A spare section sitting in a garage is not paused. It is reacting to the air around it, to whatever it is resting on and to the residue left on it from the last time it ran. Corrosion on a stored part usually starts before the part is put away, not during storage.
The good news is that the same factors control almost every case: contamination on the surface, moisture in the air, contact with a dissimilar material, and no air movement. Control those and a steel section will keep for a long time.
Exhaust parts come off the car carrying carbon, road film and the residue of whatever was sprayed near them. Those deposits hold moisture against the surface and, in the case of salt film, actively drive attack. A part put away dirty will corrode where the dirt sits, in patches.
Wash and dry before storage, and pay attention to the inside of the pipe, where condensate collects and where nobody looks. A stainless steel exhaust muffler with a damp, sooty interior is the classic case: sound on the outside, staining inside.
Still, humid air is worse than moving air at the same humidity, because the surface never dries. A sealed plastic bag around a damp part traps exactly the conditions that cause trouble. A breathable cover in a ventilated space is usually the better choice.
Temperature swings matter as much as the average. A cold garage that warms during the day will condense moisture onto cold metal, and the part will be wet every morning without any leak being present. Keeping the part off a concrete floor helps, since floors stay cold and hold damp.
Two different metals touching in the presence of moisture form a small corrosion cell, and the less noble one loses. Storing stainless against plain steel, or hanging a part from a mild steel hook, gives corrosion a place to start that has nothing to do with the part itself.
Rest aluminum and titanium parts on wood or plastic rather than on steel. Foam and cardboard are also a problem, because both hold moisture against the surface for long periods. A part should touch as little as possible, and what it touches should be inert and dry.
Clean it, dry it thoroughly, and give bare or lightly finished surfaces a thin film of oil or a wipe of a suitable protectant. Plug or cap open ends so air does not circulate through the inside, and hang the part rather than stacking it so nothing rests against a finished surface.
Flanges are the most vulnerable area, because they are machined flat and any rust there ruins the seal. Keep them off the ground and out of contact with anything that can hold water. Shielding material stored alongside should be kept flat and dry, since the same moisture that attacks steel degrades the facing of an exhaust heatshield.
Before fitting, look for tea-coloured staining rather than red rust on stainless, since stainless usually stains before it pits. Run a finger inside the flange face and around welds, where moisture collects. Any roughness on a sealing surface means the part needs attention before it is fitted.
Check that the bore is still clear and that nothing has made a home in a stored pipe, which is more common than owners expect. If a stored section has pitted at a weld or a flange, it is no longer a spare. It is stock for repair, and it should be treated as such rather than fitted and hoped over.
