Electrochemical studies on the alternating current corrosion of mild steel under cathodic protection condition in marine environments

Alternating current (AC) corrosion of mild steel in marine environments under cathodic protection (CP) condition was studied. Electrochemical studies at the two protection potentials namely −780 and −1100 mV versus SCE were examined by different techniques. DC polarization study was carried out for mild steel in natural seawater and 18.5 g/L NaCl solution to evolve corrosion current density. The corrosion rate determination, pH of the end experimental solution and surface morphology of the mild steel specimens under the influence of different AC current densities were studied. The amount of leaching of iron into the solution was estimated using inductively coupled plasma spectrometry. All these techniques revealed that AC influences the corrosion of mild steel in the presence of marine environments even though CP was given. Surface micrographs revealed that spreading of red rust products noticed on the mild steel surface. At −780 mV CP, red rusts are visually seen when the AC source was above 10 A/m² in both the media but red rusts are appeared after 20 A/m² in the case of −1100 mV CP. Weight loss measurements coupled with surface examination and solution analysis is a effective tool to characterize and quantify the AC corrosion of mild steel in marine environments.     

AlN and intermetallic compound layers formed between aluminum and austenitic stainless steel using barrel nitriding

Recently, a new nitriding process was proposed to produce the aluminum nitride on an aluminum surface using a barrel. After barrel nitriding, AlN nitride layer is formed on the aluminum surface and the surface hardness can be improved remarkably. In this study, barrel nitriding was performed to investigate the interface between aluminum substrate, with SUS304 austenitic stainless steel used for a physical catalyst. The barrel nitriding was carried out at 893 K for 18 ks, 25.2 ks and 36 ks, respectively with aluminum and aluminum–magnesium alloy powder. After barrel nitriding, aluminum nitride layer and Fe–Al intermetallic compound layers were formed at the interface between pure aluminum and austenitic stainless steel at the same time. The thickness of the aluminum nitride layer and intermetallic layer was increased by increasing the treatment time.     

Preparation and photoelectrochemical characterisation of electrosynthesised titanium dioxide deposits on stainless steel substrates

TiO₂ films on stainless steel 304 substrates have been prepared from acidic aqueous solutions of TiOSO₄ and H₂O₂ by room temperature potentiostatic cathodic electrosynthesis. Coatings of varied thickness were produced by repeating the deposition step two or three times accompanied by drying steps in between. The resulting gel films were annealed at 400 °C to obtain crystalline TiO₂ (anatase) films with loadings in the 0.1-1 mg cm⁻² range. The deposits had a macro-particulate structure and adhered well on stainless steel 304. The electroactive surface area of the composite electrodes was estimated by cyclic voltammetry in the dark, while their photoelectrochemical behaviour was investigated by photo-voltammetry under UV illumination, both in the presence and absence of oxidizable organics. The effect of deposit thickness on photocurrent and an indicative comparison of electrosynthesised TiO₂/stainless steel 304 photoelectrodes with thermal and particulate TiO₂/Ti electrodes are also presented.     

3D porous polymeric conductive material prepared using LbL deposition

In this work a 3D porous polymeric conducting material is derived from a multi-percolated polymer blend system. The work has focused on the preparation of low surface area porous substrates from polymer blends followed by the deposition of polyaniline conductive polymer (PANI) on the internal porous surface using a layer-by-layer (LbL) technique. The approach reported here allows for the percolation threshold concentration of polyaniline conductive polymer (PANI) to be reduced to values of no more than 0.19%. Furthermore, depending on the amount of PANI deposited, the conductivity of the porous substrate can be controlled from 10⁻¹⁵ S cm⁻¹ to 10⁻³ S cm⁻¹.Ternary and quaternary multi-percolated systems comprised of high-density polyethylene (HDPE), polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) are prepared by melt mixing and subsequently annealed in order to obtain large interconnected phases. Selective extraction of PS, PMMA and PVDF result in a fully interconnected porous HDPE substrate of ultra-low surface area and highly uniform sized channels. This provides an ideal substrate for subsequent polyaniline (PANI) addition. Using a layer-by-layer (LbL) approach, alternating poly(styrene sulfonate) (PSS)/PANI layers are deposited on the internal surface of the 3-dimensional porous polymer substrate. The PANI and sodium poly(styrene sulfonate) (PSS) both adopt an inter-diffused network conformation on the surface. The sequential deposition of PSS and PANI has been studied in detail and the mass deposition profile demonstrates oscillatory behavior following a zigzag-type pattern. The presence of salt in the deposition solution results in a more uniform deposition and more thickly deposited PSS/PANI layers. The conductivity of these samples was measured and the conductivity can be controlled from 10⁻¹⁵ S cm⁻¹ to 10⁻⁵ S cm⁻¹ depending on the number of deposited layers. In the case of a porous sample which can be crushed, applying a load to the substrate can be used as an additional control parameter. In that sample a high load results in higher conductivity with values as high as 10⁻³ S cm⁻¹ obtained. The work described above has focused on very low surface area porous substrates in order to generate a conductive device with the lowest possible concentration values of polyaniline, but high surface area substrates can also be readily prepared using this approach.     

