That bay should be relatively resistant to moisture intrusion, Steve. Perhaps the failed filler/cap neck resulted in some fluid presence where it shouldn’t be, but the vapor point and pH of that I’d not think a corrosion threat, but maybe so. Be certain that servicing techs cleaned up any spilled fluid in that compartment. Check that the bay door seals properly when closed. The Hydronics unit gets warm and can vaporize fluids or water present in the bay proper., so it can get to the control boards.
There is a product called Corrosion-X that can be used on basic electrical connectors and buss’, but I’m not sure I’d use it on circuit boards because it’s protective residue might be problematic for future repairs and tests. Even in our formerly leaking front electrical bay that was often flooded, circuit boards didn’t corrode but barrel connectors, screws, and fasteners did. The circuit board in our HydroHot bay has been clean and dry for 16 years.
Another protective product I use on finer connections and circuit board repairs is “Deoxit”. It comes in various forms for certain uses, but for just protective care not requiring pre-cleaning I’d use Deoxit Shield. Spray cans, bottles, and pens aren’t cheap, for sure, but a little can go far if sparingly used; which is why I prefer the brush in a bottle version so I can delicately just dab at only metals and solder points.
https://caig.com/deoxit-shield-s-series/Check for sales online, and be aware that the micro-brush bottle is quite a bit tinier than its photo reveals. Too small for me.
Joel