1. Characteristics
(1) BH-88 is an environmentally acceptable chromium-plating technology without hexavalent- chromium pollution.
(2) The plating bath is based on sulfates, preventing equipments from corrosion and eliminating the hazard of gaseous chlorine to personnel when using a chloride system.
(3) The plating bath has good covering power and the produced coatings feature high brightness and good corrosion resistance.
2. Formulation and technological conditions
Optima
Conducting salt 280-330 g/L 330 g/L
Starting agent 12 0-150 g/L 140 g/L
Auxiliary agent 9-12 ml/L 10 ml/L
Wetting agent 2-3 ml/L 3 ml/L
Temperature 45-55°C 50°C
pH 3.0-3.7 3.5
Cathodic current density 3-10 A/dm2 5 A/dm2
Duration 2-5 min 3 min
Potential below 12 V
Agitation slight air agitation
Filtration circulating filtration
Air draft required
Anode DSA coated anode
3. Equipment required for starting
(1) Rectifier power supply
(2) PVC lined steel tank or PP electroplating tank
(3) Filter (continuous adsorption treatment with activated carbon is highly recommended)
(4) Titanium heating tube or quartz heating tube
(5) Medium-intensity agitator with air treated by oil removal
(6) Exhauster and automatic temperature-control device
(7) pH meter
4. Preparation of plating bath
(1) Fill a clean tank with 10% (volume fraction) sulfuric acid, release the sulfuric acid solution after a few hours, and wash the tank with pure water.
(2) Add pure water in an amount of 60% of required volume, heating to ca. 65°C.
(3) Add accurately-weighed conducting salt under stirring. Add starting agent with a calculated amount after the complete dissolution of conducting salt. Add pure water to nearly final volume, stirring well.
(4) Add a solution of 20% NaOH very slowly under intensive stirring while maintaining temperature at 45-50°C (the pH of the solution is not allowed to be higher than 3.8, otherwise the performance of the solution is reduced). Fill up to final volume with pure water.
(5) Hold temperature at ca. 55°C for at least 12 hours.
(6) Add auxiliary agent and wetting agent as required and stir well.
(7) Apply an electric current with a density of 4-6 A/dm2 for at least 2 Ah/L before normal use of the plating bath. Check the pH and adjust to 3.5 with 20% H2SO4 or 20% NaOH solution.
5. Roles of additives and make-ups
(1) Conducting salt: ensures the conductance of the bath and stabilizes pH. Its concentration is determined by analyzing the conducting salt used or by examining the solution density.
(2) Starting agent: offers chromium and is used for starting the plating process. It should be supplied to plating bath based on the analysis of its concentration in routine maintenance. The addition of 20 ml/L starting agent increases the concentration of Cr by 1 g/L.
(3) Auxiliary agent: the carrier of brightening agent, producing bright coatings. It should be added based on analysis and controlled at ca. 1%.
(4) Make-up 1#: maintains the concentration of Cr in the range of 5.0-7.5 g/L. The suitable feeding rate is 400-800 ml/kAh.
(5) Make-up 2#: ensures the brightness of coatings. The suitable feeding rate is 80-120 ml/kAh.
(6) Make-up 3#: a complexing agent. It should be added in an amount of 5-10 ml/L in case of over-high concentration of Cr caused by the addition of Make-up 1# or by the volume lost through removing workpiece from plating bath.
(7) Wetting agent: reduces the surface tension and improves the throwing power in low current density range, maintaining the surface tension lower than 35 dynes/cm. The suitable feeding rate is 30-82 ml/kAh.
(8) Impurity-removing agent: eliminates the contamination of metal ion impurities.
6. Process flow (for reference only)
Pretreatment → strike plating → Ni plating → reclaiming → washing with pure water for 3 times → Cr plating (immersing for 15 s before applying electric current) → reclaiming → rinsing twice → washing with pure water → washing with hot pure water → drying.
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