Improvement of the electrochemical behavior of steel surfaces using a TiN[BCN/BN]n/c-BN multilayer system

The aim of this work is to improve the electrochemical behavior of AISI 4140 steel substrates by using a TiN[BCN/BN]_n/c-BN multilayer system as a protective coating. We grew TiN[BCN/BN]_n/c-BN multilayers via reactive r.f. magnetron sputtering technique, systematically varying the length period (Λ) and the bilayer number (n), maintaining constant the total thickness of the coating and all other growth parameters. The coatings were characterized by FTIR spectroscopy that showed bands associated to h-BN bonds, and c-BN stretching vibrations centered at 1385 cm^− 1 and 1005 cm^− 1, respectively. Film composition was studied via X-ray photoelectron spectroscopy where typical signals for C1s, N1s and B1s are shown. The electrochemical properties were studied by electrochemical impedance spectroscopy and Tafel curves. In this work, the maximum corrosion resistance for the coating with (Λ) equal to 80 nm was obtained, corresponding to n = 25 bilayers. The polarization resistance and corrosion rate were around 10.1 kOhm cm² and 0.22 mm/year; these values were 83 and 15 times higher, respectively, than uncoated AISI 4140 steel substrate (0.66 kOhm cm² and 18.51 mm/year). Optical microscopy was used for surface analysis after corrosive attack. The improvement of the electrochemical behavior of the AISI 4140 coated with this TiN[BCN/BN]_n/c-BN multilayer system can be attributed to the presence of several interfaces that offer resistance to diffusion of Cl⁻ of the electrolyte toward the steel surface.     

Effects of nanocrystallization on the corrosion behavior of 309 stainless steel

Nanocrystallized (NC) 309 stainless steel (309SS) coating has been fabricated on glass substrate by DC magnetron sputtering. The coating, with an average grain size less than 50 nm, had ferritic (bcc) structure rather than the austenitic (fcc) structure of the bulk steel. The electrochemical corrosion behavior of the NC coating and the bulk steel in solutions of 0.25 M Na₂SO₄ + 0.05 M H₂SO₄ and 0.5 M NaCl + 0.05 M H₂SO₄ was investigated by using potentiodynamic polarization, potentiostatic polarization and AC impedance techniques. The results showed that the corrosion behavior of the NC 309SS coating and 309SS bulk steel depended on the composition of the solutions. In the Na₂SO₄ solution there was only a little difference between the corrosion resistance of the passive films on the NC coating and the bulk steel. However, in the solution with chloride ions, the localized corrosion resistance of 309SS was greatly enhanced by nanocrystallization due to the formation of a compact and stable passive film on the NC coating. The electronic structure of the passive film formed on the NC coating and on the bulk steel was analyzed by means of capacitance measurements, and a corrosion mechanism is proposed.     

Molecular simulation, quantum chemical calculations and electrochemical studies for inhibition of mild steel by triazoles

The inhibition performance of three triazole derivatives on mild steel in 1 M HCl were tested by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The adsorption behavior of these molecules at the Fe surface was studied by the molecular dynamics simulation method and the quantum chemical calculations. Results showed that these compounds inhibit the corrosion of mild steel in 1 M HCl solution significantly. Molecular simulation studies were applied to optimize the adsorption structures of triazole derivatives. The iron/inhibitor/solvent interfaces were simulated and the charges on the inhibitor molecules as well as their structural parameters were calculated in presence of solvent effects. Aminotriazole was the best inhibitor among the three triazole derivatives (triazole, aminotriazole and benzotriazole). The adsorption of the inhibitors on the mild steel surface in the acid solution was found to obey Langmuir's adsorption isotherm.     

