Suitable Stainless Steel Selection for Exhaust Line Containing a Selective Catalytic Reduction (SCR) System 2011-01-1323

Due to the evolution of emission control standards, new pollution control systems will be necessarily used for off-road vehicles and trucks exhaust systems and in the near future for passenger cars. Indeed, the will to reduce NOx emission through Euro 5 (2009) and then to Euro 6 (2014) and American EPA Tier 4 (2008-2015) imposes the implementation of a new after-treatment system within the exhaust line. One of the most promising technologies takes advantage of the reduction feature of ammonia (NH₃) on NOx. This system called Selective Catalytic Reduction (SCR) couldn't be developed by storing directly ammonia as a reduction agent on the vehicle due to its high toxicity and flammability. It is why urea is used as an ammonia generator through thermolysis reaction.

During this process, the stainless steel wall of the exhaust line undergoes sprinkling of urea decomposition products - among which ammonia - at the upstream injection point of the catalyst and also cyclic heat treatment due to exhaust gas. The combination of both needs to be studied in further detail to get a better understanding of mechanisms involved in the exhaust material durability under such conditions. With the aim of simulating this particular hot corrosion process two dedicated laboratory benches were developed at the ArcelorMittal Stainless Steel Research Center (Isbergues, France).

The simulated tests consist in spraying urea solution on cyclic heated stainless steel in the temperature range of the application (from 200°C to 600°C). We evidenced nitriding mechanisms due to the contact between urea decomposition products and the hot stainless steel surface and also highlighted different performance between austenitic and ferritic grades and even noticeable behavior variation among every stainless steel family. Thus, ferritic grades exhibit better performance among which K41X (441 - 1.4509) and K33X (molybdenum stabilized ferritic close to 436Ti grade - 1.4513) show the best behavior, in particular as compared to 304 austenitic grade.

This paper will review the test set-up, the results obtained and will discuss the most suitable stainless steel grade selection for the SCR application in both mixing zone and the downstream area.