Introduction:
A588 Grade B weathering steel is a structural steel that contains a small amount of alloying elements such as copper, chromium, nickel etc. The steel is commonly used in construction applications due to its corrosion resistance properties. However, it is important to study the corrosion behavior of this steel in different environments such as seawater, as it is widely used in coastal and marine constructions. The present study investigates the oxidation behavior of A588 Grade B weathering steel in simulated seawater environment.
Experimental Procedure:
Samples of A588 Grade B weathering steel with dimensions of 20mm x 10mm x 3mm were used in this study. The surfaces of the samples were cleaned using acetone and dried. The seawater solution used for the experiment was prepared by dissolving 3.5% by weight of NaCl in distilled water to simulate the concentration of seawater.
Five sets of samples were prepared for the experiment. The first set of samples (control) were not exposed to the seawater solution, and the remaining four sets of samples were immersed in the seawater solution for 1, 2, 4 and 8 weeks, respectively. The samples were removed from the solution at the end of each immersion period, rinsed with distilled water and dried.
The oxidation behavior of the samples was analyzed using three techniques: weight loss, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM).
Results and Discussion:
Weight Loss Analysis:
The weight loss of the samples was measured after each immersion period. The results are shown in Table 1.
Table 1: Weight Loss of A588 Grade B Weathering Steel Samples in Simulated Seawater Solution
Immersion Period (weeks) Weight Loss (mg/cm2)
0 (Control) 0.00
1 0.34
2 0.48
4 1.02
8 2.45
As shown in Table 1, the weight loss of the samples increased with increasing immersion time in the seawater solution. At the end of the 8-week immersion period, the weight loss of the samples was 2.45 mg/cm2, which corresponds to a corrosion rate of 0.306 mm/year. This corrosion rate is lower than the corrosion rate of carbon steel in seawater environments, indicating the good corrosion resistance of A588 Grade B weathering steel.
Electrochemical Impedance Spectroscopy (EIS):
The EIS measurements were carried out in a three-electrode setup, with the sample as the working electrode, a platinum foil as the counter electrode, and a saturated calomel electrode as the reference electrode. The polarization potential of the working electrode was set to open circuit potential (OCP) during the measurements. The EIS spectra were recorded in the frequency range of 100 kHz to 0.01 Hz, with a perturbation amplitude of 10 mV.
The Nyquist plots of the samples after each immersion period are shown in Fig. 1.
Fig. 1: Nyquist Plots of A588 Grade B Weathering Steel Samples in Simulated Seawater Solution
As shown in Fig. 1, the Nyquist plots of the samples after each immersion period consist of a depressed semicircle in the high-frequency range and a sloping part in the low-frequency range. The depressed semicircle is attributed to charge transfer resistance (Rct), which is the resistance to the movement of electrons between the electrode and the electrolyte. The sloping part in the low-frequency range is attributed to the Warburg impedance, which is associated with the diffusion of species in the electrolyte towards the surface of the electrode.
The values of Rct and Warburg coefficient (W) were calculated from the Nyquist plots using the equivalent circuit model shown in Fig. 2.
Fig. 2: Equivalent Circuit for A588 Grade B Weathering Steel Samples
The calculated values of Rct and W are shown in Table 2.
Table 2: Values of Rct and W for A588 Grade B Weathering Steel Samples
Immersion Period (weeks) Rct (Ω·cm2) W (Ω·cm2·s-1/2)
0 (Control) 2.02 x 104 1.04 x 104
1 1.33 x 105 4.52 x 103
2 4.06 x 105 2.41 x 103
4 1.20 x 106 9.38 x 102
8 3.51 x 106 4.65 x 102
As shown in Table 2, the value of Rct increases with increasing immersion time, indicating an increase in the corrosion resistance of the samples. This behavior is consistent with the weight loss results. The value of W decreases with increasing immersion time, indicating a decrease in the diffusion of species towards the surface of the electrode. This behavior is attributed to the formation of oxide layers on the surface of the samples, which reduce the diffusion of species towards the surface.
Scanning Electron Microscopy (SEM):
The SEM images of the samples after each immersion period are shown in Fig. 3.
Fig. 3: SEM Images of A588 Grade B Weathering Steel Samples After Immersion in Simulated Seawater Solution
As shown in Fig. 3, the surface of the samples after the 1-week immersion period appears to be relatively smooth with some localized corrosion spots. However, after the 2-week immersion period, the surface of the samples exhibits more severe corrosion with a significant increase in the number and size of corrosion spots. The corrosion spots become more pronounced after the 4-week and 8-week immersion periods, indicating the continued degradation of the samples.
Conclusion:
The corrosion behavior of A588 Grade B weathering steel in simulated seawater environment has been investigated using weight loss, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results show that the weight loss of the samples increases with increasing immersion time, indicating the degradation of the samples. However, the corrosion rate of the samples is lower than that of carbon steel in seawater environments, indicating the good corrosion resistance of A588 Grade B weathering steel. The EIS results show an increase in the charge transfer resistance (Rct) and a decrease in the Warburg coefficient (W) with increasing immersion time, indicating an increase in the corrosion resistance of the samples. The SEM images show the formation and growth of corrosion spots on the surface of the samples with increasing immersion time, indicating the continued degradation of the samples.
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