Microbial electrocatalysis with Geobacter sulfurreducens biofilm on stainless steel cathodes

Stainless steel and graphite electrodes were individually addressed and polarized at −0.60 V vs. Ag/AgCl in reactors filled with a growth medium that contained 25 mM fumarate as the electron acceptor and no electron donor, in order to force the microbial cells to use the electrode as electron source. When the reactor was inoculated with Geobacter sulfurreducens, the current increased and stabilized at average values around 0.75 A m⁻² for graphite and 20.5 A m⁻² for stainless steel. Cyclic voltammetry performed at the end of the experiment indicated that the reduction started at around −0.30 V vs. Ag/AgCl on stainless steel. Removing the biofilm formed on the electrode surface made the current totally disappear, confirming that the G.sulfurreducens biofilm was fully responsible for the electrocatalysis of fumarate reduction. Similar current densities were recorded when the electrodes were polarized after being kept in open circuit for several days. The reasons for the bacteria presence and survival on non-connected stainless steel coupons were discussed. Chronoamperometry experiments performed at different potential values suggested that the biofilm-driven catalysis was controlled by electrochemical kinetics. The high current density obtained, quite close to the redox potential of the fumarate/succinate couple, presents stainless steel as a remarkable material to support biocathodes.     

Orientation of graphitic planes during annealing of “dip deposited” amorphous carbon film: A carbon K-edge X-ray absorption near-edge study

Annealing effect of amorphous carbon thin films on Si(1 0 0) substrates is studied by normal incidence and angle dependent carbon K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The angle dependence of the XANES signal shows that the graphitic basal planes are oriented perpendicular to the surface when the film is annealed at 1000 °C. Micro-Raman spectroscopy reveals two well-separated bands the D band at 1355 cm⁻¹ and G band at ∼1600 cm⁻¹, and their I_D/I_G intensity ratio indicates the formation of more graphitic film at higher annealing temperatures. X-ray diffraction pattern of 1000 °C temperature annealed film confirms the formation of graphite structure.     

Accumulation of uranium on austenitic stainless steel surfaces

The surface contamination by uranium in the primary circuit of PWR type nuclear reactors is a fairly complex problem as (i) different chemical forms (molecular, colloidal and/or disperse) of the uranium atoms can be present in the boric acid coolant, and (ii) only limited pieces of information about the extent, kinetics and mechanism of uranium accumulation on constructional materials are available in the literature. A comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system are presented and discussed. The surface excess, oxidation state and chemical forms of uranium species sorbed on the inner surfaces of the stainless steel tubes of steam generators have been detected by radiotracer (alpha spectrometric), ICP-OES and XPS methods. In addition, the passivity, morphology and chemical composition of the oxide-layers formed on the studied surfaces of steel specimens have been analyzed by voltammetry and SEM-EDX. The experimental data imply that the uranium sorption is significant in the pH range of 4-8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ_sample = 1.22 μg cm⁻²U ≅ 4 × 10⁻⁹ mol cm⁻² UO₂(OH)₂) exceeds a monolayer coverage.     

Argon plasma treatment techniques on steel and effects on diamond-like carbon structure and delamination

