Rechromating old castings
You must first clean the existing chromate off, this is best done by blasting with aluminium oxide grit (do not use sand,glass,steel or cast iron media) or boil in caustic soda (nasty). Use gloves as finger marks sometimes stops the chromating working.
Chromating can be done cold between 4 to 12 hours but if you heat the solution to about 50 degrees C (do not boil) the process takes about 15 minutes but do it in a well ventilated area. The mix will 'go active' when it is working with small bubbles coming to the surface and even produce an orange froth depending on the preparation of the casting. The colour should be dark brown to black when the process is complete,the castings must be washed well to remove the chromating liquid if you leave the castings in too long you will get a black powdery coating on the casting which will need hot water cleaning with a stiff brush to remove. After some time the mix will loose its effect especially if done hot, in which case throw away and wash the container for a fresh mix.
The information given above is our own version based on the information from, www.magnesium-elektron/com/data/downloads/ds256suo/pdf .
Bearing in mind the use of our products in a racing environment and the need to strip and crack test after any accident and at yearly intervals, to use the full recommended surface corrosion given in the Magnesium Elekton data sheet would not be economic or practical.