Numerical simulation of the failure behavior of steam‐side oxide scale considering oxide creep and physical defects

et al. 2020

Self Cite

This paper proposes a method for predicting the creep rupture life of superheater tubes, considering the continuous increasing metal temperature and stress as a result of oxide formation. The steam-side oxide growth was assessed by the parabolic oxidation rate law, and the temperature profiles of superheater tubes was calculated combining the 3D computational fluid dynamics simulation and the 1D analytical models. Based on the estimated temperature and maximum elastic hoop stress, the parametric extrapolation technique was utilized to assess the creep rupture life. The validity of this method was confirmed by measuring the oxide thickness and the steam temperature in the field. The results found that T91 alloy in the steam outlet region and T23 alloy at the connection point was susceptible to creep rupture failure, which had the greatest cumulative damage and thickest oxide scale. The cumulative damage of superheater tubes varied significantly with tube count. Results suggested that replacement of the particular T91 alloy tubing section having the minimum creep rupture life with a better heat-resistant steel would avoid mechanical failure by creep for the subject boiler system.

Section: Introductionmentioning

confidence: 99%

Creep rupture life assessment of superheater tube with oxide formation in power plant

et al. 2020

Self Cite

The role of Cr atom in the early steam oxidation of Fe‐based alloys: An atomistic simulation

Liang

et al. 2020

Self Cite

This study employed the density functional theory to capture the atomic‐level dissociation processes of steam and investigate the ions migration on Fe(001) and FeCr(001) surfaces, revealing the role of Cr atom in the early oxidation. Various coadsorption structures with different steam‐derived species have been systematically examined to find the most energetically favored surface site. The results showed that the steam dissociation on the alloy surface underwent two steps. First, H2O molecule on the top site was dissociated into OH group and H atom, which further combined with metal atoms on the bridge site and hollow site. Second, the OH group was decomposed into O and H atoms, which moved to two adjacent hollow sites and generated an oxide. On further oxidation, the Fe atom migrated outward and formed an outer Fe oxide, whereas the Cr oxide could only grow inward as O atom passed through oxide. It was found that the presence of Cr atom on the surface was thermodynamically beneficial, which could promote steam oxidation. The Cr atom could effectively block ion diffusion across the oxide scale and protect the underlying substrate from further oxidation. These results were in good agreement with experimental observation.

Reactive molecular dynamics study of the oxidation behavior of iron‐based alloy in supercritical water

2021

Self Cite

A theoretical study of the oxidation behavior of iron-based alloy in the supercritical water (SCW) has been carried out based on ReaxFF force-field molecular dynamics simulation. An atomic model has been proposed to simulate the initial chemisorption reactions and atoms diffusion behavior across the oxide layer. Simulation results imply that Cr addition has an important effect on the oxidation behavior of iron-based alloy. In the initial stage of oxidation, H 2 O prefers to adsorb on the Cr atom, and some species in the form of Cr(OH) 4 are observed on the FeCr alloy surface. Once an initial oxide layer is formed, further oxidation is controlled by the migration of vacancy. The O vacancies are formed at the oxide/FeCr alloy interface and migrate toward the steam, whereas Fe vacancies are formed at the oxide/steam interface and migrate toward the FeCr alloy. Attributed to the stronger binding energy of O-Cr bond than O-Fe bond, the Cr diffusivity in the oxide is less than Fe atoms. Thus, double oxide layers, including the inner Fe-Cr-O layer and outer Fe-O layer, are formed on the FeCr alloy, which is in good agreement with previous experimental observation.