Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

Long-term anticorrosion behaviour of polyaniline on mild Steel

Anticorrosion performances of polyaniline emeraldine base/epoxy resin (EB/ER) coating on mild steel in 3.5% NaCl solutions of various pH values were investigated by electrochemical impedance spectroscopy (EIS) for 150 days. In neutral solution (pH 6.1), EB/ER coating offered very efficient corrosion protection with respect to pure ER coating, especially when EB content was 5-10%. The impedance at 0.1 Hz of the coating increased in the first 1-40 immersion days and then remained constant above 10⁹ Ω·cm² until 150 days, which in combination with the observation of a Fe₂O₃/Fe₃O₄ passive film formed on steel confirmed that the protection of EB was mainly anodic. In acidic or basic solution (pH 1 or 13), EB/ER coating also performed much better than pure ER coating. However, these media weakened the corrosion resistance due to breakdown of the passive film or deterioration of the ER binder.     

Corrosion and passivation mechanism of chromium diboride coatings on stainless steel

The corrosion resistance of fully crystalline CrB₂ coatings magnetron sputtered onto AISI 316L stainless steel was tested in acidic solutions. CrB₂ coatings showed excellent corrosion protection, but suffered a breakdown when an anodic potential of greater than about +1 V (SHE) was applied to the surface in a 1 M HCl electrolyte. The coating failure at high potentials is attributed to transpassive dissolution of the coating at volume defects, enabling the electrolyte to reach the underlying 316L substrate, resulting in its rapid corrosion and subsequent fracturing of the coating. Electrochemical data and potential-pH (Pourbaix) diagrams, constructed from thermodynamic data, indicate that the corrosion resistance of CrB₂ is due to the formation of a Cr(III) oxide passive film in the absence of activation corrosion.     

Inhibitory effect of some amino acids on corrosion of Pb-Ca-Sn alloy in sulfuric acid solution

The present article describes the inhibition effect of amino acids cysteine (Cys), methionine (Met) and alanine (Ala), towards the corrosion of lead-alloy (Pb-Ca-Sn) in H₂SO₄ solution by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurement and scanning electron microscopy (SEM) methods. The influence of inhibitor concentration, temperature and time on inhibitory behavior of the amino acids was investigated. The corrosion data including corrosion current density (I_corr), corrosion potential (E_corr) and charge transfer resistance (R_ct) were determined from Tafel plots and EIS. Recording impedance spectra showed that the charge transfer resistance is increased by increasing adsorption time. The SEM micrographs revealed that the corroded surface area is decreased in the presence of amino acids. Meanwhile, the inhibition efficiency (IE) was found to be depending on the type of amino acid and its concentration. The IE for 0.1 M Cys in 0.5 M H₂SO₄ is greater than 96%. Adsorption isotherms were fitted by Langmuir isotherm.     

Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy studied in 0.01 M Na₂SO₄ + x M NaCl (x = 0-0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states.     

Corrosion prevention of passivated iron in 0.1 M NaCl by coverage with an ultrathin polymer coating and healing treatment in 0.1 M NaNO3

An ultrathin, ordered and two-dimensional polymer coating was prepared on a passivated iron electrode by modification of 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Subsequently, the electrode was healed in 0.1 M NaNO₃. Protection of passivated iron against passive film breakdown and corrosion of iron was examined by monitoring of the open-circuit potential and repeated polarization measurements of the polymer-coated and healed electrode in an aerated 0.1 M NaCl solution during immersion for many hours. Localized corrosion was markedly prevented by coverage with the polymer coating and the healing treatment in 0.1 M NaNO₃. Prominent protection of iron from corrosion in 0.1 M NaCl was observed before the breakdown occurred. The electrode surface covered with the healed passive film and polymer coating was analyzed by contact angle measurement, X-ray photoelectron spectroscopy and electron-probe microanalysis.     

A room temperature cured sol–gel anticorrosion pre-treatment for Al 2024-T3 alloys

The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion protection system. Historically these corrosion protection systems have been based upon the use of chemicals containing Cr(VI) compounds. Organic–inorganic hybrid silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In the present study, a hybrid epoxy–silica–alumina coating with or without doped cerium nitrate has been prepared using a sol–gel method. The hybrid coatings were applied by a dip-technique to an Al–Cu alloy, Al 2024-T3, and subsequently cured at room temperature. The anticorrosion properties of the coatings within 3.5% NaCl were studied using electrochemical impedance spectroscopy (EIS), and conventional DC polarisation. An exfoliation test method involving immersion in a solution of 4 M NaCl, 0.5 M KNO₃ and 0.1 M HNO₃ was also used. The cerium nitrate doped sol–gel coating exhibited excellent anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The resistance to corrosion of the sol–gel coating was also evaluated by analysing the morphology of the coating before and after corrosion testing using scanning electron microscopy.     

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.     

