Showing papers in "Corrosion Science in 2003"

AC and DC study of the temperature effect on mild steel corrosion in acid media in the presence of benzimidazole derivatives

Abstract: The effect of benzimidazole derivatives on mild steel corrosion in 1 M HCl at five different temperatures has been studied. Impedance, polarization resistance, polarization curves measurement and gravimetric methods have been used. The inhibiting efficiency and the apparent activation energy (Ea) have been calculated in the presence and in the absence of the organic inhibitors. The comparative investigations carried out in 1 M H2SO4 and 1 M H2SO4+1×10−4 M KI verify the assumption that the halogen ions facilitate the inhibitors adsorption. The electrostatic absorption of the compound cation form seems more probable. The behaviour of 2-mercaptobenzimidazole (2-SH-BI) contrasts that of the rest of the substances investigated. Chemisorptive adsorption is more probable in this case. The lower value of activation energy Ea in the presence of 1-benzyl-benzimidazole when compared to that in 1 M HCl is explained with a partial compensation effect.

An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II.: EIS interpretation of corrosion behaviour

Abstract: In Part I, of this work the equivalent circuits for electrochemical impedance spectroscopy (EIS) modelling of PVD coated steels in 0.5 N NaCl solution were established. In this paper, Part II, the EIS spectra of such coated systems are modelled using the equivalent circuits. The circuit parameters obtained are correlated with the dielectric characteristics, and microstructure of steels and PVD hard coatings. Coating porosity and localised corrosion with exposure time have also been determined using the corrosion potential difference (ΔEcorr) between mild steel and PVD coatings and polarisation resistance Rp, which was obtained through EIS modelling using equivalent circuits. In addition, diffusion rates of the reactants (e.g. oxygen) through ‘permeable’ defects (e.g. pores) are studied by introducing the diffusion impedances W and O in EIS modelling. It has been found that the usage of impedances W and O is closely related to the crystallite features of PVD coatings. Warburg impedance (W) is most suitable for columnar crystallites, while the co-tangent-hyperbolic diffusion impedance (O) is best for the equiaxed crystallite structure. Finally, visual inspection, SEM examination, and the scanning reference electrode technique were employed to observe the corrosion progress of PVD coated steels with immersion time, in order to validate the EIS interpretation.

Inhibition of aluminum corrosion using Opuntia extract

Abstract: The inhibitive action of the mucilage extracted from the modified stems of prickly pears, toward acid corrosion of aluminum, is tested using weight loss, thermometry, hydrogen evolution and polarization techniques. It was found that the extract acts as a good corrosion inhibitor for aluminum corrosion in 2.0 M HCl solution. The inhibition action of the extract was discussed in view of Langmuir adsorption isotherm. It was found that the adsorption of the extract on aluminum surface is a spontaneous process. The inhibition efficiency (IE) increases as the extract concentration is increased. The effect of temperature on the IE was studied. It was found that the presence of extract increases the activation energy of the corrosion reaction. Moreover, the thermodynamic parameters of the adsorption process were calculated. It was found also that the Opuntia extract provides a good protection to aluminum against pitting corrosion in chloride ion containing solutions.

Sequence of steps in the pitting of aluminum by chloride ions

Abstract: Corrosion pit initiation in chloride solutions is given by an electrode kinetic model which takes into account adsorption of chloride ions on the oxide surface, penetration of chloride ions through the oxide film, and localized dissolution of aluminum at the metal/oxide interface in consecutive one-electron transfer reactions. A previous model has been extended here to consider that penetration of chloride ions can occur by oxide film dissolution as well as by migration through oxygen vacancies. Pit initiation occurs by chloride-assisted localized dissolution at the oxide/metal interface. The electrode kinetic model leads to a mathematical expression which shows that the critical pitting potential is a linear function of the logarithm of the chloride concentration (at constant pH), in agreement with experiment. The model also predicts that the critical pitting potential is independent of pH (at constant chloride concentration), also in agreement with experiment. Corrosion pit propagation leads to formation of blisters beneath the oxide film due to localized reactions which produce an acidic localized environment. The blisters subsequently rupture due to the formation of hydrogen gas in the occluded corrosion cell. Calculation of the local pH within a blister from the calculated hydrogen pressure within the blister gives pH values in the range 0.85 to 2.3. Published by Elsevier Science Ltd.

