Effect of accelerators and stabilizers on the formation and characteristics of electroless Ni–P deposits

The effect of thiourea, succinic acid and lead acetate on the formation and characteristics of electroless Ni–P deposits obtained from an acidic hypophosphite reduced electroless nickel bath is addressed in this paper. The rate of deposition of electroless Ni–P coating is found to be a function of concentration of these additives. Thiourea accelerates the rate of deposition up to 0.8 ppm and started to inhibit at a concentration of 1 ppm. Similarly, succinic acid increases the plating rate up to 12 g l−1 and exhibit an inhibiting effect at higher concentrations of the order of 15 g l−1. Addition of lead acetate inhibits the rate of deposition even at a concentration of 0.5 ppm and the extent of inhibition is increased when the concentration is higher than 1 ppm. Addition of these additives also caused a change in phosphorus content of the deposits; lead acetate (1 ppm) tends to increase the phosphorus content whereas thiourea (0.8 ppm) and succinic acid (12 g l−1) tends to decrease the phosphorus content. The X-ray diffraction patterns of electroless Ni–P coatings obtained in the absence of additives and in presence of 1 ppm of lead acetate exhibit a single broad peak centered at 44.5◦ 2θ, indicating the amorphous nature of these coatings. The peak broadening suggests a greater tendency to form amorphous structure when lead acetate is used as the additive. In contrast, for electroless Ni–P coatings obtained from thiourea and succinic acid containing baths, besides the reflection from Ni (1 1 1) plane, a weak reflection from Ni (2 0 0) plane is also observed. The X-ray diffraction patterns of electroless Ni–P coatings after annealing at 400 ◦C for 1 h exhibit the formation of fcc nickel and bct nickel phosphide (Ni3P) phases in all the cases with Ni3P (2 3 1) as the most intense reflection. Electroless Ni–P coatings obtained in presence of thiourea and succinic acid exhibit a nodular feature with a typical cauliflower like structure. The size of the nodules is relatively less in the latter case. In contrast, the electroless Ni–P coating obtained in the absence of additives and in presence of 1 ppm of lead acetate is relatively smooth. However, the deposit obtained in the absence of additives reveals the presence of fine particulates, attributed to the precipitation of Ni3P phases in the absence of stabilizers. The DSC traces of electroless Ni–P coatings exhibit a single well-defined exothermic peak in the temperature range studied in all the cases, which could be attributed to the precipitation of metallic nickel phase and formation of nickel phosphide (Ni3P) phase. The variation in the peak temperature and the energy evolved during the phase transition is due to the slight variation in the phosphorus content caused by the addition of thiourea, succinic acid and lead acetate. The study recommends that the choice of accelerators and stabilizers should be made only after a careful study

Pulsed electrodeposition of nanocrystalline Cu–Ni alloy films and evaluation of their characteristic properties

The preparation of nanocrystalline Cu–Ni alloy films by pulsed electrodeposition process and their structural, morphology, thermal characteristics and magnetic properties are addressed. The study reveals that the film composition, lattice constant and magnetic properties of the films could be controlled by the applied current density and duty cycle. Energy dispersive X-ray analysis (EDX) confirms that the Cu–Ni alloy film has a stoichiometry of Cu0.98Ni0.02, Cu0.95Ni0.05, Cu0.89Ni0.11, Cu0.77Ni0.23, Cu0.56Ni0.44 and Cu0.38Ni0.62 that are obtained at 2.5, 5, 7.5, 10, 15 and 20 A/dm2, respectively. The X-ray diffraction (XRD) measurements confirm that all the six Cu–Ni alloy films of the present study possess the f.c.c. structure. The lattice constant is found to decrease with increase in nickel content of the Cu–Ni alloy. The crystallite size lies in the range of 15 to 46 nm for as-plated alloys and increases from 20 to 114 nm, following vacuum annealing at 400 °C for 1 h. The differential scanning calorimetry (DSC) trace indicates a broad exothermic peak characteristic of nanocrystalline materials. The vibrating sample magnetometer (VSM) study reveals that, among the six types of Cu–Ni alloy films, the films obtained at 2.5 and 5.0 A/dm2 are diamagnetic; the one obtained at 7.5 A/dm2 is weakly ferromagnetic, whereas those obtained at 10, 15 and 20 A/dm2 are ferromagnetic. The saturation magnetization increases with increase in nickel content of the Cu–Ni alloy film

Corrosion resistance of electroless Ni–low B coatings

The corrosion resistance of electroless (EL) Ni–low B coatings, obtained using an alkaline borohydride-reduced electroless plating bath, with varying concentrations of NaBH4 (0?2–1?0 g L21), in 3?5%NaCl, was evaluated. The rate of deposition, boron content and the size of the nodules of the EL Ni–low B coatings were increased while the crystallinity of the coating was decreased with increasing concentration of NaBH4. The change in chemical composition and decrease in crystallinity did not seem to have any influence on the corrosion resistance of the EL Ni–low B coatings of the present study, as opposed to the nodular growth with a columnar structure which had a profound effect. The results of polarisation and electrochemical impedance spectroscopy (EIS) studies confirm penetration of the corrosive medium through the columnar nature of the coating and ascertain its dominating influence on the corrosion resistance of EL Ni– low B coatings over other factors. The results of the present study again confirm the poor corrosion protective ability of EL Ni–B coatings and justify the selection of EL Ni–P coatings for applications that warrant high corrosion resistance