Corrosion behavior of Cr/Cu-coated Mg alloy (AZ91D) in 0.1 M H2SO4 with different concentrations of NaCl

An electroplating process was proposed for obtaining a protective Cr/Cu deposit on the two-phase Mg alloy AZ91D. The corrosion behavior of Cu-covered and Cr/Cu-covered AZ91D specimens was studied electrochemically in 0.1 M H₂SO₄ with different NaCl concentrations. Experimental results showed that the corrosion resistance of an AZ91D specimen improved significantly after Cr/Cu electrodeposition. The corrosion resistance of Cr/Cu-covered AZ91D decreased with increasing NaCl concentration in 0.1 M H₂SO₄ solution. After immersion in a 0.1 M H₂SO₄ with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-covered AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-covered AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials.     

Film properties and stability influence on impedance distribution during the dissolution process of low-carbon steel exposed to modified alkaline sour environment

In this work, we characterized the anodic dissolution and the hydrogen transport within carbon steel (SAE 1018) samples immersed in alkaline sour solutions (CN⁻, polysulfide-base inhibitor and H₂S(aq)). The evolution of interfacial and transport processes could be quantified by Electrochemical Impedance Spectroscopy (EIS) and hydrogen permeation measurements. EIS experimental data were analyzed and fitted by using Transmission Line Model (TLM); this latter helped to propose the mechanisms through the porous layer of the corrosion products formed. The area influencing the dissolution and the mass transfer process was quantified by the pores number, pores thickness and the interfacial passive electrical elements describing the mechanisms in different regions within the pores of the corrosion product layer. The TLM was used to analyze the active-mass transport processes occurred at different spatial positions of the porous layer, such as the mass transfer at the wall and the active-mass transfer at the base of the cylindrical pore of the non-stoichiometric Fe_xS_y.     

A study on mechanism of corrosion protection of polyaniline coating and its failure

Polyaniline (PANI) coatings were electrochemically deposited on substrates of stainless steel and platinum in solutions of 0.2 M H₂SO₄ and 0.1 M aniline by cyclic voltammetry. The corrosion protection of the PANI coatings and their failure were investigated in 0.2 M H₂SO₄ solution. It was observed that the corrosion protection ability of the coating to steel substrate was increased with the increase of the coating thickness. The corrosion protection ability was mainly attributed to the passivating effect of PANI due to its oxidizing ability in its emeraldine state. During its operation, the PANI coating in emeraldine state tended to gradually lose its corrosion protection ability. This gradual failure of the PANI coating, but faster than expected, was confirmed to be related to a gradual reduction of the emeraldine PANI and a gradually increased resistance between the PANI coating and the stainless steel substrate. These findings lead to a new mechanism for the corrosion protection of PANI coating and its failure.     

Analysis of the electrochemical reaction behavior of alloy AZ91 by EIS technique in H3PO4/KOH buffered K2SO4 solutions

The EIS technique was used to analyze the electrochemical reaction behavior of Alloy AZ91 in H₃PO₄/KOH buffered K₂SO₄ solution at pH 7. The corrosion resistance of Alloy AZ91 was directly related with the stability of Al₂O₃ · xH₂O rich part of the composite oxide/hydroxide layer on the alloy surface. The break down of the oxide layer was estimated to occur mainly on the matrix solid solution phase in Alloy AZ91. The mf capacitive loop arose from the relaxation of mass transport in the solid oxide phase in the presence of Al₂O₃ · xH₂O rich part and from Mg⁺ ion concentration within the broken area in the absence of Al₂O₃ · xH₂O rich part in the composite oxide structure on the alloy surface. The lf inductive loop had tendency of disappear when the dissolution rate of the alloy decreased as a result of the formation of the protective oxide layer.     

Corrosion behavior of equal-channel-angular-pressed pure magnesium in NaCl aqueous solution

Effect of microstructure change on corrosion behavior of equal-channel-angular-pressed (ECAPed) pure Mg was investigated. The ECAPed sample after 6 passes obtained finer grains (50-100 μm) compared with as-cast one (800-1500 μm). The strain-induced grain refinement with more crystalline defects weakened corrosion resistance of pure Mg, resulting in more and deeper pits after in-situ corrosion, higher mass-loss rate immersed in NaCl solution, larger I_corr values in polarization curves and lower fitted R_t values in EIS plots. However, the enhanced initial OCP values indicate better weather resistance. Furthermore, corrosion improvement can be expected by reducing defects via subsequent annealing.     

Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 °C and examined in 0.1 M Na₂SO₄ without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.     