An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part I. Establishment of equivalent circuits for EIS data modelling

Abstract: Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique, which can provide a wealth of information on the corrosion reactions, the mass transport and the electrical charge transfer characteristics of physical vapour deposition (PVD) ceramic coated steels in an aqueous solution. Although a huge amount of potentially useful data can be generated using the EIS technique, these data need to be carefully interpreted. This is usually done using an ‘equivalent circuit’ which comprises an assembly of electrical circuit elements that model the physicoelectric characteristics of the electrode/solution interface. A systematic study of PVD ceramic (TiN and CrN) coated mild steel and AISI 316L stainless steel was carried out using the EIS technique as the coated systems were immersed in 0.5 N NaCl solution. The relevant equivalent circuits (ECs) are developed by non-linear least square curve fitting to the exponential data to build up a description of the influence of different coatings deposited on steels on the temporal evolution of corrosion in such systems. Constant phase elements describing the non-ideal (e.g. capacitive) characteristics of the electrochemical interface, designated as Q , are introduced to achieve a more accurate simulation of electrochemical corrosion. The mass transport behaviour is also dealt with, through the introduction of diffusion-related elements such as Warburg (designated as W ) and cotangent–hyperbolic (designated as O ) impedance. The use of these elements significantly improves the quality of fit of the simulation to the EIS data. Finally, the physical validity of the proposed models is discussed in terms of the current–frequency response of the coated steel electrode, during extended corrosion degradation over a period of immersion of up to one week.

Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in sodium chloride solution.: Part 2: mechanism for corrosion protection

Abstract: The corrosion protection of AA 2024-T3 by films of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane) was studied in a neutral 0.6 M NaCl solution using potential transient, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results showed that a highly crosslinked or dense interfacial layer that developed between the silane film and the aluminum oxide is the major contribution to the corrosion protection of AA 2024-T3. The formation of this interfacial layer heavily restricts pit growth underneath via retarding the transport of corrosion products, as well as effectively blocks a number of cathodic sites available for cathodic reactions.

Oxidation behavior of Ti3AlC2 at 1000–1400 °C in air

Abstract: The isothermal oxidation behavior of bulk Ti3AlC2 has been investigated at 1000–1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti3AlC2 has excellent oxidation resistance. The oxidation of Ti3AlC2 generally followed a parabolic rate law with parabolic rate constants, kp that increased from 4.1×10−11 to 1.7×10−8 kg2 m−4 s−1 as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al2O3. The outer layer changed from rutile TiO2 at temperatures below 1200 °C to a mixture of Al2TiO5 and TiO2 at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al2TiO5 and, predominantly, α-Al2O3, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al2O3 layers.

The isoxazolidines: a new class of corrosion inhibitors of mild steel in acidic medium

Abstract: The cycloaddition of the cyclic nitrone-1-pyrroline-1-oxide with a variety of alkenes, phenyl isocyanate and phenyl isothiocyanate afforded a multitude of cycloaddition products (isoxazolidines). One of the cycloadducts on reaction with benzyl and propargyl chloride gave quaternary ammonium salts. All these new isoxazolidines are tested for corrosion inhibition of mild steel in 1 M HCl at 60 °C by gravimetric and electrochemical methods. The inhibition efficiency of this class of compounds are determined for the first time. All compounds have shown excellent corrosion inhibition efficiency (IE%) in acidic solution; IE% in the range 86.7–99.5 are measured by the gravimetric method. Comparable results were obtained by the electrochemical method using Tafel plots for the inhibition efficiency of some of the selected synthesized compounds.

