Electrochemical injection of organic corrosion inhibitors into carbonated cementitious materials: Part 2. Mathematical modelling

A mathematical model is presented for simulation of changes in the pore solution phase chemistry of carbonated hardened cement paste when aqueous solutions of organic base corrosion inhibitors are applied to the surface of the material and constant current densities in the range of 1-5 A/m² are passed between anodes placed within the inhibitor solutions and steel mesh cathodes embedded within the paste. The model, based on the Nernst-Planck equation, is used to predict the concentration profiles of electrochemically injected inhibitors and the major ionic species present within the pore electrolyte. For two specific organic base inhibitors with widely different pK_a values, viz. ethanolamine (pK_a 9.5) and guanidine (pK_a 13.6), the model predictions are compared with experimental results described in Part 1 of the investigations.     

Electrochemical injection of organic corrosion inhibitors into concrete

Two organic bases (ethanolamine and guanidine) that are known to act as corrosion inhibitors for steel in aqueous media were introduced into saturated specimens of carbonated and non-carbonated concrete from external electrolytes under the influence of an electrical field applied between embedded steel cathodes and external anodes. The cathodic current density was galvanostatically controlled at values in the range 1-5 A/m² for periods of 3-14 days. Control experiments, in which the corrosion inhibitors were applied to similar saturated concrete surfaces from external electrolyte without current, were also conducted. After treatment, the specimens were sectioned and analysed to determine the concentration profiles of the corrosion inhibitors within the concrete. It was found that the efficiency of injection of both ethanolamine and guanidine under the applied field was far higher in carbonated concrete than in non-carbonated concrete and that, in the carbonated specimens, the inhibitors became concentrated near the embedded steel. In non-carbonated concrete, guanidine penetration was accelerated to a modest extent by the applied field but ethanolamine penetration was not significantly enhanced by the field. These findings were explicable in terms of the influence of the pH values of the pore solutions in the various specimens on the degrees of ionisation of the organic bases concerned and hence on their tendencies to migrate and neutralise cathodically-generated hydroxyl ions.     

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.     

Influence of steel-concrete interface condition on galvanic corrosion currents in carbonated concrete

This paper presents specific experiments which were developed in order to assess galvanic currents in macrocell corrosion specimens involving active steel in carbonated concrete and passive steel in sound concrete. The influence of the steel-concrete interface condition on the galvanic current was also experimentally investigated. To focus on macrocell corrosion rate assessment, the initiation time of the corrosion process (concrete carbonation) was accelerated. FEM simulations were carried out in order to enhance the physical comprehension of these corrosion experiments. It was found that, in realistic condition, the electrical coupling of active and passive steel areas leads to high galvanic currents and consequently high corrosion levels according to RILEM recommendation. Moreover, steel-concrete interfacial defaults significantly increase the macrocell driving potential and, therefore, the galvanic corrosion current.     

High-throughput channel arrays for inhibitor testing: Proof of concept for AA2024-T3

A high-throughput test has been developed for screening aqueous corrosion inhibitors on aluminium AA2024-T3. The method adapts basic microfluidic technology to create multiple channels in polydimethylsiloxane, which allow solutions to flow over the surface of the alloy, causing severe corrosion within hours if no inhibitor is present. In three-channel experiments under various channel conditions, corrosion rates were accelerated up to 15 times when compared to standard immersion tests. In addition, 10-channel experiments were conducted to simultaneously test 10 different inhibitors, and the results were compared visually and to actual corrosion results obtained quantitatively via solution analyses.     

The effect of temperature on the corrosion of steel in concrete. Part 1: Simulated polarization resistance tests and model development

The effect of temperature on the corrosion rate of steel corrosion in concrete is investigated through simulated polarization resistance experiments. The simulated experiments are based on the numerical solution of the Laplace’s equation with predefined boundary conditions of the problem and have been designed to establish independent correlations among corrosion rate, temperature, kinetic parameters, concrete resistivity and limiting current density for a wide range of possible anode/cathode (A/C) distributions on the reinforcement. The results, which successfully capture the resistance and diffusion control mechanisms of corrosion as well as the effect of temperature on the kinetic parameters and concrete/pore solution properties, have been used to develop a closed-form regression model for the prediction of the corrosion rate of steel in concrete.     

2-Mercaptobenzimidazole as a copper corrosion inhibitor: Part II. Surface analysis using X-ray photoelectron spectroscopy

In Part I of this study, the high corrosion inhibition effectiveness of 2-mercaptobenzimidazole (MBIH) in 3 wt.% NaCl solution was unambiguously proven. In Part II, using angle-resolved X-ray photoelectron spectroscopy (XPS), the surface chemical structure and composition of adsorbed MBIH on Cu from the same solution was investigated. It was found that MBIH molecules are directed toward the Cu surface through their N and S atoms. The MBIH layer thickness is 1.9 ± 0.5 nm, as determined from a detailed analysis of the background in the XPS spectra. Tentative MBIH orientations on Cu were suggested based on the XPS measurements.     

