Showing papers in "Corrosion Science in 2008"

The use of quantum chemical methods in corrosion inhibitor studies

Abstract: Quantum chemical methods are particularly significant in the study of electrochemistry and provide researchers with a relatively quick way of studying the structure and behaviour of corrosion inhibitors. The originality of this review article is based on the fact that it is the first and unique general reference for all those interested in the use of quantum chemical methods in corrosion inhibitor studies. It begins with a concise summary of the most used quantum chemical parameters and methods and then summarizes the results of research articles in corrosion science over the past 20 years.

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Abstract: Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH) 2 corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg 17 Al 12 ) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.

Influence of the β-phase morphology on the corrosion of the Mg alloy AZ91

Abstract: The influence of the microstructure, particularly the morphology of the β-phase, on the corrosion of Mg alloys has been studied using AZ91 as a model Mg alloy. The corrosion behaviour was characterized for five different types of microstructure produced by heat treatment of as-cast AZ91. The influence of microstructure can be understood from the interaction of the following three factors: (i) the surface films can be more or less effective in hindering corrosion and more or less effective in controlling the form of corrosion as uniform corrosion or localised corrosion, (ii) the second phase (the β-phase in AZ91) can cause micro-galvanic acceleration of corrosion and (iii) the second phase can act as a corrosion barrier and hinder corrosion propagation in the matrix, if the second phase is in the form of a continuous network. It is expected that these factors are important for all multi-phase Mg alloys because all known second phases have corrosion potentials more positive than that of the α-phase. A particular example of the corrosion barrier effect is provided by the fine (α + β) lamellar micro-constituent; when a β-phase plate nucleates this micro-constituent, the β-phase plate acts as a corrosion barrier. In contrast, nano-sized β precipitates, produced by aging, caused micro-galvanic corrosion acceleration of the adjacent α-phase. However, it is an important finding that the corrosion rate of the α-phase was decreased by the aging treatments that caused the precipitation of the nano-sized β particles.

Inhibitory action of Phyllanthus amarus extracts on the corrosion of mild steel in acidic media

Abstract: The inhibitive action of leaves (LV), seeds (SD) and a combination of leaves and seeds (LVSD) extracts of Phyllanthus amarus on mild steel corrosion in HCl and H 2 SO 4 solutions was studied using weight loss and gasometric techniques. The results indicate that the extracts functioned as a good inhibitor in both environments and inhibition efficiency increased with extracts concentration. Temperature studies revealed an increase in inhibition efficiency with rise in temperature and activation energies decreased in the presence of the extract. A mechanism of chemical adsorption of the plants components on the surface of the metal is proposed for the inhibition behaviour. The adsorption characteristics of the inhibitor were approximated by Temkin isotherm.

Pitting corrosion behaviour of austenitic stainless steels – combining effects of Mn and Mo additions

Abstract: Mn and Mo were introduced in AISI 304 and 316 stainless steel composition to modify their pitting corrosion resistance in chloride-containing media Corrosion behaviour was investigated using gravimetric tests in 6 wt% FeCl3, as well as potentiodynamic and potentiostatic polarization measurements in 35 wt% NaCl Additionally, the mechanism of the corrosion attack developed on the material surface was analysed by scanning electron microscopy (SEM), X-ray mapping and energy dispersive X-ray (EDX) analysis The beneficial effect of Mo additions was assigned to Mo6+ presence within the passive film, rendering it more stable against breakdown caused by attack of aggressive Cl− ions, and to the formation of Mo insoluble compounds in the aggressive pit environment facilitating the pit repassivation Conversely, Mn additions exerted an opposite effect, mainly due to the presence of MnS inclusions which acted as pitting initiators

Electrochemical and theoretical investigation on the corrosion inhibition of mild steel by thiosalicylaldehyde derivatives in hydrochloric acid solution