Corrosion protection by multilayered conducting polymer coatings

Abstract: Multilayered coatings, consisting of combinations of the conducting polymers polyaniline (Pani) and polypyrrole (Ppy), were galvanostatically deposited on to both carbon steel and stainless steel. Potentiodynamic polarisation was used to assess the ability of these copolymers to provide an effective barrier to corrosion in chloride environments. For carbon steel the performance of these multilayered coatings on carbon steel were not sufficiently better than for single Pani coatings to justify their more complicated deposition procedures. However, in the case of stainless steels the new multilayered coatings proved to be significantly better than previously reported single Pani coatings, especially at protecting against pitting corrosion. It was found that the degree of protection was a function of the deposition order of the copolymer, with films consisting of a Pani layer over the top of a Ppy layer yielding the best results. Scanning electronic microscopy observations and adhesion measurements, along with the electrochemical data suggested that the ability of a conducting polymer film to act as electronic and chemical barriers were more important in providing corrosion protection than its ability to act as a physical barrier.

The effect of neutral red on the corrosion inhibition of cold rolled steel in 1.0 M hydrochloric acid

Abstract: The corrosion inhibition of neutral red on the corrosion of cold rolled steel in 1.0 M hydrochloric acid (HCl) was studied using weight loss method and potentiodynamic polarization method. Results obtained revealed that neutral red is an effective inhibitor. It was found that the adsorption of neutral red could prevent steel from weight loss and the adsorption accorded with the Langmuir adsorption isotherm. Potentiodynamic polarization studies showed that neutral red mainly acted as a mixed-type inhibitor for steel in 1.0 M hydrochloric acid. Thermodynamic parameters such as adsorption heat, adsorption entropy and adsorption free energy were obtained from experimental data of the temperature studies of the inhibition process at four temperatures ranging from 30 to 45 °C. The most suitable range of inhibitor concentration was discussed. The kinetic data such as apparent activation energies and pre-exponential factors at different concentrations of the inhibitor were calculated, the effect of the apparent activation energies and pre-exponential factors on the corrosion rates of cold rolled steel was discussed. The inhibitive action was satisfactorily explained by using both thermodynamic and kinetic models. Synergism between chloride ion and neutral red was proposed. The results obtained from weight loss and potentiodynamic polarization were in good agreement.

Impedance spectroscopic study of corrosion inhibition of copper by surfactants in the acidic solutions

Abstract: The inhibitive action of the four surfactants, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate, sodium oleate and polyoxyethylene sorbitan monooleate (TWEEN-80), on the corrosion behavior of copper was investigated in aerated 0.5 mol dm(-3) H2SO4 solutions, by means of electrochemical impedance spectroscopy. These surfactants acted as the mixed-type inhibitors and lowered the corrosion reactions by blocking the copper surface through electrostatic adsorption or chemisorption. The inhibitor effectiveness increased with the exposure time to aggressive solutions, reached a maximum and then decreased, which implies the orientation change of adsorbed surfactant molecules on the surface. CTAB inhibited most effectively the copper corrosion among the four surfactants. The copper surface was determined to be positively charged in sulfuric acid solutions at the corrosion potential, which is unfavourable for electrostatic adsorption of cationic surfactant, CTAB. The reason why CTAB gave the highest inhibition efficiency was attributed to the synergistic effect between bromide anions and positive quaternary ammonium ions. C16H33N(CH3)(4)(+) ions may electrostatically adsorbed on the copper surface covered with primarily adsorbed bromide ions. On the basis of the variation of impedance behaviors of copper in the surfactant-containing solutions with the immersion time, the adsorption model of the surfactants on the copper surface was proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.

A corrosion study of the main constituent phases of AZ91 magnesium alloys

Abstract: The different constituents of an AZ91 alloy (α,β, and MnAl phases) were synthesized and their corrosion resistance was studied by electrochemistry in ASTM D1384 water, pH 8.3. The pure phases were characterised through the corrosion potential, the polarisation resistance, and polarisation curves, then systematically coupled to assess the galvanic corrosion occurring in the AZ91 alloy. The aluminium content of the oxide film was obtained by X-ray photoelectron spectroscopy measurements. The corrosion rate of the α solid solution alloys depends closely on their Al content. Aluminium enhances the corrosion resistance of the α-phase through the formation of an Al enriched superficial layer. The β-phase is 150 mV nobler than the α-phase, but their corrosion rates are similar. The galvanic currents are low (below 20 μA cm−2) whatever the implemented couples and close to the corrosion current previously measured for the AZ91 alloys.

