All steels corrode under the conditions prevailing on the outer pipe surfaces in a boiler: 20-600°C + air + chlorides + water.Steels in boilers of garbage incineration plants corrode particularly quickly because the incineration of garbage produces extremely much of different chlorides.However, stainless steels corrode differently than non-alloyed steels (low carbon steels): stainless steels tend to pitting corrosion and crevice corrosion on welds. This type of corrosion is much more dangerous than a large-scale corrosion of a non-alloyed steel and can hardly be calculated.Especially for this reason (not predictable corrosion process) stainless steels are used very little in the boiler industry. There is also a high price of stainless steels.A protective layer of rust slows down the corrosion of steels, so corten steel corrodes slower at the beginning than the same steel without rust. Unfortunately, with long-term operation, this difference is negligible.
Corrosion condition in boilers is stongly dependent to nature of fuels used and furthur products over heating surface. So many factor may affect corrosion by several mechanisms mostly in form of chloride and sulfate attack in all phases. It is essential to use a material could withstand against corrosive agents at desired temperature and environment.As far as I know, higher than 350-400 corrosion damage acclerate significantly. Therefore, a material with ability to form a protective/integrate/fully covered oxide layer should be considered in designation.For instance, dense-chromia forming material could witstand against chlorine attack in oxidising environment up to 550-600 C where presense of deposites including alkali chlorids lead to breakdown of protective chromia scale by formation of chromate. Furthremore, highly alloyed Ni-based materials are more resistant material compared to Fe-based ones. I assume that fromed scale over weathering steel including A 588 & A242 mostly Fe2O3 which is less protective compared to Cr2O3 or Al2O3.In addition as mentioned by Mr. Vadim, cost issue is another decisive role in material selection. Ni based material or dense protective oxide layer forming material are more expensive ones but less susceptible to corrosion damages.