Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.     

Effect of carburization on electrochemical corrosion behaviours of TiAl alloy

Electrochemical corrosion behaviours of the untreated and the carburized of Ti-46.5Al (mol %) alloy were investigated. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were applied to characterize the carburized layer. Potentiodynamic polarization curve, electrochemical impedance spectroscopy (EIS) and SEM morphology of the corroded surface were used to evaluate corrosion resistance of both carburized and untreated TiAl alloy in 1 mol/L HCl. The outer layer of the carburized TiAl alloy is a continuous Ti₂AlC scale. Polarization curve and electrochemical impedance spectroscopy (EIS) of the carburized TiAl alloy present a nobler corrosion potential, a more positive pitting potential and a higher polarization resistance, respectively, compared with the untreated sample. After anodic corrosion or immersion corrosion, a deposited layer can be observed on the surface of the carburized titanium aluminide alloy. By contrast, pitting and crevasse corrosion occur on the surface of the untreated TiAl alloy after anodic corrosion and some corrosion products and slight corrosion appear on the surface of the untreated TiAl alloy after immersion corrosion.     

Evaluation of benzotriazole as corrosion inhibitor for carbon steel in simulated pore solution

In this investigation, benzotriazole (BTAH), a well known corrosion inhibitor for copper, has been evaluated as a possible corrosion inhibitor of a carbon steel (CA-50) used as reinforcement in concrete. BTAH was added to a simulated pore solution of an aged concrete with addition of 3.5 wt% NaCl to imitate marine environments. The effect of BTAH in a concentration of 1.5 wt% on the corrosion resistance of CA-50 carbon steel was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The improvement of the corrosion resistance due to BTAH addition was superior to that associated with nitrite in similar concentration, suggesting that BTAH is a potentially attractive alternative to nitrites for inhibiting corrosion of reinforcement steel in concrete.     

Inhibition of mild steel corrosion in sulfuric acid by some newly synthesized organic compounds

The corrosion inhibition of mild steel in 0.5 M sulfuric acid solutions by some new synthesized organic compounds namely (E)-2-acetyl-3-(butyl amino)-N-phenyl buten-2-thioamide (compound A), (E)-3-(4-(dimethyl amino) phenyl amino)-2-acetyl-N-phenyl buten-2-thioamide (compound B) and (E)-3-(2,3-dimethyl phenyl amino)-2-acetyl-N-phenyl buten-2-thioamide (compound C) was investigated using weight loss and potentiostatic polarization techniques. These measurements reveal that the inhibition efficiency obtained by these compounds increased by increasing their concentration. The inhibition efficiency follows the order A > B > C. Polarization studies show that these compounds are of the mixed-type but dominantly act as an anodic inhibitors for steel in 0.5 M H₂SO₄ solutions. These inhibitors function through adsorption following Langmuir isotherm. Activation energy and Gibbs free energy for adsorption of inhibitors are calculated. Molecular modeling has been conducted to correlate the corrosion inhibition properties with the calculated quantum chemical parameters.     

Corrosion inhibition of carbon steel and antibacterial properties of aminotris-(methylenephosphonic) acid

In this work, the corrosion inhibition property and the antibacterial activity of the aminotris-(methylenephosphonic) acid (ATMP) have been studied. ATMP has been evaluated as a corrosion inhibitor for carbon steel in 1 M HCl solution using weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. According to the experimental results, the inhibition efficiency increased with increasing inhibitor concentration. Tafel polarization study showed that the ATMP acts as a mixed inhibitor. Data, obtained from EIS measurements, were analyzed to model the corrosion inhibition process through appropriate equivalent circuit models. Adsorption of ATMP on the carbon steel surface obeyed the Langmuir adsorption isotherm. The calculated ΔG_ads value showed that the corrosion inhibition of the carbon steel in 1 M HCl is mainly controlled by a physisorption process. In addition, the effect of immersion time on the corrosion of carbon steel was also studied in this work using ac impedance technique. The corrosion inhibition mechanism of ATMP was discussed. This inhibitor can be also used as biocide in aqueous environments. Our results showed that ATMP have an antibacterial effect against both Gram positive and Gram negative bacteria. The lowest MIC (0.2 mg l^?1) was measured for Pseudomonas fluorescens while the highest MIC was measured for Escherichia coli (3.1 mg l^?1). In addition, the results showed that the MIC of ATMP against Listeria innocua in a buffered medium (pH 6.5) was of ca. 4-fold higher than MIC measured in unbuffered medium. Thus, our findings showed that the antibacterial activity of ATMP is a result of a combined effect of the pH solution and the chemical nature of the used phosphonate molecule.     

