Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

The transpassive dissolution mechanism of 316L stainless steel

The transpassive dissolution mechanism of AISI 316L stainless steel was studied using electrochemical impedance spectroscopy (EIS). A generalized model of the transpassivity is proposed. The transpassive film is modeled as a highly doped n-type semiconductor-insulator-p-type semiconductor (n-i-p) structure. Injection of negative defects at the transpassive film/solution interface results in their accumulation as a negative surface charge. It alters the non-stationary transpassive film growth rate controlled by the transport of positive defects (oxygen vacancies). The model describes the process as dissolution of Cr as Cr(VI) and Fe as Fe(III) through the transpassive film via parallel reaction paths.     

The anodic dissolution of zinc and zinc alloys in alkaline solution. I. Oxide formation on electrogalvanized steel

The polarization behaviour of zinc in alkaline solution has been investigated using atomic emission spectroelectrochemistry. By independently measuring the oxidation rate of zinc (electrical current) and the rate of Zn²⁺ dissolution (partial elemental current) it is possible to calculate the amount of insoluble zinc cations produced at any instant. Assuming the insoluble cations are present as a zinc oxide film, the growth of this film as a function of potential and time was determined. On the basis of kinetic evidence, it was found that at least three forms of zinc based oxide/hydroxide films form during polarization experiments. Type I oxide formation occurs when the metal/electrolyte interface becomes locally saturated with Zn²⁺ ions. Type II oxide forms on the metal surface underneath the film of Type I oxide but has little inhibiting effect on zinc dissolution. Type III oxide is produced in much smaller quantity and results in a transition to the passive state. This may be due to a potential induced transition of Type II → Type III oxide.     

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.     

Corrosion and passivation behaviors of some stainless steel alloys in molten alkali carbonates

The corrosion and passivation behaviors of two types of stainless steel alloys (ferritic and austenitic steels) in ternary molten Li₂CO₃-Na₂CO₃-K₂CO₃ mixture at different temperatures (475-550 °C) were studied using galvanostatic polarization and cyclic voltammetry. The galvanostatic polarization curves of the investigated alloys illustrate the passivation and passivity breakdown of the alloys. The passivation potential range for the three investigated steel alloys is about 1.15-1.3 V. During this potential range different oxide and spinels are formed, the nature of which depends on the type of alloy and the anodization potential. At high anodic potentials the decomposition of carbonate takes place, leading to passivity breakdown and oxygen evolution. The values of corrosion parameters (R_p, i_o and i_corr) were calculated. The calculated values indicate that the corrosion resistance of the austenitic stainless steel is higher than that of the ferritic steel. The activation energy of the corrosion process was found to be equal to about 70 kJ mol⁻¹. The results of the cyclic voltammetric investigations indicate that the behavior of the austenitic steels is about the same and differs from that of ferritic steel. The corrosion tests in 0.2 M HCl solutions have shown that the oxide scales formed on the surface of the austenitic stainless steels are multilayered, whereas those formed on the ferritic alloy are uniform.     

Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions

Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na₂SO₄ solutions of different concentrations at 25 ± 0.2 °C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i_corr and i_c increases with increasing either Cl⁻ or SO₄²⁻ concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl⁻ or SO₄²⁻ concentration.     

Formation and composition of nanoporous films on 316L stainless steel by pulsed polarization

Nanoporous films have been formed on 316L stainless steel in 5 mol dm⁻³ sulphuric acid at 60 °C by square wave pulse polarization between active or transpassive and passive potentials, and characterized using glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, Rutherford backscattering spectroscopy and nuclear reaction analysis. The films are shown to be chromium- and molybdenum-rich relative to the substrate, and to consist mainly of sulphates, oxides and hydroxides. The morphology and composition of the films are discussed in relation to the polarization conditions and mechanism of film formation.     

