Properties of Pack Aluminized Steels

Oxidation Resistance. The action of pack aluminized steels under heat is similar to that of unprocessed stainless steels. Stainless steels owe their oxidation resistance to the formation of a thin chromium-rich oxide (Cr2O3) film which protects the underlying steel. A similar reaction occurs with aluminized steels, which form an even more protective oxide. Al2O3, from the aluminum in the coating. This oxide grows at a lower rate than Cr2O3 and does not exhibit volatility in the presence of oxygen above about 927 °C (1700 °F) as does Cr2O3. While a type 304 stainless steel scales excessively at temperatures of > 870 °C (> 1600 °F) in an oxidizing environment, a pack aluminized carbon steel exhibits only slight discoloration of the surface under these conditions. Figure 9 compares aluminized carbon steel with bare carbon steel, 5% Cr steel, 12% Cr steel, and 18Cr-8Ni steel with respect to the temperature at which scaling is less than 10 mg/cm2 for oxidation in air. High-strength, highly alloyed stainless steels are also frequently aluminized to improve their oxidation resistance in elevated-temperature applications.

Steel type

Fig. 9 Oxidation of steels in air at the temperature at which scaling is less than 10 mg/cm2. Source: Ref 60

Steel type

Fig. 9 Oxidation of steels in air at the temperature at which scaling is less than 10 mg/cm2. Source: Ref 60

Sulfidation Resistance. Pack aluminized steels have remarkable resistance to the corrosive attack of gases such as hydrogen sulfide (H2S), sulfur dioxide (SO2), and sulfur trioxide (SO3) as well as many other sulfur-bearing atmospheres at temperatures exceeding 230 °C (450 °F). The diffusion zone typically contains a minimum of 20% Al while the alloy surface is about 50 at.% Al and is far more resistant to high-temperature sulfide corrosion than stainless steels containing as much as 28% Cr. The maximum temperature at which these materials can be used in sulfur-bearing environments is limited by the high-temperature mechanical properties of the base steel. Figure 10 provides experimental data on the relative corrosion rates of bare and aluminized 9Cr-1Mo steel in H2S environments.

Fig. 10 Relative corrosion rates of 9Cr-1 Mo alloy steel in 5 mol% H2S at 3550 kPa (515 psi) for 300 h. Source: Ref 61

Fig. 10 Relative corrosion rates of 9Cr-1 Mo alloy steel in 5 mol% H2S at 3550 kPa (515 psi) for 300 h. Source: Ref 61

Carburization Resistance. When exposed at high temperatures to carbon-rich atmospheres, both stainless and alloy steels will carburize, become extremely brittle, and lose their heat- and corrosion-resistance properties. In certain environments, metal dusting associated with carburization will rapidly destroy the steel. Pack aluminized steels, however, suffer negligible deterioration from either carburization or metal dusting in high-temperature, carbon-rich atmospheres, thus greatly lengthening the service life of the steels in these environments. Figure 11 compares the carburization resistance of bare and pack aluminized austenitic stainless steels.

100 200 300 400 SOD 600 700 SCO 900 1000 Q Exposure time, h

Fig. 11 Carburization resistance of bare and aluminized stainless steels at 925 °C (1700 °F). Source: Ref 62

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