Effect of high temperature ageing on the corrosion behaviour of nitrogen-added AISI type 316L stainless steel

This paper deals with the effect of microstructural changes that occur on high temperature aging of AISI type 316LN stainless steel (SS) on its corrosion behaviour. Aging at 923, 973 and 1023 K for time durations up to 1000 h caused the following changes in the microstructure of type 316LN SS: (a) precipitation of Cr2N, carbide/carbonitride (C/CN) and sigma-phase at the austenite grain boundaries, (b) changes in the amount, size and shape of sigma-phase, (c) depletion of solid solution strengtheners, such as Cr, Mo, C, N, in the austenite matrix. Corrosion behaviour of annealed and aged base metals was studied in acidified 1M NaCl solution by electrochemical polarisation technique. Copious sigma-phase precipitation on aging for 1000 h at 1023 K caused significant rise in critical current density (i(CCD)), while at other temperature-time combinations, the changes in i(CCD) were minimum. Aging caused the critical pitting potential (E-pit) and repassivation potential (E-rep) to decrease;. the decrease being maximum on aging for 1000 h at 1023 K. The Electrochemical Potentiokinetic Reactivation (EPR) tests in 0.01M KSCN + 0.5M H2SO4 solution showed high intergranular depletion of chromium and molybdenum on aging at 923 K and no depletion of these elements at other temperatures.