A novel accelerated corrosion test for exhaust systems by means of power ultrasound

Cycling corrosion tests have been performed to simulate corrosion conditions in the cold end of an automotive exhaust system. A middle range 1.4512 (AISI 409) stainless steel is submitted to a conventional dip dry test (DDT) parallel to a similar test but including an additional external stress thanks to an ultrasonic transducer. This new ultrasonic test (so called UST) is expected to reduce the diagnostic time of the corrosion test by the combined action of the chemical corrosion process and of mechanical degradations. Both corrosion tests are performed in two different media in order to simulate internal corrosion due to exhaust gas condensate and external cosmetic corrosion, greatly enhanced by road salt during winter. It respectively concerns a synthetic gas condensate, the composition of which is derived from what is obtained from motor gasoline combustion, and NaCl solution. In both electrolytes the stainless steel suffers from pitting corrosion. Samples are examined by optical micrography and surface profilometry. The degradation state is quantified according to three parameters: maximum pit depth, average pit diameter and porosity rate. The efficiency of each test is then related to the time of immersion. As expected, use of ultrasound allows pits growth to be achieved from the beginning of the exposure time, so that the maximum pit depth recorded after 180 h of immersion is twice than with the classical dip dry test. It seems that it does not modify the pit initiation mechanism but only increases growth kinetics.