In-depth distribution of rusts on a plain carbon steel and weathering steels exposed to coastal-industrial atmosphere for 17 years

Abstract: In-depth distribution of rusts on two weathering steels and a plain carbon steel exposed to atmosphere for 17 years under a bridge at a coastal + industrial region in Japan were studied. In the rust layer on all specimens, α-FeOOH, β-FeOOH, γ-FeOOH, Fe3O4 and so-called amorphous rust were found. Within rust layers, there were thick parts and thin parts, which were finely and complicatedly distributed on steels. Among these rust species, α-FeOOH was dominant on all specimens. α-FeOOH appeared almost homogeneously through the rust layer. Its concentration was higher on weathering steels than on plain carbon steel. β-FeOOH was found mainly at thick parts and was scarce at thin parts of rust layers. Concentration of α-FeOOH was higher and that of γ-FeOOH was lower on weathering steels than on plain carbon steel. Amorphous rust was located at the bottom of the rust layer irrespective of steel types. Concentration of magnetite was negatively correlated with concentration of β-FeOOH.

Inhibition of steel corrosion by polyaniline coatings

Abstract: Polyaniline (PANI) coatings were electrosynthesised on steel samples (13% and 4.44% Cr) using sulphuric and phosphoric acids as supporting electrolytes. Protective properties of PANI coatings in the supporting electrolytes were investigated by monitoring the open-circuit potential vs. time, and by applying electrochemical impedance spectroscopy. PANI layers have been found to provide corrosion protection. Thicker PANI layers at 530 mV vs. Ag/AgCl (3 mol dm−3 KCl) exhibit pure capacitive behaviour at low frequencies, and in addition a small resistance at high frequencies. Thinner layers at 530 mV exhibit a much higher resistance attributed to a higher degree of PANI-free electrode surface and/or to a significant amount of PANI transformed from emeraldine to leucoemeraldine form. The layer deposited in a phosphate solution appears to have better protective properties than the layer deposited in a sulphate solution. Therefore, PANI from phosphate solution was tested also in 0.1 mol dm−3 HCl. However, in the chloride-containing solution, the time of protection was significantly shorter.

Synergism and antagonism in mild steel corrosion inhibition by sodium dodecylbenzenesulphonate and hexamethylenetetramine

Abstract: The inhibition effects of sodium dodecylbenzenesulphonate (SDBS) and hexamethylenetetramine (HA) on the corrosion of mild steel in sulphuric acid solution have been studied using weight loss, electrochemical impedance and Tafel polarisation measurements. For HA, a monotonous increase in inhibition efficiency is observed as a function of concentration. For SDBS, however, an optimum in the inhibition efficiency is observed for a concentration close to 250 ppm, which is ascribed to the formation of hemi-micellar aggregates that provoke inhibitor desorption from the metal/solution interface at higher concentrations. Upon mixing HA and SDBS, concentration regions showing synergistic and antagonistic inhibition behaviour are identified, and it is concluded that electrostatic interactions between adsorbate ions are likely responsible for both phenomena. Langmuir and Frumkin isotherms were tested for relevance in describing the adsorption behaviour of both HA and SDBS.

Crystallization of anodic titania on titanium and its alloys

Abstract: Crystallization of amorphous anodic films grown at constant current density on sputtering-deposited titanium, and Ti–Si and Ti–Al alloys, in ammonium pentaborate electrolyte, has been examined directly by transmission electron microscopy. In the case of titanium, anatase develops at relatively low voltage in the inner film region, formed by inward migration of oxygen species. In contrast, the outer film region, formed at the film/electrolyte interface, is composed of amorphous oxide only. Oxide crystals are particularly found near the plane, separating the two regions, which is located at a depth of 35–38% of the film thickness. Oxide zones, of size ∼ 1 nm, with a relatively ordered structure, developed at the metal/film interface, are considered to lead to transformation of the inner region structure. The incorporation into the film of either aluminium or silicon species suppresses the formation of crystalline oxide to much increased voltages. However, eventually nanocrystals form at ∼40% of the film thickness, probably originating from pre-cursor nuclei in the air-formed on the as-deposited alloy.

