Electrical and electrochemical measurements as predictors of corrosion at the metal—organic coating interface

Five types of electrical and electrochemical measurements are reviewed: D.C. electrical properties, A.C. resistance, capacitance, corrosion potential, and polarization curves. When the coating resistance decreases with time and falls below 10⁶ to 10⁷ ohms/cm², corrosion beneath the coating is occurring at a significant rate. A low electrical resistance requires (a) ion and water penetration into the coating, (b) ready motion of ions through the coating, and (c) proceeding ionic/electron transfer reactions (corrosion) at the polymer—metal interface. Care must be taken when making electrical measurements that the measurement itself does not affect the corrosion rate.     

The inverse oxidation phenomenon

The oxidation rate of some “as manufactured” graphites increases with increased distance from the surface. The preponderance of the data supports the conclusion that this phenomenon is due to increased catalytic activity of one or more of the impurities in the graphite, and that the increase in catalytic activity is due to changes in the chemical state of the impurities.     

Kinetics of electrochemical corrosion of polytetrafluorethylene by alkali metal amalgams

Alkali metals contained in amalgams react with polytetrafluorethylene at room temperature. An electrochemical mechanism of the reaction

was suggested. A theoretical relation (dl/dt = Kl^?1) was derived and verified by experiment for the rate of penetration of the reaction into the depth of polytetrafluorethylene. The system alkali metal —polytetrafluorethylene forms a typical corrosion galvanic cell with a solid electrolyte. The electrolyte exhibits selective ionic conductivity for Li⁺.     

Kinetics of bio-oxidation of metal sulphides

The microbiological oxidation of cadmium, cobalt, nickel and zinc sulphides has been investigated using a pure strain of Thiobacillus ferrooxidans. Kinetic parameters (V_m and K) have been derived regarding the effect of initial total surface area of these substrates. A relationship is suggested to exist between the rate of sulphide oxidation and the solubility product of the respective metal sulphide.     

表面活性剂用作金属缓蚀剂的进展

表面活性剂的研究、生产和应用已成为一门迅速发展的科学和技术,在工业生产中被广泛采用．介绍了具有缓蚀作用的表面活性剂的特点及分类情况,综述了近年来国内外对具有缓蚀作用的表面活性剂的研究及应用情况．此外,还指出了用于金属缓蚀的新型表面活性剂的开发、研究方向．     

High-throughput syntheses of nano-scaled mixed metal sulphides

The use of metal and mixed metal sulphides as heterogeneous catalysts is often hampered by an undefined composition formed by in situ sulphidation of oxides. Controlled synthesis methods are serial and thus slow, because high-throughput (HT) methods for the well-directed syntheses of exactly defined metal sulphides and especially mixed metal sulphides are still lacking. The present contribution presents the development of HT synthesis methods for a broad range of nano-scaled sulphides for various applications, such as heterogeneous catalysts or photocatalysts. The nano-scale regime for the particle sizes offers the possibilities of high surface area, appearance of quantum size effects and the possibility of modification of their properties as well as stabilisation of the particles by surface coating. Several precursor molecules have been tested, which hydrolyse under specific conditions and release sulphide ions for metal and mixed metal sulphide precipitation. Particle size confinement was achieved by w/o microemulsions using non-ionic surfactants. In a first step for the parallel synthesis of sulphides a 9-fold reactor assembly was developed, which offers the possibility of synthesis by precipitation and separation of the resulting materials by filtration in one setup.     

Inferring the distribution of iron oxidation species on mineral surfaces during grinding of base metal sulphides

Electrochemistry plays an important role in the flotation of base metal sulphide minerals. During grinding a galvanic interaction occurs between minerals and grinding media and controls the iron contamination on mineral surfaces, which depresses mineral flotation significantly. In this study, the galvanic interaction was quantified by measuring the iron oxidation species originated from grinding media by ethylene diamine-tetra acid (EDTA) extraction in single mineral and mixed mineral systems. It was found that the extent of galvanic interaction between minerals and grinding media was intimately associated with the electrochemical reactivity of minerals. The nobler the mineral, the stronger the galvanic interaction with grinding media, and the higher the amount of iron oxidation species from grinding media. For both galena and chalcopyrite a linear relationship was observed between the amount of iron oxidation species and flotation recovery in single mineral systems. This relationship was able to predict the iron oxidation species on galena and chalcopyrite surfaces when they were mixed with pyrite separately. The distribution of iron oxidation species onto the two minerals in the mixture changed with the ratio of the mineral surface areas and was correlated with mineral flotation recovery. The major cathodic mineral in the mixture was dictated by the combination of mineral surface area and reactivity and drew iron oxidation species from the grinding media.     

