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Article

Electrochemical Analysis on Intergranular Corrosion of Austenitic Stainless Steel Weld in Molten Nitrate Salt

by
Noparat Kanjanaprayut
1,
Thamrongsin Siripongsakul
2,*,
Panya Wiman
2,
Wannapha Issaard
2,
Thanasak Nilsonthi
2 and
Piyorose Promdirek
2
1
Corrosion Technology Department, Thai-French Innovation Institute, King Mongkut’s University of Technology North Bangkok, 1518, Pracharat 1 Road, Wongsawang, Bangsue, Bangkok 10800, Thailand
2
High Temperature Corrosion Research Centre, Department of Materials and Production Technology Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, 1518, Pracharat 1 Road, Wongsawang, Bangsue, Bangkok 10800, Thailand
*
Author to whom correspondence should be addressed.
Metals 2024, 14(11), 1284; https://doi.org/10.3390/met14111284
Submission received: 28 September 2024 / Revised: 1 November 2024 / Accepted: 8 November 2024 / Published: 12 November 2024
Browse Figures
Versions Notes

Abstract

:
An investigation of intergranular corrosion (IGC) sensitization in molten nitrate salts of austenitic stainless steel welds of AISI 304, AISI 304H, and AISI321 produced by GTAW with ER 308L and ER 347 fillers was performed. The degree of sensitization (DOS) to IGC was assessed using a double loop electrochemical potentiokinetic reactivation and pitting potential. It was found that DOS levels in weld zones were quite low, not exceeding 15%, while those in HAZs were up to 60% after exposure at 600 °C for 300 h. The low DOS levels were due to low carbide precipitation. However, another cause of DOS was the delta-ferrite to sigma transformation in weld zones. Linear sweep voltammetry was used to quantify the sigma phase.

1. Introduction

Currently, the global warming phenomenon has become a crisis, since energy consumption is mainly derived from fossil fuels, which result in an increase in greenhouse gases. A number of alternative and clean energy sources are being developed and applied throughout the world, such as hydrogen fuel cells, wind, tidal energy, solar photovoltaics, and solar thermal energy [1,2,3,4]. As one of the most sustainable sources of energy, concentrated solar power (CSP) has become an increasingly interesting research topic. Solar energy is converted into thermal energy by tracking and concentrating solar radiation to a receiver. The heat will be kept by thermal energy storage (TES) materials or exchanged by heat transfer fluid (HTF) to drive a turbine to generate electricity [5]. In 2022, the Shouhang Dunhuang 100MW solar power plant and the Qinghai SUPCON Delingha solar power plant achieved records for continuous operation of 263 h and 292.7 h, respectively [6]. In demonstrations, CSP plants connected to the power grid demonstrated a number of advantages, including rapid response, synchronization, frequency regulation, voltage control, and the suppression of low frequency oscillations [6]. In 2025 to 2030, various sustainable power plants will surpass coal-fired power plants [7].
Molten nitrate salts, such as solar salt, are used as TES and HTF materials in CSP plants. The corrosion process becomes more violent as the operating temperature increases. In particular, cracks often occur in furnaces and peripheral equipment at temperatures around 700 °C, as a result of intergranular corrosion (IGC), which can cause significant damage to the equipment. As well as power plants that use molten nitrate salts, the operating temperature in such plants can be raised up to 600 °C, which falls within the range of sensitized temperatures for IGC. IGC is explained by the precipitation of chromium carbide at the grain boundaries. In view of these conditions, there is a deficiency in chromium and close proximity to the site of IGC. With the use of electrochemical analysis techniques, IGC has been evaluated in terms of the degree of sensitization (DOS) [8,9]. Recently, it was reported that stainless steels exposed to molten salts for extended periods of time exhibited a significant amount of IGC in terms of DOS. This finding was compared to those exposed to air at 600 °C [10].
In power plants and petrochemical plants, AISI 304H, AISI 321H, and AISI 347H stainless steel grades are frequently used in containers, tubes, and heat exchangers. As a result of their strength, toughness, weldability, and corrosion resistance, they are expected to withstand extreme temperatures and corrosive environments [11,12]. There is, however, a well-known issue with stainless steel welds: they are prone to intergranular cracking in the heat affected zone (HAZ) [13,14]. In a paper by Arivarasu, dissimilar weldments of AISI 4340 and AISI 304L were tested in K2SO4 with 60% NaCl. This experiment was performed under molten salt conditions [15]. Ramkumar also reported that the weld’s corrosion resistance was better than the base metal when tested with 40%K2SO4–60%NaCl at 650 °C [16]. Vilchez also reported that the corrosion resistance of austenitic stainless steel welds was significantly lower than that of a non-processed base metal, as well as the fact that laser surface melting treatment could delay the IGC of the weld joints [17]. In spite of the fact that welding is a common method of building components and facilities for power plants, there have been few studies that have examined the effects of welding on IGC when exposed to molten salt. Thus, it is very pertinent to understand the corrosion behavior of stainless steel welds when exposed to molten salts in CSP plants.
For CSP plants, austenitic stainless steels are an excellent choice in terms of efficiency and economy. This study investigates the effects of heat on phase formation in the microstructure and the IGC of stainless steel welds in terms of DOS. This step is accomplished by exposing them to molten nitrate salts at 600 °C for up to 300 h. A scanning electron microscope is utilized to analyze the microstructure, as well as linear sweep voltammetry (LSV), an electrochemistry analysis method used to determine the alloy phase in the welds. To evaluate the DOS, double loop electrochemical potentiokinetic reactivation (DL-EPR) and pitting potential measurements are employed.