On the high-temperature oxidation of nickel

Abstract: This paper summarizes on some of the extensive experimental data and corresponding models suggested to account for the oxidation mechanism of Ni in the temperature range 500–1400 °C. In addition it reports on in-house experimental data from investigations related to the oxidation of high-purity Ni from 500 to 1300 °C in the oxygen pressure range 1×10 −4 –1 atm based on TG, measurements of surface kinetics, two-stage oxidation, scanning electron microscopy, atomic force microscopy, secondary ion mass spectroscopy etc. The main part of this paper focuses on the more complex models suggested to account for experimental observations of the oxidation kinetics and the oxide morphology below 1000 °C.

Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in neutral sodium chloride solution. Part 1: corrosion of AA 2024-T3

Abstract: This study consists of two parts. In the first part, the corrosion of 2024-T3 aluminum alloy (AA 2024-T3) was studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the anodic S phase (Al2CuMg) particles dealloyed Al and Mg during the 3.5 h of immersion in a neutral 0.6 M sodium chloride (NaCl) solution; with the dealloying of Mg being the most severe. Simultaneously, a heavy dissolution was also observed for the surrounding Al matrix of the S phase particles. This Al dissolution is likely to be caused by a local alkalization resulting from the coupled cathodic reaction (water and/or oxygen reduction). Such corrosion in AA 2024-T3, however, can be inhibited efficiently after the treatment of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane). The associated studies on bis-sulfur silane treated AA 2024-T3 will be presented in the second part.

On the akaganéite crystal structure, phase transformations and possible role in post-excavational corrosion of iron artifacts

Abstract: The crystal structure of akaganeite and the akaganeite to hematite transition has been studied by means of conventional and synchrotron X-ray and neutron powder diffraction. The chemical formula of akaganeite can be written as FeO0.833(OH)1.167Cl0.167. The crystal structure does not contain free water. Heating below 200 °C will not alter the akaganeite structure. Initial water loss can be attributed to a large amount of adsorbed water due to a very small particle size; 0.15 μm by 0.03 μm. Chloride is released from the structure only in connection with the transformation to hematite. Due to its stability, the presence of akaganeite does not in itself posses a threat to iron artifacts, but it is rather a symptom of the presence of high concentrations of chloride in an acidic environment.

Corrosion inhibitors for iron in hydrochloride acid solution by newly synthesised pyridazine derivatives

Abstract: Inhibition by some newly synthesised pyridazine compounds of corrosion of pure iron in 1 M HCl solution has been studied using weight loss measurements, polarisation and impedance spectroscopy methods. The inhibiting action is more pronounced with stet-5-benzyl-6-methylpyridazin-3-yl thioethanoic (P1) and (5-benzyl-6-methylpyridazin-3-yl) ethyl thioethanoate (P3) and their inhibition efficiency increases with concentration to attain the maximum value of 85% and 81% at the 10−4 M, respectively. We note good agreement between gravimetric and electrochemical methods (potentiodynamic polarisation and impedance spectroscopy (EIS)). Polarisation measurements show also that the pyridazines act essentially as cathodic inhibitors. The cathodic curves indicate that the reduction of proton at the pure iron surface happens with an activating mechanism. The presence of the sulphur atom increases the inhibition efficiency changing the adsorption mechanism.

The inhibition of pitting corrosion of stainless steels by chromate and molybdate ions

Abstract: The inhibitive effects of chromate and molybdate on pitting corrosion in stainless steel AISI 304 and AISI 316 were studied in acidified chloride solution. The results presented show that these known inhibitors affect both the nucleation of pitting and metastable pitting by reducing the numbers and sizes of these events. This makes attainment of stable pit growth more difficult.

