Exhaust hangers

On both the Jimny and now our new to us 2008 Vitara I have found the exhaust hangers corroded to the point of no longer connected to the exhaust pipes. Now the exhaust pipes themselves are sound on both vehicles but the spigots are corroding at the weld connections. Why is that I wonder? In the Jimnys case I had the exhaust system removed so repairing that was quite easy. But in the Vitaras case we need it working so it is a tad more awkward to repair. Anybody else come across this?   

Yes a bit with our Jimny. The first hanger on the first section after the CAT had completely corroded away. I made a bolt on repair section using an exhaust U clamp as the mount rather than welding on my back. This was later followed by wasting and cracking of the rear silencer mount, causing the exhaust to hit the body on rough roads.
Fixed all this by having a custom made stainless steel CAT back system a few weeks ago, should see my Jimny out!

Exhausts are never painted because they get very hot during normal operation. Bare steel will rust over time as we all know. But welds are always corrode faster than the base steel.
Here is why (Google):
'The weld metal compositions are usually optimized to enhance their mechanical properties; this makes them more anodic than the base steel, causing them to corrode at higher rates compared to their base metal'

Before fitting new exhaust system I always spay them with heat resistant paint (up to 650C) so they last longer than untreated ones.  

You can paint them and there is a brand designed for them. Heat resistant to 560 C. I have and have had all makes of cars and these are the only ones I have had this problem with!