Test Method Development for Material Selection of Diesel Exhaust Line

The evolution of emission control standards on particulate matter and NO_x has led to a significant increase of complexity of the diesel exhaust line which includes catalytic converter, particulate filter and selective catalytic reduction systems. The exhaust line is no longer a component that customers can change easily; its durability has to be studied for longer lifespan and if possible to be predicted. From a corrosion point of view, emission control systems have led to more and more severe conditions for stainless steel material used in the exhaust line. In particular, mufflers are exposed to higher temperature during the regeneration of the particle filter and also to acidification of gas condensates due to high sulphur content that can be found in diesel. To assess material performance in these severe conditions, a test method was developed to simulate the environment of the inner part of a muffler through corrosion cycles composed of oxidation steps in a furnace and dipping steps in a synthetic condensate. This dip/dry test simulates the most severe kind of corrosion encountered in mufflers, the crevice corrosion. In that fully automatic test, different cycles (oxidation/ immersion/ emission) can be applied according to the studied driving environment; urban or highway. Furthermore, the impact of the various parameters (sulphur content and pH, impact of biodiesel condensates) on the aggressiveness of the condensate can also be also studied. After the test, the depth of the pits is measured on each sample and extreme value statistics method is applied to predict corrosion kinetic and perforation probability. In such tests, 17% Cr ferritic steels offer a good behaviour. Among them, stabilized ferritic with molybdenum AISI 444 EN 1.4521 (K44X) and AISI 436Ti EN 1.4513 (K33X) are good technical and economical alternative materials to austenitic AISI 316L EN 1.4404 and AISI 304 EN 1.4301 respectively.