Aluminum castings darken a few weeks after blasting
Have a condition where aluminum 380 alloy castings are deburred with a zinc shotblast media. after 2-3 weeks castings are becoming darker in shade and customer is concerned. Does anyone have a explanation for this condition?Greg Utterback
light metal products
Wouldn't this be normal, Greg? With the exception of gold (and other precious metals), I don't think I can think of any metal that doesn't tarnish.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
My nonscientific guess is that you are leaving a very thin film of zinc on the part which will be moderately bright at first and then will darken like aged galvanizing. I would not be surprised if there was a chemical reaction going on because they are fairly far apart on the EMF chart (if I remember correctly). The attack on the aluminum most likely will depend on its environment. I would doubt if you could determine if it was zinc with spot testing. A friend or vendor with a SEM or XRF could tell you very rapidly if it was. I hope one of the scientific folks can answer question of effect on the base aluminum other than being ugly.James Watts
- Navarre, Florida
These castings, because they are being left unfinished, are oxidizing. They should be anodized .David A. Kraft
- Long Island City, New York
For my opinion you can solve the problem with two options:
A. Don't make your blast with such reactive shot like zinc , you can use some more inert media like glass or even sand.
B. Do passivation treatment after blasting . I know that this is slightly difficult but if you want more detail about such process I can help you.
YB Plating Engineering and Quality - Haifa Israel
WHEN YOU ARE BLASTING WITH ZINC YOU CREATE A SMALL REACTION BETWEEN THE ALLOYS. 380 ALUM. IS NOT TOO HAPPY WITH ZINC INTERACTION, MUCH LIKE TOO LONG OF A QUENCH TIME. THERE IS NO HELP W/ZINC! YOU SHOULD REALLY TRY WALNUT SHELLS! CHEAP AND HIGHLY EFFECTIVE, OR TRY MEDIA BLAST!Mark Floyd
gmti lester division former itt automotive
This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site