Failure Mechanisms in Automotive Exhaust Decouplers

Automotive exhaust decouplers (flexible couplings) are a critical component that must remain leak free in order to meet extended warranty and pollution control requirements. The key component of a decoupler is the bellows, which allows easier fit-up during exhaust system assembly and absorbs vibration during service. This bellows is most often constructed of austenitic stainless steel. Though known for improved corrosion resistance, there are many possible corrosion related failure mechanisms for stainless steels. Nineteen decouplers, taken from standard endurance track tests and from normal highway use, have been destructively examined in order to document which failure mechanisms may cause premature failure and compare the performance of various component materials of construction. As a result of exposure to road salt in track tests and normal winter driving conditions, the decoupler bellows have been found to fail by stress corrosion cracking (SCC), pitting, crevice corrosion, hot salt attack, molten salt attack, accelerated oxidation and fatigue or corrosion-fatigue. Failure analysis results were correlated to laboratory testing results. Also the performance of stainless steel decoupler bellows were compared to units fabricated from more corrosion resistant high nickel alloys.