We demonstrate alteration in diamond-like carbon (DLC) film structure, chemistry and adhesion on steel, related to variation in the argon plasma pretreatment stage of plasma enhanced chemical vapour deposition. We relate these changes to the alteration in substrate structure, crystallinity and chemistry due to application of an argon plasma process with negative self bias up to 600 V.Adhesion of the DLC film to the substrate was assessed by examination of the spallated fraction of the film following controlled deformation. Films with no pretreatment step immediately delaminated. At 300 V pretreatment, the spallated fraction is 8.2%, reducing to 1.2% at 450 V and 0.02% at 600 V. For bias voltages below 450 V the adhesion enhancement is explained by a reduction in carbon contamination on the substrate surface, from 59 at.% with no treatment to 26 at.% at 450 V, concurrently with a decrease in the surface roughness, R_q, from 31.5 nm to 18.9 nm. With a pretreatment bias voltage of 600 V a nanocrystalline, nanostructured surface is formed, related to removal of chromium and relaxation of stress; X-ray diffraction indicates this phase is incipient at 450 V. In addition to improving film adhesion, the nanotexturing of the substrate prior to film deposition results in a DLC film that shows an increase in sp³/sp² ratio from 1.2 to 1.5, a reduction in surface roughness from 31 nm to 21 nm, and DLC nodular asperities with reduced diameter and increased uniformity of size and arrangement. These findings are consistent with the substrate alterations due to the plasma pretreatment resulting in limitation of surface diffusion in the growth process. This suggests that in addition to deposition phase processes, the parameters of the pretreatment process need to be considered when designing diamond-like carbon coatings.     

Polyaniline for corrosion prevention of mild steel coupled with copper

Polyaniline emeraldine base/epoxy resin (EB/ER) coating was investigated for corrosion protection of mild steel coupled with copper in 3.5% NaCl solution. EB/ER coating with 5-10 wt% EB had long-term corrosion resistance on both uncoupled steel and copper due to the passivation effect of EB on the metal surfaces. During the 150 immersion days, the impedance at 0.1 Hz for the coating increased in the first 1-40 days and subsequently remained constant above 10⁹ Ω cm², whereas that for pure ER coating fell below 10⁶ Ω cm² after only 30 or 40 days. Immersion tests on coated steel-copper galvanic couple showed that EB/ER coating offered 100 times more protection than ER coating against steel dissolution and coating delamination on copper, which was mainly attributed to the passive metal oxide films formed by EB blocking both the anodic and cathodic reactions. Salt spray tests showed that 100 μm EB/ER coating protected steel-copper couple for at least 2000 h.     

Electrochemical activity of Geobacter sulfurreducens biofilms on stainless steel anodes

Stainless steel was studied as anode for the biocatalysis of acetate oxidation by biofilms of Geobacter sulfurreducens. Electrodes were individually polarized at different potential in the range −0.20 V to +0.20 V vs. Ag/AgCl either in the same reactor or in different reactors containing acetate as electron donor and no electron acceptor except the working electrode. At +0.20 V vs. Ag/AgCl, the current increased after a 2-day lag period up to maximum current densities around 0.7 A m⁻² and 2.4 A m⁻² with 5 mM and 10 mM acetate, respectively. No current was obtained during chronoamperometry (CA) at potential values lower than 0.00 V vs. Ag/AgCl, while the cyclic voltammetries (CV) that were performed periodically always detected a fast electron transfer, with the oxidation starting around −0.25 V vs. Ag/AgCl. Epifluorescent microscopy showed that the current recorded by chronoamperometry was linked to the biofilm growth on the electrode surface, while CVs were more likely linked to the cells initially adsorbed from the inoculum. A model was proposed to explain the electrochemical behaviour of the biofilm, which appeared to be controlled by the pioneering adherent cells playing the role of “electrochemical gate” between the biofilm and the electrode surface.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Corrosion of carbon steel in sodium methanoate solutions

The behaviour of steel electrodes in sodium methanoate solutions was studied by coupling electrochemical techniques (voltammetry, OCP vs. time) with in situ micro-Raman spectroscopy analyses of the corrosion products. The polarisation curves depended strongly on the methanoate concentration. For the smallest concentration (10⁻³ mol L⁻¹), the current density increased regularly with the applied potential. So the behaviour of the electrode was typical of an active material. In contrast, for the largest concentration (10⁻¹ mol L⁻¹), the curves obtained were typical of a passive material. Methanoate ions favoured growth and stability of a passive oxide film more likely by adsorbing on its surface. The polarisation curve obtained for the intermediate concentration (10⁻² mol L⁻¹) was unusual and testified of an imperfect passivation of the steel surface. Finally, steel electrodes were left at the open circuit potential in the methanoate solutions. In any case, the passivity was rapidly lost and a general corrosion of the surface took place. In situ Raman spectroscopy analyses at the early stage of the corrosion process demonstrated that the first product to form was a green rust, GR(HCOO⁻). It was oxidised later into γ-FeOOH (lepidocrocite) by dissolved O₂. The process is then typical of what is usually observed in neutral or alkaline media, whatever the anions present and responsible of the GR formation. A new and detailed characterisation of GR(HCOO⁻) by X-ray diffraction was performed and a crystal structure is proposed.     

