Electrochemical corrosion parameters of as-cast Al-Fe alloys in a NaCl solution

The aim of this study is to evaluate the general electrochemical corrosion resistance of Al-Fe alloys in the range of hypoeutectic compositions, Al-0.5 wt.% Fe and Al-1.5 wt.% Fe alloy. EIS plots, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters in a 0.5 M NaCl solution at 25 °C. It is shown that for an Al-0.5 wt.% Fe alloy, coarse cells tend to improve the corrosion resistance mainly due to the reduction in cellular boundaries and for an Al-1.5 wt.% Fe alloy, an opposite trend has been detected.     

Pitting behavior of a bulk Ni-18 wt.% Fe nanocrystalline alloy

Three electrodeposited Ni-18 wt.% Fe samples were annealed at 400 °C for 3 h (hrs), 8 h, and 24 h to study the effects of grain size on the electrochemical properties of bulk Ni-18 wt.% Fe in 3.5 wt.% NaCl. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). Of the four materials studied, the as-received nanocrystalline alloy less sensitive to localized corrosion.     

Corrosion behaviour of low temperature plasma carburised 316L stainless steel in chloride containing solutions

The electrochemical corrosion behaviour of the carburised (expanded austenite) layer on 316L austenitic stainless steel produced by low temperature plasma carburising has been studied in 0.5 M NaCl and 0.5 M HCl + 0.5 M NaCl solutions. The present work focuses on the variation of the corrosion behaviour of the carburised layer with depth from the surface and the effect of carbon concentration on electrochemical behaviour. The results show that the carburised layer has excellent resistance to localised corrosion. There exists a critical carbon concentration, above which the expanded austenite possesses excellent resistance to both metastable pit formation and pit growth.     

Corrosion behavior of iron-based alloys in the LiBr + ethylene glycol + H2O mixture

The corrosion resistance of 1018 carbon steel, 304 and 316 type stainless steels in the LiBr (55 wt.%) + ethylene glycol + H₂O mixture at 25, 50 and 80 °C has been studied using electrochemical techniques which included potentiodynamic polarization curves, electrochemical noise and electrochemical impedance spectroscopy techniques. Results showed that, at all tested temperature, the three steels exhibited an active-passive behavior. Carbon steel showed the highest corrosion rate, since both the passive and corrosion current density values were between two and four orders of magnitude higher than those found for both stainless steels. Similarly, the most active pitting potential values was for 1018 carbon steel. For 1018 carbon steel, the corrosion process was under a mixed diffusion and charge transfer at 25 °C, whereas at 50 and 80 °C a pure diffusion controlled process could be observed. For 316 type stainless steel, at 25 and 50 °C a species adsorption controlled process was observed, whereas at 80 °C a diffusion controlled mechanism was present. Additionally, at 25 °C, the three steels were more susceptible to uniform type of corrosion, whereas at 50 and 80 °C they were very susceptible to localized type of corrosion.     

Electrochemical corrosion behavior of Mg-5Al-0.4Mn-xNd in NaCl solution

The electrochemical corrosion behavior of Mg-5Al-0.4Mn-xNd (x = 0, 1, 2 and 4 wt.%) alloys in 3.5% NaCl solution was investigated. The corrosion behavior of the alloys was assessed by open circuit potential measure, potentiodynamic polarization, and electrochemical impedance spectroscopy. The electrochemical results show the intermetallic precipitates with Nd behave as less noble cathodes in micro-galvanic corrosion and suppress the cathodic process. During corrosion, Al₂O₃ and Nd₂O₃, in proper ratio, is incorporated into the corrosion film, and enhances the corrosion resistance.     