Evaluation of the corrosion resistance of electroless Ni-P and Ni-P composite coatings by electrochemical impedance spectroscopy

Electroless Ni-P composite coatings have gained a good deal of popularity and acceptance in recent years as they provide considerable improvement of desirable qualities such as hardness, wear, abrasion resistance, etc. The disagreement among researchers on the corrosion behaviour of these coatings warrants a thorough investigation. Among the various techniques available for the determination of corrosion resistance, electrochemical impedance spectroscopy (EIS) is considered to be superior as it provides not only an assessment of the corrosion resistance of different deposits but also enables the mechanistic pathway by which the deposits become corroded to be determined. The present investigation focuses on the evaluation of the corrosion resistance of electroless Ni-P and Ni-P-Si3N4, Ni-P-CeO2 and Ni-P-TiO2 composite coatings produced using an acidic hypophosphite-reduced electroless nickel bath, using EIS. The study makes evident that the same fundamental reaction is occurring on all the coatings of the present study but over a different effective area in each case. The charge transfer resistance of electroless Ni-P and Ni-P composite deposits are in the range 32,253–90,700 Ω cm2, whereas the capacitances of these coatings are in the range 11–17 µF/cm2. The improved corrosion resistance obtained for electroless Ni-P and Ni-P composite coatings is due to the enrichment of phosphorus on the electrode surface, which enables the preferential hydrolysis of phosphorus over that of nickel. The better corrosion resistance obtained for electroless Ni-P composite coatings can be ascribed to the decrease in the effective metallic area prone to corrosion. Among the three electroless Ni-P composite coatings, the corrosion resistance is in the following order: Ni-P-CeO2=Ni-P-Si3N4>Ni-P-TiO2

Preparation and characterization of Nanocrystalline nickel based alloy deposits

Nanostructured materials are experiencing a rapid development in the last two decades due to their potential applications in a variety of technological areas such as electronics, aerospace, catalysis, ceramics, magnetic data storage, biomedicine, etc. Electro- and electroless deposition processes have received much attention in the fabrication of nanostructured materials due to their unique advantages, high deposition rate, simplicity in operation, cost-effectiveness and controllable patterning. The present thesis addresses the synthesis and evaluation of the characteristic properties of Ni-P and Ni-B alloy deposits obtained by the electroless deposition technique, Cu-Ni and Co-Ni-Fe alloy deposits obtained by the pulsed current electrodeposition technique and Ni-Mn-P alloy deposits obtained by direct current electrodeposition technique. These nickel based alloy systems are chosen because of their unique characteristics, better magnetic properties and good corrosion resistance. These nickel based alloy systems also find widespread industrial applications. An attempt is also be made to synthesize CoNiFe nanorods using anodized aluminum oxide membrane as template by pulsed current electrodeposition technique. These nanorods will find application in the area of magnetic storage devices

Deposition of electroless Ni–P graded coatings and evaluation of their corrosion resistance

Electroless Ni–P coatings provide high hardness and excellent resistance to wear and abrasion. The present work aims to study the formation of electroless Ni–P graded coatings, with varying nickel and phosphorus contents of the individual layers and to evaluate their corrosion resistance by polarization and electrochemical impedance spectroscopic studies. The possibility of preparing electroless Ni–P graded coatings by sequential immersion in three different plating baths is discussed. The study reveals that electroless Ni–P graded coatings offer better corrosion resistance than non-graded Ni–P coatings

Formation of electroless Ni–B coatings using low temperature bath and evaluation of their characteristic properties

The formation of electroless Ni–B coatings obtained using a low temperature bath and evaluation of their characteristic properties are addressed in this paper. An alkaline bath having nickel chloride as the source of nickel and borohydride as the reducing agent was used to prepare the electroless Ni–B coatings. The influence of concentration of sodium borohydride in bath on the plating rate and the nickel/boron content of the resultant Ni–B coatings was studied. Selected coatings were characterized by X-ray diffraction (XRD), differential scanning lorimetry (DSC) and vibrating sample magnetometer (VSM), respectively, for assessing the phase content, phase transformation behaviour and magnetic properties. XRD patterns reveal that the structure of electroless Ni–B coatings in as-plated condition is a function of the boron content of the coating: higher the boron content, greater the amorphous nature of the coating and vice-versa. DSC traces exhibit two exothermic peaks around 300 and 420 °C, corresponding to the phase transformation of crystalline nickel and Ni3B phases at 300 °C and the transformation of a higher phase compound to Ni3B at 420 °C. VSM studies indicate that the magnetic properties of the coating is also a function of the boron content of the coating: higher the boron content, lesser the saturation magnetization