The corrosion-inhibiting effect of polypyrrole films doped with p-toluene-sulfonate,benzene-sulfonate or dodecyl-sulfate anions,as coating on stainless steel in NaCl aqueous solutions

This article presents a study of the conditions for electro-synthesis of polypyrrole (PPy) films on stainless steel, in the presence of the anions p-toluene-sulfonate (pTS), benzene-sulfonate (BS) or dodecyl-sulfate (DS). Cyclic voltammetry (CV) was used in the synthesis of the polypyrrole films on the stainless steel (SS). These polymeric films were characterized by IR and UV–vis spectroscopy and their morphology and thickness were analyzed by scanning electron microscopy (SEM). Their performance as protective films against corrosive processes presented by the SS/PPy-pTS, SS/PPy-BS or SS/PPy-DS systems was evaluated in 0.1 M NaCl aqueous solution. The study of the corrosion processes of the stainless steel/polymer systems was conducted through measurements of open circuit potential (E_OCP), polarization curves (PC) and electrochemical impedance spectroscopy (EIS). The results showed that the protective capacity of these polymeric systems on stainless steel, mainly with regard to pitting, depends on the nature of the anion dopant used during electro-synthesis of the PPy film. The best performance was seen with the dopants pTS and BS.     

Selective synthesis of diamond and CNT nanostructures directly on stainless steel substrates

Without surface pretreatment or applying additional interlayer, diamond films have been directly synthesized on an Fe-25Cr-5Al steel substrate by a hot filament chemical vapor deposition method from an H₂-1vol.% CH₄ gas mixture. Due to an effective removal of intermediate graphite phase from the diamond-steel interface, the coated diamond films were continuous and adherent well to the steel substrate. Aligned conical diamond structures were also achieved on this steel substrate by negatively biasing the substrate holder and inducing a glow discharge. The deposition behavior of carbon on Fe-Cr-Ni steel substrate was different. A graphite-rich carbon film incorporated with diamond particles grew in the absence of biasing, then aligned carbon nanotube bundles were formed in the presence of negative biasing and glow discharge. The different deposition behavior of carbon on the two kinds of steel substrates was addressed in terms of the effect of their chemical compositions.     

硝化锅不锈钢蛇管腐蚀的原因与对策

针对硝基苯装置硝化锅换热蛇管腐蚀的原因进行了讨论与分析,提出了防止措施和改进意见。     

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.     

Micro-electrochemical characterization of the effect of applied stress on local anodic dissolution behavior of pipeline steel under near-neutral pH condition

Local anodic dissolution behavior of smooth and pre-cracked X70 pipeline steel specimens under applied stress were investigated by micro-electrochemical measurements and numerical simulation. The results demonstrated that the anodic dissolution rate of steel is enhanced by applied tensile stress. Corrosion of the stressed steel is accompanied with the formation of a layer of corrosion product deposit on electrode surface. However, the deposit layer does not provide effective protection to the underlying steel for corrosion attack due to its loose, porous structure. Under small tensile stress, the stress-enhanced dissolution of steel is not significant. With the increase of stress level up to 80% of yielding strength of steel, the activity of the steel increases remarkably, resulting in a significant enhancement of the anodic dissolution of steel. A stress concentration is developed at the crack-tip and enhances significantly the local anodic dissolution of steel. The stress effect factor at the crack tip is as high as 3.6 when a tensile force of 3000 N is applied on the pre-cracked specimen, while that for the low-stress area is 1.102 only. The local dissolution rate at the crack-tip is accelerated with the test time, which would be attributed to the fact that, along with continuous propagation of the crack, the stress concentration at the crack-tip is further increased, which, again, enhances local dissolution. This result reflects exactly the interaction of stress and anodic dissolution at crack-tip during SCC of steel.     

Electrodeposition of poly(N-methylpyrrole) on stainless steel in the presence of sodium dodecylsulfate and its corrosion performance

Poly(N-methylpyrrole)-dodecylsulfate (PNMPy-DS) coating was electrosynthesized by potentiodynamic method on a stainless steel in oxalic acid solution containing sodium dodecylsulfate for the first time. The effects of electrochemical synthesis parameters, such as applied potential, scan rate and cycle number, on the protective behaviors of PNMPy-DS films were investigated and the optimum synthesis conditions were determined. The PNMPy-DS coating was characterized by the cyclic voltammetry, FT-IR spectroscopy and SEM methods. Corrosion protection behavior of this polymer-coated steel was investigated in 0.5 mol L⁻¹ HCl solution by potentiodynamic polarization and EIS methods. The results show that the PNMPy-DS coating provides effective protection for the stainless steel against to corrosion due to the fact that the large negatively charged dodecylsulfate dopant in the polymer structure electrostatically repels corrosive chloride ions and delays their access to metal surface.     

