Inhibition of the corrosion process of alloy AA5083 (Al‐Mg) in seawater by cerium cations. An EIS study

The principal corrosion process of the alloy AA5083 in aerated NaCl solutions mainly affects the zones occupied by Al(Mn,Fe,Cr) cathodic precipitates, over which the oxygen reduction reaction takes place. Thus, the design of an effective protection system could be based on the use of cathodic inhibitors. In previous papers, the inhibition efficiency of various lanthanide chlorides, particularly CeCl₃, has been proved. These compounds act as cathodic inhibitors, blocking these cathodic sites by means of the precipitation of a lanthanide oxide or hydroxide film. In this paper, EIS has been used to detect the blocking effect of the cathodic sites. This fact is reflected in the electrical response of the system through the minimisation of the values of those elements of the electrical loop that are related to the response of the cathodic intermetallics. In addition, the dielectric properties of the anodic film have been evaluated by measuring the value of the associated capacitance. From these values, it can be concluded that the thickness of the film produced over the matrix when the inhibitor is added is much lower than that produced in the bare solution. This finding is in good agreement with the results obtained with other techniques and may be related to the minimisation of the anodic reaction as a consequence of the decrease in the intensity of the cathodic process by the action of CeCl₃.     

Inhibitoren der Korrosion 23 (1) – Gibt es eine Struktur- Wirkungs-Beziehung bei organischen Inhibitoren der Korrosion von Aluminium?

Corrosion inhibitors 23 (1) – Does there exist a structure-efficiency relation in the organic inhibitors of aluminium corrosion? Over 400 organic compounds have been routinely tested as potential inhibitors for the corrosion and dissolution of aluminium in In HCl/2,5% NaCl solution, using 10^?2 m/l as the concentration of the organic compound being tested. It was attempted to derive a relationship between efficiency of the inhibitor with molecular structure. Particular attention was devoted to the following classes of compound: organoarsenic; phosphonic and phosphinic acids; aromatic aldehydes and ketones; aromatic and aliphatic carboxylic acids; dihydric phenols; tetrazolium salts and formazan compounds; sulphoxides and aromatic sulphonic acids; sulphonamides and sulphones. For a selected number if inhibitors, the dependence of concentration and the effect of an oxygen-containing atmosphere were studied. Compounds which show good inhibition in the corrosion of aluminium are consistently even better in the protection of zinc surfaces. The mechanism of the corrosion inhibition is not clear, but the results suggest that the total molecular structure of the inhibitor must be considered, with particular importance paid to the nature and the spatial relationship of the different functional groups.     

Electrochemical studies on the effect of (2E)‐3‐amino‐2‐phenylazo‐but‐2‐enenitrile and its derivative on the behaviour of copper in nitric acid

The objective of this work is to provide additional insight on the influence of (2E)‐3‐amino‐2‐phenylazo‐but‐2‐enenitrile and its derivative as corrosion inhibitors for copper in 0.5 M HNO₃. Electrochemical techniques (potentiodynamic polarization, polarization resistance and impedance spectroscopy) as well as weight loss measurements have been employed to study the corrosion inhibition. The investigated compounds have shown inhibition efficiency in 0.5 M HNO₃. Inhibition efficiency of these compounds has been found to vary with the concentrations of the compounds. The adsorption of these compounds on the copper surface from the acid solution has been found to obey Langmuir adsorption isotherm. The results revealed that the compounds are mixed type inhibitors. The effect of temperature on the inhibition efficiency was studied.     

Film formation on stainless steel in a solution containing chromic and sulphuric acids

The paper discusses the kinetics of the spontaneous reaction occurring during the formation of a coloured interference film on stainless steel in hot CrO₃ + H₂SO₄ aqueous solution. The film behaves as a porous membrane, permitting liquid phase diffusion of the dissolution product of the steel through it. Film growth is by precipitation at the film/solution interface of a spinel oxide to which metal ions formed by both anodic dissolution of the steel and reduction of chromic acid—the accompanying cathodic process—contribute. The kinetics of the reaction follow an exponentially increasing growth law, which is unusual and possibly novel among film-forming reactions.     