2. Experiment

2.1. Materials

In this study, a gas tungsten arc welding (GTAW) technique with solid filler material and argon shielding gas was applied. The welding of austenitic stainless steels AISI 304, AISI 304H, and AISI 321H was carried out using two types of filler rods, ER 308L (GEMINI, Samutprakarn, Thailand) and ER 347 (Xiang Welding Industrial, Tianjin, China). ER 308L was used to produce weld-joints of AISI 304-AISI 304 and AISI 304H-AISI 304H, while ER 347 was used for AISI 321H weld-joints. Table 1 shows the chemical compositions of the filler rods, while those of the stainless steels are available elsewhere [10].
AISI 304, AISI 304H, and AISI 321H plates with a thickness of 10 mm were prepared in a single-V butt shape for welding. The root gap and thickness were 2.6–3.0 mm and 1 mm, respectively, with a groove angle of 60°. Using the V-butt geometry, three-layer welding was applied to form the weld bead in the schematic shape shown in Figure 1a. The welding parameters are shown in Table 2. An example of a weld produced by three-layer welding is shown in Figure 1b.
The first and third layers had small volumes of welding, so the current and applied potential was set at 100 A and 11.0–12.0 V. There was more volume in the second layer, so the parameters became 120 A and 12.5–13.0 V. The travel speeds of 60–100 mm/min were used to weld with the filler rods ER 308L, while the higher traveling speeds of 100–150 mm/min were used for ER 347. Due to the low traveling speeds, the temperatures at the start of the second and third layers were higher but not exceeding 145 °C for AISI 304 and AISI 304H welds. The starting temperatures for AISI 321H were lower and were limited to no more than 100 °C.

2.2. Molten Nitrate Salt Exposure

After welding, the stainless plates were cleaned with pickling paste. Samples were cut to 20 × 40 × 10 mm3. The cut samples were exposed to molten nitrate salt in an induction furnace at 600 °C, for 1, 10, 100, and 300 h. The molten nitrate salt contained 60 weight percent NaNO3 and 40 weight percent KNO3 of laboratory quality. Before and after exposure to molten nitrate salt, samples were investigated on delta-ferrite contents, microstructure, and potentiodynamic polarization. Prior to the investigation, each sample was prepared by surface finishing with emery papers and fine finishing using alumina powder 0.3 µm in diameter and degreasing with acetone.