Anodic decomposition of citric acid on gold and stainless steel electrodes: An in situ-FTIR-spectroscopic investigation

The electrochemical decomposition of citric acid on gold and stainless steel AISI 304 (18% Cr, 10% Ni) electrodes was investigated using the technique of in situ InfraRed Reflection Absorption FTIR Spectroscopy (IRRAS) in combination with cyclic voltammetric measurements. The applied potential sweep starts from −0.265 up to +2.5 V on gold and from +0.4 up to +2.4 V on steel electrodes. The initial potentials of the anodic decomposition of citric acid could be observed on both electrode materials. Carbon dioxide was detected as decomposition product. Using stainless steel electrodes, the decarboxylation of citric acid and the forming of citrate complexes were observed. The consumption of citric acid is both due to its anodic decomposition and the generation of soluble citrate complexes of iron, nickel and chromium, resulting from the transpassive dissolution of the electrode material. At potentials more positive than +500 mV both processes are occurring simultaneously.     

Marine microbial fuel cell: Use of stainless steel electrodes as anode and cathode materials

Numerous biocorrosion studies have stated that biofilms formed in aerobic seawater induce an efficient catalysis of the oxygen reduction on stainless steels. This property was implemented here for the first time in a marine microbial fuel cell (MFC). A prototype was designed with a stainless steel anode embedded in marine sediments coupled to a stainless steel cathode in the overlying seawater. Recording current/potential curves during the progress of the experiment confirmed that the cathode progressively acquired effective catalytic properties. The maximal power density produced of 4 mW m⁻² was lower than those reported previously with marine MFC using graphite electrodes. Decoupling anode and cathode showed that the cathode suffered practical problems related to implementation in the sea, which may found easy technical solutions. A laboratory fuel cell based on the same principle demonstrated that the biofilm-covered stainless steel cathode was able to supply current density up to 140 mA m⁻² at +0.05 V versus Ag/AgCl. The power density of 23 mW m⁻² was in this case limited by the anode. These first tests presented the biofilm-covered stainless steel cathodes as very promising candidates to be implemented in marine MFC. The suitability of stainless steel as anode has to be further investigated.     

Mechanism of electropolymerisation of methyl methacrylate and glycidyl acrylate on stainless steel

An aqueous based technique for producing uniform, thin, passive films of poly(methyl methacrylate) (PMMA) and poly(glycidyl acrylate) (PGA) on stainless steel electrodes has been developed. A cathodic free radical polymerisation mechanism is proposed based upon the results of cyclic voltammetry (CV), mechanistic electro-polymerisation and gel permeation chromatography (GPC) experiments. The polymerisation yield was increased by a synergistic relationship between sulphuric acid and potassium persulphate initiators, proposed to involve the formation of long-lived radical species. Termination reactions are believed to be inhibited by the formation of co-ordination complexes between the growing polymer chain and sulphuric acid. The mechanism accounts for the broad molecular weight distribution, cross-linking and post-electrolysis polymerisation, where polymer continued to form after the current flow ceased.     

The electrochemical copolymerization of pyrrole and bithiophene on stainless steel in the presence of SDS in aqueous medium and its anticorrosive performance

For the first time, the electrosynthesis of poly(pyrrole-co-bithiophene) copolymers (P(Py-co-BT) I, II, III) was carried out using the potentiostatic technique on stainless steel (SS) electrode from aqueous oxalic acid solutions containing fixed concentration of bithiophene (BT) and different concentrations of pyrrole (Py) in the presence of sodium dodecylsulfate (SDS). Corrosion protection behaviors of these copolymer-coated steels were investigated in 3.5% NaCl solution by potentiodynamic polarization, Tafel test technique and electrochemical impedance spectroscopy (EIS). Among the protective copolymer coatings, the P(Py-co-BT) II, which was obtained from polymerization solution containing 0.025 M Py, exhibited the best protection against corrosion. Hence, only this copolymer was characterized by cyclic voltammetry, FT-IR, UV–vis, conductivity measurement and differential scanning calorimetry (DSC) by comparing with those of the polybithiophene (PBT) and polypyrrole (PPy) homopolymers. It was determined that this polymer was the polypyrrole-based copolymer. The incorporation of BT units into PPy chains not only increased dry conductivity but also developed the toughness of polymer.     

