Effect of Element Diffusion Through Metallic Networks During Oxidation of Type 321 Stainless Steel

Abstract

A detailed study was conducted on localized oxidation on Type 321 stainless steel (321ss) using synchrotron x-ray nanobeam analysis along with Raman microscopy. The results showed the presence of metallic nanonetworks in the oxide scales, which plays an important role in the continued oxidation of the alloy at 750°C. A mechanism is proposed to explain the rapid oxidation of 321ss in complex gaseous environments at elevated temperature. Neutral metal atoms could diffuse outward, and carbon atoms could diffuse inward through the metallic nanonetworks in oxide layers. Alternately, diffusion tunnels can dramatically affect the phase composition of the oxide scales. Since the diffusion rate of neutral metal and carbon atoms through the metallic nanonetworks can be much faster than the diffusion of cations through Cr 2 O 3 , the metallic nanonetwork provides a path through the protective Cr 2 O 3 layer for the rapid outward diffusion of metallic chromium and iron atoms to the nonprotective spinel layer. This diffusion process affects the solidstate reaction near the alloy-oxide boundary, and a dense Cr 2 O 3 protective layer does not form. The classic stable structure of the oxide scales, with a dense Cr 2 O 3 layer at the bottom, is damaged by the rapid diffusion through the tunnel at the reaction front, resulting in locally accelerated oxidation. This process can subsequently lead to ''breakaway'' oxidation and catastrophic failure of the alloy.

Oxidation behavior of 316L austenitic stainless steel in high temperature air with long-term exposure

“…For samples exposed to air at 400°C, Fe-rich, Cr-rich and Ni-rich oxides will be nucleated at the beginning of oxidation, while the number of Cr-rich oxides is significantly more than that of Fe-rich and Ni-rich oxides due to the lower Gibbs free energy to form Cr-rich oxides than that of Fe-rich oxides and higher concentration in matrix in comparison of Ni element. Meanwhile, the size of nucleated oxide particles gradually grows with increasing exposure periods at different temperatures, and then reacts with each other to form new oxides, such as FeCr 2 O 4 , NiFe 2 O 4 and NiCr 2 O 4 , forming an oxide film on the surface of samples and hindering the outward diffusion of metal ions [26,36]. After that, a great number of Fe-rich oxide particles will be formed on the surface of Cr-rich oxide films due to the high diffusion coefficient for Fe than Cr and Ni in this oxide layer [32], as shown in figure 4.…”

High‐Temperature Initial Oxidation Behavior in LDX 2101

“…On the austenite surface, the peaks at 219, 237, 287, 402, and 493 cm −1 were characteristic of α ‐Fe 2 O 3 . The peak at 602 cm −1 was assigned to Cr 2 O 3 , and that at 650 cm −1 was responsible for (Fe,Cr,Mn) 3 O 4 . On the ferrite surface, the peaks at 316 and 369 cm −1 were characteristic of Mn 2 O 3 .…”

Effect of Simulated Annealing Conditions on Scale Formation and Neutral Electrolytic Pickling

“…[ 17 ] In the internal oxidation process, the oxidation of chromium and iron forms (Fe,Cr) x O y spinels due to the inward diffusion of oxygen, [ 32 ] whereas nonoxidized nickel and iron‐rich areas remain in the spinel structure, forming an internal, net‐like scale structure. Zeng et al [ 33 ] proposed that the outward diffusion of chromium and iron atoms through the net‐like structure occurs via a network of Fe‐ and Ni‐rich particles and the inward diffusion of oxygen via the oxidized area network.…”