Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

Abstract

It is essential for materials used at high-temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high-temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low-Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni-base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550 8C in the case of low-alloy steels, 750 8C in the case of an austenitic steel (TP347) and up to 1000 8C in the case of the Ni-base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low-Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short-circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr 2 O 3 with some FeCr 2 O 4 at localized sites, while for specimens with larger grain size a non-protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni-base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high-angle grain boundaries.

High Temperature Steam Electrolysis Materials Degradation: Preliminary Results of Corrosion Tests on Ceramatec Electrolysis Cell Components

“…In addition, the corrosion microstructures observed for the uncoated FSS specimens (Figure 16, Figure 23), in which there appears to be a thicker layer of scale along the metal grain boundaries, has been observed previously for high-chrome steels 11 , and has been attributed to Cr-rich phases forming on the surface at the grain boundaries. It has not yet been confirmed in this work if the thicker phases at the grain boundaries in these specimens are Cr-rich.…”

Computer-based simulation of high-temperature corrosion phenomena

“…The high Cr concentration in the alloy IN625 causes the formation of an external chromium oxide scale (Cr 2 O 3 ) during exposure to an oxidizing atmosphere at temperature range between 850 o C and 1000 o C. 8 Beside the superficial Cr 2 O 3 scale, internal oxides can be observed (see Figure 4a). The internal oxidation zone is composed of intergranular aluminum oxide (Al 2 O 3 ).…”

Oxidação Intergranular De Ligas De Níquel Em Altas Temperaturas Usando Ebsd

“…Uma subcamada de SiO 2 , mesmo que parcial sob camadas de Cr 2 O 3 tem mostrado muito eficiente na proteção contra corrosão em altas temperaturas [2]. No entanto, mesmo com a presença de tais camadas óxidas, corrosão interna (oxidação e/ou nitretação) comumente ocorre em ligas de níquel, tendo em vista que os elementos não metálicos como oxigênio e nitrogênio conseguem difundir através da camada ou permear através de defeitos (trincas/poros) da camada e atingem o substrato com potencial químico suficiente para reagirem com menos nobres, como alumínio e titânio [3]. É conhecido que difusão ao longo de contornos de grãos e no grão apresentam comportamentos diferentes.…”