What is this gunk?

[Peel]

Originally Posted by Boatyarddog

I'd bet dollars to donuts...

Unfortunately that's not as good a bet as it once was. Most donuts these days (at least the good ones) are a going to cost you more than a dollar.

[Lepke]

You have at least 2 dissimilar metals and water carrying dissolved iron. Leaks over time have deposited the rust when the water evaporates. Clean it up, check the aluminum for corrosion. Do the engines have zincs?
Anywhere you have a salt water system where there are dissimilar metals, you need protection and bond the various metals together. Unprotected metals, separated by hose, need separate protection or bonding to a protected metal in the system.

[MicHughV]

with all that dissimilar metal there and a salt water environment the alum could be acting as a sacrificial anode. the problem seems to be originating at the welds, a microscopic pinhole or any kind of impurity in the metal or welding rod/wire is enough to start the process.
It doesn't look look like normal "rust" as one would associate with steel. I think " rust" is the wrong term here.

[thinwater]

Originally Posted by boatpoker

Where do you see iron ? I see only brass fittings into the aluminum spuds on the tank.

PS. What the heck is "iron bacteria" ?

Really? A boat poker that didn't know that one?
https://en.wikipedia.org/wiki/Iron-oxidizing_bacteria


Yes, that is what I'm seeing. In fact, there may be very, very little iron in the "gunk."

[thinwater]

Originally Posted by boatpoker

Read that more carefully. It refers to dissolved iron ... like the iron in your blood. Nothing to do with corrosion.

No. You need to read further into the topic. Bacterial induced corrosion is one of the leading causes of fuel tank failure.



https://en.wikipedia.org/wiki/Microbial_corrosion


In fact, there are several things going on at the same time, so most of the posters are at least partly right.

[MicHughV]

I think it's clear that the general consensus appears so to be bacterial corrosion, and I go along with that.
The question is where, how and why did it originate in that location.....followed by...what to do next.
As a retired structural marine engineer, I've seen my share of the weird and unexplained and I'm curious to know where do we go from here.