M. Sarret

Complexing agents for a Zn–Ni alkaline bath

Abstract Zinc–nickel alloys were obtained from alkaline electrolytes. Four amines, all of which are good complexing agents for nickel, were tested to identify their effects on the deposition process and final plate. The influence of THEED, an additive commonly used with alkaline electrolytes, was also examined. The electrochemical analysis showed some differences between acid and alkaline deposition processes, but no great influence was observed for the amine type. All four amines gave a consistent nickel content throughout the range of currents tested. The main difference was obtained in the structure of the alloys since some of the coatings showed an η-phase with an unusually high Ni content.

Dependence of coating characteristics on deposition potential for electrodeposited Zn–Ni alloys

Abstract The electrodeposition of zinc–nickel alloys from ammonium chloride baths was studied on different substrates under potentiostatic and galvanostatic conditions, from low potentials/current densities where the codeposition was normal to the conditions where anomalous deposition took place. Anodic linear sweep voltammetry (ALSV) and ex situ techniques were used to characterize the coatings obtained in each condition. The results indicated that three deposition potential zones could be defined corresponding to practically pure nickel, α- and γ-phase domains. It was observed that the abrupt change in alloy composition and current efficiency that occurred during potentiostatic deposition did not correspond to the transition from normal to anomalous codeposition. In our conditions, this change always coincided with the appearance of γ-phase in the coatings.

Some peculiarities in the codeposition of zinc–nickel alloys

Abstract The electrodeposition of zinc–nickel alloys from ammonium chloride baths was studied at intermediate current densities, where some peculiarities were observed both during the deposition and during the oxidation of the alloys. In this interval, although the whole deposit had a composition corresponding to α-phase zinc–nickel alloy, electrochemical and surface analyses indicated that the deposition took place with the formation of two structures: a first nickel-like crystalline deposit followed by a second-stage typical α-phase microcrystalline structure deposit. The high hydrogen evolution on this second deposit promoted the inclusion of zinc oxidized species on the coating. During the potentiodynamic oxidation of the alloys obtained at these current densities, a reduction process was observed that was attributed to the deposition of the α-phase together with hydrogen evolution.

Properties of Zn-Ni alloy deposits from ammonium baths

Zinc-nickel alloys have been electrodeposited from a simple bath containing only zinc, nickel and ammonium chlorides. The composition, morphology, structure and corrosion resistance of the alloys obtained with this bath have been studied and the influence of some additives, normally used to reduce the stress of the deposits, has also been analysed. Among these additives it has been found that the aromatic sulfonimide does not significantly modify the deposit characteristics but improves the corrosion resistance of the alloys. Moreover, the applicability of two statistical methods to a deposition process has been tested, to obtain the main factor determining the deposit composition.

ZnNi/SiC composites obtained from an alkaline bath

Abstract Composite coatings were prepared by electrodeposition from a zinc–nickel alkaline electrolyte containing silicon carbide microparticles. Electrochemical results indicate that the alloy electrodeposition process was hardly affected by the presence of the particles, although some properties, such as the coating morphology and structure, were clearly modified. The influence of the particle concentration, stirring rate and current density on the amount of particles embedded was examined and the value of each parameter necessary to achieve the maximum carbide incorporation was determined.

Electrodeposition of zinc-nickel alloy coatings: influence of a phenolic derivative

The electroplating of Zn−Ni alloy films from a chloride bath has been studied under different plating conditions, both in the absence and presence of a phenolic derivative. Under the conditions examined, the electrodeposition of the alloys belonged to the anomalous type. The morphology and composition of the deposits varied with current density, temperature, bath composition and additive concentration. The results show that the additive modifies the structure and surface topography of the deposits to a large extent and produces smoother deposits. The corrosion resistance of the alloys has been analyzed by means of salt-spray tests.

Chemical and phase compositions of zinc + nickel alloys determined by stripping techniques

Zinc-nickel alloys were obtained from a bath with a high concentration of ammonium chloride that was previously used to prepare the alloys under industrial conditions. These industrial alloys were characterized with the usual ex situ techniques, while in the present case potentiodynamic and galvanostatic stripping methods were used. Three substrates with different characteristics were selected and in all cases the stripping techniques were shown to be a reliable method for fast determination of both the chemical and phase composition of electrodeposited zinc-nickel alloys. With the plating conditions used in the present work, the oxidation potentials of α- and η-phases coincided with those observed in other cases, while the γ-phase was oxidized at a significantly more positive potential. This fact could be attributable to a very low value of the exchange current density of zinc from this phase. Moreover, the analysis of the composition of the alloys with different thicknesses revealed that the deposition of the nickel-rich α-phase was favoured in the initial stages of the process.

Zinc-nickel coatings: Relationship between additives and deposit properties

The electroplating of zinc-nickel alloys from a chloride bath containing two brighteners (a phenolic derivative and an unsaturated aromatic compound) and a levelling agent (an aromatic carboxylate) has been studied under different plating conditions. The composition and morphology of the alloys depended on the concentration of all the additives and also on the temperature. As a general effect, these additives smooth the deposit and refine the grain size. By means of scanning electron microscopy, it was possible to classify the deposit morphologies according to the type and concentration of the additives. The resistance of the alloys to corrosion was studied by means of a neutral salt-spray test.

Corrosion resistance and mechanical properties of zinc electrocoatings

Zinc electrocoatings obtained from an industrial bath were characterized by analyzing properties such as morphology, structure, microhardness and residual stress. The influence of plating variables (additives, current density, deposit thickness) on these properties was also studied. The corrosion behaviour of the deposits was analyzed by salt-spray and electrochemical methods and a good correlation between the results obtained with both techniques was observed. For all properties studied, the most influential parameter was the presence of a tensioactive used as an additive in the plating bath, and a relationship was established between some of the coating characteristics and the corrosion resistance.

Some Considerations on the Influence of Voltage in Potentiostatic Two-Step Anodizing of AA1050

The two-step potentiostatic anodizing process of a 1050 aluminum alloy (AA1050) in an oxalic electrolyte was studied. Special attention was paid to the effect of the applied anodic voltage on the dimensions, homogeneity, and order of the anodic alumina porous layer obtained after a second step. The experimental conditions for obtaining homogeneous and ordered anodic aluminum oxide layers were defined. The effect of symmetric (the same applied voltage in the two anodizing steps) and asymmetric signals (different applied voltages in the first and second anodizing step) on the structure and geometry of the porous layer was studied. When the first anodizing voltage was higher than the second, the pore/cell ratio was greater than 1, meaning that the structure had more than one pore per cell.