2.3. Microstructure

Microstructure observation was performed to identify the existence of distinguishing phases in stainless steels AISI 304, AISI 304H, and AISI321H. Sodium hydroxide (NaOH) solution was prepared by using a concentration of NaOH 20 g per DI water 100 mL. A positive polarity of 2.5 volts was applied to the specimen for 15 s to the specimen. This step revealed a dendritic structure for delta-ferrite phase in a matrix of austenite phase. After surface finishing, the weld shape was revealed. This step allowed for the placement of a 2 mm diameter ferritescope probe at the center of a weld zone and a HAZ for each sample. The measurements were conducted after the measurements of the standard samples in the test kit were verified. The observation was further carried out using a standard scanning electron microscope, TESCAN MIRA3 series, (TESCAN, Brno, Czech Republic), with energy dispersion spectroscopy (EDX, Oxford Instrument, Abingdon, UK) to identify the existence of the sigma (σ) phase proximity to the delta-ferrite phase.

2.4. Electrochemical Analysis

Potentiodynamic polarization analyses were performed using DL-EPR pitting potential (Epit) and linear sweep voltammetry (LSV) measurement techniques. The testing system was performed using a VSP300 potentiostat Electrochemical Workstation (BioLogic, Seyssinet-Pariset, France). There were three electrodes in the test cell: a saturated calomel electrode (SCE) for the reference electrode, a platinum plate for the counter electrode, and a sample for the working electrode. DL-EPR and pitting potential measurements were used to evaluate the DOS level and the correlation to pitting potentials. The details of these analyses by these methods are described elsewhere [10]. The LSV measurement system uses selective oxidation of the alloy elements chromium and molybdenum to evaluate the quantity of the sigma phase [18,19]. A sample was exposed to a 20% KOH solution through a 6 mm diameter circular window in the test cell. As a pretreatment, a potential of −0.818 VSCE was applied to the sample for 60 s, after which the potential was scanned to the anodic zone at a speed of 3.42 mV/s. A stop point was set at the potential where the electrical current reaches 1 mA/cm2.

3. Experimental Results

3.1. Corrosion Behavior in a Weld Zone and a HAZ

All welds show delta-ferrite dendrites in an austenite matrix. There are differences in the microstructure between AISI 321H and AISI 304 or AISI 304H in carbide formation at the HAZ. In AISI 321H, Ti-C can be observed at HAZ, while, in AISI 304 and AISI 304H, Cr-C is found. Figure 2a,b shows AISI 321H at the HAZ and weld zone, respectively, after being exposed to molten nitrate salt at 600 °C for 300 h. In the HAZ, there are a large number of black contrast spots dispersed near the surface of the sample. The location of the black spots indicates that they contain 1–2 wt % more carbon than the base metal. This observation indicates that they are carbide precipitates formed after exposure to molten salt. As well as carbide precipitates, titanium-nitride is also confirmed in the gray square-shaped spot. The detection of Ti-C precipitates at the HAZ can be confirmed as shown in Figure 2c, while Ti and C mapping images are shown in Figure 2d,e, respectively. The surface of stainless steel shows signs of pitting corrosion in Figure 2a. Upon prolonged exposure to molten salt, pitting corrosion occurs at the HAZ where carbide precipitates appear at the surface. When this action occurs, it develops into IGC. In Figure 2b, the dendritic microstructure at a weld zone displays delta-ferrite, skeletons, or lacy patterns. In the weld zone, little precipitation of carbon dioxide is observed.

