3.1 Weight loss Analysis
Figure 4 depicts the corrosion rate of carbon steel welds after 85 days of immersion in seawater environment. The corrosion rate of the samples increases drastically for the first 51 days, with an optimal corrosion rate of 0.804 mm/yr as experienced by the as-welded sample at the first 17 days. Then the corrosion gradually reduces as the time of exposure increases; this change is due to the aggressiveness of the chemical constituents, transport mechanisms, and temperature of the seawater environment. This resulted in the formation of a reddish-brown colouration that forms a protective layer; this layer inhibited the surface, thereby reducing the corrosion rate [22]. This is in accordance with the findings of Zhang et al.[23], who observed that when protective barrier layers developed on the steel surfaces over time, the corrosion rate of carbon steel in general decreased. Further results from figure 4 also revealed that the PWT-7000C sample exhibited the lowest corrosion rate as compared to the other post tempered samples (as-welded, PWT-550 0C, and PWT-650 0C).
3.2 Electrochemical measurements
The electrochemical measurement was investigated with open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy.
i. Open circuit potential (OCP)
Figure 5 depicts the variation of Eocp with time obtained for as-welded and post weld tempered UNS G10400 carbon steel weld in seawater at 25°C. The figure also shows that Eocp in the as-welded part started at -0.523 v and then shifted anodically,and the steady state was reached after 300 s. This reveals that the initial dissolution of the air formed oxide on the samples and then began the attack on the metal surface. The post-weld tempered samples (PWT-550 0C,PWT-650 0C, and PWT-700 0C), Eocp, started at a relatively positive potential with respect to the as-welded samples, then shifted anodically. This result further reveals that there is an increasing shift of the Eocp in the positive direction as experienced by the post-weld-tempered samples. This shift is due to the formation of a protective oxide film, which restrains the attack of Cl−ions present in the seawater [24].
ii. Potentiodynamic Polarization (PDP)
The potentiodynamic polarization test was conducted to evaluate the protective performance of as-welded and post-weld tempered UNS G10400 carbon steel, as well as the cathodic and anodic corrosion behavior of the carbon steel [25]. Tafel curves of as-welded and post-weld-tempered samples of UNS G10400 carbon steel in seawater are shown in Fig. 6. It is observed from the post-weld tempered (PWT-550 0C, PWT-650 0C, and PWT-700 0C) curves that the cathodic current was highly suppressed as compared to the anodic current. Electrochemical corrosion parameters, namely corrosion densities (jcorr), cathodic tafel slopes (βa and βc), and corrosion potential (Ecorr), were obtained from the tafel plot, and the results are presented in Table 3. As observed, the post-weld tempered samples have lower corrosion densities ((jcorr) compared to the as-welded samples. This is due to the effect of the tempering process, which minimizes the ion exchange within the metal's anodic and cathodic sites [26].The corrosion rates of the as-welded and post-weld tempered samples are 0.13510, 0.06331, 0.06271, and 0.01578 mm/yr, respectively. The low corrosion values of the tempered samples could be attributed to the formation of a protective film on the metal surface [11].
Table 3
Tafel data for the as-welded and post weld tempered UNS G10400 Carbon steel samples in seawater.
Samples
|
βa (mVdec− 1)
|
βc(mVdec− 1)
|
Ecorr(V)
|
jcorr(A/cm²)
|
Corrosion Rate (mm/yr)
|
As-welded
|
19.396
|
2.68
|
-4.114
|
5.814
|
0.1351
|
550 0C
|
5.533
|
4.22
|
-2.72
|
2.725
|
0.06331
|
650 0C
|
5.464
|
3.464
|
-2.055
|
2.699
|
0.06271
|
700 0C
|
1.021
|
1.659
|
-2.007
|
0.679
|
0.01578
|
iii. Electrochemical impedance spectroscopy (EIS)
Figure 7 shows the impedance spectra for as-welded and post-weld-tempered samples of UNS G10400 carbon steel in seawater. The plot depicts a single depressed capacitive semicircle between the frequency range. The plots for the as-welded and post-weld tempered samples are similar, indicating similar corrosion mechanisms [27]. However, the smaller size of the semicircle for as-welded in seawater suggests lower charge transfer resistance (Rct), as depicted in Table 4, hence higher susceptibility to corrosion. The post-weld tempered samples exhibited better corrosion resistance, as shown from the data presented in Table 4. It is clear that Rct for post-weld tempered samples increased with an increase in tempering temperature. In other words, the lower double-layer capacitance (Cdl) value as observed in Table 4 shows that Cdl for the post-weld tempered samples decreases with the tempering process. The difference in electrochemical characteristics of the post-weld tempered steel can be attributed to the change in mechanical properties and micro-structural re-crystallization that occur as a result of post-weld tempering [27].
Table 4
Nyquist data for as-welded and post-welded tempered samples
Samples
|
Rs(Ωm2)
|
Cdl(µF/cm²)
|
n
|
Rct(Ωm2)
|
As-welded
|
0.00016
|
52.52
|
0.9987
|
1724
|
550 0C
|
0.001
|
31.72
|
0.9818
|
3036
|
650 0C
|
0.0012
|
15.51
|
0.7532
|
3172
|
700 0C
|
1.378
|
7.67
|
0.7167
|
3726
|
3.3 Surface Morphology
The microstructure of the as-welded sample was composed of grain boundary ferrite [28] as shown in Fig. 8(a). In contrast to the post-weld tempered samples (PWT-550 0C, PWT-6500C, and PWT-700 0C) as depicted in Fig. 8(b-d), these samples have an increased grain size, forming a tempered martensite. These results are due to re-crystallization and grain growth resulting from post-weld tempering. Generally, grain size increases with an increase in tempering temperature [29]. The results obtained reveal that post-weld tempering had significant impact on corrosion resistance of the material and that the bigger the grain size of the micro-structure of the martensite structure, the more the corrosion resistance of the samples increases; hence, the PWT-7000C exhibited a more superior resistance as a result of the increased grain size of the samples [30].
The Energy-Dispersive X-ray Spectroscopy (EDX) studies, as shown in Fig. 8 (e), show that Fe(Iron) in the as-welded samples exhibited the highest Fe peak, while Fig. 8 (h), the EDX of PWT-700 0C, shows that the peak value of Carbon (C) exhibited the highest increases when compared relatively to Fig. 8 (f-g) of the other post-weld tempered samples. The increase in the peak value of C that occurred in the PWT-7000C sample is due to the overlaying protective film [31].This result is in agreement with those previous results obtained from weight loss and electrochemical measurements, which suggest that the surface film protected the metal dissolution and further retarded the rate of corrosion [32, 33].