Hi,I am going to replace my stock Y pipe with a magnaflow Y pipe.

Does anyone know if it is necessary to use any sealant in these joints?
Thanks in advance

I have that Magnaflow Y pipe, the guy I had do the exhaust had to use a little sealant at the manifolds on one side IIRC.

I used RED hi temp Permatex on the joints on my Toyota Camry and even a catalytic converter flange.
It's still there and hasn't burned away. At the very least, it could help prevent moisture from getting into the joints and rusting.

Santiago, Chile?! Winter all year 'round? Brrrrr...

It mounts-up with ball/flange connections, just like the stock Y-pipe, so you shouldn't need any sealants if you get it lined up properly. It will be a tight fit, so it might not be easy to line it up right. But don't give up at "good enough." Keep at it until you get it right, and you shouldn't need any sealants. I installed the MAC Y-pipe many years ago, same factory-style connections, no sealants, no leaks.

With the MAC pipe, I had to keep the stock spacer in the passenger side. I can't say if that will be necessary with the Magnaflow pipe, but if it doesn't seem to be fitting on the passenger side, then you'll probably need to insert that spacer. If you don't know the spacer I'm talking about, then you'll see when you remove the stock pipe, and a chunk of iron falls out. So don't have your head right underneath when you drop the pipe!

Your bigger problem will be finding that Y-pipe. It's been discontinued and out of stock everywhere, supposedly no longer in production. It's still listed for sale from several vendors, but you're likely to find that it's not available anymore. That's according to reports from people here on TGO who have been shopping for one recently. Hopefully you already have one. The MAC Y-pipe, and all MAC products, are gone. MAC is no more. Closed its doors during COVID.