Joachim Walter Schultze

Structure and properties of plasma-nitrided stainless steel

Abstract A series of plasma-nitriding experiments has been conducted on AISI 304L austenitic stainless steel at temperatures ranging from 400 to 600 °C using a pulsed d.c. plasma with various pulse duration/repetition ratios in an N2H2 gas mixture. The structure and composition of the plasma-nitrided surface layer were analysed by means of X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), optical microscopy and microhardness testing. The corrosion behaviour of the S phase was also investigated. The maximum Knoop hardness after plasma nitriding is about 1400 HK 0.01 and the maximum thickness of the compound layer is 34 μm. XRD patterns show that the surface layer consists of the S phase only. The corrosion performance was tested in 0.05 M Na2SO4 solution at pH 3.3 and in neutral 3.5% NaCl solution at ambient temperature. Potentiostatic and potentiodynamic experiments yielded slightly higher passive corrosion currents for plasma-nitrided 304L. Pitting corrosion in neutral electrolytes containing chloride ions was observed only for untreated 304L. The passive layers formed on both types of samples were similar in constitution and thickness as determined from XPS sputter profiles.

Principles of electrochemical micro- and nano-system technologies

Abstract The paper gives an introduction to this Special Issue and corresponding book ‘ Electrochemical Micro- and Nano-System Technologies’ (Elsevier). Interdisciplinary aspects are demonstrated by microgalvanics, engineering, electrochemical materials science, electroanalysis and biology. Moreover, the continuous scaling down to molecular systems is described. Experimental parameters like topography, current densities, field strengths and hydrodynamics are characterised in double logarithmic plots. Types of reactions and materials properties determine the possibility of localisation and production rate. The EMT-number is introduced to differentiate maskless processes: delocalising 2D conditions (EMT>1) can be distinguished from localising 3D processes (EMT≪1). Nanoscopic localisation can be achieved using microelectrode arrays or micromechanical devices. Microscopic local elements play an important role in unwanted corrosion or intended structuring like phosphating. Examples of technical EMST applications are given and, for the case of phosphating, characterised in a flow-diagram and the Pourbaix-diagram.

Growth of polyaniline films under pulse potentiostatic conditions

Abstract Strong activation of the growth of polyaniline (PANI) films is found to occur when a pulse potential programme consisting of periodic cathodic and anodic rectangular pulses is applied. The potential interval for the cathodic pulses where growth activation is observed coincides with the potential interval where the polymerized material is almost fully reduced but is still conducting. It is found that the pulse programme does not influence the formation of the first nuclei but markedly affects the early stage of growth of the PANI films, i.e. the stage where different processes such as oligomerization, further nucleation and growth up to the formation of a full coverage layer take place on the electrode surface. The further growth of the compact PANI layer seems to be unaffected by the pulse procedure. It is shown that the activated polymer growth should not be related to the influence of the pulse procedure on the overoxidation (termination) reaction of the polymer chains. Different reasons for the activated PANI growth observed under pulse conditions, such as continuous redistribution of the redox centres in the PANI nuclei, periodic stretching and relaxing of the polymer chains etc., are discussed briefly.

EQCM study of electropolymerization and redox cycling of 3,4-polyethylenedioxythiophene

Polyethylenedioxythiophene films were electropolymerized potentiostatically (E=1.1–1.3 V/SHE) at a gold electrode covering an EQCM from solutions containing monomer, lithium perchlorate and non-ionic surfactant (polyoxyethylene-10-laurylether). The ion exchange was studied by EQCM during redox cycling at a scan rate of 20–50 mV s−1 in 0.5 M LiClO4+0.5 M LiCl+0.5 M Na toluenesulfonate solutions in the potential range −0.5<E<0.8 V (SHE). Mass versus charge curves display a hysteresis. During the reduction scan the ion content of the film is in equilibrium and is defined by the potential, whereas during oxidation the mass lags behind the charge due to slow water exchange. The reduction scan can be described by equilibrium theory supposing that about 40% of counter-ions remain bound in the film. During polymerization at E≥1.2 V, the mass gain calculated from the frequency change slows down more rapidly than is estimated from the current. This may be attributed to the viscosity of the growing film with acoustic decay length of 2.45 μm.

Single crystal experiments on grains of polycrystalline materials

Passivation on polycrystalline Zr (hcp) and Ta (bcc) depends on the crystallographic orientation of the individual grains, determined by electron back scattering diffraction (EBSD). Microelectrochemical experiments yield characteristic data, such as oxide formation potentials, formation factors, capacities, and thicknesses, of local oxide formation on single grains. Strong differences occur on Zr, for Ta only a small influence of crystallographic orientation is observed. The rate of oxide formation, determined from potentiodynamic and capacitive measurements, and EBSD pattern quality, increases with decreasing surface atom density. The dominating influence can be described by the Euler angle phi, the second angle phi2 has a smaller influence. The differences in surface atom density lead to a specific current-dependent potential drop during oxide growth and therefore to a shift in the oxide formation potential. Furthermore the crystallinity and epitaxy of the oxides can depend on the orientation of the base metal causing a variation in ion conductivity.

