Chemical structure and corrosion behaviour of S phase coatings

Abstract

The chemical structure and corrosion behaviour of stainless steel coatings with nitrogen contents up to 40 at.-% have been investigated using X-ray photoelectron spectroscopy and electrochemical tests, respectively. X-ray photoelectron spectroscopy revealed that nitrogen existed in a high binding energy form (N1s≈400 eV) at the very surface of the passive film and a nitride-like state (N1s=397·2 eV) within the coatings. For Cr, an additional peak was observed (at Cr2p=574·7 to 574·8 eV) with a binding energy that fell between those for metallic Cr and Cr nitrides. All of the coatings possessed corrosion resistance superior to that of bulk 316L in 1·0 mol L⁻¹ HCl. The passive corrosion current density initially decreased with increasing nitrogen content up to 8 at.-% above which the current density was below the resolution of the potentiostat (<0·001 mA cm⁻²). The breakdown potential for the coatings did not change with increasing nitrogen content and was due to transpassive dissolution rather than localised corrosion. Significant substrate corrosion was observed at defects and flaws within the coatings.