The oxidation of dispersed refractory metal compounds and their behavior as carbon oxidation catalysts

The oxidation behavior of a number of finely dispersed refractory metal borides, carbides, nitrides and silicides has been studied in flowing air at temperatures up to 1000°C. Most of the materials investigated were rapidly converted to oxides under these conditions, but the carbide, nitride and silicide of chromium were more oxidation resistant. Several of these compounds (particularly those of V and Mo), were very active catalysts for the oxidation of graphite in this temperature range, but the presence of boron oxide reduced gasification rates in most cases. Compounds of Zr, Si, Al, Ti, Hf and Nb had little or no catalytic activity for graphite oxidation, whereas the borides of Zr, Si, Cr and Al exerted an inhibiting effect.     

The electrochemical oxidation of phenylenediamines

Five p-phenylenediamines were oxidized electrochemically at a glassy carbon rotating ring-disk-electrode assembly. The overall reaction comprises the transfer of two electrons per molecule. At low to intermediate potentials the rate determining step for all the five compounds is the acid dissociation of the semiquinonediimine radical cation formed in a pre-equilibrium involving the transfer of one electron. Only at higher potentials can the radical cation be oxidized in parallel to the neutral semiquinone radical to form the quinonediimines. In contrast, the neutral semiquinonediimine radicals are oxidized at the same or even a lower potential than the parent phenylenediamines. The dissociation rate constants of the radical cations are in the range of 10-300 s⁻¹. The effect of the substituents on the oxidation potentials is as expected by their property to donate electrons to the mesomeric system. Their effect on the dissociation rates, however, seems to rather correlate with the inductive effect and might influence not only the electronic structure of the molecule itself but also the structure of the ions and the solvent around them. The semiquinonediimine radical cations are found to be slightly more acidic than their protonated parent phenylenediamine cations with pK_a values somewhat below 6.     

Influences of metal oxides on carbon corrosion under imposed electrochemical potential conditions

Graphite electrodes, whose surface is partially covered with metal oxides, are prepared by the electrochemical deposition of MoO_x, WO_x, and SnO₂ particles, and their corrosion behaviors under the imposed electrochemical potential conditions are investigated. Scanning electron microscopy observation shows that highly oriented pyrolytic graphites (HOPGs) modified with MoO_x and WO_x particles (MoO_x|HOPG and WO_x|HOPG) suffer from carbon corrosion and show pits formation on the surface of HOPG after an electrochemical potential cycling in a range of 0.8–1.0 V. In contrast, SnO₂-modified HOPG (SnO₂|HOPG) exhibits superior tolerance to electrochemical oxidation. Cyclic voltammograms show that the SnO₂ modification prevents an increase in the redox currents for quinone/hydroquinone reactions and the electrochemical double layer capacitances on the HOPG electrode. These results indicate that the influence of metal oxides on carbon corrosion varies depending on the kind of metal oxides, and that SnO₂ is a proper choice to protect the surface of HOPG from carbon corrosion, under electrochemical potentials.     

The electrochemical oxidation of manganous hydroxide

Manganous hydroxide layers have been oxidized anodically by constant potential steps. The charge uptake and current—time transients have been correlated with changes of composition and crystal structure as revealed by X-ray diffraction. Two oxidation stages have been observed, and the process is compared with the anodic oxidation of nickel hydroxide.     

Anodic oxidation of aryl sulphides—IV. Thianthrene

Anodic oxidation of thianthrene in 80% (v/v) acetic acid-water mixtures containing HClO₄ at 1·15 V vs Ag/AgCl sat'd. KCl (approx. 1·10 V vs sce) gives the monoxide in near quantitative yield. At 1·6 V, thianthrene and its monoxide give a mixture of products consisting of 44% cis-, 28% trans-dioxide, 13% sulphone, 10% trioxide and 5% tetroxide.     

A DC electrochemical method for studying the inhibition of metal corrosion by chromate-containing paint

A DC potentiodynamic curve method is used to observe the effect of a pigmented organic coating on the passivity of a metal substrate. In this method, a painted metal coupon, which is scribed to expose metal, functions as the working electrode (WE) of an electrochemical cell; the cell includes a graphite bar counter electrode (CE), a saturated calomel reference electrode (RE), and aqueous electrolyte. Increasingly oxidizing potentials are imposed on the WE by a potentiostat, and current between the WE and CE is measured at each applied potential. The resulting current–voltage (I–V) curve indicates effects of the coating on the metal's open circuit potential and passivity. The passivating effect of a chromate-containing primer paint on mild steel is demonstrated and compared to the passivity of 316 stainless steel.     