Corrosion and corrosion inhibition of Cu-20%Fe alloy in sodium chloride solution

The electrochemical behavior of copper (Cu), iron (Fe) and Cu-20%Fe alloy was investigated in 1.0 M sodium chloride solution of pH 2. The effect of thiourea (TU) addition on the corrosion rate of the Cu-20%Fe electrode was also studied. Open-circuit potential measurements (OCP), polarization and electrochemical impedance spectroscopy (EIS) were used. The results showed that the corrosion rates of the three electrodes follow the sequence: Cu < Cu-20%Fe < Fe. Potentiostatic polarization of the Cu-20%Fe electrode in the range −0.70 V to −0.45 V (SCE), showed that iron dissolves selectively from the Cu-20%Fe electrode surface and the rate of the selective dissolution reaction depends on the applied potential. At anodic potential of −0.45 V, thiourea molecules adsorb at the alloy surface according to the Langmuir adsorption isotherm. Increasing thiourea concentration (up to 5 mM), decreases the selective dissolution reaction and the inhibition efficiency η reach 91%. At [TU] > 5 mM, the dissolution rate of the Cu-20%Fe electrode increases due to formation of soluble thiourea complexes. At cathodic (−0.6 V), the inhibition efficiency of thiourea decreases markedly owing to a decrease of the rate of the selective dissolution reaction and/or desorption of thiourea molecules. The results indicated that thiourea acts mainly as inhibitor of the selective dissolution reaction of the Cu-20%Fe electrode in chloride solution.     

Effects of heat-treatment on the extent of chromium depletion and caustic corrosion resistance of Alloy 690

To quantify the extent of chromium depletion, electrochemical potentiokinetic reactivation tests were performed on solution-annealed (1393 K) and solution-annealed followed by ageing at 973 K for 4 h or 16 h samples of austenitic Alloy 690. The electrochemical studies indicated very high equilibrium chromium concentration at the chromium carbide/matrix interface for the aged samples. A new electrochemical test parameter that is the ratio of peak activation to maximum passive current density (defined by I_a/I_f) was considered to express the extent of chromium depletion. The alloy, in three different heat-treated conditions, revealed unstable passivity in deaerated 5% and 10% NaOH solutions at 295 K. A comparatively higher passive current density of the aged samples in deaerated caustic environments than the annealed one could be attributed to reactivity of Cr-carbides with the caustic. Microscopic studies indicated that the TiN inclusion/matrix interface may provide a preferred site for pit initiation in caustic environments, which could be attributed to large degree of misfit between TiN inclusion and austenite matrix.     

An electrochemical study of the effect of Li on the corrosion behavior of Ni3Al intermetallic alloy in molten (Li + K) carbonate

A study of the effect of lithium content (1, 3 and 5 wt.%) and heat treatment (400 °C during 144 h) on the corrosion behavior of Ni₃Al alloy has been carried out in a 62 mol.%Li₂CO₃-38 mol.%K₂CO₃ mixture at 650 °C using electrochemical techniques. Employed electrochemical techniques included potentiodynamic polarization curves, linear polarization resistance, LPR, electrochemical impedance spectroscopy, EIS, and electrochemical noise, measurements EN. Results have shown that the alloys exhibited an active-passive behavior regardless of the heat treatment. For alloys without heat treatment, the most corrosion resistant was the Ni₃Al base alloy, but when they were heat treated, the most corrosion resistant was the alloy containing 3%Li. EIS results showed that for short immersion tests, the corrosion process was under diffusion control, but for longer exposure times, the presence of a protective scale was evident. All the alloys were highly susceptible to a localized type of corrosion according to EN measurements and supported by SEM micrographs.     

Effects of pretreatment on the aluminium etch pit formation

The effect of chemical pretreatments on the electrochemical etching behavior of aluminium was investigated with the topographic studies of surface and the analysis of initial potential transients. Two-step pretreatments with H₃PO₄ and H₂SiF₆ result in a high density of pre-etch pits on aluminium surface by the incorporation of phosphate ion inside the oxide film and the removal of surface layer by aggressive fluorosilicic acid solution. It generates a high density of etch pits during electrochemical etching and results in the capacitance increase of etched Al electrode by expanding the surface area, up to 61.3 μF/cm² with the pretreatment solution of 0.5 M H₃PO₄ at 65 °C and 10 mM H₂SiF₆ at 45 °C.     

A study of the corrosion behavior of gamma titanium aluminide in 3.5 wt% NaCl solution and seawater

Gamma titanium aluminide (γ-TiAl) was evaluated for corrosion resistance in 3.5% NaCl and seawater using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Polarization plots indicate very low corrosion rates although in both electrolytes the control samples lost passivity at low polarization potentials. Surface modification treatments were employed by oxidizing the samples in air at 500 °C and 800 °C with the purpose of improving corrosion resistance. While the surface treatments rendered γ-TiAl passive in the polarization experiments in both electrolytes, EIS tests showed that the oxide formed at 800 °C was not protective. EEC models are proposed to explain the EIS results for the three surface conditions tested.     

