Study of the electroplating mechanism and physicochemical proprieties of deposited Ni-W-Silicate composite alloy
Introduction
The electrodeposition of Ni-W alloys was anomalous in nature, with the co-deposition resulting in a higher amount of Ni in the final deposit [1], [2], [3]. Therefore, deposit characteristics of Ni-W as compared to the conventional nickel include benefits of extended corrosion resistance and significantly harder coatings. Moreover, the presence of tungsten imparts a good barrier resistance to the coating. However, most previous papers reported that the increase of W content into layer produce amorphous structure. Indeed, crystal lattices of films could be distorted due to larger atomic radii of W (0.1270 nm) which substituted the Ni atom with the atomic radii of 0.1245 nm. This crystal structure increases the appearance of pores and imperfections on final Ni-W coating [3], [4], [5]. Attempts were made to enhance the Ni-W alloy compactness by either introducing adsorbed species in the bath or by developing a ternary alloy [6], [7], [8].
The use of additives in electrodeposition bath is extremely important due to their influence on the growth and structure of the resulting deposited films. The presence of Diethanolamine and Triethanolamine such as additives during Ni-Zn electroplating had been shown to influence physical and mechanical properties of the electrodeposits (grain size, brightness, internal stress, pitting and even chemical composition) [9]. In addition, the codeposition of phosphorous along with Zn–Ni improves the corrosion [10] and hydrogen permeation [11] characteristics of the coating.
On the other hand, composite coatings based on silicate were also quoted to provide a higher barrier resistance and better stability than others composites [12], [13], [14], [15]. Previous studies [12], [13], [14], [15] had shown that the presence of silicates into the electroplating bath of zinc allowed the formation of a physical barrier preventing the penetration of aggressive ions. Moreover, the reactions of the film formation were reported incomplete during electroplating and continued during immersion in NaCl solution resulting in a decrease of the corrosion rate [16]. Furthermore, the addition of silicate content into electrolyte bath during DC electroplating of Ni-Zr coating increase its wear resistance [17]. While there is wealth of information on the effect of Silicate species on the electroplating of various metals specially Ni, information on Ni-W electroplating is absent. Hence, it was felt that it would be interesting to carry out a comprehensive study in an extended area, regarding the effect of sodium silicate addition in the plating bath on the properties of the produced Ni-W coatings.
Thus, the objective of the current work is to study the effect of Silicate addition on the electroplating mechanism and corrosion behavior of Ni-W alloy obtained from citrate-ammonia bath and electrodeposited onto copper substrate.
Section snippets
Optimization
The pH of the electroplating bath and silicate content (CSil) are very important to synthesize a resistant composite layer against chloride ions penetration and make the electroplating successful in the shortest time possible.
A Doehlert design, with 9 experiments for 32 (3 levels and 2 factors), was carried out as a screening approach to optimize those experimental conditions using NEMROD software program (L. P. R. A. I, Marseille, France). The polarization resistance values Rp of Ni–W alloy
Results and Discussion
Fig. 1 translates the evolution of Rp of Ni-W-Sil composite coatings with the interaction between Silicate content (CSil/mg L−1) and pH of the electroplating media. The exam of Fig. 1 shows that optimum conditions relative to the highest Rp values were CSil =5 mg L−1 and pH = 6,2 ±0,1. Based on this result, we kept these values constant in all electrodeposition experiences.
Conclusions
The present study shows that Ni-W composite coatings can be electroplated from silicate-citrate-ammonia media to improve the morphology and the corrosion behavior of copper substrate.
- 1)
The adsorption of silicate does not only happen through the layer but also on copper surface before the beginning of electroplating which decreases the dissolution rate of Ni-W coating.
- 2)
Ni-W coating presents a rough and porous aspect with big pores and imperfections, while Ni-W-Sil composite layer reveals smooth and
Acknowledgments
The authors would like to acknowledge the financial support provided by “Action Intégrée Franco-Tunisienne du Ministère des Affaires Etrangères et Européennes français et du Ministère de l’Enseignement Supérieur, de la Recherche Scientifique et de la Technologie tunisien”.
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