Electroplating of CE alloy
Before attempting to apply electroplated coatings to Osprey® controlled expansion alloys (CE alloys), you are advised to discuss your plating requirements with Sandvik. The plating procedure given below is intended as an aide memoire for those who have already received a full briefing by Sandvik.
The preparatory steps in the plating of Osprey® CE alloys of silicon-aluminium, including those with a silicon content greater than 50% (by weight) silicon, are given below.
Outline of the processing steps are as follows:
- 1. Surface preparation
- 2. Application of an initial thin zinc coating
- 3. Electroless deposition of a nickel coating
- 4. Consolidation heat-treatment
- 5. Application of the final coatings by electroplating
Note on surface preparation
Good surface preparation is the key to obtaining satisfactory adhesion of the electroplated layer. Surfaces must be free from friable material and porosity. Provided that the initial electroless nickel layer is subsequently activated correctly any metals capable of being electro-deposited can then be applied.
There is no significant degradation of the surface finish due to the pre-treatment process. No blistering of the electroplated coatings has been experienced at temperatures up to 450ºC (842ºF), when the processing is correctly applied.
Cleaning and plating chemicals
The following materials have been found to be satisfactory in the coating operation but it may be possible to substitute similar commercial products. Trials should, however, be carried out to determine whether the alternatives produce a satisfactory result.
Bondal zincating solution
|Macdermid Electronic Solutions||UK (Birmingham):
US (Waterbury, CT):
|Circuitprep 40||Enthone Inc. (Cookson Electronics)||UK (Woking, Surrey):
US (West Haven, CT):
|Electroless nickel - e.g. Nichem CS||Atotech||UK (West Bromwich):
US (Rock Hill, SC): 803-817-3500
Preparation of parts and operation of solutions
A. Surface preparation
1. Wire the parts to a holder using aluminium wire.
2. Vapour degrease.
3. Immerse in Minco cleaner at 55°C (131°F) using ultrasonic agitation for 5 minutes or until all loose surface material (including residual Si-Al dust from the machining operations) is removed. Do not exceed 5 minutes of ultrasonic agitation and care must be taken not to raise the ultrasonic power to a level where it generates instant cavitation of the CE alloy.
4. Rinse thoroughly in running water.
5. Immerse in Kelco cleaner at 35°C (95°F) for 30 secs.
6. Rinse thoroughly in running water.
7. Immerse in 70% nitric acid containing 30 g/l ammonium bifluoride at room temperature for 60 - 90 secs.
8. Rinse thoroughly in running water.
B. Application of an initial thin zinc coating
9. Immerse in Bondal zincating solution at 30°C (86°F) for 20–30 secs.
10. Rinse thoroughly in running water.
11. Repeat step 7.
12. Rinse thoroughly in running water.
13. Repeat steps 9 to 12.
C. Electroless deposition of a nickel coating
14. Immerse in the electroless nickel solution for the required time. It is recommended that at least 5 microns of electroless nickel be applied to minimize porosity.
15. Rinse thoroughly in running water.
16. Rinse in de-ionised water.
D. Consolidation heat-treatment
18. Heat-treat at 200°C (392°F) for 2 hours in a protective (oxygen-free) atmosphere.
E. Application of the final coatings by electroplating
19. Wire the parts to the cathode using copper wire.
20. Immerse in Circuitprep 40 solution for 1 minute.
21. Rinse thoroughly in running water.
22. Electroplate with the required metals (gold, silver, etc) using standard procedures.