Corrosion behaviour of nitrogen ion implanted AISI type 304L stainless steel in nitric acid medium

The effects of nitrogen ion implantation on corrosion behaviour of 304L stainless steel in 1 N HNO₃ medium were investigated using surface analytical and electrochemical techniques. Nitrogen ion was implanted at 70 keV in the dose range of 1 × 10¹⁵, 1 × 10¹⁶, 1 × 10¹⁷ and 2.5 × 10¹⁷ N⁺/cm², respectively. Grazing incidence X-ray diffraction results for unimplanted and up to dose of 1 × 10¹⁶ N⁺/cm² showed co-existence of γ-Fe and α′-Fe and, at higher doses (1 × 10¹⁷ and 2.5 × 10¹⁷) preferential formation of chromium nitride was observed. X-ray photoelectron spectroscopy investigation confirmed the formation of chromium nitride at higher doses. Electrochemical corrosion investigation revealed nobler open circuit potential, decrease in corrosion current densities, passive current densities and increase in polarization resistance with increase in dose rate. Surface morphology analysis after polarization study using atomic force microscope showed grain boundary dissolution for unimplanted specimens and resistance to surface dissolution with increase in dose rate for implanted specimens.     

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm⁻¹) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm⁻¹) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field.     

Effect of heat treatment on anodic activation of aluminium by trace element indium

The presence of trace elements in Group IIIA-VA is known to activate aluminium anodically in chloride environment. The purpose of this paper is to investigate the surface segregation of trace element In by heat treatment and resulting surface activation. Model binary AlIn alloys, containing 20 and 1000 ppm by weight of In, were characterized after heat treatment at various temperatures by use of glow discharge optical emission spectroscopy, electron microscopy and electrochemical polarization. Heat treatment for 1 h at 300 °C gave significant segregation of discrete In particles (thermal segregation), which activated the surface. Indium in solid solution with aluminium, obtained by 1 h heat treatment at 600 °C, also activated by surface segregation of In on alloy containing 1000 ppm In, resulting from the selective dissolution of the aluminium component during anodic oxidation (anodic segregation). The effect of anodic segregation was reduced by decreasing indium concentration in solid solution; it had negligible effect at the 20 ppm level. The segregated particles were thought to form a liquid phase alloy with aluminium during anodic polarization, which in turn, together with the chloride in the solution destabilized the oxide.     

Electrochemical behaviour of Ni-base alloys exposed under oil/gas field environments

The electrochemical behaviour of Ni-base alloys (Inconel 625, Inconel 718, G3 and Incoloy 825) is carried out at 80 °C in CO₂/H₂S corrosion environments using cyclic potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The passivity mechanisms are analysed and discussed. In addition, some significant characterisation parameters such as E_corr, I_pass, E_pit, E_pp, ΔE and I_pass in cyclic polarisation curves are analysed and compared to reveal the corrosion resistance of various Ni-base alloys. The equivalent circuit model and ZsimpWin software are utilised to discuss the Nyquist plots of various Ni-base alloys. The diffusion mechanism in EIS measurement is discussed. The result shows that the corrosion resistance of the Ni-base alloys to CO₂ corrosion or CO₂/H₂S corrosion follows the sequence: Inconel 625 > G3 > Inconel 718 > Incoloy 825. H₂S works as a cathodic depolariser with accelerating initiation of the corrosion process.     

Corrosion inhibition of carbon steel in tetra-n-butylammonium bromide aqueous solution by benzotriazole and Na3PO4

The corrosion inhibition behavior of benzotriazole, Na₃PO₄ and their mixture on carbon steel in 20 wt.% (0.628 mol l⁻¹) tetra-n-butylammonium bromide aerated aqueous solution was investigated by weight-loss test, potentiodynamic polarization measurement, electrochemical impedance spectroscopy and scanning electron microscope/energy dispersive X-ray techniques. The inhibition action of BTA or SP or inhibitors mixture on the corrosion of carbon steel is mainly due to the inhibition of anodic process of corrosion. The results revealed that inhibitors mixtures have shown synergistic effects at lower concentration of inhibitors. At 2 g l⁻¹ BTA and 2 g l⁻¹ SP showed optimum enhanced inhibition compared with their individual effects.     