3.2. Electrochemical Analysis Results

Figure 3a–f shows DL-EPR curves used to evaluate IGC in a term of degree of sensitization (DOS) of AISI 304, AISI 304H, and AISI 321H after exposure to molten salt at 600 °C for 1, 10, 100, and 300 h. A DL-EPR curve has a forward scan starting from the cathodic potential of −0.4 VSCE to the anodic potential of 0.3 VSCE and a subsequent reverse scan back to −0.4 VSCE. The ratio of maximum current density of the reverse scan to that of the forward scan is calculated as a function of a DOS level in percent. The forward scan at the anodic zone has low current density due to passivation layer formation at the surface of the specimen. This property results in a lower maximum current density of the reverse scan, which indicates damage to the formed passivation layer. Because damage often occurs in the intergranular regions, the DOS level can be evaluated.
Figure 3a–c represents the DL-EPR curves of AISI 304, AISI 304H, and AISI 321H welds. The reverse current densities were relatively low, ranging from 0.0 to 5.3 mA/cm2, compared with the high maximum forward current densities of 35.6 to 47.5 mA/cm2. There is a rising tendency with exposure time for the reverse current density, but such a trend was never found for the forward current density. Reverse current densities of all specimens increased, resulting in DOS levels increasing from 0.6% to 7.8% for AISI 304, from 0.6% to 7.8% for AISI 304H, and from 0.0% to 10.3% for AISI 321H. These results are also shown in Figure 4a–c.
On the other hand, the DL-EPR curves of AISI 304, AISI 304H and AISI 321H in HAZ regions are shown in Figure 3d–f. The maximum forward current densities of AISI 304 at the HAZs are slightly lower than those of welds. However, they decreased by about 46% and 8% for the HAZs of AISI 304H and AISI 321H. This decrease is due to the effect of the Cu addition in austenitic stainless steels, as reported in the previous literature [20,21]. According to previous work [10], this result reproduces well. The weld does not exhibit this trend because the alloy formed at the weld consists partially of a base metal and a filling rod with less than 0.3% Cu. AISI 304 and AISI 304H specimens showed similar reverse current densities, resulting in high DOS levels at HAZs: from 5.1% to 57.1% for AISI 304 and 0.5% to 62.2% for AISI 304H. It should be noted that only AISI 321H showed a slight reduction after exposure to molten salt for 300 h. After 100 h, the DOS level of 52.4% dropped to 44.1% after 300 h. Figure 4a–c summarizes DOS levels based on exposure time. A comparison is made between these results and the previous results that reported a decrease caused by a recovery effect caused by alloying elements such as Ti [10]. This effect, however, is not significant in this present work, likely due to sensitization during the three-layer welding process. Tokunaga et al. simulated Cr-depleted zones in the Fe-Cr-C system and reported that, at temperatures between 700 and 800 °C, Cr depletion occurred with less than 1 h of exposure when C composition was 0.1% [22].
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Figure 3. DL–EPR curves of AISI 304 (a,d), AISI 304H (b,e), and AISI 321H (c,f) after exposure to molten salt at 600 °C for 1, 10, 100, and 300 h. The Figures in the upper row are DL–EPR results measured at weld zones, and those in the lower row are at HAZs.
Figure 3. DL–EPR curves of AISI 304 (a,d), AISI 304H (b,e), and AISI 321H (c,f) after exposure to molten salt at 600 °C for 1, 10, 100, and 300 h. The Figures in the upper row are DL–EPR results measured at weld zones, and those in the lower row are at HAZs.
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Figure 4a–c shows DOS levels based on exposure time at weld zones and HAZs of AISI 304, AISI 304H, and AISI 321H in comparison to those of base metals and super 304H exposure to air reported in the literature [10,23], respectively. It can be seen that DOS levels increase much less when exposed to air than when subjected to molten salt at the same temperature. The reason for this is that molten salt is a better heat transfer fluid than air, which results in a greater amount of heat being transferred. Base metals such as AISI 304 and AISI 304H exhibit higher DOS levels than their HAZ. However, DOS levels become about 60% nearly equal to base metals after a prolonged exposure of 300 h. When exposed to molten salt at 600 °C for more than 100 h, the base material, AISI 321H, exhibits significantly lower DOS levels than its HAZ due to self-healing. As a result of Ti, Ti-C is formed, which prevents Cr from depletion. Compared to 321H at HAZ, recovery was barely detected. As shown in Figure 2a,c, it is due to a large amount of carbide precipitates at the HAZ generated by multi-layer welding, different from the base material. As a result, Ti is unable to form Ti-C further and instead precipitates Cr-C. This action results in Cr-depleted zones in the material, which increase DOS levels and reduce self-healing ability. The DOS levels of AISI 321H at the HAZ have improved slightly after prolonged exposure but remain above 40%.