Ethoxylated fatty acids as inhibitors for the corrosion of zinc in acid media

The inhibition effect of ethoxylated fatty acids were used as inhibitors for the corrosion of zinc metal in 1.0 M hydrochloric and 1.0 M sulfuric acid solutions at various temperatures ranging from 25 to 55 °C is investigated by weight loss measurement and electrochemical methods. The protection efficiency depends upon the type and concentration of the inhibitor and the nature of the acid medium. In both acid solutions the protection efficiencies of the inhibitors decrease with the increase in temperature. The inhibition was assumed to occur via the adsorption of the fatty acid molecules on the metal surface. The thermodynamic functions of dissolution and adsorption processes were calculated and discussed.     

Influence of chloride ion concentration and temperature on the electrochemical properties of passive films formed on a superduplex stainless steel

Polarization measurements were conducted to monitor the corrosion behavior of superduplex stainless steel ASTM A995M-Gr.5A/EN 10283-Mat#1.4469(GX2CrNiMo26-7-4) when exposed to a) an electrolyte containing 22,700 parts per million (ppm) of chloride ions at seven different temperatures and b) an electrolyte at 25 °C and different chloride ion concentrations (5800, 22,700, 58,000 and 80,000 ppm of Cl^?). The polarization curves indicate that the passive films formed are only slightly affected by NaCl concentration, but the pitting potential decreases drastically increasing the temperature, in particular > 60 °C. The image analysis of the microstructure after potentiodynamic polarization showed that the pitting number and size vary in function of the temperature of the tested medium. Nyquist diagrams were determined by electrochemical impedance spectroscopy to characterize the resistance of the passive layer. According to Nyquist plots, the arc polarization resistance decreases increasing the temperature due to a catalytic degradation of the oxide passive films.     

Influence of temperature on the pitting corrosion behavior of AISI 316L in chloride–CO2 (sat.) solutions

The paper discusses the pitting corrosion behavior of AISI (American iron and steel institute) 316L stainless steel in aerated chloride solutions (0.1–2 M NaCl) at 25, 50 and 80 °C using potentiodynamic polarization technique. A comparison is made with CO₂-saturated chloride solutions. The results have revealed that pitting potential decreased in a logarithmic relationship with the chloride concentration, and decreased linearly with temperature. The influence of CO₂ on the chloride pitting of AISI 316L stainless steel is quite complex and found to be dependent on chloride concentration and test temperature. At 25 °C the presence of CO₂ appears to have insignificant effect on E_p irrespective of chloride concentration. As the temperature is raised to 50 or 80 °C the additions of CO₂ has caused marked negative shifts in pitting potential. The detrimental effect of CO₂ increases with NaCl concentration and temperature. The results indicate that pitting potential (E_p) is influenced by a synergy between chloride, CO₂ and temperature, and that this synergy depends on the chloride concentration and test temperature.     

Corrosion inhibition of aluminum in hydrochloric acid solution by alkylimidazolium ionic liquids

The effects of newly synthesized three alkylimidazolium ionic liquids—1-butyl-3-methylimidazolium chlorides (BMIC), 1-hexyl-3-methylimidazolium chlorides (HMIC) and 1-octyl-3-methylimidazolium chlorides (OMIC)—on the corrosion of aluminum in 1.0 M HCl were investigated using potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss methods. All measurements showed that the inhibition efficiency increased with increase in the concentration of inhibitor and the effectiveness of these inhibitors was in the order of OMIC > HMIC > BMIC. Polarization curves revealed that the studied inhibitors were mixed type inhibitors. The adsorption of the inhibitors on the aluminum surface obeyed Langmuir adsorption isotherm and had a physical mechanism. The effect of temperature on the corrosion behavior in the presence of 5 × 10^?3 M of inhibitors was studied in the temperature range of 303–333 K. The associated activation energy of corrosion and other thermodynamic parameters such as enthalpy of activation (ΔH), entropy of activation (ΔS), adsorption equilibrium constant (K_ads) and free energy of adsorption (ΔG_ads) were calculated to elaborate the mechanism of corrosion inhibition.     