Porous FeAl intermetallics fabricated by elemental powder reactive synthesis

Fabrication of porous FeAl intermetallics has been realized through the Fe and Al elemental powder reactive synthesis. The swelling behavior, synthesis process and microstructure of the porous FeAl intermetallics fabricated by reactive synthesis have been systematically studied. The pore structural parameters as a function of the sintering temperature have also been systematically investigated. It has been confirmed that the pore evolution in the porous FeAl intermetallics is attributed to the following four steps: the inter-particle pores formed during the pressing procedure, the Kirkendall pores formed during the solid-state sintering, pore formed through the liquid Al reaction, and the phase transformation during the high temperature sintering.     

Synthesis of nanoporous copper by dealloying of Al-Cu-Mg amorphous alloys in acidic solution: The effect of nickel

Dealloying of Al-Cu-Mg based amorphous alloys in 1 M HCl solution and the effect of a minor amount of Ni in the precursor alloy were explored. A crystalline, Cu-rich, nanoporous structure with a pore diameter of 10–30 nm was formed as a result of the selective dissolution of Al and Mg and the reorganization of the remaining Cu. The entire specimen was fully dealloyed after prolonged exposure to a HCl solution. The nanoporous structure is finer (e.g. 10 nm) in the alloy containing Ni, likely because Ni suppresses the surface diffusion of Cu. The decrease in surface diffusion, in turn, suppresses the coarsening of the ligaments in the dealloyed layer enabling tunable porosity development. Electrochemical reactivity after the synthesis of nanoporous Cu was enhanced by a factor of 150 with Ni and 50 without Ni compared to a planar Cu electrode of the same overall dimensions.     

Steel corrosion in concrete: Determinist modeling of cathodic reaction as a function of water saturation degree

The prediction of the long-term behavior of reinforced concrete structures involved in the nuclear waste storage requires the assessment and the modeling of the corrosion processes of steel reinforcement. This paper deals with the modeling of the cathodic reaction that is one of the main mechanisms of steel rebar corrosion. This model takes into account oxygen reduction and oxygen diffusion through a diffusion barrier (iron oxide and/or carbonated concrete) as a function of water saturation degree. It is demonstrated that corrosion rate of reinforcement embedded in concrete with water saturation degree as low as 0.9 could be under oxygen diffusion control. Thus, transport properties of concrete (aqueous and gaseous phase, dissolved species) are key parameters that must be taken into account to model electrochemical processes on the reinforcement.     

Use of vapour phase corrosion inhibitors in packages for protecting mild steel against corrosion

Some aspects of the passivation of mild steel in the presence of selected vapour phase corrosion inhibitors (VCIs) were considered. In particular their ability to vapourize was evaluated by sublimation tests and their role in the inhibition mechanism of mild steel was investigated by electrochemical methods such as open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. In the presence of some carboxylates, amines and azoles alone and as mixtures a protective layer can be formed on mild steel in neutral and alkaline solution. It was shown that the passivation mechanism strongly depends on the pH of the solution. In addition the influence of contaminants from industrial alkaline cleaning baths on the protective properties was analyzed.     

Corrosion behaviour of Cu-10Sn bronze in aerated NaCl aqueous media - Electrochemical investigation

Investigation of the electrochemical behaviour of Cu-10Sn (wt.%) alloy has been conducted in aerated aqueous chloride solutions and compared to that of pure Cu and Sn. Cathodic and anodic bronze behaviours have been investigated in function of the chloride concentrations (0.1-0.001 M), the rotation speed of the electrode and the potential sweep rate after 1-h immersion time in open-circuit conditions. At the corrosion potential, the corrosion behaviour appears determined by the charge-transfer controlled reduction of oxygen and the mixed charge- and mass-transfer controlled electrodissolution. In 0.1 M NaCl solution, three anodic regions are evidenced corresponding to: (I) a dissolution part from Eoc value, (II) a maximum current region corresponding to peaks formation and (III) a large current plateau. A partial protective effect of the corrosion product layer is observed at high anodic potential. It has been interpreted as the result of a 3D growth mechanism involving the formation of stable tin species in the layer according to a “pore resistance model”. Results on synthetic alloy are compared with those obtained on an archaeological bronze with a similar composition immersed in the same medium. Equivalent interface behaviour is evidenced between the synthetic and the archaeological bronzes.     

Determination of corrosion potential of coated hollow spheres

Copper hollow spheres were created on porous iron particles by electro-less deposition. The consequent Ni plating was applied to improve the mechanical properties of copper hollow micro-particles. Corrosion properties of coated hollow spheres were investigated using potentiodynamic polarisation method in 1 mol dm⁻³ NaCl solution. Surface morphology and composition were studied by scanning electron microscopy (SEM), light microscopy (LM) and energy-dispersive X-ray spectroscopy (EDX). Original iron particles, uncoated copper spheres and iron particles coated with nickel were studied as the reference materials. The effect of particle composition, particularly Ni content on the corrosion potential value was investigated. The results indicated that an increase in the amount of Ni coating layer deteriorated corrosion resistivity of coated copper spheres. Amount of Ni coating layer depended on conditions of Ni electrolysis, mainly on electrolysis time and current intensity. Corrosion behaviour of sintered particles was also explored by potentiodynamic polarisation experiments for the sake of comparison. Formation of iron rich micro-volumes on the particle surface during sintering caused the corrosion potential shift towards more negative values. A detailed study of the morphological changes between non-sintered and sintered micro-particles provided explanation of differences in corrosion potential (E_corr).     

