Influence of the electrolyte composition and temperature on the transpassive dissolution of austenitic stainless steels in simulated bleaching solutions

The transpassive corrosion of highly alloyed austenitic stainless steels—UNS N08904, UNS S31254 and UNS S32654—was investigated at 20 and 70 °C in a range of simulated bleaching solutions with conventional and rotating ring-disc electrode voltammetry, as well as electrochemical impedance spectroscopy. The overall transpassive oxidation rate of UNS S32654 was found to be much higher than that of the other two alloys. The general features of the impedance spectra demonstrate that transpassive dissolution is favoured for UNS S32654 and secondary passivation predominates for the two other steels. The addition of oxalic acid resulted in a significant increase of the transpassive oxidation rate at both temperatures. At room temperature, the addition of diethylenetriaminopentaacetic acid (DTPA) led to a decrease of the transpassive oxidation rate, especially at pH 3. Conversely, the addition of DTPA to the pH 3 solution at 70 °C has been found to increase the transpassive oxidation rate. A kinetic model of the process is proposed, featuring a two-step transpassive dissolution of Cr via a Cr(VI) intermediate species and taking into account the dissolution of Fe(III) through the anodic film. The model has been found to be in quantitative agreement with the steady-state current versus potential curves and the impedance spectra. The kinetic parameters of transpassive dissolution have been determined and the relevance of their values is discussed.     

Investigations on bromine corrosion associated with mercury control technologies in coal flue gas

Mercury control technologies are often associated with adding halogens to the flue gas to enhance oxidation of elemental mercury. The present research was to evaluate the corrosion characteristics of iron in a flue gas containing bromine to simulate mercury control applications in coal-fired utility plants. An AISI 1008 cold rolled steel was exposed to a synthetic flue gas (7.1 vol% O₂, 14.3 vol% CO₂, 2.0 vol% H₂O, 51 ppmv HBr, 510 ppmv SO₂, 51 ppmv NO_x, and the balance N₂). Exposure times ranged from 30 days to 6 months. Metal coupons were exposed with simulated flue gases at 300°, 150°, and 80 F (149°, 66°, and 27 °C), respectively. The corroded coupons were analyzed using scanning electron microscope and micrometer measurements to determine the deposit chemistry and corrosion loss. The corrosion products consisted mainly of iron oxides and iron bromide. A mechanism for HBr corrosion is suggested. Bromine dew point corrosion took place on metal surfaces at temperatures below or close to the dew point of HBr, while active oxidation occurred at higher temperatures.     

Electropolishing of metals in alcoholic solution of sulphuric acid

The potentiodynamic polarization curves of iron, nickel, cobalt, molybdenum, copper and iron-nickel and cobalt-nickel alloys were measured in the polishing solution of 1M H₂SO₄ in CH₃OH. The above mentioned metals and alloys may be divided into three groups: metals and alloys with high dissolution rate in the active state in which the polishing follows immediately behind the active dissolution region (Fe, Co and Ni-Fe and NiCo alloys with a low nickel content), metals and alloys with low passivating current density that become passive at first and the polishing takes place behind the region of localized corrosion (nickel and NiFe and NiCo alloys with a high nickel content) are metals that are polished in the transpassive region (molybdenum).     

The improvement of the corrosion resistance of 309 stainless steel in the transpassive region by nano-crystallization

The effect of nano-crystallization on the corrosion behavior of 309 stainless steel in the transpassive region was investigated in 0.5 M Na₂SO₄ (pH 2) solution. Three parts defined as transpassive dissolution, secondary passivity and oxygen evolution can be observed in the transpassive potential region of the anodic polarization curves. In the whole transpassive region the nano-crystalline coating has a smaller corrosion current density than the bulk steel, which indicates the transpassive dissolution rate is decreased by nano-crystallization. In addition, there is an obvious difference in the surface micrographs of the two materials at transpassive potentials, as scanning electron microscopy (SEM) shows. The electron probe microanalysis (EPMA) reveals that nano-crystallization improves the homogeneity of Cr on the surface. Mott-Schottky plots displays that the carrier density of the oxide film in the transpassive region is decreased by nano-crystallization. Thus, the interfacial reactions are decelerated and the corrosion resistance of the stainless steel in the transpassive region has been greatly improved by nano-crystallization.     

