Real time measurement of elemental dissolution rates by atomic emission spectroelectrochemistry under alternating current (AC) perturbations of ZnAlMg alloy was used to precise mechanisms controlling selective dissolution as a function of pH in chloride containing solutions in the presence of NH4+. At pH values of 8.5 and 10.5 Zn and Mg dissolution currents (jZn, jMg) followed the total current je* respecting the stoichiometry of Zn2Mg at the maximum of anodic current in solution at pH 8.5. Al dissolution current (jAl) followed je* at pH 10.5 and 8.5 with a phase shift of 180° further called cathodic Al dissolution. No correlation between AC perturbations and Al dissolution was seen at pH 8.5. At pH 13 anodic Zn dissolution, cathodic Al dissolution and no Mg dissolution were observed. The elemental dissolution rates of Al at pH 8.5 and Mg at pH 13 did not vary with AC perturbations. The detected concentrations of Al at pH 8.5 and Mg at pH 13 correlated well with the saturation concentrations of Al3+ and Mg2+ in studied electrolytes expected from solubility constants. Thus, their dissolution was controlled by the solubility of the surface films observed by infrared and Raman spectroscopy. The finding implies the separation of cathodic Al dissolution, and anodic Zn and Mg dissolution in the appropriate equivalent circuit for an adequate analysis of conventional EIS data.
Local Electrochemical Impedance Mapping (LEIM) methodology was adopted to quantify the propagation of electrochemically active regions with a micrometric precision. The method consisted in the use of the gradient modulus of the admittance map as a parameter for the spatial quantification. Numerical simulations were used to optimize the experimental conditions, namely the AC frequency, the distance between the local bi-probe and the working electrode, and the distances between the probes for the local bi-probe used for the local current mapping. This analysis was reinforced by experimental verifications on coated electrodes. The quantitative LEIM methodology was successfully applied to follow the delamination kinetics on Zn coated with the polyvinyl butyral polymer in NaCl solutions. At 1 kHz, the LEIM response only reflected the position of the anodic front beneath the polymer because oxygen reduction reaction was diffusion limited and hence, independent of the applied potential. This novel LEIM methodology completes the set of usual tools used to investigate the delamination mechanisms on metal substrates.
Protection mechanisms of layered double hydroxide (LDH) anticorrosion coatings on galvanized steel were investigated. On the first step, the mechanisms of the action of MoO42-, L-cysteine and L-phenylalanine as water soluble inhibitors were verified. MoO42- showed the best performance associated with the rapid (less than 2 s) formation of Mo(V)-rich films on Zn surface in alkaline solutions. On the second step, factors affecting the release of MoO42- from Zn2Al/-LDH were identified. Presence of only Cl- in the solution resulted in a less than 40 % release of MoO42- after 24 h of the immersion with the Fick’s diffusion inside LDH controlling the overall process. Addition of carbonates resulted in the complete release after 1 h with the change of the release kinetics attributed to the surface controlled reaction. The immersion tests of coated samples showed slight inhibiting effect of MoO42-/LDH anticorrosion coatings in Cl- and high in CO32- solutions coherent with higher level of MoO42- released.
Various α-amino acid (αAA) molecules were screened for their ability to retard the corrosion of AA2024 aluminum alloy substrate. The αAAs screened were l-arginine (l-Arg), l-asparagine (l-Asn), l-cysteine (l-Cys), l-cystine, l-histidine (l-His), l-methionine (l-Met), l-phenylalanine (l-Phe), l-serine (l-ser), l-tryptophan (l-Trp), and l-tyrosine (l-Tyr). From their performances compared with that of the reference additives chromate and 2-mercaptobenzothiazolate (MBT), l-Cys and l-Phe were selected, and their layered double hydroxide (LDH) interleaved derivatives were further scrutinized. Different LDH phases, namely, LiAl2, Mg2Al, MgZnAl, and Zn2Al, were tested as hosts for the inhibitor αAA substances, and the materials were characterized by XRD, FTIR spectroscopy, and SEM. The efficiencies and durable performances of the hybrid materials as an anticorrosion agents for aluminum alloy 2024 (AA2024) were demonstrated through direct current (DC) polarization measurements, and the evolution of the polarization resistance was recorded. The mechanism of inhibition focused on the most promising hybrid material LDH/l-Cys and was tentatively explained as anion exchange and dissolution of the inorganic framework at the cathodic and anodic corrosion zones, respectively, with the particular occurrence of Cu-rich intermetallic zones. The obtained results evidence that the LDH/l-CYS assembly embedded in the polymer coating retards the corrosion process of the AA2024 substrate after a prolonged immersion time.
