In this work, friction stir welding (FSW) was employed to alloy 625 grade I (soft annealed) sheets. Therefore, solid-state based welding was undertaken with a tool rotational speed of 200 rpm and welding speed of 1 mm/s. Microstructural features were analyzed by light optical and scanning electron microscopy (SEM). Moreover, microhardness measurements were performed. The susceptibility to intergranular corrosion was verified by the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test. Complementary, intergranular corrosion was evaluated by ASTM G28 Method A. FSW promoted grain refinement, increased microhardness, and reduction in the degree of sensitization. Finally, the mean corrosion rate observed in the ASTM G28A test was 0.4406 mm/year, which suggests a good weld quality.
Research Article
Friction Stir Welding: Mitigating the Susceptibility to Intergranular Corrosion of Alloy 625
https://doi.org/10.21203/rs.3.rs-712605/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 03 Mar, 2022
You are reading this latest preprint version
Friction stir welding
alloy 625
intergranular corrosion
double loop electrochemical potentiokinetic reactivation
ASTM G28 Method A.
Alloy 625 (UNS N06625) is employed in the aerospace, nuclear, and oil and gas industries due to its high mechanical properties and exceptional corrosion resistance [1, 2]. In these fields, joining methods applied to nickel alloys are usually based on fusion (arc welding) such as shielded metal arc welding (SMAW), tungsten inert gas (TIG), amongst others. Nickel-based alloys are usually highly alloyed so that traditional fusion welding processes and heat treatments can lead to precipitation of metal-rich carbides (metal often being Cr and Mo), Laves and Delta phases, and promote grain growth [1, 2, 3], which tend to affect their corrosion resistance and mechanical properties.
Alloy 625 sheets are typically supplied in two grades: soft annealed (grade I) and solution annealed (grade II), with differences in their properties. It is interesting to note that the alloy 625 may be chosen for low or high temperature applications [4, 5]. For example, grade I might have higher mechanical properties than grade II. Also, alloy 625 grade I soft annealed has M(C, N) carbonitrides dispersed in the matrix as well as M6C and M23C6 carbides (normally rich in Mo and Cr) at some grain boundaries [2]. Moreover, although alloy 625 (also called Inconel 625®) presents a solid-solution hardening mechanism; it is well known that carbide precipitation plays an important role for improving mechanical properties [6]. In consequence, nickel-based alloys may have increased mechanical properties due to carbide precipitation [7, 8, 9]. On the other hand, carbides can deteriorate the corrosion properties. In this context, when alloy 625 is typically exposed to a temperature range between 600 to 900°C [10], it could exhibit instability and sensitization. Therefore, the sensitization phenomena can be understood as the precipitation of metal-rich carbides (usually Cr and Mo) preferentially at grain boundaries (area of the most free energy [11]). Thus, the neighboring regions may become depleted of these metals and consequently susceptible to intergranular corrosion.
Friction stir welding (FSW) is a high-quality, solid-state joining method to overcome typical issues of the conventional joining techniques in nickel-based alloys since it produces sound joints at relatively low temperatures, but sufficient for recrystallization and small-sized grains to occur. Therefore, improved properties can be achieved by FSW [1]. In this process, a rotating tool performs a movement that promotes hot work and material in the plasticized state for the weld processing. In general, filler material is not needed, and the joint is processed autogenously with chemical composition of the base material [12]. However, FSW for high plasticizing and melting point alloys is not well developed as it is worldwide recognized for aluminum alloys (low-melting point). In this context, few works have reported FSW in nickel-based alloys. It remains unknown in investigating the intergranular corrosion properties of friction-stir-welded alloy 625, as indicated in [1].
The intergranular corrosion of various alloys with a passive layer has been analysed over time by distinct corrosion tests [13]. The first electrochemical tests were named after their inventors (Streicher, Strauss and Huey), as mentioned in [14]. Aiming at substituting the long-term assays, the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test was grateful developed and accepted [14, 15, 16]. The main advantage of the DL-EPR test is to obtain fast results for the degree of sensitization (DOS). In this sense, nickel-based alloys have also been evaluated by DL-EPR, with some improvements in the test [13, 17]. However, the DL-EPR test in friction-stir-welded alloy 625 has not been found. Still, the intergranular corrosion though ASTM G28 Method A remains to be explored in friction-stir-welded alloy 625, as proposed in the current investigation.
An understanding of the phenomena that occur on the top surface of friction-stir-welded alloy 625 is of great importance as this region is exposed to the environment; consequently, its integrity might be related to corrosion, wear, and residual stresses. In addition, as intergranular corrosion is the most important degradation mechanism of alloy 625 [5]; it must also be evaluated in friction-stir welds of this nickel-based alloy. Therefore, the current work focuses on mitigating the susceptibility to intergranular corrosion of alloy 625 grade I by FSW.
Friction-stir-welded alloy 625 sheets (300 mm × 80 mm × 3.2 mm) were investigated in this work, with their chemical composition given in Table 1 (data from the supplier). This nickel-based alloy is often chosen for cladding of rigid pipes due to its outstanding corrosion properties. Moreover, the alloy 625 evaluated was in the soft annealed condition (grade I) [2, 18].