Hydrogen reduction of tungsten oxides: Alkali additions,their effect on the metal nucleation process and potassium bronzes under equilibrium conditions

The addition of alkali compounds influences W particle growth during the reduction of tungsten oxide using hydrogen. Existing literature does not reveal the mechanism and clear mode of interaction. To improve understanding of the reduction process, lithium, sodium and potassium compounds were added to a highly pure tungsten oxide prior to reduction, and interrupted reduction experiments were carried out at 750 °C. The resulting powders were investigated by metallographic and chemical methods: X-ray diffraction, SEM and EDX analysis.The experiments confirmed that intermediately formed tungsten bronzes play an important role in the early stages of reduction. Further experiments showed that bronzes are formed from potassium containing compounds in equilibrium with mixtures of W and WO₂ or WO₂ and WO_2.72. Furthermore it is demonstrated that the kinetics of the reduction sequence differs significantly between doped and undoped tungsten oxide powders, as demonstrated by the on-line measurement of reaction water in the hydrogen process gas.The results help to understand how the presence of alkali compounds effects the reduction of tungsten oxides. Investigations in production scale with intentionally contaminated tungsten oxide showed that bronzes can play a role in industrialised production.     

Aminic nitrogen-bearing polydentate Schiff base compounds as corrosion inhibitors for iron in acidic media: A quantum chemical calculation

Quantum chemical calculations based on DFT method were performed on three polydentate Schiff base compounds (PSCs) used as corrosion inhibitors for iron in acid media to determine the relationship between the molecular structure of PSC and inhibition efficiency. The structural parameters, such as the frontier molecular orbital energy HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), the charge distribution of the studied inhibitors, the absolute electronegativity (χ) values, and the fraction of electrons (ΔN) transfer from inhibitors to iron, were also calculated and correlated with inhibition efficiencies. The results showed that the inhibition efficiency of PSCs increased with the increase in E_HOMO and decrease in E_LUMO − E_HOMO; and the areas containing N atoms are most possible sites for bonding the metal iron surface by donating electrons to the metal.     

Azole Compounds as Corrosion Inhibitors: Part I

The inhibitory effect of 3-amino-1,2,4 triazole (I), 2-aminothiazole (II), and 3-amino-5-mercapto-1,2,4 triazole (III) on the corrosion of steel in 2 M HCl solution has been studied using chemical technique and electrochemical techniques. The inhibition efficiencies (η) obtained from all the methods employed are in good agreement. Results reveal compound (III) to be the most efficient inhibitor with an inhibition efficiency of 91%. The inhibition efficiency increases with increasing concentration in all three compounds. Studies clearly indicate that these compounds act as a mixed type inhibitor without altering the cathodic reaction. The inhibitors are adsorbed on the surface according to the Langmuir adsorption isotherm. Quantum chemical parameters such as highest occupied molecular orbital energy (E _HOMO), lowest unoccupied molecular orbital energy (E _LUMO), energy gap (ΔE), and dipole moment (μ) have been calculated for 3-amino-1,2,4 triazole (I), 2-aminothiazole (II), and 3-amino-5-mercapto-1,2,4 triazole (III) using semi-empirical PM3 and 6-311G(d,p) DFT methods.     

Shot Noise Analysis to Investigate the Corrosion Inhibition of AZ91D Magnesium Alloy in Sulfuric Acid Solution

The corrosion inhibition of AZ91D magnesium alloy in 0.01 M H₂SO₄ by a Schiff base compound was investigated using Potentiodynamic polarization, Electrochemical Impedance Spectroscopy and Electrochemical Noise methods. It is found that the Schiff base acts as mixed-type inhibitor and the inhibition efficiency increases with concentration. Electrochemical impedance spectroscopy results showed that the Schiff base acts by adsorption at the metal/solution interface. Shot noise analysis showed that the inhibitor addition decreases the number of charge transfer events instead of the value of charge transfer at each electrochemical corrosion event. The corrosion inhibition effect was approved by suitable surface analysis.     

