I am guessing , you changed the belts and not the tensioner as a set , right? Common mistake , always change tensioner along with belts. And is some cases , water pump also ...(with the one with metallic parts inside.)
In most cases changing the belts and pulleys is OK but with a rusted tensioner you never know. I don't think they are included in the standard sets? Pulley and metal holder are but there is no need to change that metal part, you can press the pulley off and on.
The rust on the spring has no effect really as the spring is static once the 17mm nut is tightened , what I would look at is why some water got in there (previous leaking water pump ?) and most likely into the bearing as well .. as it is the bearing that has seized that has caused this failure .. or was the belt tension set to tight (a common mistake) stressing the bearing ? .
The bearings on the tensioners do wear out and can fail. I replaced the tensioner wheels, along with the belts on both my cars. The ones I took off were mostly starting to feel rough, and it would have just been a matter of when they failed, not if.....
I have loads of these tensioners, all brand new. Can anyone tell me the difference between the two versions I have? One has plastic roller, as pictured above, whereas the other has a plain metal roller. Is it possible to use either, or are they type-specific?