High temperature oxidation and NaCl-induced accelerated corrosion of hot-dip aluminized 9Cr-1Mo and 310 stainless steel

摘要

The behaviors of high temperature corrosion on hot-dip aluminized on 9Cr-1Mo and 310 stainless steels when catalyzed by NaCl and cyclic heating environment were studied experimentally. The corrosion behavior and morphological development were investigated by weight gain kinetics, metallographs, depths of attack, metal losses, and X-ray analyses. The results of 310SS deposited with salt mixtures show that weight gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 hr, while the kinetics with salt deposits display multi-stage growth rates. NaCl is the main corrosive specie in high-temperature corrosion involving mixtures of NaCl/Na2SO4 and is responsible for the formation of internal attack. Uniform internal attack is the typical morphology of NaCl-induced hot corrosion, while the extent of intergranular attack is more pronounced as the content of Na2SO4 in the mixture is increased. The thermal-cycling test results of 310SS deposited NaCl and coated 7wt%Si/93wt%Al show that the aluminized layers have good corrosion resistance during the first four cycles of testing, while degradation occurs after testing for five cycles. The reason for degradation of aluminized layers is attributed to the formation of interconnecting voids caused by aluminum inward diffusion, chloridation/oxidation cyclic reactions and the penetration of molten NaCl through the voids into the alloy substrate. The 9Cr-1Mo steels coated with 7wt%Si/93wt%Al oxidized at 750, 850, and 950?C in static air show that oxidation kinetics followed a parabolic rate law at 750 and 850 ?C. The cracks propagated through the FexAly layer due to the growth of brittle FeAl2 and Fe2Al5 at 750 and 850?C. The voids condensed in the interface of intermetallics and substrate are attributed to the Kirkendall effect. At 950?C, the fast growing aluminide layer has a different expansion coefficient than oxide scale, leading to scale cracking, oxygen penetration, and internal oxidized, evidenced by a rapid mass gain.