THE EFFECT OF COMPOSITION MODIFICATION AND GRAIN SIZE REFINEMENT ON THE OXIDATION BEHAVIOR OF STAINLESS STEEL COATINGS PRODUCED BY CRYOMILLING AND SPS

摘要

The modification of the composition of 316LSS combined to a grain refinement to the nanometer level was investigated to determine the potential oxidation resistance improvement on coatings produced using the Spark Plasma Sintering (SPS) technique. Three different SS coating systems were studied: conventional and nanostructured SS316L, and nanostructured SS316L containing 6 wt% Al. Oxidation kinetics was conducted for the three feedstock alloys, namely conventional SS, nSS, nSS6Al with a TGA at 500 °C, 800 °C and 1000 °C for 48 hr. Figure 1 shows the oxidation kinetic traces for the three alloys at 800 °C oxidation. For all tested temperatures, nSS6Al showed a lower weight gain than the conventional SS and nSS. The oxidation rate constant for the feedstock was calculated for all three systems for all tested temperatures and the results are plotted using Arrhenius oxidation kinetic equation as shown in Figure 2. The oxidation rate constants of nSS6Al alloy is lower than the rate constant of the conventional SS and nSS at all tested temperatures, which is caused by the outward diffusion of Al and slower oxidation kinetics once the initial Al_2O_3 layer is formed. The calculated activation energies of oxidation from the slope of Figure 2 for the nanostructured alloys are 97 kJ/mol for nSS and 85 kJ/mol for the nSS6Al. The values are all within the same order of magnitude and approximately half the calculated activation energy of181 kJ/mol obtained for the conventional SS. The reduced activation energy for the nanostructured alloys can be explained by the higher diffusivity arising from a higher volume fraction of grain boundaries for the nanostructured alloys which act as fast diffusion paths for elemental diffusion.