Abstract
In order to enhance corrosion resistance of stainless steel (SS) 316LN at high temperature environments, surface modification was carried out by Si deposition and subsequent heat treatment at 900 °C for 1 h. This resulted in the formation of Fe5Ni3Si2 phase on the surface region. The surface-modified alloy was exposed to high temperature S-CO2 (650 °C, 20 MPa) and steam (650 °C, 0.1 MPa) for 500 h and evaluated for its corrosion behavior in comparison to the as-received alloy. In S-CO2 environment, the as-received SS 316LN showed severe oxide spallation and thick Fe-rich oxide formation, while the surface-modified alloy formed a continuous and adherent Si- and Cr-rich oxide layer. In steam, as-received SS 316LN formed very thick duplex Fe- and Cr-rich oxide layers. On the other hand, surface-modified SS 316LN formed notably thinner oxides, which could be attributed to the formation of Si-rich oxide under outer Fe-rich oxides on the surface-modified alloy. Thus, in view of the weight changes, oxide thickness, and morphologies of the two conditions, it was found that Si diffusion coating was effective in improving the corrosion resistance of SS 316LN in both S-CO2 and steam environments.
Highlights
With increasing operating temperatures of generation power plants for greater efficiency, Ni-base alloys with high creep and corrosion resistance are likely to be used for critical load-bearing components such as turbine blades
Fe, Ni, and Cr contents in this layer are about 45, 18, and 14 wt.%, respectively. This indicates that interdiffusion of the deposited Si layer and elements in the matrix occurred during heat treatment at 900 ◦ C, and formed Fe5 Ni3 Si2 phase
TEM analyses performed on the Si-rich oxide formed(not on the surfaceafter steam corrosion in this study revealed it to be amorphous
Summary
With increasing operating temperatures of generation power plants for greater efficiency, Ni-base alloys with high creep and corrosion resistance are likely to be used for critical load-bearing components such as turbine blades. It has been reported that stainless steel (SS) 316LN showed significantly higher weight gains compared to other alloys after exposure to supercritical-carbon dioxide (S-CO2 ) at 650 ◦ C (20 MPa) for up to 3000 h [1]. This was due to spallation of initially formed continuous Cr-rich oxide layer followed by growth of thick Fe-rich oxides. Severe oxide spallation was reported for SS 316L when exposed in steam at 650 ◦ C (0.1 MPa) for up to 5000 h [2]
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