Is this corrosion of my fuel bowls

Yes, what fuel are you using?

91 octane - from Chevron.

julian van santen wrote:

91 octane - from Chevron.

E-10 or Alcohol-Free???

What Aircraft?   What Year?

My understanding is that Chevron 91 octane can contain up to 10% ethanol.

This is a Vans RV12 that I built and completed July 2017.  Rotax 912ULS was brand new then.

The white powder residue is usually a product of water sitting on aluminum for extended periods.

I was going to suggest that you run a tank of 10% ethanol gas through the tank to eliminate any water in the system but you are already doing that.

10 gallon of E-10 gas will absorb over 6 ounces of water.

So unless you park the plane outside and your gas caps leak water in during rain events, there should not be any measurable water in the system.

Has there been a history of water in the tank sumps in the past?

A tank flush-out and new fuel filters may be in order.