Corrosion behaviour of nitrogen bearing austenitic stainless steel surface diffusion alloyed with Ti/Al precoated films

The present work investigates the aqueous corrosion behaviour of type 316L stainless steel (SS) containing various matrix nitrogen contents (0(.)015, 0(.)1, 0(.)2 and 0(.)56%N), surface modified by diffusion annealing of a precoated film of titanium/aluminium. Type 316L SS specimens were precoated with a Ti/Al multilayer by the electron beam deposition method and surface diffusion annealed at 1173 K for I h in vacuum. X-ray diffraction analysis indicated the formation of Ti3Al, Al5Ti2, Al2Ti and Al13Fe4 intermetallic phases. Nitrides such as Ti2N were also observed, particularly in high nitrogen steels. The interaction between the titanium/aluminium coating and the matrix constituents, particularly with nitrogen, was characterised by secondary ion mass spectrometry (SIMS). The nitrogen content at the modified surface increased with increase in the nitrogen content of the substrate matrix. SEM observation of cross-sectionally mounted surface modified alloys indicated the formation of thick adherent layers. The role of such intermetallic phases in corrosion resistance in both 0(.)5 M H2SO4 and 0(.)5 M NaCl is discussed in detail based on open circuit potential-time measurements, potentiodynamic polarisation studies and electrochemical impedance spectroscopy (EIS) investigations. The role of matrix nitrogen in the formation of intermetallic coatings and its role in corrosion resistance in acidic and chloride media are investigated.