Abstract: Inhibitory effect of three Schiff bases 2-{[(2-sulfanylphenyl)imino]methyl}]phenol (A), 2-{[(2)-1-(4-methylphenyl)methylidene]amino}-1-benznethiol (B), and 2-[(2-sulfanylphen-yl)ethanimidoyl)]phenol (C) on corrosion of mild steel in 15% HCl solution has been studied using weight loss measurements, polarization and electrochemical impedance spectroscopy (EIS) methods. The results of the investigation show that the compounds A and B with mean efficiency of 99% at 200 mg/L additive concentration have fairly good inhibiting properties for mild steel corrosion in hydrochloric acid, and they are as mixed inhibitor. All measurements show that inhibition efficiencies increase with increase in inhibitor concentration. This reveals that inhibitive actions of inhibitors were mainly due to adsorption on mild steel surface. Adsorption of these inhibitors follows the Langmuir adsorption isotherm. Thermodynamic adsorption parameters ( K ads , Δ G ads ) of studied Schiff bases were calculated using the Langmuir adsorption isotherm. Activation parameters of the corrosion process such as activation energies, E a , activation enthalpies, Δ H ∗ , and activation entropies, Δ S ∗ , were calculated by the obtained corrosion currents at different temperatures. Obvious correlation was found between the corrosion inhibition efficiency and the calculated parameters. The obtained theoretical results have been adapted with the experimental data.

Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41

Abstract: The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)2 in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.

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

Abstract: 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 (chi) values, and the fraction of electrons (Delta N) 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. (C) 2007 Elsevier Ltd. All rights reserved.

Evaluation of the inhibitive effect of some plant extracts on the acid corrosion of mild steel

Abstract: Corrosion inhibition of mild steel in 2 M HCl and 1 M H 2 SO 4 by extracts of selected plants was investigated using a gasometric technique at temperatures of 30 and 60 °C. The studied plants materials include leaf extracts Occimum viridis ( OV ), Telferia occidentalis ( TO ), Azadirachta indica ( AI ) and Hibiscus sabdariffa ( HS ) as well as extracts from the seeds of Garcinia kola ( GK ). The results indicate that all the extracts inhibited the corrosion process in both acid media by virtue of adsorption and inhibition efficiency improved with concentration. Synergistic effects increased the inhibition efficiency in the presence of halide additives. Inhibition mechanisms were deduced from the temperature dependence of the inhibition efficiency as well as from assessment of kinetic and activation parameters that govern the processes. Comparative analysis of the inhibitor adsorption behaviour in 2 M HCl and 1 M H 2 SO 4 as well as the effects of temperature and halide additives suggest that both protonated and molecular species could be responsible for the inhibiting action of the extracts.

Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments

Abstract: High-entropy alloys (HEAs) are a newly developed family of multi-component alloys. The potentiodynamic polarization and electrochemical impedance spectroscopy of the Al x CrFe 1.5 MnNi 0.5 alloys, obtained in H 2 SO 4 and NaCl solutions, clearly revealed that the corrosion resistance increases as the concentration of aluminium decreases. The Al x CrFe 1.5 MnNi 0.5 alloys exhibited a wide passive region, which extended >1000 mV in acidic environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. SEM micrographs revealed that the general and pitting corrosion susceptibility of the HEAs increased as the amount of aluminium in the alloy increased.

Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

Abstract: The hydrogen trapping efficiency in different microstructures is compared, and the critical hydrogen flux for hydrogen induced cracking (HIC) is determined for API X65 grade linepipe steel. By controlling the start cooling temperature (SCT) and the finish cooling temperature (FCT) in thermomechanically controlled process (TMCP), three different kinds of microstructure such as ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) are obtained. A modified ISO17081(2004) standard method is used to evaluate the hydrogen trapping by measuring the permeability (JssL) and the apparent diffusivity (Dapp). Microstructures affecting both hydrogen trapping and hydrogen diffusion are found to be DP, AF, BF and martensite/austenite (M/A) constituents. The hydrogen trapping efficiency is increased in the order of DP, BF and AF, with AF being the most efficient. HIC is initiated at the local M/A concentrated region when the steel has such microstructures as F/AF or F/B. Although the trapping efficiency of bainite is lower than that of AF, bainite is more sensitive microstructure to HIC than to AF.

