The development of silicon‐containing oxides during the oxidation of iron‐chromium‐base alloys

Abstract

The influence of silicon on the oxidation of Fe‐14% Cr and Fe‐28% Cr has been studied at high temperature, with particular emphasis on the development and nature of the healing SiO2 layer. In general, silicon is a less effective addition than aluminium to these alloys in improving oxidation resistance because SiO2 grows at a lower rate than α‐Al2O3. Hence, silicon is a less successful oxygen secondary getter and development of a complete healing layer of SiO2 is less rapid than that of α‐Al2O3 on a corresponding aluminium‐containing alloy. Nonetheless, the addition of only 1% Si to Fe‐28% Cr causes a marked reduction in the overall oxidation rate, particularly by facilitating development of the Cr2O3 scale. Precipitates of SiO2 form at the alloy/scale interface. These grow inwards and laterally until they eventually link up to establish a continuous healing layer at the interface after several hundred hours exposure at 1000°C. Similar features are observed for Fe‐14% Cr‐3% Si but the healing SiO2 layer develops after a much shorter time for Fe‐14% Cr‐10% Si, due to the high silicon availability. In every case, the healing layer has been shown to be amorphous SiO2. Although this phase is very protective during isothermal oxidation, it is a site of weakness during cooling and scale spallation is very extensive from specimens where the SiO2 is continuous, with failure occurring cohesively within that layer. Ion implantation of silicon into Fe‐14% Cr and Fe‐28% Cr gives a reduced oxidation rate due to facilitation of a more rapid establishment of a Cr2O3 scale. Similar implantation of yttrium into the ternary alloys assists in development of the silicon‐containing oxide layer, possibly associated with an influence on the nucleation of the oxide precipitates in the early stages of exposure.

Oxidation Behavior of Fe–25Cr–20Ni–2.8Si During Isothermal Oxidation at 1,286 K; Life-time Prediction

“…the AES results discussed above) is fully amorphous (in accordance with results reported in Ref. 22). …”

“…Although small additions of Si to Cr 2 O 3 -scale-forming steel are known to improve its high-temperature corrosion resistance (in particular, it enhances the reformation of a protective Cr 2 O 3 scale on the barealloy surface after oxide spallation [3,10,[12][13][14][15]), a fundamental understanding of this effect lacks [15][16][17][18]. The Si content corresponding with the maximum oxidation resistance (i.e., the longest breakaway oxidation time at constant temperature) appears to depend strongly on the steel microstructure [3,10,13,[19][20][21][22].…”

Oxidation Behavior of Co-15wt% Cr Alloy Containing Dispersed Oxides Formed by Internal Oxidation

“…An amorphous silica layer, which was believed to act as a diffusion barrier to reduce iloxidation rates, has been detected [20][21][22] at oxide/alloy interfaces in alloys con_ning Si, and the presence of such a silica layer also increased scale spallation [21]. In the present study, the silica layer was never found to be continuous.…”

Long-Term Cyclic-Oxidation Behavior of Selected High Temperature Alloys

“…The former was inferred to be SiO 2 . Addition of Si is known to improve the oxidation resistance of chromia-forming alloys [10][11][12]; however, it is also known to decrease scale spallation resistance [12][13][14]. Apparently, the nominally 2.75 wt% Si addition in the HR-160 alloy forms a continuous SiO 2 layer at the chromia/alloy interface and this in turn may have considerably decreased scale adhesion particularly at 982 o C. It is seen from Table III that HR-160 alloy underwent considerable metal loss (~ 43 µm) at 982 o C even though it contains sufficient amount of Cr to establish a protective oxide scale.…”