G. Bech-Nielsen

Improved corrosion resistance of pulse plated nickel through crystallisation control

When electrodeposition of nickel is used for corrosion protection of steel two aspects are important: the porosity of the coating and the resistance against corrosion provided by the coating itself. Using simple pulsed current (PC) plating, the size of the deposited crystals can be significantly smaller, thereby reducing porosity correspondingly. This usually also leads to improved hardness of the coating. Introducing pulse reversal (PR) plating, the most active crystals are continuously dissolved during the anodic pulse, providing a coating with improved subsequent corrosion resistance in almost any corrosive environment. This correlation between film texture and corrosion resistance will be discussed.

The anodic dissolution of iron—VII: A detailed kinetic model for the two coupled, parallel anodic reactions

Abstract Over the last few years it has been demonstrated beyond doubt that the anodic dissolution of iron takes place via two different reactions, I 1 at lower potentials and I 2 at higher potentials. A model for the transition between these two reactions at intermediate potentials has been developed in our laboratory and tested for a wide range of conditions. On the basis of this experience, the great amount of information regarding the I 1 reaction and the more scarce information regarding the I 2 reaction, a detailed model covering both reactions is suggested. The model takes into account the influence of the state of the metal surface (cold-worked or annealed), the influence of pH and species dissolved in the electrolyte solution (anions or molecules, which will combine with iron atoms forming reaction intermediates) and the influence of adsorption laws for reaction intermediates together with the influence of potential. It is demonstrated that the present model, interpreting the essential features of the I 2 reaction in terms of a Freundlich adsorption isotherm for an Fe(II)-intermediate, predicts some simple correlations between Tafel slope and the reaction orders wrt pH and anions or molecules. These correlations are shown to be reasonably consistent with several sets of experimental data, while the entire range for the kinetic parameters is predicted and found to be unusually wide. The model is compared with a recent model for the I 2 reaction proposed by Lorenz and co-workers. The part of the present model dealing with the I 1 reaction is in principle rather similar to the well-known Bockris-model, but deviates from the latter in some details, thereby allowing a consistent interpretation of some seemingly conflicting observations. The suggestion of a change in some cases from a Langmuir to a Freundlich adsorption of a reaction intermediate extends the range of applicability for this model to conditions of higher Tafel slopes. The theoretical basis for the interpretation of the phenomenon of coupled, parallel reactions is new and may turn out to be applicable to other electrode reactions, eg the anodic dissolution of cobalt.

The anodic dissolution of iron—III. Coverage on iron in the active and passive states in acid carboxylate solutions☆

Abstract In several electrolyte systems the current/potential curve of iron exhibits two anodic maxima. In the case of acetate solutions at rather low pH both maxima are analysed on the basis of a procedure described in previous publications. It is thereby shown that each of the maxima can be accounted for by assuming an adsorption of a layer on the metal surface. The equilibrium coverage behaviour is demonstrated by simple relations between degree of coverage, potential, pH and anion concentration. The possible composition of the layer is discussed. Analysis of the results is clear and simple only for the case of constant coverage. The type of adsorption isotherm is discussed, and it is shown that a Langmuir-type combined with some increase of Tafel slope for the uninhibited process in the range of adsorption, probably gives the best explanation of the deviations from ideal shape that are observed with the directly calculated isotherms.

The anodic dissolution of iron—V. Some observations regarding the influence of cold working and of annealing on the two anodic reactions of the metal

Abstract Under conditions, where two anodic current regions, I 2 and I 2 , were distinct in the Tafel plots of the iron electrode, the reaction orders with respect to pH were determined. In solutions containing chloride or bromid, pH ca 3–5, the reaction order for I 1 ( p 1,pH ) was close to 1, while that for I 2 ( p 2,pH ) assumed positive or negative values or even values changing from positive to negative over the pH-range. The numerical values were small, 0·2–0·3. Only lightly cold-worked iron was used in this group of experiments. In another group of experiments only involving acetate containing solutions both cold-worked and annealed electrodes were employed. Until the influence of acetate became negligible at low pH, the effect of the treatment was largely limited to dissolution rates. The characteristic parameters at pH ca 4 were: for the I 1 reaction: b 1 = 30 mV, p 1,pH = 1·7 and p 1,Ac 1 (the reaction order for Ac) = −0·7; for the p 2 reaction: b 2 = 110 to 130 mV, p 2,pH = 0·2 and p 2,Ac = −0·2 to −0·3. At lower pH the influence of acetate gradually disappeared, and higher values of b 1 and lower values of p 1,pH were then noted for the annealed metal. The I 2 reaction could not be followed here. The transition between the I 1 and the I 2 reaction was studied at pH ca 4, and some deviation from earlier experience was noted for strongly cold-worked iron.

The initial behaviour of freshly etched copper in moderately acid, aerated chloride solutions

Abstract When freshly etched samples of various types of copper were exposed in moderately acid, aerated chloride solutions, two phenomena were observed. First the corrosion potential and the pH of the solution decreased over a shorter time, then the potential increased over a long period (600–1500 min), following an s-shaped pattern. Increase in pH during the second stage was avoided using a pH-stat. The corrosion rate increased little or not at all over the entire period. A tentative interpretation of the short-term behaviour is presented with some reservation. The long-term development of the potential suggests phase formation or transformation following the Avrami pattern. By suitable derivations it was possible to fit the development of potentials to the Avrami equation. Subsequent examinations by Auger spectroscopy proved the presence of thin layers of Cu 2 O on the copper surfaces, increasing in thickness with exposure time. The dissolution kinetics can be described in terms of two parallel electrochemical reactions and a simultaneous non-electrochemical dissolution reaction.

