P-91 ID Rust Prevention

PMoon, Thank you. You've been most informative, and enlightening.

P=91 is not prone to rust - most times an argon purge (0% O2), A Hydrogen removal cycle (a extended soak after welding at preheat temperature or even an elevated temperature (600, 700 degrees F.) for an hour per inch or there abouts. This is more important if your are cooling to ambient before RT. It is recommended that you go to ambient before PWHT when welding P91

The anti spatter is not made really for preventing spatter from stickling to the work piece it is really made to use on nozzles and tips although it doesn't hurt to apply[ it anywhere spatter may stick. Anti spatter should not contaminate the weld but it may trap dust and other contaminates that will if the bevels are not very clean when it is applied.I do not understand why you are trying to prevent rust in a steam line other than just covering the ends of your pipe after prepping the ends for welding. The normal way to do more do more to prevent rust would be to apply the Bloxide brand weldable paint to the prepped area before covering the pipe. Bloxide brand paint is one coating that made for this and it is weldable and although the welders will b***h about it and may even buff it off before making a weld it does work and I have made many hundreds of x-ray quality welds with it on the bevels. The thing is that you should be doing a series of steam blows on any steam line after it is installed (but before you install the flow element and tie in and use the line) to remove any debris left in the pipe like welding rod stubs, slag, drink cans and anything else that someone drops in the pipe during construction and this will get all of the rust and scale as well. The blow targets will tell you when it is clean and although you can prevent excessive rust and debris from entering the pipe during construction I have never seen it happen 100%.