Dear Shakir, The so called EPMA thin film analysis is under certain conditions able to determine passivation thicknesses down to the sub-nm range. It bases on the determination of the mass-thickness of a film element by WDS and using a evaluation software as e.g. STRATA (http://www.samx.com/microanalysis/products/stratagem_us.html). It is non-destructively and fairly precise. We used it already for thin oxide layers or e.g. Al2O3 or Cr2O3 layer on different substrates, with thicknesses from below 1nm up to 500nm.