Nickel electroplating of metals is one of the most important industrial processes in the area of metal finishing. Organic compounds in small concentrations are added to nickel plating baths for a variety of purposes as for example, improving their macro- and micro-throwing power (TP), which is essential from an economic point of view as well as assuring the performance of the coatings. Among these organic compounds, certain leveling agents and brighteners have found extensive use in the galvanotechnic industry.1 They are all able to fill in macroscopic scratches, to fill in the microvalleys (the recesses) rather than add to the micropeaks, to reduce the grain size of the deposits and to give a mirror-like luster to the surface of the specimens. A number of papers have been published on the improvement of different properties of nickel electrodeposits from Watts baths with or without organic additives.2-6
In this work an attempt was made to use the cationic dye Nile Blue (C.I. 51180) as an additive in the Watts bath. The dye is an oxazine derivative which has a characteristic positive charge, a fraction of which is on the two auxochromes.7 It moves under the influence of the electric field, at rates depending on the applied voltage, to the cathode, which is anodized aluminum, penetrates the oxide layer and it is localized mainly at the bottom of the pores, according to the Buchmeier-Brodalla patent.8 Besides, it was found in the paper of Farmer and Muller9 in our previous papers10-12 that it influences the electrochemical reactions during electrolytic coloring processes.
The influence of the dye Nile Blue (NB) during the electroplating of nickel on brass was investigated under the optimum working conditions (i.e., voltage, bath agitation, appropriately elevated temperature, cathode-anode separation and Watts bath composition). Potentiostatic, galvanostatic and potentiodynamic electrochemical methods were used for this purpose. Scanning electron microscopy (SEM) with EDAX facilities was also used in order to study the quality and the grain size of the deposits. The thickness of the nickel deposits was measured by the use of x-ray fluorescence (XRF) and a Fischer Permascope ES (magnetic type). An Erichsen mini-Glossmaster was also used for the measurement of the brightness of the deposits.
Brass specimens (alloy 65% Cu - 35% Zn) of dimensions 3 × 5 cm underwent the following pretreatments: immersion in a 1:1 HNO3 solution for 60 sec at room temperature, degreasing in acetone for 60 sec at room temperature and etching in a solution containing 85% H3PO4 and 15% HNO3 for 30 sec at a temperature of 80°C.1 After rinsing in deionized water and drying in a stream of air at room temperature, the specimens were stored in a desiccator.
Nickel was plated from a Watts type bath containing 270 g/L NiSO4•6H2O, 70 g/L NiCl2•6H2O and 40 g/L H3BO3 in deionized water. The simple Watts bath as well as Watts baths containing different amounts of the additive Nile Blue A Sulfate (C.I: 51180) were studied.
Nickel coatings were deposited under potentiostatic or galvanostatic conditions, using a constant voltage (3.0 V) or constant DC current (40 mA/cm2) applied by a computer-controlled DC power supply (Delta Elektronika E 015-2), while the current-time or voltage-time transients, respectively, were recorded by the computer via a multimeter (Keithley 2000, ±0.025%), at 50±0.5°C under agitation with a magnetic stirrer (SB-162 Stuart). For five measurements the standard deviation of the method was ±2%.
As working electrodes, the pretreated specimens were used. A nickel metal plate (99.9% purity) was used as the anode. Both electrodes were vertically set in a cell 500 mL in volume.
The pH of the Watts bath was adjusted to 3.5 (if it was necessary) by addition of either concentrated H2SO4 or NH4OH. It was measured by a pH meter (827 lab Metrohm).
Potentiodynamic measurements were conducted with an EG&G Μ352 Versastat Galvanostat/Potentiostat in Watts baths with and without the additive, with a scan rate of 25 mV/sec, at pH 3.5 and temperature 50 ± 0.5°C. The reference electrode (Ag/AgCl) was connected in a convenient cell with a working electrode area 3.14 cm2 using a Luggin capillary and two rods of graphite as counterelectrodes.
SEM observations of the surface of the pretreated specimens before and after electroplating for 1.0, 3.0 and 20 min were conducted using a JEOL JSM-5600 SEM with EDAX facilities (Oxford Link Isis 300 EDS system) with the suitable software (Oxford SEM-Quant). The standard deviation of the quantitative analysis of the specimen surfaces was ±15%.
An Erichsen Mini-Glossmaster 507-M 60° (±2%) was used for the measurement of the brightness of the nickel deposits, according to DIN 67530, ISO 2813, BS 3900-D5, ASTM D523-08 and ELOT 725. The standard deviation of the method, using three experimental measurements, was ±5%.
XRF was used for the measurements of the thickness of thin nickel deposits (0.5 - 5.0 μm).13 A Helmut Fischer Permascope ES was used for the measurement of the thickness of thick nickel deposits (8 - 30 μm).
The cathodic current efficiency CCE% was calculated from the Faraday’s equation: