Cold starts with ambient temps less than 50 degrees without getting the engine to at least near max operating temp condensing and collecting in an exhaust system not brought to a sustained max temp is the number one offender of rotting out these exhaust. In other words, short jaunts rots pipes. I'd propose a ride of at least 25 miles would be needed to get the engine and the exhaust system to near max temp to fully "cook out" the products of combustion and related condensation. I remember my Dad saying, at 32 degrees (ambient) the center of a cranksahft is still cold after the engine is run for 30 minutes. (of course, that is the crankshaft of a liquid cooled engine.)
