Muhammad Ali Abro

High temperature corrosion of hot-dip aluminized steel in Ar/1%SO2 gas

Carbon steels were hot-dip aluminized in Al or Al-1at%Si baths, and corroded in Ar/1%SO2 gas at 700-800 °C for up to 50 h. The aluminized layers consisted of not only an outer Al(Fe) topcoat that had interdispersed needle-like Al3Fe particles but also an inner Al-Fe alloy layer that consisted of an outer Al3Fe layer and an inner Al5Fe2 layer. The Si addition in the bath made the Al(Fe) topcoat thin and nonuniform, smoothened the tongue-like interface between the Al-Fe alloy layer and the substrate, and increased the microhardness of the aluminized layer. The aluminized steels exhibited good corrosion resistance by forming thin α-Al2O3 scales, along with a minor amount of iron oxides on the surface. The interdiffusion that occurred during heating made the aluminized layer thick and diffuse, resulting in the formation of Al5Fe2, AlFe and AlFe3 layers. It also smoothened the tongue-like interface, and decreased the microhardness of the aluminized layer. The non-aluminized steel formed thick, nonadherent, nonprotective (Fe3O4, FeS)-mixed scales.

High-temperature oxidation of AZ91-0.3%Ca-0.1%Y alloy in air

AZ91 magnesium alloys containing 0.3% Ca and 0.1% Y were cast, and their oxidation behavior was investigated between 425 and 600 °C in atmospheric air to examine roles of Ca and Y during oxidation. During casting, Ca formed Al2Ca particles intergranularly, and reduced the amount of Al12Mg17 particles, while most of yttrium existed as Al2Y particles inter- and intra-granularly in the alloy. The AZ91 alloy oxidized fast above 425 °C, leading to complete ignition. By contrast, AZ91-0.3Ca-0.1Y alloy oxidized very slowly up to 550 °C. Calcium, which is more active than Mg, preferentially oxidized to CaO at the surface of the MgO-rich oxide to suppress the oxidation, evaporation and diffusion of Mg during the initial oxidation stage. Such suppression was due to the quite low vapor pressure and high stoichiometry of CaO. Calcium also suppressed the formation of less oxidation-resistant Al12Mg17 through forming oxidation-resistant Al2Ca in the alloy from the initial oxidation stage.

Corrosion of Fe-(9~37) wt.%Cr Alloys at 700-800 oC in (N2, H2O, H2S)-Mixed Gas

Fe-(9, 19, 28, 37)wt.% Cr alloys were corroded at 700 and 800 oC for 70 h under 1 atm of N2, 1 atm of N2/3.2%H2O-mixed gas, and 1 atm of N2/3.1%H2O/2.42%H2S-mixed gas. The corrosion rate of Fe-9Cr alloy increased with the addition of H2O, and furthermore with the addition of H2S in N2 gas. Fe-9Cr alloy was always nonprotective. In contrast, Fe-(19, 28, 37) wt.%Cr alloys were protective in N2 and N2/H2O-mixed gas because of formation of the Cr2O3 layer. They were, however, nonprotective in N2/H2O/H2S-mixed gas because sulfidation dominated to form the outer FeS layer and the inner Cr2S3 layer containing some FeCr2S4.

High-Temperature Corrosion of NiCrAlY/YSZ Coatings in Ar-1%SO2 Gas

The thermal barrier coating that consisted of ZrO2-8Y2O3 top-coat and Ni-22Cr-10Al-1Y bond-coat corroded in Ar-1%SO2 gas at 1000-1200°C for up to 300 h. The top-coat and bond-coat consisted primarily of β-ZrO2 and (γ-Ni, α-Al2O3), respectively. During corrosion, Al oxidized preferentially to α-Al2O3 to protect TBC, and sulfidation occurred to a small extent.

Corrosion of Fe-9%Cr-1%Mo Steel at 600 and 700℃ in N₂/(0.5, 2.5)%H₂S-mixed Gas

The T91 steel (Fe-9%Cr-1%Mo) was corroded at 600 and 700℃ for 5 - 70 h in the N₂/(0.5, 2.5)%H₂Smixed gas at one atm. It was corroded fast, forming the outer FeS layer and the inner (FeS, FeCr₂O₄)-mixed layer. The formation of the outer FeS layer facilitated the oxidation of Cr to FeCr₂O₄ in the inner layer. Since the nonprotective FeS scale was present over the whole scale, T91 steel displayed poor corrosion resistance.

High Temperature Corrosion of Al Hot-Dipped Low Carbon Steels in N2/H2S-Mixed Gases

A low carbon steel was hot-dip aluminized, and corroded in the N2/0.4%H2S-mixed gas at 650-850°C for 20-50 h in order to find the effect of aluminizing on the high-temperature corrosion of the low carbon steel in the H2S environment. A thin Al topcoat and a thick Al-Fe alloy layer that consisted primarily of Al5Fe2 and some FeAl and Al3Fe formed on the surface after aluminizing. The corrosion rate increased with an increase in temperature. Hot-dip aluminizing increased the corrosion resistance of the carbon steel through forming a thin protective α-Al2O3 scale on the surface. The α-Al2O3 scale was susceptible to spallation. During corrosion, internal voids formed in the Al-Fe alloy layer, where the Al5Fe2, AlFe, and Al3Fe compounds gradually transformed through interdiffusion.