Inhibiting effects of some quinolines and organic phosphonium compounds on corrosion of mild steel in 3M HCl solution and their adsorption characteristics

Abstract

The inhibiting effects of quinoline, 8-hydroxyquinoline, benzo(f)quinoline, quinoline-2-thiol, triphenylbenzyl, and tetrabenzyl phosphonium chloride on the corrosion of mild steel (0.26 wt-%C) in deaerated 3M HCl solution have been studied using the determination of polarisation curves as well as linear polarisation measurements. Adsorption isotherms and thermodynamic parameters for the adsorption process of the inhibitors were also determined and are discussed. Except for benzo(f)quinoline, which acts as an accelerator of corrosion, the other quinolines act as corrosion inhibitors. The inhibition was found to be predominantly anodic with quinoline and 8-hydroxyquinoline, while quinoline-2-thiol is a mixed inhibitor. The increase in inhibition efficiency with temperature, the resulting Langmuir adsorption isotherm and the high negative values of the standard free energy of adsorption ΔG_a° denoted chemisorption. Triphenylbenzyl and tetrabenzyl phosphonium compounds were found to be inhibitors of the mixed type. High negative values of the standard free energy of adsorption and the resulting Temkin isotherm indicated chemisorption of the phosphonium compounds. The positive values of the standard enthalpy and entropy of adsorption indicated that adsorption of the inhibitors is associated with the desorption of H₂O molecules from the electrode surface. High values of the standard energy of activation E_a* of the inhibited corrosion process were interpreted in terms of deactivating coverage.     

咪唑类化合物自组装膜对 316 不锈钢的缓蚀作用

通过电化学阻抗谱、极化曲线和量子化学方法,研究了在0.5 mol/L的硫酸溶液中,咪唑、十二烷基咪唑、苄基咪唑、苯并咪唑四种缓蚀剂自组装膜对316不锈钢的缓蚀性能。研究结果表明:四种缓蚀剂均为阴极型缓蚀剂,对不锈钢具有较好的缓蚀作用,在1×10-3mol/L的浓度下,随着组装时间的延长,自组装膜对不锈钢的缓蚀效率也相应增强,它们的缓蚀能力主要由最高占据轨道能EHOMO的大小决定,缓蚀能力由大到小依次为苯并咪唑>十二烷基咪唑>苄基咪唑>咪唑。     

The corrosion inhibition of Ti-alloy (VT-9) in mixture of H<Subscript>2</Subscript>SO<Subscript>4</Subscript>-NaF by organic compounds

This investigation deals with corrosion behavior of high strength titanium alloy in concentrated sulphuric acid solution containing different concentrations (500, 1000, 1500 ppm) of fluoride ion (F⁻) using various organic compounds (MPA, L-OH, NFP) as inhibitor, potentiodynamically. The open circuit potential values noted before and after each experiment, varied appreciably. These values were negative before polarization but after completion of the experiment turned positive and remained stable over long period of time. It is observed that cathodic current density values increase with increasing cathodic potential (more negative) and fluoride ion. The values of cathodic Tafel slopes derived from the curves (∼110 − 140 mV/dec I) indicate hydrogen evolution reaction (h.e.r). The corrosion potential (E _corr) varied slightly with addition of inhibitors. The corrosion current densities (I _corr) increased with increasing fluoride ion concentration, but these values decreased appreciably when inhibitor (MPA) was used. SEM micrograph reveals reduction of pits in the presence of inhibitor (MPA). So this concludes that organic compound was used in this case acts as a good inhibitor. The article is published in the original.     

Influence of the alkyl chain length of 2 amino 5 alkyl 1,3,4 thiadiazole compounds on the corrosion inhibition of steel immersed in sulfuric acid solutions

The corrosion inhibition efficiency of 2 amino 5 alkyl 1,3,4 thiadiazole compounds with different alkyl chain lengths, namely: 2 ethyl, 3 n propyl, 5 n penthyl, 7 hepthyl, 11 undecyl and 13 tridecyl, was evaluated in the system steel/1 M H₂SO₄. These compounds were synthesized, characterized by FT-IR and NMR spectroscopy analysis and evaluated using electrochemical impedance spectroscopy and SEM analysis. The results showed that the inhibition mechanism involves blockage of the steel surface by the inhibitor molecules by a Langmuir-type adsorption process and that the alkyl chain length plays an important role in the inhibition efficiency of the synthesized inhibitors.     

