Shigeo Nagaura

Coverage of iron surface by organic compounds and anions in acid solutions

Abstract The electrical double layer capacities at the interface between iron and acid solutions containing organic inhibitors and halide ions were measured as a function of inhibitor concentrations. The degree of surface coverage as calculated from the change in capacity due to adsorption of inhibitors was compared to inhibitor efficiency. These showed a linear relationship which might be used as a means for evaluation of inhibitors. Peaks in coverage-concentration or inhibitor-efficiency-concentration curves are interpreted in terms of a change of mechanism from adsorption to film-forming inhibition. Synergism of amines and halide ions is discussed from a viewpoint of surface charge. A model of co-adsorption using halide ions and organic cations suitably interprets the phenomenon.

A contribution to the understanding of the synergistic effect of anions for the corrosion inhibition of Fe by amines

Synergistic and antagonistic effects of organic or inorganic anions with alkyl or cyclic amines were observed in the corrosion inhibition of Fe in HClO4. Anions specifically adsorbed on Fe attract amine cations electrostatically into the Helmholtz electrical double layer. If the electron availability of the amines is insufficient, the cationic amines simply cover the anion layer rather than chemisorb directly on the Fe surface. In the case of weak anion adsorption, however, the cations often tend to withdraw the adsorbed anions into the HClO4. Synergistic adsorption is, therefore, classified into two types plus a mixture of the two : (1) specific co-adsorption of anions and amines, as reported in a previous paper5; and (2) ionic or physical overlap adsorption of amine cations over the anion-covered Fe surface. In the latter case, increase of amine concentration may cause withdrawal of the anions from the Fe surface into the solution. The surface coverage thus decreases and, as a result, an antagonistic effect should be observed.

Impedance of the oxygen-evolution reaction on noble metal electrodes☆

Abstract The impedances of the oxygen-electrode reaction on platinum, palladium, gold and silver electrodes in acidic and alkaline solutions were measured with an ac bridge in the range of frequency from 10 Hz to 20 kHz, under dc polarization. For these electrodes, plots of impedances on the complex plane give semi-circles whose diameter is proportional to the dc being passed through the cell. By analysing these plots and impedance/dc curves, it is shown that the oxygen-evolution reaction on these electrodes is controlled by a charge-transfer process, except for the case of platinum in perchloric acid solution. In the latter system, a chemical reaction on the electrode surface is rate-determining.

Impedance of electrode reactions under dc polarization

Abstract The impedance of electrode reactions under polarization with direct current (idc) is theoretically analysed for the reaction where a first- or second-order chemical reaction on the electrode surface follows the charge-transfer process: A → νB + n e−, B → C. Discussion is developed upon the dependence of impedance on ac frequency and dc cd or electrode potential, for the cases of either of the above steps being rate-determining. It is shown that the plot of reactance (1/ωCs) υs resistance (Rs), which are the components of impedance in the series equivalent circuit, gives a semi-circle on the complex plane, irrespective of the rate-determining step in the reaction. The value of RT/αnF, the rate constant of the surface chemical reaction, and also the double-layer capacitance, can be determined by analysing the semi-circle. The dependence of angular frequency corresponding to the maximum mf 1/ωCs on idc can be taken as a criterion for the determination of the rate-determining step.

Differential capacity curves of iron in perchloric acid in the presence of anions

Abstract Influences of inorganic anions on the differential capacity ( C ) vs. electrode potential ( E ) curves of iron in perchloric acid solution system have been measured by the single pulse method. Three capacity maxima for HClO 4 alone are interpreted as being due to: (1) OH −1 desorption or, alternatively, reorientation of water dipole molecules; (2) adsorption of ClO 4 poststaggered− ; and (3) adsorption of hydrogen evolution precursors. Heights of the capacity maxima and potentials at which they take place depend greatly on the perchloric acid concentration. Addition of Cl − , Br − , or SO 4 poststaggered− to the HClO 4 solution shifted the peaks and suppressed the curve over the entire potential region. Furthermore, new peaks attributable to the adsorption or desorption of added ions were observed. With l − the peaks were lacking and the curves were depressed, implying less reversible sorption of iodide ion than for the other halide ions. Adsorb ability of inorganic anions examined in these experiments was in the order: F − 4 poststaggered− 2 − − − .

Differential capacity vs. electrode potentialcurves for Fe-HClO4 in the presence of organic anions

Abstract The influence of acetate, propionate and citrate anions on the differential capacity vs.electrode potential curves of the Fe-HClO 4 system was examined. Each of these organic anions introduced a new adsorption-desorption peak, while both the water-reorientation peak and the ClO 4 − desorption peak were suppressed by the addition of the organic acids. The potential of the H-adsorption peak was shifted to more anodic values as a result of the adsorption of the negatively charged organic anions, but this peak was eventually suppressed as the concentration of the additives increased. Addition of ClO 4 − to the Fe-HClO 4 system containing acetate or propionate anions produced a broad peak. This is interpreted in terms of the co-adsorption of ClO 4 − and the organic anions. Such a peak was not appreciable in the case of citrate. The order of the degree of adsorbability of the organic anions, estimated from the extent of the suppression of C-E curves, was acetate

Anodic Oxidation of Acetate and Electroinitiated Polymerization of Acrylic Acid in Glacial Acetic Acid

氷酢酸溶媒中におけるアクリル酸の電解重合を行なった。酢酸-酢酸カリウムの系を電解すると陽極で,通常,Kolbe 反応として知られているようにエタン,二酸化炭素を主として生成し,副生成物としてメタンが生成した。この系にアクリル酸を共存させて電解すると,アクリル酸の重合物が得られた。p-ペンゾキノンによる重合の抑制効果から,アクリル酸の重合開始は,Kolbe反応の中間過程に生成するラジカル種によるものであると考えられた。アクリル酸共存によるKolbe反応生成物の電流効率の低下からアクリル酸濃度1mol/l以上の電解では,電解で生成したアセトキシルラジカルの70%以上がアクリル酸の重合反応に寄与し,残りの30%は脱炭酸してメチルラジカルになり,そのうち75%が重合反応に消費されることがわかった。また,アクリル酸濃度の変化による重合物の収率の変化から,この系における重合反応は不均一表面反応であることが推定された。さらに,電流-電位曲線の解析から,アクリル酸の電極表面への吸着が予想され,重合開始反応にかなり影響しているものと考えられた