Reaction Penetrating Metal Sheet
Depending on the material that the casing is made out of, you could have a mercury or gallium payload.  They both are capable of significantly weakening the structures of some metals (such as aluminum).  See the following video of gallium weakening a coke can.  Mercury can also weaken the structure, especially in conjunction with gallium as seen here.  Mercury will form an amalgam with aluminum and significantly weaken it, as will gallium.  You might conceivably have nanobots bring a payload of the two metals to weaken the structure, especially since they are not nearly as reactive with other things.

Unfortunately, this would only work if the structure you are trying to damage is primarily aluminum, but might give a starting place for non-acidic solutions.