Hello everyone. My first question is Why do you want to plate Silver over Chromium? What is the final result you wish to obtain? I do electroplating for a living, and these are the questions I always ask my clients. Lets assume for a moment you wish to make the Chrome finished part, electricaly conductive for subsequent soldering and you are not that concerned about cosmetics.
Wire the part using electrical grade copper wire. Durring the process be sure not to touch the surface of the part as the oils in your skin will contaminate the surface. ALWAYS USE PERSONAL SAFETY EQUIPMENT (GLASSES, GLOVES, ETC...)
1) Strip Chrome using Reverse DC and 10 oz/gal NaOH solution. Steel cathodes are frequently used. Only takes a few seconds (20- 40). The Chrome layer is vary thin about 40 microns.
2) Rinse the part vary well in clean running water.
3) Clean again using soap cleaner (No current)
4) Rinse again in running DI Water.
5) Activate in 20%v/v H2SO4 for 30-60 seconds.
6) Rinse again. Be sure ALL ACID IS REMOVED. DO NOT TOUCH THE PART.
There are two methods to use;
1) Brush Plating. Using a cotton swab. Swab a solution of 1oz/gal Silver Cyanide solution on to the surface of the part to be coated. Coating will occure almost imediatly. continue over entire surface. Rinse with running water. REMOVE ALL TRACES OF CYANIDE, BY WASHING IN RUNNING WATER!!!!!
2) Submerge part in a 1 oz/gal solution of Silver Cyanide and apply 0.5 a DC current for about 30 - 120 seconds (more time the greater the thicknes).
NOTES: ALWAYS RINSE PARTS THOUROGHLY!!!! WHEN YOU THINK YOU'VE RINSED ENOUGH.....RINSE ONE MORE TIME!
"My spel check is not working yet"....sorry everyone....I will fix it ASAP.
What your experiencing is Ionic contamination. You should use NaOH with a SMALL Revers current to strip the chrome. Then go into your electrocleaner. Avoid using HCL. Use 10 -30 % H2SO4. Then silver plate. Just a small amout of Chromium contamination will prevent the silver from sticking.
What I have done in the past is after stripping the chrome and cleaning it was to place it back in Bright nickel tank for a few minutes. Then continue with silver plating.