Localized corrosion (pitting): A model of passivity breakdown including the role of the oxide layer nanostructure

Abstract: A model of passivity breakdown including the role of the inter-granular boundaries of the barrier oxide layer on the redistribution of the potential at the metal/oxide/electrolyte interfaces in the passive state is presented. Different mechanisms of breakdown at the oxide grain boundaries are considered, depending on which interface governs the potential drop: (i) local thinning and dissolution of the oxide layer, (ii) metal voiding or (iii) particle growth at the metal/oxide interface followed by rupture of the barrier layer. The role of chloride ions is discussed in each case. The key experimental observations made at the nanometre scale and validating the model are discussed.

Molecular dynamics and density functional theory study on relationship between structure of imidazoline derivatives and inhibition performance

Abstract: The inhibition performance of two imidazoline derivatives, 3-ethylamino-2-undecyl imidazoline (EUI) and chloride-3-ethylamino-3-(2,3-two hydroxyl) propyl-2-undecyl imidazoline sodium phosphate(CEPIP), for Q235 steel in CO2 saturated solution at 298 K have been tested by weight loss experiment and electrochemical techniques. The adsorption behavior of the two inhibitors on Fe surface has been studied using molecular dynamics (MD) method and density functional theory. The results indicated that the two imidazoline derivatives could both adsorb on the Fe surface firmly through the imidazoline ring and heteroatoms, the two inhibitors both have excellent corrosion inhibition performance.

Quantitative analysis of iron oxides using Fourier transform infrared spectrophotometry

Abstract: In this study, a systematic approach based on the application of Fourier transform infrared spectrophotometry (FTIR) was taken, in order to quantitatively analyze the corrosion products formed in the secondary cycle of pressurized water reactors (PWR). Binary mixtures of iron oxides were prepared with known compositions containing pure commercial magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), and hematite (α-Fe 2 O 3 ) for calibration purposes. Calcium oxide (lime) was added to all samples as a standard reference in obtaining the calibration curves. Using regression analysis, relationships were developed for intensity versus concentration for absorption bands corresponding to each of the phases in their corresponding FTIR spectrum. Correlation coefficients, R 2 , of 0.82, 0.87, and 0.86 were obtained for maghemite–magnetite, hematite–magnetite, and hematite–maghemite systems, respectively. The calibration curves generated were used to quantify phases in multi-component unknown field samples that were obtained from different components (moisture separators, condensers, and high- and low- pressure heaters) of the two units (units 1 and 2) of the secondary cycle of the Comanche Peak PWR.

Inhibition of mild steel corrosion in acidic medium using synthetic and naturally occurring polymers and synergistic halide additives

Abstract: The corrosion inhibition of mild steel in H2SO4 in the presence of gum arabic (GA) (naturally occurring polymer) and polyethylene glycol (PEG) (synthetic polymer) was studied using weight loss, hydrogen evolution and thermometric methods at 30–60 °C. PEG was found to be a better inhibitor for mild steel corrosion in acidic medium than GA. The effect of addition of halides (KCl, KBr and KI) was also studied. Results obtained showed that inhibition efficiency (I%) increased with increase in GA and PEG concentration, addition of halides and with increase in temperature. Increase in inhibition efficiency (I%) and degree of surface coverage (θ) was found to follow the trend Cl− < Br− < I− which indicates that the radii and electronegativity of the halide ions play a significant role in the adsorption process. GA and PEG alone and in combination with halides were found to obey Temkin adsorption isotherm. Phenomenon of chemical adsorption is proposed from the trend of inhibition efficiency with temperature and values ΔGads0 obtained. The synergism parameter, SI evaluated is found to be greater than unity indicating that the enhanced inhibition efficiency caused by the addition of halides is only due to synergism.

Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Abstract: Effects of pH solution and chloride (Cl − ) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy. The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E pit and corrosion E cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits. Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).

Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings

Abstract: The corrosion performance of WE43-T6 and AZ91D magnesium alloys with and without treatment by plasma electrolytic oxidation (PEO) was investigated by electrochemical measurements in 3.5 wt.% NaCl solution. For untreated WE43-T6 alloy, formation of a uniform corrosion layer (Mg(OH) 2 ) was accompanied by initial pits around magnesium-rare earth intermetallic compounds. The AZ91D alloy disclosed increased corrosion susceptibility, with localized corrosion around the β-phase, though the β-phase network phase acted as a barrier for corrosion progression. PEO treatment in alkaline phosphate electrolyte improved the corrosion resistance of WE43-T6 alloy only at the initial stages of immersion in the test solution. However, PEO-treated AZ91D alloy revealed a relatively high corrosion resistance for much increased immersion times, contrary to the relative corrosion resistances of the untreated alloys. The improved performance of the PEO-treated AZ91D alloy appears to be related to the formation of a more compact coating.

The role of rusts in corrosion and corrosion protection of iron and steel

Abstract: The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe 2+ and OH − ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH − ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.

Corrosion mechanism of model zinc–magnesium alloys in atmospheric conditions

Abstract: Recently, superior corrosion properties of zinc coatings alloyed with magnesium have been reported. Corrosion behaviour of model zinc–magnesium alloys was studied to understand better the protective mechanism of magnesium in zinc. Alloys containing from 1 to 32 wt.% magnesium, pure zinc, and pure magnesium were contaminated with sodium chloride and exposed to humid air for 28 days. Composition of corrosion products was analyzed using infrared spectroscopy (FTIR), ion chromatography (IC), and Auger electron spectroscopy (AES). The exposure tests were completed with scanning Kelvin probe (SKP) and electrochemical measurements. Weight loss of ZnMg alloys with 1–16 wt.% magnesium was lower than that of pure zinc. Up to 10-fold drop in weight loss was found for materials with 4–8 wt.% Mg in the structure. The improved corrosion stability of ZnMg alloys was connected to the presence of an Mg-based film adjacent to the metal surface. It ensured stable passivity in chloride environment and limited the efficiency of oxygen reduction.

Corrosion protection with polyaniline and polypyrrole as anticorrosive additives for epoxy paint

Abstract: Accelerated immersion tests have been carried out in the laboratory to investigate the performance of polyaniline emeraldine salt and polypyrrole composite with carbon black as additives of an epoxy paint coating. The steel panels coated with paint modified with polyaniline emeraldine salt presented the best protection after 720 h of exposure in 3.5% NaCl solution. The results indicated that this conducting polymer might works as both corrosion inhibitor and adhesion promoter. The protective mechanism imparted by conducting polymers is also discussed.

Corrosion protection of magnesium alloys by cerium, zirconium and niobium-based conversion coatings

Abstract: A new Ce, Zr and Nb-based conversion coating was designed for AZ91 and AM50 magnesium alloys. The corrosion protection provided by this coating was evaluated by electrochemical measurements (polarization curves, electrochemical impedance spectroscopy) in Na 2 SO 4 electrolyte, and accelerated atmospheric corrosion tests (humid, SO 2 polluted air, and salt spray). Its chemical composition was characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements showed that Mg alloys treated during 24 h in the Ce–Zr–Nb conversion bath exhibit: (i) increased corrosion potential, (ii) decreased corrosion and anodic dissolution current densities, and (iii) increased polarization and charge transfer resistances. The accelerated corrosion tests revealed excellent atmospheric corrosion resistance for all Ce–Zr–Nb-treated samples, with or without an additional layer of epoxy–polyamide resin lacquer or paint. XPS analysis showed that the coating includes CeO 2 , Ce 2 O 3 , ZrO 2 , Nb 2 O 5 , MgO, and MgF 2 as main components. No significant modification of the chemical composition was observed after cathodic and anodic polarization in Na 2 SO 4 . This new coating provides improved corrosion resistance, and excellent paint adhesion. It offers an alternative to the chromate conversion coating for magnesium alloys.

Inhibition effects of some Schiff’s bases on the corrosion of mild steel in hydrochloric acid solution

Abstract: The corrosion inhibition effect of 4-{[(1Z)-(2-chloroquinolin-3-yl)methylene]amino} phenol (CAP), N -[(1Z)-(2-chloroquinolin-yl)methylene]- N -(4-methoxyphenyl)amine (CMPA) and N -[(1E)-(2-chloroquinolin-3-yl)methylene]- N -(4-nitrophenyl)amine (CNPA) on mild steel in 1.0 M HCl has been investigated using mass loss, polarization and electrochemical impedance techniques at 300 K. The inhibition efficiencies increased with increase in inhibitors concentration. Polarization studies showed that the inhibitors are of predominantly cathodic character. Among the three compounds studied, CAP exhibited the best performance giving more than 97% IE. Some samples of mild steel were examined by SEM. All the inhibitors were found to adsorb on the mild steel surface according to the Langmuir adsorption isotherm.