Semiconducting and passive film properties of nitrogen-containing type 316LN stainless steels

The semiconducting property of passive films of nitrogen-containing type 316LN stainless steels in different electrolytic media (0.5 M NaCl, borate buffer and borate buffer + 0.5 M NaCl) was investigated by electrochemical impedance spectroscopy (EIS). The nitrogen effect on the chemical composition of the passive films was investigated using X-ray photoelectron spectroscopy, (XPS). Based on capacitance results, the semiconducting parameters obtained from the Mott-Schottky plots indicated a decrease in the donor and acceptor density (N_D and N_A) with increase in nitrogen content, and variation in the flat band potential (E_FB), depending on the electrolytic media. Thus indicating that the oxide layers of the passive film are modified by nitrogen addition. The presence of nitrogen and in the passive film was confirmed by the XPS analysis of the passive film. Cyclic polarization for pitting and repassivation corrosion studies indicated a decrease in hysteresis loop with increase in nitrogen content in 0.5 M NaCl solution. In the highest nitrogen-containing alloy (0.556 wt.% N), the hysteresis loop was small and negligible indicating that the pit initiation is minimum in this alloy. Based on the results obtained, an attempt was made to correlate the semiconducting nature of the passive films with pitting corrosion resistance.     

Evolution of dissolution processes at the interface of carbon steel corroding in a CO2 environment studied by EIS

The evolution of interfacial phenomena during CO₂ corrosion of C1018 carbon steel was characterized by EIS (Electrochemical Spectroscopy Impedance) and LPR (Linear Polarization Resistance). Turbulent conditions were simulated by a channel flow cell with deoxygenated 3 wt.% NaCl solution at 80 °C and pH 6 during 158 h. EIS helped in the characterization of the dynamic mechanism during the formation of the unprotective porous Fe₃C layer, and subsequent precipitation of the protective FeCO₃ layer inside the cementite. The experimental response of the active states at the interface was characterized by electrical passive elements with constant phase parameter analogs showing good agreement with the experimental results.     

Corrosion characterization of new tin-silver binary alloys in nitric acid solutions

Electrochemical techniques were used to characterize the corrosion behavior of four new binary alloys xSn-Ag (x = 26, 50, 70 and 96.5 wt%) alloys and their individual metal components in nitric acid solutions. The experimental data were collected by using open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Over the concentration range studied (0.075-4.5 M), each of the two corrosion parameters (E_corr and i_corr) shows a regular dependency on both the alloy composition and the solution concentration. In general, for all studied samples, especially pure Ag and those with lower Sn contents (26 and 50 wt%), increasing the acid concentration increases i_corr, meanwhile causes a shift of the corresponding E_corr towards more positive values. This is probably due to the increase in the effect of cathodic depolarizer as the nitric acid concentration is increased. EIS results at the free corrosion potential confirmed well this behavior, where at concentrations ?1.5 M the thickness of the surface film increases while its resistance decreases with increasing tin wt%, indicating formation of less protective thicker film. However, at higher concentrations all samples exhibit identical behavior.     

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.     

Comparative studies on the corrosion properties of a Fe-Mn-Al-Si steel and an interstitial-free steel

Recent developments in a group of super ductile Fe-Mn-Al-Si steels with high-manganese content demands for more research in the corrosion behavior of such steels. The corrosion properties of the Fe-Mn-Al-Si steel was studied in acidic (0.1 M H₂SO₄), alkaline (0.1 M NaOH) and chloride-containing (3.5% NaCl) environments, using immersion and polarization experiments and compared with that of interstitial-free (IF) steel. In acidic solution, the Fe-Mn-Al-Si steel exhibited significantly lower corrosion resistance than that of IF steel. Though the Fe-Mn-Al-Si steel showed lower corrosion resistance as compared to IF steel in chloride solution, the difference was not as substantial as observed in acidic medium. However, in alkaline solution, the Fe-Mn-Al-Si steel showed no significant difference in the corrosion resistance in comparison with that of IF steel, and moreover exhibited substantially high corrosion resistance than in acidic and chloride solution. The post-corrosion characterization studies showed higher corrosion attack of the Fe-Mn-Al-Si steel exposed to acidic solution as compared to that in alkaline and chloride solutions, which is consistent with the corrosion measurement data.     

Corrosion control of mild steel using 3,5-bis(4-methoxyphenyl)-4-amino-1,2,4-triazole in normal hydrochloric acid medium

The inhibition performance of the 3,5-bis(4-methoxyphenyl)-4-amino-1,2,4-triazole (4-MAT) on mild steel in normal hydrochloric acid medium (1 M HCl) at 30 °C was tested by weight loss, potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. This organic compound inhibits the acidic corrosion even at very low concentration, reaching a value of inhibition efficiency up to 98% at a concentration of 3 × 10⁻⁴ M. The results obtained from the different corrosion evaluation techniques are in good agreement. Polarisation curves indicate that 4-MAT is a mixed inhibitor, affecting both cathodic and anodic corrosion currents. Data, obtained from EIS measurements, were analyzed to model the corrosion inhibition process through appropriate equivalent circuit model, a constant phase element (CPE) has been used. The adsorption of 4-MAT on the steel surface, in 1 M HCl solution, obeys to Langmuir’s isotherm with a very high negative value of the free energy of adsorption ΔG°_ads (chemisorption). X-ray photoelectron spectroscopy (XPS) was carried out to establish the mechanism of corrosion inhibition of mild steel in 1 M HCl medium in the presence of 3,5-bis(4-methoxyphenyl)-4-amino-1,2,4-triazole (4-MAT).