Try this. That rod looks like it might be treated with something. Take that dremel tool and cutting wheel, and grind the part you want to solder. I put my cutting wheel at a 45 degree angle, and just go back and forth. It starts to shine when you get off the surface coating, down to the good metal. Do this all the way around. It's worked a bunch of times for me. After that, flux the rod and tubing and solder away. Let me know if you get it

Alex