Table 1. Chemical composition of alloy 625 grade I (% by weight).
|
|
Ni |
Cr |
Fe |
Mo |
Nb |
Co |
Mn |
Al |
Ti |
Si |
C |
|
Alloy 625 |
Bal. |
21.7 |
4.7 |
8.6 |
3.38 |
0.03 |
0.09 |
0.13 |
0.18 |
0.18 |
0.015 |
|
Standard UNS N06625 |
>58 |
20-23 |
<5 |
8-10 |
3.15-4.15 |
<1 |
<0.5 |
<0.4 |
<0.4 |
<0.5 |
<0.1 |
Alloy 625 sheets were prepared by cleaning the surfaces with abrasive sanding and acetone, aiming at the removal of grease, oxides, and impurities. This step was recommended to increase the surface quality of the joints [2]. Afterwards, the sheets were fixed and aligned on the welding table.
Welded joints were processed in the butt joint configuration. FSW was carried out on a rigid gantry machine, which has servomotors and automated control systems. It was used a polycrystalline cubic Boron Nitride (pcBN) Q70, with W-Re (binder phase), shoulder of 25 mm diameter, and probe of 3 mm long. Furthermore, the tool was tilted 1.5° to the vertical axis to obtain the welded joint. Also, welding was performed with an argon atmosphere to reduce oxidation.
For microstructural analysis, the samples were cut by electrical-discharge machining (EDM), prepared by the metallography practices (sanding and polishing), and etched with Glyceregia (10 ml HCl, 10 ml HNO3, and 0.5 ml glycerine). Thus, the microstructural features were observed by light optical microscopy (LOM) and scanning electron microscopy (SEM).
Local mechanical properties were evaluated in terms of microhardness. The Vickers microhardness profile measured on the top surface was obtained with a 500gf load and distance between indentations of 0.2 mm.
The investigation of the degree of sensitization was based on the DL-EPR test, which is schematically shown in Figure 1. The parameters of this electrochemical test were taken from [13]. In this test, the regions chosen were firstly anodically polarized to the metal passivation zone and then scanned in the reverse direction. In other words, when the region is anodically polarized, it is theoretically covered by a passive layer. Next, when the scan is reversed, the passive film breaks in the metal-depleted regions. These metal-depleted areas (diminished Cr or Mo content) tend to be preferentially attacked due to anodic polarization effects that can create uneven surfaces [19] or even a thinner and less protective oxide layer [20], so that these act as preferential sites for the depassivation. Hence, the selected regions investigated were as follows: base material (BM) and stir zone (SZ).
The susceptibility to intergranular corrosion through weight loss was carried out in accordance with ASTM G28 Method A [21]. Two samples (12.5 mm x 50 mm x 3.2 mm) were machined by EDM, sanded and polished. The weight change was verified after the tests. A solution composed of 25 g FeSO4, 236 ml H2SO4, and 400 ml deionized water was used. The duration of the tests was 120 hours (starting from the time that the solution reached the boiling point). Hence, the corrosion rate was calculated via equation 1, as follows:
where the constant was 86.760 and the density of alloy 625 was 8.44 g/cm³ [5, 21]. Furthermore, the samples tested were observed by LOM.
A schematic sketch (Figure 2) is therefore detailing the samples for both corrosion tests (DL-EPR and ASTM G28 Method A).
Base material
The base material microstructure is shown in Fig. 3. Therefore, the alloy 625 grade I (soft annealed) presents an amount of MC carbonitrides (Fig. 3a)), M6C (Fig. 3b)), and M23C6 carbides (Fig. 3c)). In this context, the MC carbonitrides were mainly located in the matrix, with the metal usually being Nb or Ti. Furthermore, the M6C and M23C6 carbides, rich in Mo and Cr, respectively, were observed at isolated grain boundaries, as prior reported in [2, 22].
Friction-stir-welded alloy 625
The macrostructure on the top surface of friction-stir-welded alloy 625 is shown in Fig. 4, where small sized grains are verified through a darker microstructure (next detailed in Fig. 4b)). Therefore, an MC type carbonitride (metal being Ti) was noted as well as a remarkably grain refinement of the austenite microstructure, which is typically accomplished by FSW in alloy 625 [2, 22]. In addition, no significant quantity of M6C and M23C6 carbides were observed by SEM imaging.
Figure 5 presents the time-temperature-sensitization diagram of alloy 625 grade I (soft annealed) [23], along with peak temperatures measured in the current investigation (temperatures at 15 mm distance from the weld centerline). Unsurprisingly, the maximum temperature on the AS was higher than that on the RS [22]. Furthermore, it is interesting to note that FSW would not lead to considerable sensitization in alloy 625 (peak temperatures outside of the “C” curve). Otherwise, FSW tends to promote smaller grains and a cleaner microstructure compared to the alloy 625 grade I, base material (as received), thus diminishing the susceptibility to sensitization of alloy 625.