An investigation of three novel nonionic surfactants as corrosion inhibitor for carbon steel in 0.5 M H2SO4

The inhibition effect of three novel nonionic surfactants, 2-((alkylimino)methyl)phenyl bis(53-hydroxy-3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51-heptadecaoxatripentacontyl) phosphate (I–III), on the corrosion of carbon steel in 0.5 M H₂SO₄ was studied by polarization, EIS and weight loss measurements. It was found that all the inhibitors were effective inhibitors and their inhibition efficiency was significantly increased with increasing both concentration and temperature. Polarization curves revealed that the used inhibitors represent mixed-type inhibitors. Adsorption of used inhibitors led to a reduction in the double layer capacitance and an increase in the charge transfer resistance. Adsorption of used compounds was found to obey Langmuir isotherm and showed a chemical mechanism.     

Corrosion inhibition of 7000 series aluminium alloys with cerium diphenyl phosphate

Cerium diphenyl phosphate (Ce(dpp)₃) has previously been shown to be a strong corrosion inhibitor for aluminium-copper magnesium alloy AA2024-T3 and AA7075 in chloride solutions. Surface characterisation including SEM and ToF-SIMS coupled with electrochemical impedance spectroscopy (EIS) measurements are used to propose a mechanism of corrosion inhibition which appears to involve the formation of a complex oxide film of aluminium and cerium also incorporating the organophosphate component. The formation of a thin complex film consisting of hydrolysis products of the Ce(dpp)₃ compound and aluminium oxide is proposed to lead to the observed inhibition. SEM analysis shows that some intermetallics favour the creation of thicker deposits predominantly containing cerium oxide compounds.     

Thermodynamic, electrochemical and quantum chemical investigation of some Schiff bases as corrosion inhibitors for mild steel in hydrochloric acid solutions

The corrosion inhibition of mild steel in 1.0 M HCl solution by four Schiff bases was investigated using weight loss and electrochemical measurements and quantum chemical calculations. All compounds showed >90% inhibition efficiency at their optimum concentrations. The activation energy (E_a) of corrosion and other thermodynamic parameters were calculated to elaborate the mechanism of corrosion inhibition. The adsorption of the inhibitors on the mild steel surface follows Langmuir isotherm model. Polarization studies indicated that all studied inhibitors are mixed type. The computed quantum chemical properties viz., electron affinity (EA) and molecular band gap (ΔE_MBG) show good correlation with experimental inhibition efficiencies.     

On electrochemical techniques for interface inhibitor research

The electrochemical behavior related to different modes of inhibition effect is discussed for interface inhibitors. The action coefficients of an inhibitor on the anodic and cathodic reactions (f_a and f_c, respectively) of a corrosion process are defined. An equation is deduced which denotes that if the inhibition effect is caused by geometrically blocking the surface of metal electrode by adsorbed inhibitive species, the corrosion potential in the inhibitor-containing solution will be nearly equal to that in the solution without the inhibitor. In this case the inhibition efficiency (η) equals the coverage (Θ) of the adsorbed species and can be estimated by polarization resistance (R_p) measurements. In addition, the plot of η vs the “relative coverage” (μ) estimated from interfacial capacitance measurements will be a straight line through the original point of the coordinate. If the inhibition is not due to the geometric blocking effect, which can be judged by the noticeable shift of corrosion potential as the inhibitor is added into the solution, then η cannot be estimated by R_p and does not equal Θ. Instead, transfer resistance (R_t) can always be employed to estimate inhibition efficiency whatever the mode of the inhibition effect. It is argued that theoretically no Tafelian straight lines can be measured in solutions with interface inhibitors unless Θ is independent of the electrode potential E or f_a and f_c of the inhibitor change linearly with E. Equations of faradaic admittance at the corrosion potential in inhibitor-containing solution for different modes of inhibition effect are deduced. In addition to the estimation of R_t, R_p and interfacial capacitance, information on the adsorption of the inhibitive species can also be gathered by EIS measurement. In the case of the geometric blocking effect, the EIS display in the inhibitor-containing solution will be similar to that in the black solution when η is low, while a single capacitive loop will display when η is high. If the inhibition is not due to the geometric blocking effect, two time constants will be involved in the EIS display measured in the inhibitive-containing solution no matter what the displays of EIS in the blank solution are, and in this case the effect of E on Θ can be predicted based on the EIS and polarization curve measurements.     

