coral-like structure growing on aluminum diecasted part - what causes it

[Andon]

Hello Community,

I am currently performing some field tests of aluminum (AlSi10Mg) diecasted parts, which after assembly become water pump, which ,of course, is usually always floating in not salty water.
After few weeks in a nearby lake I saw it grew many coral-like structures. I can't explain to myself why is this happening, I certainly didn't have this in the past, the water quality didn't change at all.
I am suspecting our supplier changed something regarding the surface treatment.

Any advice what might be the nature of this phenomenon will be very hepful.

Thank you!

 

[btrueblood]

Galvanic corrosion between areas of the casting that solidified in different composition to other areas. This alloy doesn't have much magnesium (.25% or so?) compared to a typical boat hull material like 5052, at 2.5% magnesium, which gives it pretty good corrosion resistance in water. Of course, higher Mg means more trouble in casting due to dross (oxidation) of the Mg.