EN, EIS and polarization studies to evaluate the inhibition effect of 3H-phenothiazin-3-one, 7-dimethylamin on mild steel corrosion in 1 M HCl solution

Abstract: The corrosion inhibition effect of 3H-phenothiazin-3-one, 7-dimethylamin as a new inhibitor was studied using different electrochemical and weight loss methods. It was found that this compound acts as a strong inhibitor for mild steel in 1 M HCl even at very low concentration (1 ppm). Results showed that this compound acts as a mixed type inhibitor. As the inhibitor concentration increased, the charge transfer resistance of mild steel increased and double layer capacitance decreased. The results of EN measurements after trend removal were in good agreement with other methods results. It was found that this inhibitor acts through adsorption on the metal surface. Also, adsorption obeys the Langmuir isotherm.

Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy

Abstract: Phosphate conversion coating, which is considered as all alternative to chromium conversion coating for improving the corrosion resistance of die-cast AZ91D magnesium alloy is studied. The structure and formation mechanism of the coating was investigated in details using ESEM/EDX, XRD, EPMA, ICP and electrochemical method. It was found that the conversion coating was composed of complex phosphate containing magnesium and aluminum and showed amorphous structure. The thickness of the coating was about 10 mu m. A possible formation mechanism for phosphate conversion coating was proposed. Local pH rise due to the evolution of hydrogen in the vicinity of micro-cathode attributed to the movement of hydrolysis reaction balance for manganese dihydro phosphate in treating solution and resulted in the phosphate coating precipitated oil metal surface. Initial depositing position was related to the microstructure of substrate. Galvanic effect between alpha phase and beta phase caused the flower-like phosphate nuclei preferential depositing oil beta phase. Ball-like phosphate nuclei deposited mainly in alpha phase interior due to the micro-galvanic effect within alpha phase. The primary alpha phase was removed from the metal matrix due to the corrosion dissolution, which suggested the anodic dissolution mechanism during phosphate conversion coating growth. The inner stress in dried film resulted in random distribution of the network micro-cracks oil film surface, the crack site did not correspond to beta phase. (C) 2007 Elsevier Ltd. All rights reserved.

SRB-biofilm influence in active corrosion sites formed at the steel-electrolyte interface when exposed to artificial seawater conditions

Abstract: Electrochemical evolution of the interface formed by carbon steel exposed to artificial seawater with nutrients in the presence and absence of mixed cultures that contain sulfate-reducing bacteria (SRB) is characterized by electrochemical impedance spectroscopy (EIS). The artificial seawater in sterile conditions progressively covered the surface of the steel sample with two different layers after 30 days of exposure. An outer layer is formed by a mixture of chlorides and phosphorus-based iron corrosion products with organic compounds from the culture media, and an inner layer is formed by corrosion products mixture constituted mainly by phosphorus-base products. Alternatively, under biotic conditions there was one heterogeneous layer composed by a mixture of phosphorous and sulfur-based corrosion products and biofilm. Three time constants were observed with EIS for sterile conditions. At low frequencies one constant is associated with the charge transfer resistance related to the iron dissolution reaction and inversely proportional to the active area; the porous resistance magnitudes at medium frequencies characterized the physicochemical properties of the inner layer, and high frequency described the electrical properties of the outer mixture layer. Low carbon steel in the presence of SRB (halophilic hydrogenotrophic) showed the impedance distribution after the formation of a corrosion product thick black layer mixed with organic composites and bio-entities. The SRB-biofilm enhanced the corrosion rate and influenced the appearance of diffusion controlled mechanism process. Electrical passive analogs in terms of constant phase elements characterized the evolution of the cover films formed and the impedance of the layers with time. The mechanisms are characterized based on the impedance response for three time constants in the absence of SRB and one time constant with a finite Warburg element when SRB are present in the electrolyte. The validation of the theoretical approximation with electrical analogs was in good agreement with the experimental results.

