Corrosion properties of polished and shot-peened austenitic stainless steel 304L and 316L with and without plasma nitriding

Austenitic stainless steels exhibit good corrosion resistance due to their high chromium contents. Metastable austenitic stainless steels like 304L or 316L undergo a phase transformation to deformation-induced martensite by cold working, e.g. shot-peening. Furthermore, austenitic stainless steels suffer from quite poor mechanical properties which make them susceptible to wear. Therefore, plasma nitriding processes enhance the surface hardness up to the eight times of the original hardness value, thus improving the wear resistance. Additional, the corrosion resistance can be improved, too, if plasma nitriding is carried out below a critical temperature to avoid the precipitation of chromium nitrides. In this study, two austenitic steels - 304L and 316L - were used in a solution annealed condition with either polished or shot-peened surfaces. Furthermore, these samples were tested without plasma nitriding, with plasma nitriding or annealing and a combined nitriding and post annealing process. The microstructural properties were examined by optical microscopy (OM) and scanning electron microscopy (SEM), phases were analyzed by X-ray diffraction (XRD) and the corrosion properties were tested by means of potentiodynamic polarization testing in 5% NaCI solution. Statistical evaluations were carried out to identify significant effects between the microstructure and nitriding conditions and the corrosion properties. The results indicate, that plasma nitriding improves the corrosion resistance of both steels with polished surfaces, while shot-peening increases the corrosion rates. Surface hardness measurements show the positive influence of plasma nitriding to surface hardness. Furthermore, a correlation between specimen treatment and the corresponding microstructures, the nitriding and the annealing process and the corrosion resistance will be presented. (C) 2017 Elsevier B.V. All rights reserved.