Deposit Mitigation in SCR Aftertreatment Systems by Deposition of LotusFlo® Superhydrophobic Coatings on Exhaust Pipe Surfaces

The objective of this research was to mitigate urea-derived deposit formation within selective catalytic reduction (SCR) aftertreatment systems by application of specific superhydrophobic coatings to exhaust pipe surfaces. Coatings were applied using a plasma immersion ion processing (PIIP)-like process denoted LotusFlo™. This process utilizes high DC-impulse power waveforms in conjunction with appropriate chemical precursors to generate coatings suited to a specific application. For the present research, organosiloxane (Lotus-130) and fluorinated organosiloxane (Lotus-131) coated surfaces were evaluated. Deposit accumulation studies were executed using the Exhaust Composition Transient Operation Laboratory^TM (ECTO-Lab^TM) burner system. An in-situ imaging system was installed in the ECTO-Lab to observe differences in the deposit formation process between coated and uncoated exhaust pipes. The adhesion strength of urea to each of the LotusFlo surfaces was quantified using the Shear and Normal Adhesion Instrument for Deposits (SNAIDS). Each of the Lotus coatings were subjected to various chemical and thermal treatments according to conditions that would be encountered during in-field usage. Following such treatments, contact angle measurements were performed to assess coating durability. Results from this research verify that application of LotusFlo coatings to internal surfaces of exhaust pipes can significantly alter the distribution of deposit accumulation. At an exhaust gas temperature of 180°C, 95% of all recovered deposit mass accumulated in the uncoated pipe region, but application of LotusFlo coatings reduced the value to as low as 45%. Adhesion measurements demonstrated that the shear force required to dislodge solidified urea from the Lotus-131 surface was reduced by 79% compared to an uncoated surface. Thermal durability testing revealed that the hydrophobic surface remained mostly intact after aging at 300°C, but significant degradation occurred after aging at 450°C. Enhanced thermal resistance of superhydrophobic coatings will be required before integration into automotive exhaust streams is possible, however, the flexibility of the LotusFlo process is well suited to adapt to such challenges.