Introduction:
The A588 Grade B weathering steel plate is a low carbon steel alloy composed primarily of iron with added elements such as chromium, nickel, copper, and phosphorus. The microalloying elements of niobium (Nb) and titanium (Ti) have been shown to improve the mechanical properties of weathering steels and hinder the formation of harmful phases in welding. Corrosion is a key factor that affects the durability and lifespan of steel structures. In this study, we investigate the influence of Nb and Ti microalloying on the corrosion behavior of welded joints of A588 Grade B weathering steel plate in different environmental conditions.
Experimental Procedure:
The A588 Grade B weathering steel plate was cut into 50 mm × 100 mm × 2 mm sheets for lap welding. The welding was performed using gas tungsten arc welding with a welding current of 100 A. The welding was carried out using 1.2 mm diameter filler wire with different Nb and Ti levels. Three different filler wire compositions were used: 0.05%Nb-0.05%Ti, 0.1%Nb-0.1%Ti, and 0.15%Nb-0.15%Ti. The welding was performed in a single pass with a heat input of 0.8 kJ/cm.
The welded joints were cut into 10 mm × 10 mm × 2 mm specimens, which were then polished with SiC paper up to 2000 grit. The specimens were cleaned with acetone and dried before the corrosion tests. Electrochemical impedance spectroscopy (EIS) was carried out in 3.5% NaCl solution, pH 6.5, and pH 3.0. The polarization curves were obtained in the same solutions using a potentiostat with a scan rate of 1 mV/s. The surface morphology and elemental composition were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).
Results and Discussion:
The results showed that the addition of Nb and Ti microalloying elements had a significant influence on the corrosion behavior of the welded joints under different environmental conditions. The corrosion rate decreased with the increase in Nb and Ti content in the filler wire. The corrosion resistance of the welded joint increased with the addition of Nb and Ti because of the formation of passive films on the surface of the welded joints. The passive films formed on the surface of the welded joints act as a barrier to the diffusion of corrosive ions through the interface, slowing down the corrosion rate.
The EIS results were consistent with the polarization curves. The presence of the passive films on the surface of the welded joints caused an increase in impedance as well as a decrease in the corrosion rate. The EIS showed that the Nb and Ti microalloying elements improved the barrier properties of the passive films and increased the resistance to the diffusion of the corrosive ions into the surface of the welded joints.
The SEM and EDS analysis of the surfaces of the welded joints showed that the microalloying elements promoted the formation of a uniform, dense, and compact passive film on the surface of the welded joints. The passive films formed on the surface of the welded joints contained Nb and Ti oxides as well as Fe and Cr oxides. The presence of Nb and Ti oxides in the passive films made them more resistant to corrosion, as they provide a higher electrochemical stability.
Conclusion:
In conclusion, Nb and Ti microalloying elements significantly improved the corrosion resistance of the welded joints of A588 Grade B weathering steel plate in different environmental conditions. The addition of Nb and Ti to the filler wire used for lap welding produced a uniform, dense, and compact passive film on the surface of the welded joints. The passive films acted as a barrier to the diffusion of corrosive ions, reducing the corrosion rate. The study demonstrated that Nb and Ti microalloying elements could be used to enhance the corrosion resistance of welded joints of weathering steels and, thus, prolong the lifespan of steel structures.
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