Electrochemical and surface analytical characterization of radiation effects after N2+ implantation into Al and Al2O3

N 2 + implantation was performed into sputter-cleaned Al and anodically oxidized Al samples ( 3 nm < d oxide < 50 nm). The energies of the N 2 + ions ranged between 3 and 200keV using doses between 2 x 10 15 and 1.8 x 10 18 N/cm -2 . Changes in surface conductivity and anodic behaviour were observed after 3 keV implantation of 2 x10 17 N 2 + cm -2 as studied by electrochemical methods and X-ray photoelectron spectroscopy (XPS). Radiation effects, such as AlN formation and gaseous nitrogen inclusions within the oxide, were monitored in situ after low energy implantation. AlN formation by reactive implantation from the oxide was only observed after a reactive A1 2 O 3-x surface was produced by the N 2 + ion beam.

Modification of oxide films by ion implantation: TiO2-films modified by Ti+ and O+ as example

Abstract Oxide films can be modified by ion implantation. Changes in the electrochemical properties of the films are due to the deposition profile of the implanted ion, ie doping and stoichiometric changes, as well as to the radiation damage. The latter is due to the formation of Frenkel defects and at high concentrations to a complete amorphization of the oxide film. TiO x -films with 1 + - and O + -ions into anodic oxide films on titanium. The electrode capacity shows always the behaviour of an n -type semiconductor with an almost constant flatband potential but a strong maximum donor concentration at about 3% Ti + concentration. Oxygen implantation, on the other hand, causes a small increase of donor concentration only at high concentration of O + , Electron transfer reactions show strong modification of the electronic behaviour of the oxide film with a maximum again at 3% titanium. Photocurrent spectra prove the increasing amorphization and show interband states 2.6 eV above the VB or below the CB. During repassivation measurements at various potentials different defects formed by Ti + - and O + -implantation become mobile. A tentative model of the band structure is constructed which takes into account the interband states due to localised Ti + -and O + -ions. The modification of ion implanted oxide films is compared with the effects of other preparation techniques.

New quaternized aminoquinoline polymer films: electropolymerization and characterization

Abstract Poly(5-aminoquinoline) films (10 to 400 nm) were prepared on gold electrodes by anodic polymerization in acetonitrile using cyclic voltammetry and potential step methods. The films were found to be smooth, homogeneous and adherent. The electrochemical quartz crystal microbalance and ellipsometric measurements proved that the polymer film thickness increases linearly with increasing anodic charge of formation. The electroactivity of the polymer films was found to be very small in acetonitrile and aqueous solutions. The impedance measurements indicated that the polymer films are very resistive, with a resistivity of approximately 0.4 MΩ cm. In sulphuric acid solutions, capacity- and photocurrent-potential curves of the polymer films show semiconducting behaviour. XPS and FT-IR indicated the presence of perchlorate anions in the polymer films as well as in the isolated product of the constant potential electrolysis. It is proposed that the polymerization proceeds mainly via coupling at position 1 (leading to quaternary ammonium cation formation) and at position 8. The exchange of the counter anions with [Fe(CN) 6 ] 4− anions from neutral solutions was found to be about 10% to 18% of the total inserted anions. With a loading efficiency of about 0.15 mol l −1 , poly(5-aminoquinoline) films can find applications in the field of fixed-mediators.

The electrosorption valency and charge distribution in the double layer. The influence of surface structure on the adsorption of aromatic molecules

Abstract Electrosorption reactions of inorganic or organic systems can be analyzed in terms of the influence of the charge of the substance, charge transfer, and dipole terms. Macroscopic and thermodynamic data of the system can be correlated with molecular data. Important parameters are electronic data of the adsorbate, and the substrate and its structure, the solvent, and the electrode potential. After a short review on inorganic systems, the analysis of thermodynamic data of phenol and pyridine is given. Then, the pyridine adsorption on various crystal planes of Au and Ag is emphasized in this paper. The estimation of dipole terms and orientation is based on the literature and recent work. It can be shown that for pyridine adsorption on gold the charge transfer increases with increasing surface indices. The analysis of the charge transfer coefficient γ, its potential dependence, and the Gibbs enthalpy of adsorption is in reasonable agreement.

Impedance spectroscopy and other electrochemical in-situ investigations of the phosphating process

Equivalent circuits for the phosphate layer were obtained from electrochemical impedance spectra. The dominant time constant is due to the transfer resistance Rt and the double-layer capacitance Cdl. An initial high frequency pseudo-inductive time constant Lpseudo/R2 is followed by a capacitive time constant R2C2 as the phosphating time increases. The results obtained on varying the reference electrode position and the bath composition suggest that Lpseudo/R2 is due to the electrical coupling between cell elements while R2C2 is due to an amorphous precipitation of FePO4 in the pores of the phosphate layer. Since electrochemical impedance spectroscopy is restricted to t > 200 s, ac conductance transients were recorded continuously to determine the coverage. Corrosion potential transients were measured simultaneously. These methods were used to estimate the effect of oxidants on the phosphating kinetics with respect to the coverage. The results obtained by the various methods were compared with ex-situ measurements from a previous study.