One thing that cones to mind is electrolytic corrosion. Are the brushes left in place when the generator is shut down for extended periods? I have seen impressions left from this. Though why not the inner ring?
As an ex-submarine electrician, this scares me. As long as you are passing current through the electrolyte (H2SO4 and H2O, yes?) you are generating some amount of hydrogen and oxygen. You do generate more hydrogen and oxygen on charge than discharge, but electrolysis is electrolysis...