Electrochemical corrosion characteristics of type 316 stainless steel in simulated anode environment for PEMFC

The corrosion behavior of type 316 stainless steel in simulated anode environment for proton exchange membrane fuel cell (PEMFC), i.e., dilute hydrochloric acid solutions bubbled with pure hydrogen gas at 80 °C, was investigated by using electrochemical measurement techniques. The main purpose is to offer some fundamental information for the use of stainless steels as bipolar plate material for PEMFC. Both polarization curve and electrochemical impedance spectroscopy (EIS) measurements illustrate that 316 stainless steel cannot passivate spontaneously in the simulated environments. The absorbed (and/or adsorbed) hydrogen atoms from cathodic corrosion reactions on the steel surface may deteriorate the passivity and corrosion resistance. The oxidation of these hydrogen atoms gives rise to a second current peak in the anodic polarization curve, and the current increases with immersion time. EIS spectra also reveal that a porous corrosion product layer formed on the steel surface during the active dissolution in the test solutions. 316 stainless steel exhibits the similar corrosion behavior in sulfate ions containing dilute hydrochloric acid solution.     

Corrosion and passivation of iron and its nitrided layer in borate buffer

The aim of this work was to gain better understanding of the reasons of enhanced resistance to pitting corrosion of nitrogen-containing iron. Gas nitrided (570 °C, 4 h) and untreated Armco iron were examined in a borate solution of pH 8.4 without and with chlorides or ammonia. Enhanced pitting resistance and enhanced anodic currents were exhibited by nitrided Fe and also by untreated Fe in the solution with added ammonia. XPS showed that anodic films on nitrided Fe contained much larger amounts of iron oxides, in particular of magnetite, than those on untreated Fe. It is suggested that the anodic behaviour of nitrided Fe is determined mainly by the effect of evolving ammonia on corrosion products. Increased anodic dissolution can be explained by the formation of soluble complexes with ammonia, whereas increased amounts of magnetite can be due to the ammonia-promoted conversion of FeOOH + Fe(II) to Fe₃O₄. It is proposed that the enhanced pitting resistance of nitrided Fe results mainly from the formation of large amounts of iron oxides and from binding of chloride anions into a Fe-NH₃-Cl complex.     

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.     

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.     

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11 wt.%) and mumetal (Ni:Fe; 77:14 wt.%) by the decomposition of C₂H₄ at 600 °C. The effect of pretreatment and the addition of H₂ on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H₂ during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1-1 mg cm⁻² h⁻¹, as mainly occurs on stainless steel.     

On the role of mass transport in high rate dissolution of iron and nickel in ECM electrolytes—I. Chloride solutions

High rate anodic dissolution of iron and nickel in 5 M NaCl was studied in a flow channel cell under controlled hydrodynamic conditions. Galvanostatic experiments were aimed at investigating the influence of current density and electrolyte flow rate on anode potential, current efficiency for metal dissolution and surface texture resulting from dissolution. Active dissolution at low current densities leads to surface etching and transpassive dissolution at high current densities leads to surface brightening. Transition from active to transpassive dissolution is mass transport controlled and is accompanied by a change in anode potential, surface microtexture and in case of iron by a change in the valence of metal dissolution.     

Continuous synthesis of metal nanoparticles in supercritical methanol

Metal nanoparticles were synthesized continuously in supercritical methanol (scMeOH) without using reducing agents at a pressure of 30 MPa and at various reaction temperatures ranging 150-400 °C. Wide angle X-ray diffraction (WAXD) analysis revealed that metallic nickel (Ni) nanoparticles were synthesized at a reaction temperature of 400 °C while mixtures of nickel hydroxide (α-Ni(OH)₂) and metallic Ni were produced at lower reaction temperatures of 250-350 °C. In contrast, metallic silver (Ag) nanoparticles were produced at reaction temperatures above 150 °C while metallic cupper (Cu) nanoparticles were produced at reaction temperatures above 300 °C. Mixtures of copper oxide (CuO and Cu₂O) and metallic Cu were produced at lower reaction temperatures of 250 °C. Scanning electron microscopy (SEM) showed that the particles size and morphology changed drastically as the reaction temperature increased. The average diameters of Ni, Cu and Ag particles synthesized at 400 °C were 119 ± 19 nm, 240 ± 44 nm, and 148 ± 32 nm, respectively. The scMeOH acted both as a reaction medium and a reducing agent. A possible reduction mechanism in scMeOH is also presented.     