The time resolution of the atomic emission spectroelectrochemical (AESEC) flow cell has been investigated by numerical simulations. The results demonstrate that the time resolution of the AESEC electrochemical flow cell may be simulated numerically based on the consideration of electrolyte flow patterns and ion transport in the cell. The residence time distribution (RTD) closely approximates a log-normal distribution for both experiment and simulation. Time resolution may be improved by increasing the flow rate, however this also leads to marked heterogeneities in the flow field near the surface. An optimum flow rate of 3 cm3 min−1 was determined. The problem may be avoided somewhat by using a mask to cover all the surface except for a small portion near the center of the flow cell.
Catalytic dehydration of methanol was studied in a reactor with inert, selectively permeable walls made of NaA zeolite membranes. Zeolite layer was synthesized on the flat metal–ceramic support by the in situ crystallization method resulting in selective and thermally resistant membrane. The separation factor of the model water/methanol mixture in the temperature range of 150–250 °C varied from 22 to 5 and the permeate flux of 0.6 kg h−1 m−2 was reached. Dehydration of methanol to dimethyl ether was carried out in flow mode in the membrane reactor, using γ-alumina catalyst. The effect of temperature, WHSV, feed pressure and flow rate of a sweep gas on methanol conversion was analyzed. The maximal conversion reached in the zeolite membrane reactor (ZMR) at 250 °C was 88% which is higher than in a conventional reactor by 8%.
Zn2Al/-layered double hydroxide (LDH) with intercalated MoO42- was investigated as a potential source of soluble molybdate inhibitor in anticorrosion coatings for hot dip galvanized steel (HDG). The effect of pH, chlorides and carbonates on the release kinetics of the interleaved MoO42- ions from the LDH powder immersed in solutions containing different anions was studied by X-ray diffraction, in situ attenuated total reflectance infrared spectroscopy (ATR-IR) spectroscopy and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The effect of the solution composition on the total release and the release kinetics was demonstrated. Less than 30 % of the total amount of the intercalated MoO42- was released after 24 h of the immersion in neutral 0.005 M - 0.5 M NaCl and 0.1 M NaNO3 solutions whereas the complete release of MoO42- was observed after 1 h in 0.1 M NaHCO3 or Na2SO4 and in alkaline solutions. The in-situ ATR-IR experiments and quantification of the released soluble species by ICP-AES demonstrated the release by an anion exchange in neutral solutions and by the dissolution of Zn2Al/-LDH in alkaline solutions. The anion exchange kinetics with monovalent anions was described by the reaction order n=0.35±0.05 suggesting the diffusion control, for divalent anions n=0.70±60.05 suggested the control by a surface reaction. Dissolution of Zn from coated HDG with and without Zn2Al/-MoO42- fillers, leaching of MoO42- from the coating and the electrochemical impedance spectroscopy response of the coated systems were measured during the immersion in 0.5 M NaCl solutions with and without 0.1 M NaHCO3. Without carbonates the release of soluble MoO42- was delayed for 24 h with no inhibiting effect whereas with 0.1 M NaHCO3 the immediate releasewas accompanied by the immediate and strong inhibiting effect on Zn dissolution. The concept of controlling the inhibition performance of LDH hybrid coatings by means of the environment composition is discussed.
The corrosion inhibiting and accelerating effect of L-cysteine on Zn were correlated with the formation of Zn oxides by means of a novel coupling of impedance spectroscopy (EIS) with atomic emission spectroelectrochemistry (AESEC). A plateau of anodic Zn dissolution was discovered in the cathodic branch of the polarization curve, hidden by the large cathodic current. The effect of L-cysteine at all pH values and all concentrations was to increase the plateau current. The effect was most pronounced at high pH suggesting a synergy between hydroxide and L-cysteine. Due to the presence of Zn plateau current, Tafel extrapolation resulted in wrong estimation of corrosion currents. The contribution of the anodic Zn oxidation in plateau region to the total current was inversely proportional to the frequency of AC perturbation.
Effect of L-cysteine on the Zn dissolution in 0.5 M NaCl with initial pH of 4–12 was measured in continuously renewed electrolyte and in immersion tests. At short exposures 1 mM L-cysteine inhibited dissolution. The initial inhibition decreased with pH because of pH-enhanced dimerization of L-cysteine, confirmed by UV–vis spectroscopy. In long exposures at pH = 6–8 the inhibition turned into the acceleration. This was not observed at pH = 12. 10 mM L-cysteine accelerated Zn dissolution. The effects were explained by a competition between strong complexation of L-cysteine with Zn-ions and its weak physisorption on the substrate. The hypothesis was confirmed by surface characterizations.
The inhibitive action of soluble Na2MoO4 on the spontaneous reactivity of hot dip galvanized steel in 0.5 M NaCl was studied at pH 4–13 by a direct measurement of Zn dissolution rate in the flowing electrolyte and postmortem surface analysis. The stability and the composition of Mo-rich films depended on the solution pH and flow conditions. The inhibition efficiency of soluble Mo(VI) correlated with the composition of the films: Mo(V)-rich films were immediately formed under uniform flow at pH 6–12, and the inhibition efficiency of Mo(VI) in these conditions (>92 %) was comparable with the efficiency of Cr(VI).