Protection of type 430 stainless steel against pitting corrosion by ladder conductive polymer

Poly(o-phenylenediamine) (PoPD) was electropolymerized by cyclic voltammetry (CV) on 430 stainless steel from sulfuric acid solution containing o-phenylenediamine monomer. The formation of the polymer film is slower than that of polyaniline (PANI) film. Transparent and compact layers (∼1.0 μm) of PoPD deposited after 100 cycles, while thicker (∼3 μm), grainy and porous layers of PANI formed after 50 cycles. The PoPD layers protect the steel substrate from pitting in 3% NaCl but the layers of PANI fail, and pitting and crevice corrosion were observed on the steel surface. Both polymers keep the steel substrate in a passive state in sulfuric acid. After aging in acid solution the underlying oxides were investigated after peeling off the polymer layers; this showed an excellent passive film formed under PoPD. The passive steel was completely free from pitting after immersion in the chloride solution for 1 week.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Corrosion inhibition of mild steel by natural product compound

Corrosion inhibition of mild steel in H₃PO₄ containing chloride or sulphate ions have been studied using different electrochemical techniques. The corrosion and hydrogen evolution of mild steel alloy in 2 M H₃PO₄ acid containing 0.5 M NaCl can be effectively inhibited by addition of natural product compound, Thymol (IPMP), of different concentrations. However, in 2 M H₃PO₄ containing 0.5 M Na₂SO₄ corrosion cannot be effectively inhibited. The results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements confirm the synergistic effects which describe the increase in the effectiveness of a corrosion inhibitor in the presence of Cl⁻ ions in the corrosive medium. At any temperature, an increase in it leads to an increase of the corrosion rate and hydrogen evolution on mild steel. Polarization and EIS results are in good agreement with each other. The obtained results were confirmed by surface examination using scanning electron microscope.     

Characterisation and corrosion studies of steel electrodes covered by polypyrrole/phosphotungstate using Electrochemical Impedance Spectroscopy

Polypyrrole/PW₁₂O₄₀³⁻ hybrid material was electrosynthesised on carbon steel electrodes in acetonitrile medium. The coatings obtained were characterised by Electrochemical Impedance Spectroscopy (EIS). On free-standing polypyrrole films the electrical response was mainly due to ion–ion charge transfer resistance, with a value of 175 Ω cm². A value of 2 × 10⁻⁵ S/cm was determined for the hybrid material conductivity. A charge transfer resistance about 7000 Ω cm² was obtained due to steel/oxide interface. Corrosion tests showed an important improvement in the protection against corrosion when the carbon steel electrodes were coated by these polymeric films.     

Structure and properties of PbO2-CeO2 anodes on stainless steel

The lack of ideal anodes with excellent activity and stability is one of the critical problems in electrochemical oxidation for organic wastewater treatment. It is reported in this paper that the PbO₂-CeO₂ films electrodeposited on stainless steel were used as catalytic electrodes for treating antibiotic wastewater. The PbO₂-CeO₂ films on stainless steel were proved to be high stability, good activity and relatively low cost. Because of these properties, the films are more attractive than any other electrocatalytic materials among conventional dimensionally stable anodes (DSA). Experimental results showed that the PbO₂-CeO₂ electrode has a service life of 1100 h in 3 M H₂SO₄ solution under a current density of 1 A cm⁻² at 35 °C, compared with 300 h for PbO₂ under the same conditions. The X-ray diffraction (XRD) patterns and SEM images indicated that the PbO₂-CeO₂ films on stainless steel have a dense structure and the preferred crystalline orientation on the substrate surface was changed. Color and chemical oxygen demand (COD) of antibiotics wastewater were studied by electrolysis by using these electrodes as anode and stainless steel as cathode. The results indicated that the anodes have excellent activity in antibiotic wastewater treatment. The PbO₂-CeO₂ electrodes have high chemical stability which contributed by the superstable nature of the electrode, dense microstructure, good conductivity and the improvement of bonding with the stainless steel during electrodeposition.