Effects of 5-(3-aminophenyl)-tetrazole on the inhibition of unalloyed iron corrosion in aerated 3.5% sodium chloride solutions as a corrosion inhibitor

The effects of 5-(3-aminophenyl)-tetrazole (APT) on the inhibition of unalloyed iron corrosion in aerated 3.5% NaCl solutions as a corrosion inhibitor have been studied using open circuit potential (OCP), cyclic potentiodynamic polarization (CPP), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) measurements. The inhibited iron surface was characterized by scanning electron spectroscopy (SEM) and energy dispersive X-ray (EDS) investigations. The OCP showed positive shifts of potential in the presence of APT and the increase of its concentration. CPP and CA measurements indicated that APT molecules decrease the pitting and uniform corrosions through decreasing the pitting and absolute currents, and corrosion rate as well as shifting the corrosion and pitting potentials of iron towards the noble values. EIS plots revealed that APT increases the surface and polarization resistances of iron. SEM/EDS investigations proved that the inhibition of iron corrosion in NaCl containing APT solutions is achieved by the adsorption of APT molecules onto iron to preclude the dissolution process by blocking the active sites on its surface.     

Adsorption behavior of caffeine as a green corrosion inhibitor for copper

Electrochemical and impedance experiments were carried out to evaluate the corrosion behavior of copper in aerated 0.1 mol L^? 1 H₂SO₄ solutions in the presence of three xanthine derivatives with similar chemical structures. The corrosion rate of copper was found to increase in the presence of theophylline and theobromine and decrease in the presence of caffeine. The adsorption and inhibitory effect of caffeine on copper surfaces in aerated 0.1 mol L^? 1 H₂SO₄ solutions were then investigated in detail by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), contact angle measurements, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and fluorescence experiments. The data obtained indicate that caffeine behaves as a cathodic-type inhibitor adsorbing onto the copper surface according to the Temkin isotherm, with the negative ?G°_ads value of ? 31.1 kJ mol^? 1 signifying a spontaneous adsorption process. The corrosion inhibition efficiency increased with caffeine concentration in the range of 1.0–10.0 mmol L^? 1. Furthermore, the EIS results obtained at the open-circuit potential and surface analysis (SEM, EDS and fluorescence) clearly demonstrated the adsorption of the organic compound onto the copper electrode. The contact angle measurements revealed the formation of a hydrophobic protective film. This film covers up to 72% of the total active surface, acts as a protective barrier and prevents interaction between the metal, water and oxygen molecules.     

Electrochemical and stress corrosion cracking behavior of 67Cu–33Zn alloy in aqueous electrolytes containing chloride and nitrite ions: Effect of di-sodium hydrogen phosphate (DSHP)

A continuing challenge in materials design is the achievement of greater operational efficiency through improvements in performance criterion, particularly high strength and service life-time characteristics. In this study we report on the evaluation of di-sodium hydrogen phosphate (DSHP) as an inhibitor for the stress corrosion cracking (SCC) of 67Cu–33Zn alloy in 0.1 M NaNO₂ and 3.5% NaCl aqueous electrolytes under both open-circuit potential (OCP) and relatively high anodic potential (300 mV_NHE) using the slow strain rate (SSR) technique. DSHP has been found to inhibit SCC of the tested brass alloy which is in parallel with the results obtained from electrochemical measurements. The inhibitive effect of DSHP can be attributed to the formation of a three dimensional film of zinc phosphate on the alloy surface which in turn inhibits dezincification and subsequently the SCC of the 67/33 brass alloy. On the other hand, the addition of 0.1 M DSHP was found to change the failure mode from brittle transgranular cracking to ductile failure. The results of both electrochemical and SCC measurements were combined together to investigate the inhibitive mechanism of DSHP.     