Cerium(III) chloride and sodium octylthiopropionate as an effective inhibitor mixture for zinc corrosion in 0.5 M NaCl

It has been reported that cerium(III) chloride CeCl₃ and sodium octylthiopropionate C₈H₁₇S(CH₂)₂COONa (NaOTP) are effective inhibitors for zinc corrosion in 0.5 M NaCl. In this study, synergistic inhibition of zinc corrosion in an aerated 0.5 M NaCl solution by a mixture of these inhibitors was investigated by polarization measurements after immersion of a zinc electrode in the solution for many hours. The inhibition efficiency of 1×10⁻⁴ M CeCl₃ plus 1×10⁻⁵ M NaOTP mixture was high, 95.1% after both 3 and 120 h. X-ray photoelectron spectroscopy and electron-probe microanalysis for the inhibited electrode revealed that the zinc surface was covered with a protective film composed of an hydrated or hydroxylated Ce-rich oxide, a small amount of Zn(OH)₂ and a trace of Zn(OTP)₂ chelate. The inhibition effect of 1×10⁻⁵ M NaOTP in the NaCl solution for the zinc electrode previously treated in 1×10⁻³ M CeCl₃ for 30 min was also examined, indicating a higher inhibition efficiency, 96.3% after immersion of the electrode in the solution for 120 h.     

Electrochemical studies on the effect of pyrazolo‐containing compounds on the corrosion of carbon steel in 1 M sulphuric acid

The effect of some synthesized pyrazolo containing compounds on the corrosion of carbon steel in 1 M H₂SO₄ was investigated. The investigation involved electrochemical polarization methods (potentiodynamic, Tafel extrapolation and the determination of the polarization resistance). A significant decrease in the corrosion rate of carbon steel was observed in the presence of the investigated compounds. The results show that these compounds act as mixed type inhibitors, but the cathode is more preferentially polarized. The relative inhibition efficiency of these compounds depends on both the nature and concentrations of the investigated compounds. Compounds are found to adsorb on the carbon steel surface according to the Langmuir adsorption isotherm.     

Raman microscopy of chromate interactions with corroding aluminum alloy 2024-T3

Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K₂Cr₂O₇. Raman spectra acquired in and near corrosion pits allowed identification of two products of Cr^VI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an Al^III-Cr^VI mixed oxide formed in formerly active or metastable pits. This mixed oxide is similar to a covalent compound which can be made synthetically by adding NaOH to solutions containing Al³⁺ and CrO₄²⁻. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a Cr^III-Cr^VI mixed oxide, formed by partial reduction of Cr^VI to Cr^III by exposed Al metal or by H₂. At the pitting potential, a mixture of Al^III and Cr^III mixed oxides was observed, with the Al^III species located primarily within pits. A model for the formation mechanism of the mixed oxides is proposed, and the consequences of the findings to corrosion inhibition by Cr^VI are considered. In particular, the concentration of Cr^VI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion.     

Three pyrazine derivatives as corrosion inhibitors for steel in 1.0 M H2SO4 solution

The inhibition effect of three pyrazine derivatives of 2-methylpyrazine (MP), 2-aminopyrazine (AP) and 2-amino-5-bromopyrazine (ABP) on the corrosion of cold rolled steel (CRS) in 1.0 M H₂SO₄ solution was studied by weight loss, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. The results show that all pyrazine compounds are good inhibitors, and inhibition efficiency follows the order: ABP > AP > MP. The adsorption of each inhibitor on CRS surface obeys Langmuir adsorption isotherm. For all these pyrazine derivatives, they act as mixed-type inhibitors. The probable inhibitive mechanism is proposed from the viewpoint of adsorption theory.