may be able to slow the rust down by attaching zinc blocks to...

may be able to slow the rust down by attaching zinc blocks to the pipeline , I use to do that with my boat motor .
I trained as a commercial diver many years ago , surprising how much work can be done underwater welding and cutting , (blasting lol) its not as hard as people would imagine , providing have the right equipment ie diving bells , and crew .
One of the advantages is you never get hot welding or cutting , but I did get electrocuted, they never told me that was possible , fortunately I was young and did not even mention it to the crew.