You don't have to have electrolysis to eat away at aluminum. Cavitation causes air bubbles to come out of the water in areas with with turbulence. If you look at the area where the thermostats go into, it appears that there is a larger "open chamber" (where you have the very bad corrosion) with a restriction below, and restriction above (the actual thermostat). I think there is a problem with cavitation in that area that is aggreviating the problem. Owners of early ford power stroke diesels had the same issue with the heads corroding from the inside do to caviation in the cooling passages within the heads. The solution was a special additive and higher pressure radiator cap to keep the bubbles from coming out of solution around that certain part of the cyl heads cooling passage. This eleminated the cavitation issue, and owners stopped having problems. I think adding a liner inside this passage and epoxying up the area around the liner with JB weld would be a good solution for your problems. Use a stainless steel wire brush to clean out all the existing corrosion. you want bright shinny alum to bond the epoxy to. If necessary, use some "the works toilet bowl cleaner" to clean up the corrosion (works wonders on pontoon hulls as well ) it's a mild acid that removes the scale and corrosion but does not damage the underlying aluminum. Then find you a sleeve that will fit inside the area and around the thermostat. put some paper towels down in the water passages, then fit a piece of foam, or styrofoam board down at the bottom of the cavity. You want the sleeve to dig into this piece of foam so the epoxy won't leak past the outside of the sleeve. Then once the epoxy is hardened, you can dig out the foam and paper towels. If you have a water pressure gauge installed, then you will need to drill and tap the epoxy & sleeve to accept the barb for the water pressure gauge.