Hello, I'd greatly appreciate any guidance from an experienced professional. In doing research for a prototype tube boiler, I'm having trouble understanding the behaviors of various materials while in service as boiler tubes. Perhaps I should start with what I am seeking in boiler tubes and then follow with a few questions. I'd like to find the most resilient material that will last the longest, regardless of the cost. It seems specific inconel alloys may be best to fulfill those properties. In researching stainless steel, it seems it's not used as often as one would think, and I still do not quite understand why. It seems one of the main reasons is due to stress corrosion cracking and failure due to chloride contamination. I simply do not understand why this is such a big issue. Should it not be easy to simply use a source of water free from chlorides, even if it is distilled or deionized? The other thing I do not understand about stainless is why dissolved oxygen would be an issue. In carbon steel and iron tubes oxygen would obviously lead to corrosion, but isn't the point of stainless steel to be passivated with oxygen to form a protective layer? Does this mechanism not work at all at elevated temperatures? One would assume if using stainless steel boiler tubes, you would actually want oxygen in the water to help maintain the protective layer. This also does not address fireside corrosion issues, which may be due to gases and elevated temperatures. For that, it seems inconel alloys are well suited.

Another question is water treatment. Are there any materials that can handle just pure distilled water, like inconel or stainless? What about for a condenser? If distilled water will tend to corrode over time, what simple water treatment would mitigate this effect? I'd prefer not to just hear about some brand name treatment, but would like to understand what specific liquid or mineral would be best to use and why.

I'm looking to create a boiler tube system that will run on wood or seed oils that will last and not need replacing for years, if not decades. The max pressure would be around 400 psi, but it could be lower. I'd like to find the right balance between material, temperature, fuel source, and water source or treatment that will be within the right conditions to preserve the material while in service. If anyone can point me in the right direction to gain more knowledge on this subject, or to specific ASME standards or others, I would greatly appreciate it. Thank you.