I am comparing the stress results between original or non-corroded model to corroded model. I found that the stress value in non corroded model is lot more bigger than the corroded model, is it correct?
Maybe it is correct, maybe it isn't.
Suppose you "corrode away" a small radius that was causing a stress concentration, and it becomes a bigger radius. Quite likely the maximum stress in the structure (at that concentration) will decrease because of the corrosion. For example an "average" stress of 1.0 and a stress concentration of 10.0 might become an "average" stress of 1.1 and a stress concentration of 9.0. Whether you count that as a "decrease from 10 to 9" or an "increase from 1.0 to 1.1" is your choice.
On the other hand if you reduce the cross section of a structural member like a beam, or the thickness of a plate, the stress will probably increase - i.e. stress = load/area, and you decreased the area.