Corrosion inhibition of nickel in H2SO4 solution by alanine

The effect of alanine, as a safe inhibitor, was studied by measuring the corrosion of Ni in aerated and stagnant 1 M H₂SO₄ solution (pH ～0.2). Measurements were performed under various conditions using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and the new electrochemical frequency modulation (EFM) methods. The obtained results showed that the addition of alanine alone gives a moderate inhibition and acts as an anodic-type inhibitor. The inhibition is due to physical adsorption of alanine on the metal surface. The inhibition efficiency enhances with increasing alanine concentration and immersion time but decreases with rise in temperature. The apparent activation energy, E_a, is higher in the presence than in the absence of alanine. Addition of I^? ions greatly improves the inhibition efficiency of alanine. The synergistic effect is due to enhanced adsorption of alanine cations by chemisorbed I^? anions on the metal surface. The results obtained from polarization, EIS and EFM techniques are in good agreement indicating that EFM method can be used successfully for monitoring corrosion rate of Ni in H₂SO₄ solution with and without alanine.     

Effects of phosphate on the chloride-induced corrosion behavior of reinforcing steel in mortars

The influence of phosphate as a corrosion inhibitor on the corrosion behavior of as-received and pre-rusted reinforcing steels in mortar specimens was investigated after 360 days exposure in 3.5% NaCl solution. This involved the use of electrochemical techniques for studying the steel surface reactions and microscopic observations of the steel–mortar interface. The electrochemical methods, including electrochemical impedance spectroscopy (EIS) and measurements of corrosion potential (E_corr) and linear polarization resistance (LPR), were employed to evaluate the corrosion tendency and general corrosion rate of steel. In addition, the pitting corrosion resistance of steel was also determined by cyclic polarization (CP) measurements. The results indicate that different from nitrite, which is generally accepted as an anodic inhibitor, phosphate may be a cathodic inhibitor according to its reduced corrosion rate and more negative E_corr at the same dosage as nitrite in mortar specimens. The study also reveals that the inhibiting efficiency of phosphate against general corrosion of both as-received and pre-rusted specimens is lower than 10%, which is inferior to nitrite in some respects. However, as indicated by cyclic polarization measurements, the presence of phosphate provides slightly higher pitting corrosion resistance in comparison to nitrite. Furthermore, it suggests that the corrosion inhibition mechanism of phosphate in mortars mainly depends on a dual effect occurring at the steel–mortar interface. Furthermore, it is confirmed that phosphate has little effect on the long-term mechanical properties of mortars.     

Aqueous corrosion performance of nanostructured bainitic steel

In this study, the corrosion performance of nanostructured bainitic steel was compared with martensitic steel in chloride-containing solution using electrochemical techniques. Electrochemical impedance spectroscopy (EIS) results showed that the polarization resistance (R_p) for nanostructured bainitic steel (3400 Ω cm²) was higher than that of martensitic steel (2000 Ω cm²). Potentiodynamic polarization results showed an 85% lower corrosion current density for nanostructured bainitic steel compared to martensitic steel. Galvanostatically polarized martensitic steel revealed high localized corrosion i.e., intergranular corrosion (IGC) as well as localized attack in the grains. However, the nanostructured bainitic steel when polarized galvanostatically exhibited only a marginal selective dissolution of retained austenite. Thus, it can be suggested that nanostructured bainitic steel will perform better than martensitic steel in chloride-containing environments.     

Comparative study of fluoride conversion coatings formed on biodegradable powder metallurgy Mg: The effect of chlorides at physiological level

The development of a biodegradable metallic implant demands a precisely defined degradation profile and adequate mechanical properties. Mg has been proposed for this purpose but it has an excessively high corrosion rate and insufficient yield strength. In the present work pure Mg mechanically reinforced by a powder metallurgy (Mg(PM)) route and treated with KF was used. The effect of chlorides, at the physiological level, on four fluoride conversion coatings (F-CC) formed on Mg(PM) was evaluated comparatively. The behavior of Mg(PM) during fluoride treatments (0.01 M–0.3 M fluoride-containing solutions) before and after the addition of chlorides (8 g L^? 1 NaCl) was investigated by conventional corrosion techniques and by scanning electrochemical microscopy (SECM) complemented with SEM observations and EDX analysis. Results showed that the composition and the microstructural characteristics of the F-CCs as well as their corrosion behavior change with KF concentration and immersion time. Treatments in the 0.01 M–0.1 M KF range prove to be effective to protect Mg(PM) against corrosion in the absence of chlorides while higher KF solution concentration (0.3 M) adversely affects the corrosion resistance of this metal. In the presence of chloride ions the F-CCs progressively lose their fluoride content and their corrosion resistance at a rate that depends on the treatment conditions. Such temporary corrosion protection is appropriate for biodegradable implants.     

