Temperature Oxidation Tests Research Articles

Effect of Rare Earth on Void Band of Diffusion Layer and Properties of Aluminized Steel

Abstract The effects of the addition of rare earth (RE) elements on the void band in the diffusion layer, and the resistances to both oxidation and spalling of aluminized steel were investigated through high temperature oxidation and spalling tests. The results showed that RE had significant effects on the void band in the diffusion layer and the properties of aluminized steel. After diffusion treatment, a considerable number of the voids between the middle layer and transitional layer of pure aluminized coating, aggregated into wavy-line-shaped void bands parallel to the outer surface. For the RE-added aluminized coating, only a few voids aggregated into intermittent block shapes. During high temperature oxidation at 800 °C for 200 h, the wavy void band of pure aluminized coating aggregated further into a linear crack parallel to the outer surface, and the internal oxidation occurred within them; the open cracks perpendicular to the surface penetrated through the diffusion layer. For the RE-added aluminized coating, only a few voids aggregated into intermittent meniscus shapes. During cyclic spalling tests, the peeling, spallation, and pulverulent cracking occurred along the void band in the diffusion layer of pure aluminized coating, but only a little spallation occurred in the diffusion layer of the RE-added aluminized coating, in which cracks perpendicular to the surface were much smaller than those of pure aluminized coating and did not penetrate through the diffusion layer. It is evident that RE addition can restrain the formation and aggregation of voids and subsequently improve the resistances to oxidation and spalling. The mechanism of the RE effect on the void band in the diffusion layer is also discussed.

Pressure effects on high temperature steam oxidation of zircaloy-4

The pressure effects on the oxidation of Zircaloy-4 (Zry-4) cladding in high temperature steam have been analyzed. A double layer autoclave was made for the high pressure/high temperature oxidation tests. The experimental test temperature range was 700–900°C, and pressures were 0.1–1.5 MPa. Steam partial pressure was found to be a more important factor than total pressure. Steam pressure enhances the oxidation rate of Zry-4 exponentially and depends on the temperature, the maximum being between 750–800°C. A preexisting oxide layer decreases the enhancement by about 40–60%. The acceleration of the oxidation rate caused by high pressure steam seems to originate from the formation of cracks due to an abrupt transformation of the tetragonal phase in the oxide, where the instability of the tetragonal phase comes from the reduction of surface energy due to steam.