THE APPLICATION OF ICP-MS AND XPS TO STUDIES OF ION SELECTIVITY DURING PASSIVATION OF STAINLESS-STEELS

Samples of Fel7Cr, Fel5Cr4Mo, and a commercial 304 stainless steel were passivated in 0.1M sulfuric acid solution at constant potentials. The alloying effects of Mo and Ni on the corrosion property were studied in relation to their potentiodynamic polarization curves. A revolutionary new method of analysis in analytical chemistry, the inductively coupled plasma-source mass spectrometry (ICP-MS), was used to analyze the corrosion products in passivating solutions so as to explore the ion selectivity during dissolution and film formation processes. The selectivity factor S was defined and used to characterize the selective dissolution of Fe and/or Ni from the steels. The dissolution behavior of the Ni bearing steel changes with polarization time, from the initial selective dissolution of Ni to the final selective dissolution of Fe. Surface analyses of the passive films by x-ray photoelectron spectroscopy (XPS) were also conducted in conjunction with ion beam sputtering to obtain depth distributions of alloying components. The selective enrichment of ions in the passive films were characterized by the enrichment factor f. A model based on the selective ionization was proposed to explain the enrichment of Ni in the metallic phase underneath the passive film. The dissolution kinetics as well as the film formation kinetics are discussed in terms of the charge balance during 69h polarization. Results from surface analyses of passive films by XPS are complementary with those from solution analyses by ICP-MS. © 1990, The Electrochemical Society, Inc. All rights reserved.