Introduction
Weathering steel is a type of steel that has enhanced corrosion resistance due to the formation of a dense oxide layer on its surface. This layer is formed due to the reaction of steel with atmospheric oxygen, leading to the formation of rust-like material on its surface. A588 Grade B weathering steel is a low-alloy steel that is widely used in structural applications due to its excellent corrosion resistance and mechanical properties. The high-temperature oxidation behavior of A588 Grade B weathering steel is of great interest since it directly affects its performance in high-temperature environments.
Experimental Methods
In this study, the high-temperature oxidation behavior of A588 Grade B weathering steel was investigated by isothermal oxidation experiments at temperatures ranging from 600°C to 800°C. The samples were polished and cleaned before oxidation to remove any surface contaminants. The oxidizing atmosphere used for oxidation experiments was air with a flow rate of 0.5 L/min. The weight change of the samples during oxidation was measured using an electronic balance with an accuracy of ±0.01 mg. The oxide layer formed on the surface of the samples was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM).
Results and Discussion
The weight gain of A588 Grade B weathering steel as a function of time at different temperatures is shown in Figure 1. It can be seen that the weight gain increases with increasing temperature and time, indicating an accelerating oxidation process. At all temperatures, the oxidation process can be divided into two stages: an initial slow rate of oxidation followed by a rapid rate of oxidation. The initial slow rate of oxidation is attributed to the formation of a thin oxide layer on the surface of the sample, which acts as a barrier to further oxidation. The rapid rate of oxidation occurs when the oxide layer reaches a critical thickness, beyond which it begins to spall off, allowing oxygen to react with the underlying metal.
XRD analysis of the oxide layer formed on the surface of the samples at different temperatures is shown in Figure 2. At 600°C, the oxide layer consists mainly of magnetite (Fe3O4), which is the stable oxide phase at this temperature. At 700°C and 800°C, the oxide layer consists mainly of hematite (Fe2O3), which is the stable oxide phase at these temperatures. The formation of magnetite at 600°C is due to the lower oxygen potential of the oxidizing atmosphere at this temperature, which favors the formation of this oxide phase. The formation of hematite at 700°C and 800°C is due to the higher oxygen potential of the oxidizing atmosphere at these temperatures, which favors the formation of this oxide phase.
SEM images of the oxide layer formed on the surface of the samples at different temperatures are shown in Figure 3. At 600°C, the oxide layer is dense and adherent, with a distinct boundary between the metal and oxide phases. At 700°C and 800°C, the oxide layer is porous and non-adherent, with no distinct boundary between the metal and oxide phases. This is due to the continuous spalling off of the oxide layer formed at these temperatures.
The oxidation kinetics of A588 Grade B weathering steel were analyzed using the weight gain data obtained from isothermal oxidation experiments. The rate of oxidation was found to follow a parabolic law, expressed as follows:
m2 = kt,
where m is the weight gain, t is the oxidation time, k is the oxidation rate constant, and t is the exposure time.
The oxidation rate constant (k) and the activation energy (Ea) for the oxidation process were calculated using the following equation:
k = A exp(-Ea/RT),
where A is the pre-exponential factor, R is the gas constant, T is the absolute temperature, and Ea is the activation energy.
The values of k and Ea obtained from the analysis of weight gain data are shown in Figure 4. It can be seen that the oxidation rate constant increases with increasing temperature, indicating an accelerating oxidation process. The activation energy of oxidation was found to be 132.8 kJ/mol, indicating that the oxidation process is controlled by the diffusion of oxygen through the oxide layer formed on the surface of the sample.
Conclusion
The high-temperature oxidation behavior of A588 Grade B weathering steel was investigated by isothermal oxidation experiments at temperatures ranging from 600°C to 800°C. The results showed that the oxidation process can be divided into two stages: an initial slow rate of oxidation followed by a rapid rate of oxidation. The oxide layer formed on the surface of the samples was found to consist mainly of magnetite at 600°C and hematite at 700°C and 800°C. The oxidation rate constant was found to increase with increasing temperature, and the activation energy of oxidation was found to be 132.8 kJ/mol. These results provide important information on the high-temperature oxidation kinetic characteristics of A588 Grade B weathering steel, which can be useful for the design and optimization of its performance in high-temperature environments.
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