Electrodeposition of sol–gel films on Al for corrosion protection

Abstract: Sol–gel films were electrodeposited on aluminum electrodes following the methodology we have developed and is based on applying negative potentials. This increases the pH at the surface, causing acceleration of the polymerization. Our process follows the “two step method”, in which the monomer is first hydrolyzed in acidic solution (pH ∼ 4) and only then the negative potential is applied, which consumes protons and releases hydroxyl ions, thus enhancing the condensation. Films made of different monomers, i.e., tetraethoxysilane (TEOS), methyl trimethoxysilane and phenyl trimethoxysilane (PTMOS), were prepared, characterized and examined for their corrosion inhibition properties. Potentiodynamic polarization, electrochemical impedance spectroscopy, optical and scanning electron microscopy as well as atomic force microscopy have been used as a means of film characterization. Hydrophobic and steric silanes, such as PTMOS showed a considerable corrosion inhibition capacity as compared to the capacity exhibited by less hydrophobic and steric derivatives such as TEOS. The difference between the conventional dip-coating method and the electrodeposition approach for depositing sol–gel films was also examined, indicating a clear advantage of the latter.

Mathematical modelling of the diffusion controlled phase in marine immersion corrosion of mild steel

Abstract: In the mathematical modelling of the ‘general’ corrosion of mild and low alloy steels and of some other metals oxygen diffusion through the corrosion products to the corroding surface controls the rate of corrosion at least during some period in the corrosion–time behaviour relationship. Under slightly different theoretical assumptions various early authors derived expressions for this phase. It was usually assumed the derived relationship applied from initial immersion. Only very limited laboratory support for the derived relationships exists. They have been applied to field data for atmospheric corrosion but with constants and exponents modified empirically. In the present paper the theory is reviewed and extended. It is argued that diffusion control cannot apply from first immersion and that this provides considerably greater freedom to mathematical modelling. Allowance is made for non-uniform corrosion product density and permeability and for loss of corrosion product. Recent detailed ‘at-sea’ experimental observations obtained on the Eastern Australian seaboard are applied to test the theory for mild steel. It is shown that the most likely competing mathematical relationships produce relatively similar results. The derived relationship is fitted to literature data to deduce its temperature dependence.

Improved corrosion resistance through microstructural modifications induced by codepositing SiC-particles with electrolytic nickel

Abstract: Micron and submicron-sized SiC-particles (5 and 0.3 μm respectively) were codeposited with nickel from a Watts electrolyte. The Ni–SiC composite coatings showed a better corrosion resistance in a 0.6 M NaCl solution than nickel electrodeposited under the same conditions. The corrosion rate of Ni–SiC decreases by two orders of magnitude with respect to pure Ni coatings. This improved corrosion resistance is quite independent of the size and amount of embedded particles, except for the smallest SiC-particles investigated. In that case, the pitting corrosion potential shifts to more noble values indicating a notable reduction of the localized corrosion susceptibility. This improved corrosion resistance of Ni–SiC coatings containing submicrometric SiC-particles is linked to a change in grain morphology and texture of the coatings. That morphology evolves from columnar grains to small and equiaxed grains.

Effect of solution heat treatment on galvanic coupling between intermetallics and matrix in AA7075-T6

Abstract: Susceptibility to localised corrosion is strongly affected by heat treatments performed on Al–Zn–Cu–Mg alloys. In order to study how galvanic coupling between intermetallics and matrix is affected by solution heat treatment, AA7075-T6 and solution heat treated AA7075 have been characterised by means of scanning electron microscopy and scanning Kelvin probe force microscopy. Solution heat treatment strongly increased the Volta potential difference between the intermetallics and the surrounding matrix showing a strong increase in galvanic coupling. This is explained by Zn and Mg enrichment of the matrix caused by dissolution of strengthening particles during solution heat treatment.

Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel

Abstract: Hydroxyapatite (HAP) coatings were developed on type 316L stainless steel (SS) by electrophoretic deposition at various deposition potentials from 30 to 90 V using the stoichiometric HAP (Ca/P ratio 1.67) powder in a suspension of isopropyl alcohol. The optimum coating parameters were established at 60 V and 3 min, after vacuum sintering at 800 C. The phase purity of the coated surface was confirmed by XRD and secondary ion mass spectrometry confirmed the presence of both Ca and P on the coated layers. The electrochemical corrosion parameters Ecorr (open circuit potential) and pitting potentials, evaluated in Hank’s solution shifted towards noble direction for the HAP coated specimens in comparison with uncoated type 316L SS. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values after immersing the coated specimens in Hanks solution for 200 h. This indicates the stable nature of the coatings formed. 2002 Elsevier Science Ltd. All rights reserved.

Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn–N duplex stainless steels

Abstract: The pitting corrosion resistance of a new family of duplex stainless steels has been evaluated. These non-standard duplex stainless steels are characterised by low Ni content and high N and Mn levels. Potentiodynamic polarisation scans in NaCl solution have been carried out to determine pitting potentials. A crevice-free cell has been used to perform the electrochemical tests. An exponential equation is obtained in the regression analysis between the pitting potential and chemical composition which allows an estimate of the pitting resistance of these new duplex stainless steels.

Dependence of the Critical Pitting Temperature on surface roughness

Abstract: It has been known for many years that highly alloyed stainless steels display a critical pitting temperature (CPT), which is the lowest temperature at which the growth of stable pits is possible. In the work reported here, the effect of varying surface roughness was investigated using potentiostatic and potentiodynamic CPT measurements on 904L stainless steel in 1 M NaCl. The results demonstrated that increasing the smoothness of the sample surface causes an increase in the CPT, even though the CPT exhibits a markedly more deterministic character than does the pitting potential. Using a potentiostatic technique, the highest measured CPT was 56 °C for a surface polished to a 3 μm finish, whilst the lowest measured CPT was 46 °C for a surface ground to a 60 grit finish. These results are consistent with an explanation of the CPT proposed by Salinas-Bravo and Newman [Corros. Sci. 36 (1994) 67].

Formation of the Fe(II)–Fe(III) hydroxysulphate green rust during marine corrosion of steel

Abstract: Rust layers formed on steel sheet piles immersed 1 m above the mud line for 25 years were analysed by Raman spectroscopy, scanning electron microscopy and elemental X-ray mappings (Fe, S, O). They consist of three main strata, the inner one mainly composed of magnetite, the intermediate one of iron(III) oxyhydroxides and the outer one of hydroxysulphate green rust GR(SO42−). Simulations of GRs formation in solutions having large [Cl−]/[SO42−] ratios revealed that the hydroxysulphate GR(SO42−) was obtained instead of the hydroxychloride GR(Cl−), as demonstrated by X-ray diffraction and transmission Mossbauer spectroscopy analyses. Measurements of the [S], [Fe] and [Cl] concentrations allowed us to establish that GR(SO42−) formed along with a drastic impoverishment of the solution in sulphate ions; the [Cl−]/[SO42−] ratio increased from 12 to 240. The GR, acting like a “sulphate pump”, may favour the colonisation of the rust layers by sulphate reducing bacteria.

Copper corrosion inhibition by triphenylmethane derivatives in sulphuric acid media

Abstract: Triphenylmethane derivatives, namely fuchsin basic (FB) and fuchsin acid (FA), were studied for their possible use as copper corrosion inhibitors in 0.001 to 1.0 M sulphuric acid solutions. Benzotriazole (BTA) was also tested for comparative purposes. These inhibitors were studied in concentrations from 1×10−5 to 1×10−1 M at temperatures from 298 to 328 K. The gravimetric method was employed to study the protective effect of FB, FA and BTA. The order of inhibition efficiency was BTA>FB>FA. Up to nine adsorption isotherms were tested to fit the experimental data. The best fit was obtained using the Frumkin isotherm model. A thermodynamic/kinetic model of adsorption was also fitted. The projected molecular area of the inhibitors was calculated to elucidate inhibitor orientation in the adsorption process.