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

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

Dezincification of Brass in Sulfide Polluted Sodium Chloride Medium: Evaluation of the Effectiveness of 2-Mercaptobenzothiazole

The present paper addresses the effectiveness of 2-mercaptobenzothiazole (MBT) in preventing the dezincification of brass in sulfide polluted 3.5% NaCl solution. The dezincification behaviour of brass in 3.5% NaCl solution containing 100-1000 ppm of sulfide ions and 1 x 10-6 to 1 x 10-3 M MBT was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, current-time transient and accelerated leaching studies. The presence of sulfide ions in 3.5% NaCl solution has caused a significant increase in the extent of corrosion attack on brass. The inhibiting effect of MBT is markedly reduced in the presence of sulfide ions in the medium. The easy replacement of Cu-MBT complex by Cu2S film on the surface of brass is responsible for the inability of MBT to prevent corrosion attack in the presence of sulfide ions. The study concludes that the effectiveness of MBT in preventing the dezincification of brass is dependent on the concentration of the sulfide ions in the medium and the effectiveness is nullified at higher concentrations of sulfide ions

Electroless Ni–Co–B ternary alloy deposits: preparation and characteristics

The formation of electroless Ni–Co–B ternary alloy deposits was studied using varying bath parameters and operating conditions. Variation in metallic ratio of the bath enables the formation of electroless Ni–Co–B deposits with varying contents of nickel, cobalt and boron. The Ni–Co–B deposits were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and vibrating sample magnetometer (VSM) to assess the phase constituents, phase transition behaviour, thermal behaviour, Curie transition and magnetic properties, respectively. The study reveals that electroless Ni–Co–B alloy deposits are amorphous in the as-deposited condition. DSC traces indicate two exothermic peaks, corresponding to the separation of nickel phase from the amorphous matrix and formation of Co3B phase. TGA studies exhibit Curie transition peaks corresponding to nickel and Co3B. VSM studies show that the saturation magnetic moment increases with increase in the cobalt content of the film and with increase in annealing temperature

Electroless Ni–Co–P ternary alloy deposits: preparation and characteristics

Electroless Ni–Co–P ternary alloy deposits were prepared by varying the metallic ratio and were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and vibration sample magnetometer. The plating rate of electroless Ni–Co–P deposits is a function of concentration of sodiumhypophosphite, pH of the plating bath, plating time and the metallic ratio. With increase in metallic ratio, the cobalt content of the deposits increases with a simultaneous decrease in the nickel content, while the phosphorus content decreases slightly. The electroless Ni–Co–P deposits of the present study are amorphous in their as-deposited condition. The DSC trace exhibits three distinct exothermic peaks, corresponding to the relaxation of lattice strain during the phase separation, the phase transformation of amorphous phase to nickel and nickel phosphide phases and the transformation of metastable phases to stable nickel phosphide phase. The XRD pattern of electroless Ni–Co–P deposits confirms the formation of Ni, Ni5P2, Ni12P5 and Ni3P phases during annealing at 300 and 400 8C for 1 h. Thermomagnetic study exhibits the Curie transition of nickel and non-stoichiometric Ni3Co based alloys. Being amorphous in nature, the electroless Ni–Co–P deposits exhibit soft magnetic characteristics. The saturation magnetization, remanence and coercivity increase with cobalt content of the deposit

Cathodic electrodeposition of zinc-zinc phosphate-calcium phosphate composite coatings on pure iron for biodegradable implant applications

Faster degradation of iron based degradable implants in physiological media, particularly during the initial stages of implantation, poses difficulties in directly using them for clinical applications. The present study aims to deposit a zinc-zinc phosphate-calcium phosphate composite coating on pure iron by cathodic electrodeposition under varying current densities (2 to 5 mA cm(-2)) at 27 degrees C for 30 min to overcome the above mentioned limitation. The composite coating was characterized for the amount of coating deposited, surface morphology, elemental composition, nature of functional groups and phase contents. The corrosion behavior of composite coated pure iron in Hanks' balanced salt solution was determined by potentiodynamic polarization, electrochemical impedance and immersion studies. The bioactivity of the composite coated iron was evaluated by immersing it in simulated body fluid. The changes in the morphological features, elemental composition and nature of functional groups present on the surface layer were used to understand the extent of corrosion attack and mineralization. The findings of the study reveal that cathodic electrodeposition enables the deposition of a uniform, highly crystalline zinc-zinc phosphate-calcium phosphate composite coating on pure iron with a plate-like morphology. X-ray diffraction measurements confirm the presence of zinc, zinc phosphate and calcium phosphate phases. The zinc-zinc phosphate-calcium phosphate composite coated pure iron offered a better corrosion resistance than the uncoated one in Hanks' solution. The composite coating also exhibits good bioactivity in simulated body fluid. The study concludes that deposition of zinc-zinc phosphate-calcium phosphate composite coatings on pure iron will be useful to overcome the existing limitation of the higher rate of corrosion of pure iron during the initial stages of implantation

Development of novel multi-functional composite coatings on titanium: Evaluation of structural characteristics, bioactivity and corrosion behaviour

A multi-functional coating approach involving a sequence of surface modification is proposed to improve the corrosion resistance and bioactivity of Ti. Development of TiO2 nanotubular (TNT) arrays on Ti using ethylene glycol + 0.4 wt % NH4F + 10 wt % deionized water at 60 V for 60 min was employed as the first stage treatment. Electrodeposition of brushite over TNT arrays using 0.1 M Ca(NO3)(2) + 0.06 M NH4H2PO4 at 1 mA/cm(2) for 45 min was used as the second stage treatment. Some of the brushite coated TNT arrays were subjected to alkaline treatment using 1.0 M NaOH at 80 degrees C for 2 h to covert the brushite into hydroxyapatite (HAP) coating. Deposition of reduced graphene oxide/polycaprolactone (RGO/PCL) composite coating by dip coating method was adopted as the third and final stage treatment. The RGO/PCL composite coating is deposited over both brushite and HAP coated TNT arrays. The coatings formed at various stages were characterized for their morphological features, phase content and nature of functional groups. The corrosion behaviour of these coatings was determined by potentiodynamic polarization studies. The bioactivity of multi-functional composite coating was determined by immersing it in simulated body fluid (SBF) solution at 37 degrees C for 168 h while their long-term stability was determined by immersing it in HBSS at 37 degrees C for 21 days. The findings of the study reveal that the novel multifunctional composite coating on Ti is promising for bone tissue engineering. (C) 2020 Elsevier B.V. All rights reserved