Mechanism of steel corrosion in concentrated NaOH solutions

A study has been made of the mechanism of corrosion of carbon steel in 1–19 N NaOH at 25–80°C. The polarization curves were obtained with a rotating steel disk electrode, and the rotating ring-hemisphere electrode technique was used to identify soluble corrosion products. It was found that at the active anodic dissolution potentials, steel dissolves to form HFeO⁻₂ ions. Both the anodic and cathodic polarization curves exhibited a well defined Tafel regime, and the electrokinetic parameters were obtained for the corrosion of steel. The results were interpreted with a corrosion mechanism based on the decomposition of molecular water to hydrogen at the cathodic sites, and the active dissolution of iron to HFeO⁻₂ ions via two adsorbed intermediates, FeOH_ads and Fe(OH)_2,ads, at the anodic sites. The theoretically derived anodic Tafel slope, anodic reaction order, corrosion potential, and corrosion current density agreed quantitatively with the experimental values. At the potentials above the anodic Tafel regime and in the neighborhood of the active—passive transition potential, steel dissolved to form both the bi-valent and tri-valent iron species, HFeO⁻₂ and FeO⁻₂ ions. The thermodynamic considerations revealed that FeO⁻₂ ion was formed from the oxidation of Fe(OH)_2,ads intermediate, rather than from the oxidation of HFeO⁻₂ ion on steel surface.     

Catalytic etching of {100}-oriented diamond coating with Fe, Co, Ni, and Pt nanoparticles under hydrogen

Etching of a highly {100}-oriented diamond coating, {100}HODC, with hydrogen gas using Fe, Co, Ni, and Pt nanoparticles as a catalyst was examined at high temperatures over 700 °C by high-resolution scanning electron microscopy and Raman spectroscopy. The metal atoms vacuum-evaporated onto the {100}HODC formed nanoparticles themselves when heated at high temperatures; e.g. 700 °C, in a flowing gas mixture of H₂ (10%) + N₂ (90%). At 800 °C, short nano-channels and etch pits holding metal nanoparticles were formed by Fe, Co, and Ni. The shapes of the Co and Ni nanoparticles in the etch pits were affected by the shape of the etch pits; reversed pyramidal shape. On the other hand, the top view of the Fe nanoparticles embedded in the etch pits showed a distorted round shape, probably due to the formation of something such as iron carbide, while the carbon content was unknown. Apparently, etching of the {100}HODC by Pt nanoparticles was observed after the treatment at 1000 °C. The difference in the catalytic etching behavior among these metal particles, the potential etching mechanism of diamonds with hydrogen by metal nanoparticles, probably as melted metal nanoparticles, and the formation mechanism of vacant etch pits were discussed.     

Comparative study of Nb, Nb-10W, and Nb-16Ta-12W corrosion behavior in sodium hydroxide solutions

The electrochemical behavior of niobium, Nb-10W, and Nb-16Ta-12W alloys is investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that the three materials are spontaneously active in 10, 15, and 30 wt.% NaOH at 25, 50, and 75 °C. The anodic polarization curves show in all cases a dissolution-passivation peak followed by a current plateau, corresponding to oxides formation. The spontaneous active corrosion of the three materials mainly leads to the formation of sodium niobates, as detected by X-ray diffraction analysis of the corrosion products. The evolution of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicate that the corrosion rates of Nb, Nb-10W, and Nb-16Ta-12W alloys increase with increasing NaOH concentration and temperature. In all cases, the increasing order of corrosion resistance is: Nb<Nb-10W<Nb-16Ta-12W.     

Adhesion characteristics and corrosion stability of epoxy coatings electrodeposited on phosphated hot-dip galvanized steel

The influence of hot-dip galvanized steel (HDG) surface pretreatment with phosphate coatings on the corrosion stability and adhesion characteristics of epoxy coatings electrodeposited on HDG steel was investigated. Phosphate coatings were deposited on hot-dip galvanized steel from baths with different concentrations of NaF (0.1, 0.5 and 1.0 g dm⁻³) and at different temperatures (50, 65 and 80 °C). The influence of fluoride ion concentration in the phosphating bath, as well as the deposition temperature of the bath, on the adhesion characteristics and corrosion stability of epoxy coatings on phosphated HDG steel was investigated. The dry and wet adhesions were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test, while corrosion stability was investigated by electrochemical impedance spectroscopy (EIS).     