Potent inhibition of copper corrosion in neutral chloride media by novel non-toxic thiadiazole derivatives

Inhibiting effect of two novel non-toxic thiadiazole derivatives on copper were investigated in 3.5% NaCl solution using weight loss and electrochemical measurements. Presence of inhibitors and increase of concentration greatly decrease corrosion rate, parameters determined from polarization curves and EIS plots show that inhibitors decrease both cathodic and anodic current densities, suppressing charge transfer process by adsorption on copper surface. Thermodynamic calculation indicates chemisorption obeys Langmuir isotherm. Surface layers were characterized by SEM coupled with EDX. Raman micro-spectroscopy reveals that inhibitor molecules suppress copper corrosion via formation of thiadiazole–Cu complex. Relationships between inhibition efficiency and molecular orbital were also evaluated.     

Phosphate ions used as green inhibitor against copper corrosion in tap water

Three dosages of ions are evaluated as corrosion inhibitors of copper in artificial tap water by measuring the corrosion potential, polarization resistance, electrochemical impedance and reflectance spectra. The water is moderately hard, highly carbonated and chloride-rich. The results show that the surface film is composed by Cu₂O. When the inhibiting agent is added, the film becomes thicker, denser and more compact. This behaviour is attributed to CuO incorporating into the passive layer. The optimal dosage of inhibitor is 10 mg l⁻¹ P when the polarization resistance increases three times. The inhibitor retards the pit initiation, without hindering pit growth.     

Corrosion behaviour in alkaline medium of zinc phosphate coated steel obtained by cathodic electrochemical treatment

The present work evaluated the ability of zinc phosphate coating, obtained by cathodic electrochemical treatment, to protect mild steel rebar against the localized attack generated by chloride ions in alkaline medium. The corrosion behaviour of coated steel was assessed by open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. The chemical composition and the morphology of the coated surfaces were evaluated by X-ray diffraction and scanning electron microscopy. Cathodically phosphated mild steel rebar have been studied in alkaline solution with and without chloride simulating the concrete pore solution. For these conditions, the results showed that the slow dissolution of the coating generates the formation of calcium hydroxyzincate (Ca(Zn(OH)₃)₂·2H₂O). After a long immersion time in alkaline solution with and without Cl⁻, the coating is dense and provides an effective corrosion resistance compared to mild steel rebar.     

Inert anode composed of Ni–Cr alloy substrate, intermediate oxide film and α-Al2O3/Au (Au–Pt, Au–Pd, Au–Rh) surface composite coating for aluminium electrolysis

An inert anode composed of alloy substrate, intermediate oxide film and surface composite coating for aluminium electrolysis has been fabricated. The intermediate oxide film (ZrO₂/Y₂O₃) provides good adhesion and mutual diffusion resistance between the substrate and the surface coating which consists of α-Al₂O₃ particles embedded in Au (Au–Pt, Au–Pd, Au–Rh) matrix. The results of electrolysis test revealed that aluminium with high purity (>99.999%) can be produced. It is demonstrated that the inert anode exhibits superior erosion and corrosion resistance during aluminium electrolysis, especially in low-temperature (800 °C) electrolytes.     

Tribocorrosion of 316L stainless steel and TA6V4 alloy in H2SO4 media

Tribocorrosion of stainless steel 316L and titanium alloy TA6V4 has been conducted in a sulphuric acid solution using an apparatus designed and built for evaluating the joint action of corrosion and wear. The material electrochemical and wear behaviours have been investigated during friction tests under electrochemical potential control. The specimens have been submitted to friction against an alumina ball under cathodic, free and anodic potentials. The friction coefficient, the wear rate and the current density were measured and the obtained results were discussed in terms of passivating film stability and repassivation kinetics.     

Electrochemical behavior of a lead-free Sn–Cu solder alloy in NaCl solution

Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization techniques and an equivalent circuit analysis are used to evaluate the electrochemical corrosion behavior of Sn–Cu alloy samples in a naturally aerated 0.5 M NaCl solution at 25 °C. It has been found that a better electrochemical corrosion resistance is provided by a coarser cellular microstructure array. It has also been found that the corrosion current density (i_corr) is of about a quarter when compared with that of the finest microstructure examined. Such behavior is attributed to both localized strains between the Sn-rich phase and intermetallic (IMC) particles and the cathode/anode area ratios. The effect of copper alloying on i_corr is also discussed.     

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.