Corrosion protection of aluminium pretreated by vinyltriethoxysilane in sodium chloride solution

The corrosion protection of vinyltriethoxysilane (VTES) films on aluminium during exposure to 3% NaCl was investigated using electrochemical impedance spectroscopy (EIS), potential-time measurements and optical microscopy coupled with image analysis. Composition and thickness of films were analyzed using Auger electron spectroscopy (AES) combined with depth profiling. It was shown that films deposited from 5% solution were significantly thicker and exhibited lower porosity and better corrosion stability, as compared to films deposited from 2 vol.% solution. VTES films deposited from 5 vol.% solutions and cured for 30 min exhibited better protection properties than other investigated films.     

Electrochemical investigation and modeling of corrosion inhibition of aluminum in molar nitric acid using some sulphur-containing amines

Chemical and electrochemical measurements incorporated with quantum chemical calculations and molecular dynamics simulations were used to study the corrosion inhibition characteristics of some thiosemicarbazone derivatives on the inhibition of aluminum corrosion in 1.0 M HNO₃. Polarization curves demonstrated that the thiosemicarbazone derivatives were of mixed-type inhibitors. EIS plots indicated that the addition of thiosemicarbazone derivatives increases the charge-transfer resistance of the corrosion process, and hence the inhibition performance. The molecular dynamics simulation results show that the three thiosemicarbazone derivatives can adsorb on the A₂O₃ (1 1 1) surface through the sulphur and nitrogen atoms as well as π-electrons in the pyridyl structure.     

Tryptamine as a corrosion inhibitor of mild steel in hydrochloric acid solution

The inhibitor effect of tryptamine on the corrosion of mild steel in 0.5 M hydrochloric acid at 30 °C was investigated using linear polarization, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The inhibition efficiency increases with an increase in concentration of tryptamine. At 500 ppm tryptamine the inhibition efficiency calculated by these techniques is around 97%. Cathodic and anodic polarization curves of mild steel in the presence of different concentrations of tryptamine at 30 °C reveal that it is a mixed type inhibitor. Tryptamine follows Langmuir adsorption with adsorption free energy of −35.07 kJ mol⁻¹.     

Effect of sintering temperatures on corrosion and wear properties of sol-gel alumina coatings on surface pre-treated mild steel

Sol-gel alumina coatings were developed on the surface pre-treated (zinc-phosphated) mild steel substrate and subsequently sintered at 300 °C, 400 °C and 500 °C. The alumina sol was synthesised using aluminium iso-propoxide as a precursor material. FTIR of the boehmite (AlOOH) gel sintered at above-mentioned temperatures was employed to identify the presence of various functional groups. The microstructural features and the phase analysis of the sol-gel coated specimens were carried out using SEM and XRD respectively. The corrosion resistance of the sol-gel alumina coatings was evaluated by electrochemical measurement in 3.5% NaCl solution at room temperature. The abrasive wear behaviour of the sol-gel coated specimens was measured in two body (high stress) conditions. The experimental results revealed that the sol-gel coated specimen sintered at 400 °C has superior wear and corrosion resistance properties as compared to the sol-gel coated specimen sintered at 300 °C. However, the sol-gel coated specimen sintered at 500 °C has exhibited a very poor corrosion and wear resistance properties. Poor performance of the sol-gel coatings sintered at 500 °C could be explained to be due to (i) the presence of numerous cracks (ii) absence of organic groups in the coating.     

Electrochemical response of AlNiLa amorphous and devitrified alloys

The electrochemical response of Al_94−xNi₆La_x alloys (x = 4, 5, 6, 7) after different stages of devitrification was studied in 0.05 M Na₂SO₄ as well as in different concentrations [0.001 M, 0.01 M and 0.1 M] NaCl solutions. Complementary crystallization studies were carried out to elucidate the composition dependent phase evolution in these alloys. It was observed that the primary crystallization did not cause any deterioration in the corrosion resistance of the alloys as compared to the amorphous alloys. In the case of Al₈₇Ni₆La₇, there was actually an improvement in the passivating ability in benign media. The various primary crystalline phases in the different alloys investigated did not cause different electrochemical responses. However, the onset of secondary crystallization caused a reduction in the corrosion resistance in the NaCl media through a loss in passivating ability of all the alloys. This is due to increased galvanic activity as well as the loss of the amorphous phase.