Corrosion Behaviour of Niobium in Sodium Hydroxide Solutions

The electrochemical behaviour of niobium was investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that Nb is spontaneously active in 10, 15 and 30 wt % NaOH at 25, 50 and 75 °C. The anodic polarization curves in all cases show a dissolution/passivation peak followed by a current plateau, corresponding to Nb₂O₅ formation. The spontaneous active corrosion of Nb leads to the formation of soluble niobates that precipitate to sodium niobates. The evaluation of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicates that the corrosion rates of niobium increase with increasing NaOH concentration and temperature.     

Corrosion behaviour of NiTi alloy

In the present work, the corrosion behaviour of NiTi in Hanks’ solution at 37 °C was assessed by the use of electrochemical methods. Pure titanium and pure nickel were included in the study in order to understand the contribution of each alloying element. The results were compared with Ti-6Al-4V alloy and 316L stainless steel, materials traditionally used as orthopaedic implants. Moreover, the susceptibility of NiTi to corrosion under different conditions was examined using other physiological solutions and different pH values.It was observed that the corrosion behaviour of NiTi is much closer to Ti than to Ni, as it may be seen on the polarization curve, where the high protective character of the passive oxide film formed on NiTi is similar to that of titanium. On the other hand, comparing the different implant materials, it was possible to establish the following relation for their corrosion resistances: 316L stainless steel < NiTi < Ti-6Al-4V.     

Corrosion characteristics of aluminum alloy in bio-ethanol blended gasoline fuel: Part 1. The corrosion properties of aluminum alloy in high temperature fuels

The corrosion properties of an aluminum alloy, A384, in bio-ethanol blended gasoline fuel were examined at various ethanol contents (10%, 15% and 20%) and temperatures (60, 80 and 100 °C). Localized pitting corrosion developed at a high temperature of 100 °C. The corrosiveness of the fuel increased with increasing ethanol content (E10 < E15 < E20). However, no such coincident tendency appeared for the temperature (80 < 60 < 100 °C) due to the structural change of protective hydroxide film. The overall corrosion process was characterized by both competitive factors of corrosive ethanol and a protective oxide film at a given temperature and ethanol content.     

Nickel oxide/hydroxide nanoplatelets synthesized by chemical precipitation for electrochemical capacitors

Nickel hydroxide powder prepared by directly chemical precipitation method at room temperature has a nanoplatelet-like morphology and could be converted into nickel oxide at annealing temperature higher than 300 °C, confirmed by the thermal gravimetric analysis and X-ray diffraction. Annealing temperature influences significantly both the electrical conductivity and the specific surface area of nickel oxide/hydroxide powder, and consequently determines the capacitor behavior. Electrochemical capacitive behavior of the synthesized nickel hydroxide/oxide film is investigated by cyclic voltammetry and electrochemical impedance spectroscope methods. After 300 °C annealing, the highest specific capacitance of 108 F g⁻¹ is obtained at scan rate of 10 mV s⁻¹. When annealing temperature is lower than 300 °C, the electrical conductivity of nickel hydroxide dominates primarily the capacitive behavior. When annealing temperature is higher than 300 °C, both electrical conductivity and specific surface area of the nickel oxide dominate the capacitive behavior.     

Anticorrosive properties of polypyrrole films modified with zinc onto SAE 4140 steel

Polypyrrole (PPy) films modified with zinc were electrosynthesized onto SAE 4140 steel in presence of bis(2-ethylhexyl) sulfosuccinate (AOT). The Zn and PPy electrodeposition was realized by using cyclic voltammetry at different temperatures. The corrosion protection properties of the films were examined in chloride solution by open circuit measurements, linear polarization and electrochemical impedance spectroscopy (EIS). The obtained results indicate that the presence of Zn in the polymer matrix improves the anticorrosive performance of PPy films. The best anticorrosion efficiency was obtained for the coatings modified at 20 °C which provided anodic protection to the steel substrate for a long period of immersion in chloride solution. Cathodic protection was observed when the electrodeposition temperature was increased. Adherence and anticorrosive properties declined sharply for the coatings electrosynthesized at 5 °C.