Effect of aluminizing and nitridation on the oxidation behavior of AISI 316LN SS

The application of stainless steel at temperatures above 973 K is rendered difficult by the chromium loss due to generation of volatile CrO3 species and the consequent reduction in the capacity to maintain protective scale. We have attempted a method to circumvent this problem by modifying the surface. The surface was modified by three techniques each suiting a particular application. The first two methods involved aluminizing the surface by physical vapor deposition as the initial step. The pre-treatments were carried out in two ways viz. (i) high temperature diffusion-annealing and (ii) laser annealing of the surface. In the third method the specimen was modified by ion-nitriding. The oxidation behavior of these modified steels was compared with stainless steel in the as-received condition to exactly gauge the effect of surface modification. The oxidation experiment was carried out at 1123 K for 3.6 Ms. The oxidation was interrupted at specific time intervals to examine the mass change of the specimen and mass of the spalled oxide. The results indicated that aluminizing followed by heat-treatment and laser-treatment showed significant improvement in the adherence of the scale. On the other hand the bare and nitrided specimens showed similar behavior characterized by intermittent spallation and poor adherence. Nitriding resulted in the increase in the surface hardness. The post-oxidation examinations were carried out using SEM, EDAX and GIXRD. The uncoated specimen showed the presence of uniform oxide layer and contained oxides of iron and chromium. The aluminum-coated specimen showed the presence of adherent scale. The surface showed a nodular structure due to the formation of pits and zones of enhanced oxidation. The major part of the oxide was alumina with small fractions of chromium and iron oxides. The nodular region was enriched in the oxide of iron. The analysis of the surface by GIXRD revealed the nature of different phases formed on the surface. The oxide on the oxidized bare metal was Cr2O3, aluminized (non-oxidized) AlFe and Al2Fe, aluminized and oxidized Al2O3 and FeCr2O4, aluminized and laser-treated Al2O3, FeCr2O4, Fe2O3 and Fe3O4, nitrided Fe2O3 and FeCr2O4. In the light of the above results the mechanism of scale failure has been proposed.