Stoker-man's recomendation to control the moisture is the key. You might already know this so please humor me. The trick is to keep the acid from forming, or being created. The acid doesn't exist the ash unless water gets to it. If you can brush/vac out as much ash as possible and can keep the moisture away from the surfaces the acid can't form. In short, sulfur(from ash) + water = acid.
If you make up a soda washing solution, a large percentage of the solution is water. If you don't get the soda
concentration where it needs to be
in the right concentration
(corners, welds ...) the water in the solution causes the acid to form, the acid remains to do its bad work on the metal surface unbeknown to you until you see the results. It works but it has to be done correctly. The acid may have never formed otherwise. Humidity/water contact can be controlled. Keeping the boiler running low or using the light bulb heat trick keeps the metal surfaces above the dew point so humidity won't condense on the surface and form acid with the remaining ash. Spraying with a rust inhibitor product forms an air barrier, plus what ever additives it's formulated with, to help keep the condensing water from making contact with the sulfur. No acid formation, no problem. You know how much moisture you have in you boiler room but you have to make the easiest/right choise to keep it in check. Good luck!
Edit: More on washing in this thread:Cleaning with baking soda/limestone