic voltammograms and potentiodynamic polarization tests were complemented with micro-Raman spectroscopy and impedance spectroscopy to follow the performance of this inhibitor. Long term performance involved weight loss evaluation. Chloride contamination increases the accumulation of corrosion products on the metallic surface and promotes pitting corrosion. In contrast, pitting is inhibited when phosphate ions are incorporated in a 1:1 phosphate to chloride molar ratio, even after a 90 days exposure. Micro Raman spectra clearly show the incorporation of phosphates to the passive film. Impedance spectroscopy results can be interpreted assuming a duplex surface film formed in the presence of phosphates. In the conditions of this investigation, phosphate ions behave as mixed-type corrosion inhibitors, protecting steel against corrosion in chloride-contaminated environments. [Copyright &y& Elsevier]

Abstract: An approach for the time dependent evolution of the local chemical environment during the initial stage of localised corrosion for aluminium alloys has been developed. The approach was applied to study anodic dissolution of S phase in sodium chloride, where both metallic species, Al3+ and Mg2+, were taken into consideration. The developed model includes hydrolysis products of aluminium and species obtained as a result of homogeneous reactions between chloride and Al3+ and Mg2+ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in the homogeneous reactions are treated explicitly using kinetic constants either taken from the literature or derived using the Eigen mechanism for complex formation. It was shown that for an active dissolution process, the time needed for chemical species to achieve their steady state concentrations ranges from minutes (for solution potential and the pH value) to hours or even days. Thus, modelling predictions obtained with the steady state approximation can give inaccurate values for species concentrations. Two modelling examples are discussed: the dissolution of a large S phase and small isolated S phase particle in Al alloys. Input data for the models are supplied by microcapillary measurements. [Copyright &y& Elsevier]

Abstract: The crevice corrosion kinetics of alloys C-22, C-22HS and HYBRID-BC1 was studied in several chloride solutions at 90°C. The crevice corrosion resistance of the alloys increased with PREN (Pitting Resistance Equivalent Number), which is mainly affected by the Mo content in the alloys. The crevice corrosion kinetics of the three alloys was analyzed at potentials slightly higher than the repassivation potential. Crevice propagation was controlled by ohmic drop in the more dilute chloride solutions, and by charge transfer in the more concentrated chloride solutions. Ohmic drop was not a necessary condition for crevice corrosion to occur. [Copyright &y& Elsevier]

Abstract: The influence of initial surface conditions on the subsequent corrosion behaviour of AZ31 and AZ61 magnesium alloys in 0.6M NaCl solution has been studied using electrochemical impedance spectroscopy. For obtaining the different surface conditions, some of the specimens were immersion tested with the surface in the as-received condition, while others were tested immediately after mechanical polishing, and part of the polished specimens after six months of exposure to the laboratory atmosphere. Considering the evolution of the high-frequency capacitive arc of the Nyquist diagram, whose diameter is related to the corrosion process, a clear effect of the initial surface conditions is observed only in the early stages of testing. This effect is especially significant for the freshly polished specimens. [Copyright &y& Elsevier]

Abstract: The corrosion properties of electrodeposited pure cobalt were investigated in deaerated 0.5M Na2SO4 solutions at pH 5, 7, 10 and 13 by using polarization techniques. The effect of chloride concentration (0.01 and 0.1M NaCl) was also studied. The results showed that increasing pH shifted the anodic polarization curves to more negative potentials. At pH 5 and 7, cobalt exhibited active dissolution without any distinctive transition to passivation. However, at pH 10, the metal surface was partially passivated and anodic current increased sharply at approximately −0.2VSCE and the corroded surface revealed several well-defined pits. On the other hand, at pH 13, a clear passivation region was observed within the potential range of −0.5 to 0.6VSCE. With increasing chloride concentrations of 0, 0.01, and 0.1M at pH 10, both the anodic current and the number of pits on the cobalt surface were gradually increased. X-ray photoelectron spectroscopic investigations showed that the cobalt surface was covered mostly with cobalt hydroxide after anodic polarization at pH 10 in 0.1M NaCl. [Copyright &y& Elsevier]

Abstract: The corrosion behaviour of steel was studied in aerated near neutral citrate solutions without and with various concentrations of NaCl. The potentiodynamic anodic polarization curve in 0.1M citrate solution exhibits four anodic peaks A1, A2, A3 and A4 prior to the oxygen evolution reaction. Addition of Cl− ions to the solution enhances the four peaks currents, specially A3, which is followed by pitting corrosion. The negative going scans of the cyclic voltammograms show two anodic reactivation peaks A5 and A6 and one cathodic plateau P1. A diffusion controlled process in the potential range of A1, A2 and P1 was detected by RDE experiments. The potentiostatic current time transients, at different concentrations of NaCl and applied potentials E a >A3, were studied. The pit nucleation rate (t i −1) is found to increase with increasing the concentration of NaCl and the applied anodic potential. The impedance spectra exhibit four different behaviours depending on the potential range used. They were fitted with a single time constant circuit at E a <−700mV. However, at −700mV< E a <−480mV, they were fitted with a circuit with two time constants. At E a >−480mV, the second semicircle is replaced by negative polarization resistance which is disappeared at E a >−300mV. The electrode impedance was found to decrease with the applied potential. [Copyright &y& Elsevier]