International audienceAcidic dissolution of ferritic stainless steels is simulated using the ab initio based Monte Carlo technique. This approach aims to reach a better understanding of the effect of alloying elements on the acidic dissolution of SS at the microscopic scale. The dissolution probability of a surface atom is shown to depend drastically on the number and chemical nature of its nearest neighbors in dense emerging crystallographic planes. A "critical coordination number" (CCN) is defined, above which the dissolution probability is too low to contribute significantly to the dissolution rate, and below which the dissolution events are too fast to be experimentally detected. For pure alpha-Fe and FeCr alloys this CCN was found to be equal to 4. Increasing the Cr concentration does not change the CCN but decreases the interatomic bonding energy, making the dissolution process easier. The addition of Mo is found to involve multiple CCN, belonging to different crystallographic orientations, which results in a substantial decrease of the dissolution kinetics. (C) The Author(s) 2016. Published by ECS. All rights reserved

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Stainless steel is largely used in the car exhaust market and will be applied now for truck and off-road vehicles. In that field of application, designs are more and more complex with the integration of a catalytic converter and particle filter, consequence of more and more severe diesel depollution regulations. In particular, due to the necessity of reducing NOx emission established by Euro 5 standard (2009), Euro 6 (2014) and American Tier 4 (2014), new equipment were developed for diesel vehicles (truck as well as car). The most promising technology is called Selective Catalytic Reduction (SCR) and takes advantage of the reduction feature of ammonia (NH 3 ) on NOx. As NH 3 cannot be stored directly within the vehicle for safety reasons (toxicity & flammability of ammonia) urea in water solution was selected to initiate the reaction by means of a spraying nozzle. To get a better understanding of the involved hot corrosion mechanisms and afterward to improve material selection, a dedicated laboratory test was developed at Isbergues Research Center. The simulated test consists of spraying urea solution on cyclic heated stainless steel in a range from 200°C to 600°C. We evidenced a nitriding mechanism due to the urea decomposition on the surface of stainless steel at high temperature, and also the very different behaviours between austenitic and ferritic grades. The last one, in particular K41X (1.4509-441) and K33X (1.4513-molybdenum stabilized ferritic) grades show the best performance in particular when compared to the standard 304 austenitic grade. The paper will review the test set-up, the result obtained and will discuss the stainless steel grade selection for the SCR application.