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

Abstract: The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H2SO4 at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.

Inhibition of iron corrosion in acid solutions by Cefatrexyl: Behaviour near and at the corrosion potential

Abstract: The corrosion inhibition of iron in HCl, HClO4, H2SO4 and H3PO4 solutions (1M for each) by cefatrexyl has been studied by polarization resistance (Rp) and electrochemical impedance spectroscopy (EIS) at the corrosion potential. The results obtained at 30 °C revealed that cefatrexyl acts as a weak inhibitor in HCl solution while it shows excellent inhibition performance in the remaining acids. Adsorption of cefatrexyl in HCl solution obeys Langmuir’s isotherm with a very low value of the free energy of adsorption Δ G ads ∘ (physisorption) while its adsorption in the other acids follows Temkin’s isotherm with very high negative values of Δ G ads ∘ (chemisorption). Data obtained from EIS measurements were analyzed to model the corrosion inhibition process through appropriate equivalent circuit models. The calculated values of the apparent activation energy (Ea) and the pre-exponential factor (λ) indicate that cefatrexyl blocks nearly the whole active centers of iron surface in H3PO4 solution even at elevated temperatures. The inhibition mechanism of cefatrexyl was discussed.

Quinolin-5-ylmethylene-3-{[8-(trifluoromethyl)quinolin-4-yl]thio}propanohydrazide as an effective inhibitor of mild steel corrosion in HCl solution

Abstract: Quinolin-5-ylmethylene-3-{[8-(trifluoromethyl)quinolin-4-yl]thio}propanohydrazide (QMQTPH) was synthesized, characterized and tested as a corrosion inhibitor for mild steel in 1 M and 2 M HCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Polarization resistances calculated from the EIS measurements were in good agreement with those obtained from direct current (DC) polarization measurements. The mild steel samples were also analyzed by scanning electron microscopy (SEM). The results showed that QMQTPH is an excellent inhibitor for mild steel in acid medium. The inhibition was assumed to occur via adsorption of the inhibitor molecule on the metal surface. It acts as an anodic inhibitor. In the 30° to 60 °C temperature range, the QMQTPH adsorption follows Langmuir isotherm model. The protection efficiency increased with increasing inhibitor concentration in the range 10−5 − 10−3 M, but slightly decreased with increasing temperature.

Corrosion characterization of tin–lead and lead free solders in 3.5 wt.% NaCl solution

Abstract: The corrosion resistance of Sn–Pb and several candidate lead free solders were investigated in 3.5 wt.% NaCl solution through potentiodynamic polarisation. Results showed that in NaCl solution lead free solders had better corrosion resistance than Sn–Pb solder and the corrosion resistance of lead free solders was similar, but the corrosion resistance of Sn–Ag solder was better than that of Sn–Ag–Cu and Sn–Cu solders. The corrosion products for Sn–Pb solder had a two-layered structure with Sn-rich phases at the outer layer and looser Pb-rich phases at the inner layer. The loose Pb-rich layer was detrimental to the corrosion property. The corrosion product on the surface of all these solders was tin oxide chloride hydroxide.

Impedance and XPS study of benzotriazole films formed on copper, copper-zinc alloys and zinc in chloride solution

Abstract: The formation of protective layers on copper, zinc and copper–zinc (Cu–10Zn and Cu–40Zn) alloys at open circuit potential in aerated, near neutral 0.5 M NaCl solution containing benzotriazole was studied using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). Benzotriazole (BTAH), generally known as an inhibitor of copper corrosion, also proved to be an efficient inhibitor for copper–zinc alloys and zinc metal. The surface layers formed on alloys in BTAH-inhibited solution comprised both polymer and oxide components, namely Cu(I)BTA and Zn(II)BTA polymers and Cu2O and ZnO oxides, as proved by the in-depth profiling of the layers formed. A tentative structural model describing the improved corrosion resistance of Cu, Cu–xZn alloys and Zn in BTAH containing chloride solution is proposed.