Ruthenium–ligand complex,an efficient inhibitor of steel corrosion in H3PO4 media

The effect of a ruthenium–ligand complex (RuLC) on the corrosion of steel in 2 M H₃PO₄ has been investigated at various temperatures using electrochemical techniques (impedance spectroscopy (EIS), polarisation curves) and weight loss measurements. Inhibition efficiency (E%) increases with RuLC concentration to attain 90% at 5 × 10^− 4 M. EIS measurements show that the dissolution process of steel occurs under activation control. Polarisation curves indicate that RuLC acts as a cathodic inhibitor. E% values obtained from various methods used are in good agreement. The temperature effect on the corrosion behaviour of steel in 2 M H₃PO₄ without and with the inhibitor at various concentrations was studied in the temperature range from 298 to 338 K. Thermodynamic parameters such as adsorption heat (ΔH_ads°), adsorption entropy (ΔS_ads°) and adsorption free energy (ΔG_ads°) have been calculated. Kinetic parameters for the corrosion reaction at different concentrations of RuLC were determined. Adsorption of RuLC on the mild steel surface in 2 M H₃PO₄ follows the Langmuir isotherm model.     

Poly(aniline-formaldehyde): A new and effective corrosion inhibitor for mild steel in hydrochloric acid

In recent years, polyanilines have emerged as efficient class of corrosion inhibitors for mild steel in acidic media. The corrosion inhibition of poly(aniline-formaldehyde) on mild steel in 1.0N HCl has been evaluated by potentiodynamic polarization, linear polarization, electrochemical impedance spectroscopy and weight loss measurements. Results obtained show that poly(aniline-formaldehyde) is a mixed inhibitor and it inhibits mild steel corrosion through adsorption mechanism. It showed >90% inhibition efficiency at 10 ppm. AFM clearly reveals that surface roughness of inhibited mild steel sample is less than uninhibited mild steel.     

Inhibitory mechanism of mild steel corrosion in 2 M sulphuric acid solution by methylene blue dye

Methylene blue dye (MB) was investigated as a corrosion inhibitor for mild steel in 2 M sulphuric acid solution using gravimetric and thermometric techniques. The inhibition efficiency of MB increased with concentration and synergistically increased in the presence of the halide additives, namely KCl, KBr and KI. The trend of inhibition efficiency with temperature suggests that inhibitor molecules are physically adsorbed on the corroding metal surface at lower concentration (0.01–0.5 mM), and chemically adsorbed at higher concentration (1.0–5.0 mM). These results were further corroborated by kinetic and activation parameters for corrosion and adsorption processes evaluated from experimental data at the temperatures studied. MB was found to obey Langmuir and Frumkin adsorption isotherms in the concentration range investigated.     

Study on the corrosion residual strength of the 1.0 wt.% Ce modified AZ91 magnesium alloy

The effect of corrosion on the tensile behaviour of the 1.0 wt.% Ce modified AZ91 magnesium alloy was investigated by the immersion of the test bar in 3.5 wt.% NaCl aqueous solution for 0, 12, 40, 108, 204, 372 and 468 h with the subsequent tensile tests in this paper. The fractography was analyzed by scanning electron microscopy. The results show that pitting corrosion should be responsible for the drop of the corrosion residual strength within the testing time. The depth of the corrosion pits was statistically and quantitatively obtained by an optical microscopy and the maximal value was recorded as the extreme depth of the corrosion pit. Furthermore, the corrosion residual strength is linearly dependent on the extreme depth of the corrosion pit, which can be attributed to the loss of cross-sectional area and the emergence of stress concentration caused by the initiation and development of corrosion pits.