Comparative Corrosion Evaluation of Ferritic Stainless Steels Utilized in Automotive Exhaust Applications

The purpose of this work was to initiate a comparative evaluation of the aqueous corrosion resistance of ferritic stainless steels currently used to fabricate automotive exhaust systems. Both acid condensate and double loop electrochemical potentiokinetic reactivation (DL-EPR) testing using both as-received and heat treated test coupons prepared from Types 409, 409Al, 436 and 439 stainless steel was conducted for this purpose. A truncated version of an in-house acid condensate testing protocol revealed that Type 409Al stainless steel was the most resistant to corrosion of the four ferritic stainless steels examined, whereas Type 409 stainless steel was the least resistance to corrosion. Interestingly, an increase in the Cr content from ~11 wt.% (Type 409 stainless steel) to ~17 wt.% (Types 436 and 439 stainless steel) was not sufficient to improve the stability of the passive film against localized breakdown to a level equivalent to the corrosion resistance exhibited by Type 409Al stainless steel. The DL-EPR testing revealed that Type 409 stainless steel exhibited the highest degree of sensitization after being subjected to a 750°C-1 h heat treatment, whereas Type 439 stainless steel exhibited the lowest. A detailed examination of the grain boundary regions is required to better understand the beneficial role of alloyed Cr in lowering the degree of sensitization observed. Regardless, the preliminary corrosion testing conducted thus far point to elevated Cr contents as being beneficial in improving corrosion resistance of exhaust components.