The solubility of O2 in water is zero when the water is brought up to the boiling point. I treat my all steel steamboat boiler with Tri-sodium phosphate to get Ph 9-11. This also softens and suspends scale buildup.
Layup the boiler totally full of de-oxygenated water, this is much better than an "approximately dry" boiler, and getting any boiler fully dry is, in my experience, often not realistic. No Oxygen = no corrosion. Totally filling the boiler with de-Oxygenated water excludes any air entrance.
How to do it: At the end of running, with steam still up, blowdown to clear accumulated mud, then put a steam lance into the reserve feed tank, and bring the tank contents to a rolling boil. Now you have a tank of deaerated water. Close all boiler valves, and Open a line thru the boiler feed check valve to the deaerated water tank, and let everything cool down. When the boiler cools below 212F, a vacuum is formed, the deaerated water fills the boiler water space completely, and this will protect the boiler for long periods (except for freezing!). Next time you steam up, blow the extra water back into the reserve feed tank, it is still deareated water, nearly Ideal for feedwater.