Removing chrome oxide passivation layer from stainless nano particles

A. Sudarsan,
I've never had any experience in nano-scale applications, but I suspect that an HCl etch, especially on that scale, would not be considered clean or controlled. You might want to search this forum for previous questions and the responses concerning treatments to activate the surface of stainless for electroplating.

Preventing repassivation should be as simple as not exposing your particles to citric acid, nitric acid, or air.

A. We agree with Ray. Etching in HCl is a harsh process for nano-particles. If passive layer is removed, the particle will immediately dissolved in the acid. In order to replace the initial passive film by the new one, it is necessary to apply the process of slow passive dissolution. During such process the initial passive film will slowly dissolve in the solution, and the new passive film of required thickness and composition will grow.