STUDIES BY AUGER-SPECTROSCOPY OF PIT INITIATION AT THE SITE OF INCLUSIONS IN STAINLESS-STEEL

In this paper a high resolution scanning Auger microscope fitted with an X-ray analysis system is used to investigate the surface chemistry of inclusion groups in stainless steel 316 relative to the bulk composition. Changes in the inclusions resulting from corrosion in sodium chloride solution are observed over a sequence of several successive exposures. The work demonstrates that careful control of experimental conditions allows the progress of initiation of pits to be observed systematically. The role of inclusions as initiators of pitting corrosion has been studied from the very inception of attack. Detailed observations have been taken from separate manganese suphide as well as mixture of manganese sulphide and multi-element oxide at various pH values of solutions to trace the behaviour of these inclusions during exposures. It is suggested from this investigation that corrosive attack starts on MnS inclusions or on small MnS precipitates associated with oxide inclusions. The observations showed that sulphur precipitated on the area surrounding the inclusion group in acidic NaCl solution. It is believed that this deposition stems from the cathodic reduction of a transient oxidised species of sulphur produced during the acid hydrolysis of the MnS. Following the deposition of sulphur the formation of an iron oxide cap to the pit and inclusion is observed. Deposition of sulphur does not occur in alkaline solutions. © 1990.