3.1. Pitting Corrosion Properties of Weld Metals
The pitting corrosion test was performed in accordance with ASTM G 48 Method A requirements; the test specimens were immersed into ferric chloride solution to the equilibrium temperature of 25 ± 1 °C for 24 h, in accordance with client specifications [13
]. The initial results were obtained for calculating the area and mass loss. At the end of the 24 hours test period, all of the specimens were removed from ferric chloride solution, cleaned, rinsed, and scrubbed with a soft bristle brush under running water and dried to remove any corrosion properties. The results that are indicated in Figure 1
had displayed that sample no.1 has weakened pitting corrosion resistance by weight loss of 1.56 g/m2
and that pitting preferentially occurred at ferrite grain in the fusion zone. The minor weight loss was caused by a deterioration of chromium oxide; no evidence of pits was observed. PWHT was applied to improve the mechanical properties of substrate material of weld overlays. It could be observed that the weight loss per unit area g/m2
had gradually increased with the increment of PWHT temperatures as well as the method of cooling being applied. The sample no. 2 (350 °C AC) and no. 4 (650 °C AC) were both cooled by stirred air and indicated significant amounts weight loss at 1.87 g/m2
and 3.53 g/m2
Sample no. 3 and no. 5 were cooled in water quenching method in which had indicated that the weight loss per unit area was about 12.3–13.6% lower than at the air cooled condition; the actual values were obtained at 1.64 g/m2
and 3.05 g/m2
. The tests results had showed that all weight losses fulfill the client standard requirements, because the metal loss per unit area has not exceeded 4 g/m2
]. Samples 2 to 3 weight loses are very close to sample 1 (as-welded); these were caused by deterioration of chromium oxide, as shown in Figure 2
. Sample 4 and 5 could be associated with the presence of sigma phase precipitation formation in between delta ferrite and austenite (δ/γ) grain boundaries, resulting in the loss of corrosion resistance in the weldment.
These sigma phases aggressively precipitate at the temperature range from 550 °C to 900 °C. Table 4
tabulates the ferrite and austenite (δ/γ) phase boundaries with a high intensity of chromium and molybdenum contents. Sigma phase precipitation occurred at dislocations within the ferrite grains. After nucleating from the ferrite and austenite (δ/γ) phase boundary, the sigma grows into the ferrite. Chromium starts to diffuse from the ferrite to sigma and causes the ferritic lattice to become unstable and transform into austenite. At longer times, the presence of sigma phase grain boundary could lead to intergranular corrosion when it is exposed to marine environment conditions. Therefore, it was recommended to improve the pitting corrosion resistance of weld metal with solution annealing temperature. Additionally, at the interface, the EDX full spectrum pattern had shown the two stronger peaks of Fe and Cr similar ration, as indicated in Figure 3
. These parallel similar peaks had clearly illustrated the formation of intermetallic Fe-Cr compound beside the high Mo content.
Test samples no. 6 and 7 were initiated with hydrogen relief temperature. Similarly with test sample no. 8 and 9, both were initiated with stress relief temperature, and later all of these test samples were heat treated at solid solution annealing temperature (1050 °C) for 2 h to transform ferrite to homogenize austenite microstructure and to dissolve or eliminate intermetallic phases in the weld metal. The weld overlay microstructures were being restored and exhibited better corrosion resistance, as shown in Figure 4
f–i; the amount of weight loss for sample no. 6 to 9 has been significantly to zero, except for sample 8 with minimal loss at 0.36 g/m2
due to coating failure during handling, as highlighted earlier that the bottoms of weld overlay samples were coated with surface tolerant epoxy to prevent corrosion.
3.2. Volume Fraction of Ferrite
The effect of ferrite contents on atmospheric pitting corrosion of duplex stainless steel consumables grade 2209 was investigated with a series of various PWHT and solid solution annealing. The content of ferrites was gradually increased with incremental PWHT temperatures. Figure 5
illustrated the development of ferrite volume fraction with the effect of PWHT and solution annealing temperatures. The results had shown that the ferrite content determination in region of weld metal overlay was increased at hydrogen relief temperature and decreased at stress relief temperature. At 350 °C with the air cooled method, the ferrite counted at 20.0 ± 1.78 vol.% and water quench method, there was a slightly reduction of ferrite content at 19.4 ± 2.05 vol.%. Similarly, with stress relief temperature at 650 °C, ferrite content indicated at 17.5 ± 1.57 vol.% for air cooled and 16.3 ± 1.34 vol.% for water quenching, which is due to slow cooling that is more favorable to austenite formation. However, the test samples with solution annealing temperature at 1050 °C with water quench method had shown lower ferrite contents at 10.6 ± 1.36 vol.% to 11.9 ± 0.82 vol.%; this provided sufficient time for the formation of austenite, giving optimum equilibrium fraction in the weld and dissolve secondary phases.
indicates the microstructure evolution of weld metal overlays after successfully conducted with PWHT. Sample no. 2 and 3 (Figure 4
b,c) showed the grain boundary austenite (GBA) and widmanstatten austenite were slightly decreased with PWHT at 350 °C as compared to the as-welded Sample no.1 (Figure 4
a). Sample 4 and 5 showed that the volume fraction of ferrite were slightly deceased in weld metal, which could be due to the precipitated ferrite phase were dissolved into gamma phase. By increasing the solid solution anneal temperature at 1050 °C with 2 h of holding time, the austenite grains have not showed any favorable growth direction. Sample no. 6 to 9 (Figure 4
f–i) showed that no secondary precipitation was present and a clear profile of austenite microstructures was observed.