Mechanisms and orientation dependence of the corrosion of single crystal Cordierite by model diesel particulate ashes

Cordierite Mg2Al4Si5O18 is a material for diesel particulate filters (DPF) with high potential. Its resistance to two simplified model ashes has been tested on single crystals at temperatures up to 1050 °C, which are realistic for use under ‘worst case’ conditions. Single crystals were examined in order to investigate the orientation dependence of attack due to the strongly anisotropic nature of the Cordierite crystal lattice. A mixture of sodium carbonate and silica was used to study the attack of an alkali-rich ash composition and a mixture of the Ca, Mg and Zn orthophosphates to study an ash composed of typical main constituents of ashes found in DPF used in traffic. The sodium-rich ash formed a melt and attacked the Cordierite by dissolving it. No anisotropy in the corrosion was observed, because the attack is controlled by outward diffusion of Mg. A kinetic break occurs in the system, caused by the formation of a Nepheline product layer, slowing down corrosion. A much stronger corrosion of Cordierite occurs by the phosphate mixture at 1050 °C. Excessive melt formation from the ash causes fast dissolution of the substrate with melt saturation within minutes. Anisotropy of the dissolution process could not be detected. The initial kinetics is dominated by saturation effects, which slow down corrosion. The saturated melt attacks Cordierite by reaction processes leading to the formation of new crystalline phases. This process is much slower than the initial dissolution process but may significantly contribute to the destruction of Cordierite substrates if large contact areas between ash melt and Cordierite exist. Additionally, the formation and local growth of crystalline phases causes the extension of faults, which may eventually become critical.