Comparison of the microbiological influence on the electro-chemical potential of stainless steel between macro- and micro-areas of specimens

The microbiologically caused ennoblement appears in natural waters on all stainless steels equally and has been described in numerous publications. In addition to an external polarization of such a system, temperature, oxygen and pH-levels, supply of nutrients and/or the supply of substances to restrain biological activity have a direct influence on the rate of the potential rise. The final value of the potential is substantially regulated by the biological system and is independent of the steel composition.At the University of Applied Sciences in Konstanz, Germany, the potential rise of stainless steels has been examined in experimental set-ups. Up to recently, only effects on the open circuit potential of specimens have been measured. In order to further understand the mechanisms behind the potential rise, these specimens have been divided into micro-areas and Microelectrodes have been introduced to measure the electro-chemical potential of the micro-areas. The experimental set-up not only allows for the average mapping of the potential distribution over a specimen's surface, it further allows for learning more of the processes behind the rise of the electro-chemical potential.Influences of availability of nutrients, biocides, oxygen, and other factors on the free corrosion potential on the whole system as well as on the micro-areas have been examined and compared.It has been observed that the use of biocides resulted in a rapid decrease of the free corrosion potential, however only a very limited decrease of the potential within the micro-areas could be measured. Experiments with an external polarization of the specimens resulted in similar curves with quantitatively different potential differences. This paper is to present results of some of the conducted experiments with possible theories.     

Corrosion protection of carbon steel and copper by polyaniline and poly(ortho-methoxyaniline) films in sodium chloride medium. Electrochemical and morphological study

The aim of this work was to obtain polyaniline (Pani) and poly(ortho-methoxyaniline) (Poma) by chemical synthesis and to evaluate their corrosion protection properties on carbon steel (CS) and copper (Cu) in an aggressive media such as sodium chloride. The syntheses of the polymers were carried out by chemical oxidation of the monomers by (NH₄)₂S₂O₈ in nitric acid solutions. Under these conditions, the polymers were obtained in the oxidized form, dissolved in 1-methyl-2-pyrrolidone and casted by solvent evaporation onto the metallic substrates (carbon steel and copper) for corrosion evaluation. The morphology of the polymers was evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical behavior was determined by open circuit potential (OCP) measurements and polarization curves (PC). The best results were obtained with Pani because this polymer film participates in the formation of an oxide film at the polymer–metal interface, a phenomenon which is not observed with Poma. This oxide film increases the barrier effect that the polymeric film has by itself.     

Influence of salt content on clay electro-dewatering with copper and stainless steel anodes

Abstract

The electro-dewatering technique is an effective method to reinforce soft clay. Electro-dewatering experiments were conducted to assess the impact of salt content on electro-dewatering behaviors and effects with copper and stainless steel anodes. The changes in the current, accumulated drainage volume, and electro-osmotic flow velocity reflect the “catch point” phenomenon between the copper anode and the stainless steel anode. Moreover, the higher the salt content was, the earlier the appearance of the “catch point,” which means that the effect of the copper anode was better in the early stages and then worse in the following stages than that of the stainless steel anode. Based on photographs of anode surface, SEM-EDS (Scanning electron microscopy-Energy dispersive spectrometry) and XRD (X-ray diffraction) results, we found that the copper anode surface produced green product, which observably decreased the conductivity of the copper anode and then reduced the effective electrical potential gradient. On the other hand, there were no added elements on the stainless steel anode surface after the electro-dewatering experiments, and the conductivity of the stainless steel anode was similar to its initial status. The above two points led to the appearance of the“catch point” phenomenon. The pH value of the stainless steel anode was lower than that of copper anode, and the concentration of Cl^? had the opposite relationship. Because the effective electrical potential gradient of the stainless steel anode was higher than that of the copper anode after the “catch point,” the water content of the stainless steel anode was lower than that of the copper anode.