Mike, I had a similar problem, but the sludge color was rusty reddish brown.
My tank had been cleaned and sealed with a thick, rubbery red product long before I bought it. It eventually peeled away from the tank in large sheets (which I discovered by accident when changing a fuel sending unit). I'm not sure if ethanol caused this or not, but the end result was that I was getting sludge in my glass pre-filter bowl and a fair amount was getting past the filter and settling into the float bowl of the carb. I'm not sure if the sludge was rust particles from the tank or the disintegrated liner, or both.
The problem went away when I changed the tank.
As to your question about whether ethanol can eat away metal, I have never heard of that happening, but it can wreak havoc on many non-metal parts and coatings. And as you probably know, it attracts moisture which causes all kinds of problems including rust in the tank.
Matt