Bulk copper electrodeposition on gold imaged by in situ STM : morphology and influence of tip potential

Electrochemical measurements were carried out simultaneously with acquisition of in situ STM images of copper electrodeposition at low cathodic overpotentials and subsequent dissolution from the underlying polycrystalline gold surfaces. The morphologies of the copper deposits were examined for correlation with features of the current-voltage diagram. Copper growth is by nucleation and formation of 3D islands. During the initial stages of bulk copper growth the potentials were fixed at selected values and a balance observed between formation of polycrystalline copper nuclei and of copper crystals. After the first cycle of copper deposition and dissolution the morphology of the polycrystalline gold surface had apparently changed into a recrystallized phase of a copper-gold alloy. At a given stage of the cycle the potential of the electrode was found to depend linearly on the tip potential. In a wide range of tip potentials the onset of copper deposition and end of dissolution showed a potential separation of 59 ± 5 mV indicating a single electron process.

The anodic dissolution of iron—VI. The behaviour of variously pre-treated iron electrodes and of cold-worked cobalt electrodes in moderately acid solutions containing iodide or carbon monoxide☆

Abstract In strongly acid, iodide containing solutions the anodic Tafel diagram of iron is characterized by a restricted. Tafel region at low anodic potentials followed by a very steep increase in current at more anodic potentials. It is shown that there is another Tafel region subsequent to the steep increase in current, and that the current level at the beginning of the steep increase is not related to the rate of diffusion of iodide ions to the metal surface. In a higher range of pH (2 to 3·5) it is demonstrated that a family of variously shaped Tafel diagrams (including the one obtained at low pH) may be obtained by successive changes in pH and iodide concentrations. These features are shown to be in agreement with a model of coupled, parallel reactions. Annealing or cold-working of the electrodes gives rise to different kinetic parameters for the reaction observed at low potentials, and minor changes in the parameters of the transition between the coupled, parallel reactions are also observed. The reaction order with respect to iodide is negative for the reaction at low potentials (I 1 ), but positive for the reaction observed at higher potentials (I 2 ). At low iodide concentrations the Tafel slope of the I 1 reaction is low (30 to 60 mV), and high (100 mV) for the I 2 reaction, but with increasingly high iodide concentrations this situation becomes reversed. For the I 1 reaction the reaction order with respect to pH is +2 for cold-worked iron and ca . + 1 for annealed iron, while for the I 2 reaction it is ca + 0·2 in both cases. A phenomenon similar to passivation is occasionally apparent at very high potentials. The restricted amount of data is consistent with further oxidation of a reaction intermediate (Fe(II) to Fe(III)), further uptake of hydroxyl ions and of iodide ions, but also, instead of real passivation, the possible operation of yet another dissolution process (I 3 ) at even higher potentials. Some less detailed experiments involving iron in solutions saturated with carbon monoxide (and cobalt in solutions containing iodide or carbon monoxide) gave results in general agreement with the above description for the case of iron in iodide solutions. It is concluded that the transition between the I 1 and the I 2 reaction follows a pattern, which has been shown to be a common feature for iron dissolution in the presence of several other electrolytes. The two individual processes follow much the same general behaviour revealed in other electrolytes, although extreme cases of behaviour are observed at high iodide concentrations. On the basis of these extreme behaviours a tentative mechanism for the I 2 reaction is suggested.

Pseudo-consecutive transition between parallel reaction paths exemplified by the iron electrode in aqueous, slightly to strongly acid media

Abstract The behaviour of iron in a number of electrolytes can be explained in terms of coupled, parallel reactions. The general model for this type of interaction gives rise to a number of characteristic Tafel diagrams, most of which have been observed with iron electrodes. Some of these Tafel diagrams might easily be interpreted as arising from simple consecutive reactions, but sufficient extension of the experimental range combined with precision of measurement will provide a clear distinction.

The anodic dissolution of iron—X. Etching-dependent behaviour of annealed iron in moderately acid to neutral chloride solutions

Abstract The earlier reported observations of two reaction regions (I 1 and I′ 1 ) overlapping within a range of potentials and indicating two different states, A and B, on an etched iron surface have been confirmed with Armco iron in KCl solutions, and an analysis of the kinetic data of the two reactions has been carried out. Both reactions can be accounted for by the same reaction scheme, the differences being due to different rds and adsorption patterns of an intermediate for the two reactions. The scheme is compatible with an earlier proposed model for the anodic dissolution reactions of iron and also with the type of effects which can be expected to arise from etching.

Molybdate based passivation of zinc

SummaryIn order to reduce corrosion rates, zinc plated parts are usually chromated. Recently Chromates have caused increasingly environmental concern, for both allergic effects among workers touching chromated parts and toxic effects on fish, plants and bacteria.A molybdate based alternative has been developed to replace Chromates in several passivation applications. Depending on the environment in which the passivated parts are to be exposed, the protection that this alternative treatment provides range from less efficient to more efficient as compared to Chromate.These aspects as well as issues such as; cost, stability, layer composition and test results from several different corrosion tests are discussed.