The Vickers microhardness profile measured on the top surface can be seen in Fig. 6. Therefore, alloy 625 grade I (base material) had microhardness values ranging from 250 to around 280 HV. In the stir zone (SZ), microhardness values up to approximately 340 HV were achieved. Thus, the increased microhardness values in the weld zone are commonly explained by finer grains obtained through FSW (clearly seen in Fig. 4). Recently, we suggested that an achievement of small-sized grains due to FSW can be related to the dynamic recrystallisation coming from the interaction between two variables (plastic flow and heat generated), which are sufficient to thus drive the recrystallization process [22].
In Fig. 7, the differences between the peak activation current density (Ia) and the peak reactivation current density (Ir) indicate a distinct electrochemical behavior among the passivation and depassivation for each region evaluated. Moreover, a significant variation in the Ir/Ia ratio between the base material (BM) and the stir zone (SZ) was verified. Thus, it was noted that alloy 625 grade I achieved a degree of sensitization of 1.23. On the other hand, FSW promoted a refined and even clean resulting microstructure concerning the carbides that would influence sensitization, facts that caused a lower degree of sensitization (0.66) in SZ. This has good agreement with the findings shown in Figs. 4 and 5. Other studies reported similar outcomes, also indicating that FSW can be beneficial to mitigate the susceptibility to sensitization of distinct metals (AA 5083 aluminum alloy [24] and 304 stainless steel [25]).
The corrosion rate evaluated by ASTM G28A test is presented in Table 2. Both samples showed similar weight loss. Thus, the average corrosion rate was found to be of 0.4406 mm/y. According to [5], the error of this test is ± 0.05 mm/y. In addition, some authors mentioned that the threshold for sensitization in the industry is 1 mm/y [13], thus an acceptable friction-stir-welded alloy 625 was obtained. It is worth noting that authors [26] showed that Inconel 625, produced by wire-arc additive manufacturing (WAAM), reached an average corrosion rate of 0.609 mm/year (slightly higher than that of the current work), which was suggested to have an excellent resistance to intergranular corrosion. Furthermore, for Inconel 686, another nickel-based alloy that was reported to be processed by gas metal-arc welding (GMAW) and by gas tungsten-arc welding (GTAW), the corrosion rate was respectively 2.5 mm/y and 2.3 mm/y [27]. Overall, based on the above observations, a suitable welded joint of alloy 625 was produced by FSW.
| Sample | Area (cm2) | Initial weight (g) | Final weight (g) | Weight loss | Corrosion rate (mm/y) |
|---|---|---|---|---|---|
| I | 15.235 | 9,6966 | 9.6187 | 0.779 | 0.4422 |
| II | 16.373 | 11.0078 | 10.9247 | 0.0831 | 0.4390 |
| Average | 0.4406 | ||||
Figure 8 shows the micrographs after ASTM G28 Method A. As this assay comprises the weight loss, and the BM had a certain amount of M6C and M23C6 at some grain boundaries, and these carbides are the most relevant to intergranular corrosion, it is assumed that the BM lost more weight than the SZ. Thus, it was noted that the MB (Fig. 8a)) achieved a worse behavior than the SZ (Fig. 8b)) regarding intergranular corrosion, which agrees well with the outcomes from DL-EPR test (Fig. 7). Therefore, the BM had the highest degree of sensitization and weight loss due to intergranular corrosion. In contrast, the SZ showed improved resistance to intergranular corrosion. In other words, FSW thus mitigated the susceptibility to intergranular corrosion of alloy 625.
This work presented friction stir welding as an alternative for mitigating the susceptibility to intergranular corrosion of alloy 625 grade I (soft annealed). The results of the present investigation can be summarized as follows:
i) FSW led to the formation of small-sized grains and increased microhardness in the stir zone.
ii) Electrochemical double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests showed that FSW decreased the degree of sensitization of alloy 625, which is likely to be related to grain refinement and the absence of carbides at grain boundaries (verified by SEM) that would influence sensitization. Therefore, the susceptibility to intergranular corrosion of alloy 625 grade I was diminished by friction stir welding.
iii) The mean corrosion rate verified with the intergranular corrosion ASTM G28 Method A test was 0.4406 mm/year, which suggests a good weld quality.
Author contributions statement
Lemos, G. V. B.: conceptualization, methodology, data curation, investigation, formal analysis, writing, and editing – original draft; Farina, A. B.: corrosion tests, data curation, review, and editing; Piaggio, H.: electrochemical corrosion analysis, writing, review, and editing; Bergmann, L: friction stir welding, investigation, review; Zoppas, J.: electrochemical corrosion analysis, supervision, and review; Dos Santos, J. F.: friction stir welding, investigation, review, and editing; Voort, G. V.: metallography, review, and editing; Reguly, A.: supervision, review, and editing.
Declaration of Competing Interest
The authors declare no competing interests.
Acknowledgements
The authors are thankful to Prof. Dr Telmo Roberto Strohaecker (LAMEF-PPGE3M-UFRGS) whose presence and wisdom are deeply missed.
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No competing interests reported.