Effect of Oxidation Treatments of Steel Substrates on the Formation of MFI Zeolite Coatings

Abstract

The surface of 316L stainless steel flat substrates has been modified by annealing in an air stream, anodizing in an acid solution, and sequential anodizing first in an acid solution and then in an alkali solution. A zeolite of the MFI structure with a Si/Al ratio = 20, 80, or ∞ has been crystallized onto the initial and modified substrates under hydrothermal conditions. The morphology and chemical and phase compositions of the initial and modified substrates and the synthesized zeolite layer have been studied using XRD, SEM, EDS, and XPS methods. On the substrates anodized in the acid solution, a uniform continuous coating with zeolite crystals with a Si/Al ratios of 80 and ∞ is observed as early as after 3 h of crystallization, and further crystallization only leads to an increase in the thickness of the zeolite layer, whereas a continuous layer of the zeolite phase in the case of Si/Al = 20 is observed as late as after 20 h of crystallization. On the unmodified substrates annealed in an air stream and anodized in an alkali solution, a continuous zeolite coating is not formed after 20 h of crystallization.