Using electrochemical impedance spectroscopy to predict the corrosion resistance of unexposed coated metal panels

The goal of the current work was to determine if electrochemical impedance spectroscopy (EIS) testing of a series of coated but unexposed metal panels could predict the corrosion results of other sections of the same coated panels that were subjected to both continuous and cyclic corrosion testing. Variables included metal, pretreatment, primer, and topcoat. EIS results were shown to be strongly dependent upon the time-of-residence in the electrochemical cell prior to commencement of testing, and to the choice of electrolyte used in the cell. Good correlations between EIS and corrosion testing were seen for topcoat effects, but not for pretreatment effects. EIS results appear to relate mostly to barrier properties rather than electrochemical properties of coatings. It is suggested that the variation seen in EIS solution resistance values (R_s) can be utilized to quantify total system error. Total error was estimated by three techniques: total R_s variation, panel replicate variation, and EIS reading replication. The three approaches yielded similar results: total error for equivalent circuit components expressed in log₁₀ form was on the order of 50%, expressed as percent standard deviation.     

Formation and behaviour study of an environment-friendly corrosion inhibitor by electrochemical methods

This paper deals with the characterization of an environment-friendly corrosion inhibitor formulated for industrial cutting fluids. Polarization measurements and electrochemical impedance spectroscopy (EIS) tests reveal the formation of a 3D film at the interface metal/elecrolyte. This film blocks iron ions oxidation by forming a barrier on the metallic surface. Studies concerning, respectively, inhibitor concentration, immersion time and temperature show that effects of these three parameters are strongly linked and influence considerably the protective effect of the inhibitor. XPS analyses confirm formation of an inhomogeneous layer submitted to a water-uptake with time when inhibitor is not added in necessary concentration.     

The electrochemical deposition of polyaniline at pure aluminium: electrochemical activity and corrosion protection properties

Polyaniline films were electrodeposited at pure aluminium from a tosylic acid solution containing aniline. These polymer films exhibited similar characteristics as pure polyaniline electrosynthesized at an inert platinum electrode, when removed from their respective substrates and dissolved in NMP. Both polymers had similar molecular weights and similar UV-visible absorption spectra. However, the aluminium substrate had a considerable effect on the electrochemical activity of the films. The polyaniline films deposited at aluminium appeared to lose electroactivity and the electrochemical impedance data were governed by the oxidized aluminium substrate. This is consistent with a galvanic interaction between the polymer and the aluminium substrate, giving rise to oxidation of the aluminium and reduction of the polymer. The polyaniline deposits appeared to offer only a slight increase in the corrosion resistance of aluminium. Surface potential measurements, using a scanning vibrating probe, showed that attack initiated underneath the polymer under anodic polarization conditions, indicating that chloride anions diffuse across the polymer to react at the underlying aluminium substrate.     

The electrochemical impedance of metal hydride electrodes

The electrochemical impedance responses for different laboratory type metal hydride electrodes were successfully modeled and fitted to experimental data for AB₅ type hydrogen storage alloys as well as one MgNi type electrode. The models fitted the experimental data remarkably well. Several AC equivalent circuits have been proposed in the literature. The experimental data, however, could not always be satisfactorily approximated. The approximation model presented here exhibits smooth fit to the experimental results for all frequencies in the whole range from 10 kHz to 0.1 mHz. Equivalent circuits, explaining the experimental impedances in a wide frequency range for electrodes of hydride forming materials mixed with copper powder, were obtained. Both charge transfer and spherical diffusion of hydrogen in the particles are important sub processes that govern the total rate of the electrochemical hydrogen absorption/desorption reaction. To approximate the experimental data, equations describing the current distribution in porous electrodes were needed. Indications of one or more parallel reduction/oxidation processes competing with the electrochemical hydrogen absorption/desorption reaction were observed. The impedance analysis was found to be an efficient method for characterizing metal hydride electrodes in situ.     

A study of hydroxyethyl imidazoline as H2S corrosion inhibitor using electrochemical noise and electrochemical impedance spectroscopy

A study of H₂S corrosion inhibition of pipeline steel by hydroxyethyl imidazoline has been carried out by using electrochemical techniques. Inhibitor concentration included 5, 10, 25, 50, and 100 ppm in a H₂S-containing 3% NaCl solution at 50 °C. Techniques included linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and electrochemical noise (EN) measurements. In addition to the traditional noise in voltage and current, noise resistance (R _n) measurements were used. All techniques showed that the most efficient inhibitor concentration was between 5 and 10 ppm, but inhibitor efficiency decreased after 8 h of testing. Furthermore, EN measurements showed that steel was highly susceptible to localized corrosion at inhibitor doses lower than 10 ppm due to the establishment of a porous inhibitor film. However, with 50 or 100 ppm of inhibitor, the steel was susceptible to a mixture of uniform and localized corrosion. Hurst exponent was higher in presence of inhibitor for times shorter than 8 h, indicating a short residence time of the inhibitor. The data could not be fitted to any adsorption isotherm model, indicating a lack of strong adsorption of the inhibitor to the metal surface.