Characterization of zinc±nickel alloys obtained from an industrial chloride bath G. BARCELO Â , E. GARCI Â A, M. SARRET, C. MU È LLER* Laboratori de CieÁncia i Tecnologia ElectroquõÂmica de Materials (LCTEM), Dpt. QuõÂmica FõÂsica. Universitat de Barcelona, MartõÂ I FranqueÁs, 1, 08028-Barcelona, Spain J. PREGONAS PREMA S.A. Company. Benages 17, 08786-Capellades, Spain Received 27 October 1997; accepted in revised form 10 March 1998 Zinc±nickel electrocoatings obtained from an industrial bath containing two additives were char- acterized by analysing their morphology, structure, microhardness, residual stress and corrosion resistance. Changes in deposit characteristics following the addition of saccharin were also studied. All coating, with or without saccharin, presented a good appearance, good mechanical properties and a high corrosion resistance. Moreover, by varying certain plating conditions it was observed that the deposits maintained their properties over a wide interval of experimental conditions, which suggests their suitability for use in batch bath. Keywords: zinc±nickel electrocoatings, characterization, electrochemical corrosion, residual stress 1. Introduction In recent years, several attempts have been made at preparing zinc coatings with high corrosion resis- tance. In so doing, zinc alloys with the group 8 metals have been developed [1±3] and, among theses, zinc± nickel alloys have received most attention because of their high degree of corrosion resistance and their mechanical properties. Although slight variations have been reported, it is accepted that the highest corrosion resistance of zinc± nickel alloys can be obtained with a nickel content in the 10±15% range and various electrolytes have been reported as depositing Zn±Ni coatings with this composition [4±8]. In previous studies of this group, zinc±nickel electroplates with the desired composition and good corrosion behaviour were obtained from an acid bath containing a high concentration of NH 4 Cl [9]. However, ecological considerations have led us to design a new plating bath without complexing agents to obtain zinc±nickel coatings in a batch bath and with commercially attractive properties. Our ®rst objective is to characterize the zinc±nickel alloys ob- tained with this new acid bath by analysing their composition, morphology, structure, corrosion resis- tance and certain mechanical properties (microhard- ness, residual stress). Since most of the characteristics of the electrocoatings are structure dependent and the structure, in turn, is determined by the plating con- ditions, our second objective is to attempt to establish the relationship between electrodeposition parame- ters, structural properties and functionality. 2. Experimental details The zinc±nickel coatings were deposited under gal- vanostatic conditions at 200 A m )2 up to a constant thickness of 10 lm, using a chloride industrial bath (1.0 mol dm )3 in Zn 2+ and 0.25 mol dm )3 in Ni 2+ ) at pH 5.2 and 28 °C. The basic electrolyte contained two additives (7.0 g dm )3 of a tensioactive and 0.2 g dm )3 of an aromatic aldehyde) and saccharin (2.0 g dm )3 was added in a number of cases to ana- lyse the eect of the latter on deposit properties. In addition, alloys obtained with small variations in these basic conditions (10% of temperature, pH, bath concentration) were also characterized to de- termine the properties of the alloys that might be obtained under these conditions in industrial plant. A range of current densities in the interval 50± 500 A m )2 and of deposit thicknesses, from 5 to 30 lm, were used in studying the in¯uence of these parameters on the characteristics of the alloys. The experimental set-up for the plating processes was as described in [9]. After deposition, some of the coatings were chromated by immersion for 25 s at 25 °C in a chromating solution, before being rinsed in deionised water and dried in warm air. Deposit morphology was examined by optical microscopy (OM) and scanning electron microscopy (SEM) and ®lm composition by atomic absorption spectrophotometry and XPS techniques. X-ray dif- fractograms were obtained on a Bragg±Brentano Siemens D500 diractometer using CuK a radiation (0.1542 nm) and the analysis of texture and the de- termination of microdeformations and the size of the coherent diracting domains (CDD) were carried * To whom correspondence should be addressed. JOURNAL OF APPLIED ELECTROCHEMISTRY 28 (1998) 1113±1120 0021-891X Ó 1998 Chapman & Hall 1113