TOPIC:

I snapped thr boom off my Big Jon Capt. Pak rigger and I'm looking for a replacement. I really dont think I need three extensions with the small boat that I have but I want to have extra on hand. 
 I always swivel them 45°. I was also wondering if I could fabricate new ones from stainless? I know aluminum and stainless dont work together.
Advice is welcome.

It's been my experience as a machinist/millwright, its not a good idea to mix stainless with aluminum. They are both abrasive materials. Sonner or later most likely cause a problem of seizing together. Just my thoughts.

You can anodize the aluminum and it probably will be fine for many years.  That being said why not fabricate the extension out of aluminum?  I may have a couple laying around as I don’t use the full 4’ arm on mine and took a section out.

Biggest problem is aluminium is softer and more flexible than stainless steel. The aluminum will take all the stress and make bend or fracture.