In contrast, the DOS levels at welds are quite low for all materials. According to the DL-EPR curves for all welds, reverse current densities only increase slightly with exposure time. This increase results in low DOS levels: from 0.6% to 7.8% for AISI 304, from 0.7% to 12.6% for AISI 304H, and from 0.0% to 10.3% for AISI 321H. Welds that are exposed to molten salt without self-healing effects have moderate levels of the DOS in comparison to super 304H with air exposure [23].
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Figure 4. The DOS levels at weld zones and HAZ evaluated by DL-EPR curves as functions of exposure time for AISI 304 in (a), AISI 304H in (b), and AISI 321H in (c) after exposure to molten salt at 600 °C. The [i] and [ii] are adapted from [10,23], respectively.
Figure 4. The DOS levels at weld zones and HAZ evaluated by DL-EPR curves as functions of exposure time for AISI 304 in (a), AISI 304H in (b), and AISI 321H in (c) after exposure to molten salt at 600 °C. The [i] and [ii] are adapted from [10,23], respectively.
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To confirm these results, a pitting potential analysis was performed. Figure 5a,b show the DOS levels and Epit values as functions of exposure time for all specimens, respectively. The broken lines represent data analyzed at welds, while the solid lines represent those at HAZs. As reported by Taji et al. and Hernández et al., DOS levels and Epit values are reciprocally related [24,25]. The Epit evaluation is relevant to Cr-oxide formation on stainless steel surfaces. As a result, Epit is susceptible to pitting corrosion. In a HAZ of each stainless steel, a large amount of Cr-C was formed due to higher carbon composition combined with the multi-layer welding process. This action causes Cr to be depleted, which means the protective Cr-oxide film is vulnerable to pitting corrosion. In contrast, a weld was generated from the base material and filler rods ER 308L or ER 347. The weld carbon content is lower, thus the Epit is lower. Contrary to the DOS, this evaluation is intended to determine whether carbide formation at grain boundaries results in sensitization to IGC. As a result of the correlation with the DOS, Epit exhibits a reverse trend during molten salt exposure. After carbide formation, Cr becomes depleted or an inadequate Cr-oxide protective film is formed.
DOS levels increase with exposure time at both welds and HAZs, corresponding to a decreasing Epit trend. This correlation shows that DOS levels are higher in HAZs but lower in welds. On the other hand, higher Epit values in welds, compared to HAZs. As exposure time is more than 100 h, the DOS levels in HAZs increase above 40%, while Epit values decrease to about 300 mVSCE. Weld DOS levels are lower than 15% in all samples, while Epit values are around 400 mVSCE. The carbon composition of HAZs is much higher than that of welds produced from low-carbon filler rods, suggesting that carbide precipitation is likely to be responsible for high levels of sensitization to IGC. Further, filler rods contain more chromium than base metals, reducing chromium depletion and improving Epit or pitting resistance.
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Figure 5. A comparison between DOS levels in (a) and pitting potentials in (b) at weld zones and HAZ as functions of exposure time for AISI 304, AISI 304H, and AISI 321H after exposure to molten salt at 600 °C.
Figure 5. A comparison between DOS levels in (a) and pitting potentials in (b) at weld zones and HAZ as functions of exposure time for AISI 304, AISI 304H, and AISI 321H after exposure to molten salt at 600 °C.
Metals 14 01284 g005

3.3. Microstructure Analysis

Delta-ferrite contents in welds and HAZs are measured before and after exposure to molten salt for 1, 10, 100, and 300 h. These results are presented in Figure 6a,b for welds and HAZs, respectively. The detected delta-ferrite contents in AISI 304, AISI 304H, and AISI 321H welds are nearly the same at all time intervals. With time, exposure to molten salt at 600 °C reduces the delta-ferrite contents in the welds. According to the results, they decrease from 11–12% to 6–7% in welds. While at HAZs, delta-ferrite contents are quite low, less than 2.4%, and reduced below 1% after exposure up to 300 h. The existence of delta-ferrite in austenitic stainless steel welds can be estimated from the equivalent contents of Cr and Ni using the following equations [26,27].
Creq = %Cr + 1.5%Si + %Mo + 0.5%Nb
Nieq = %Ni + 0.5%Mn + 30%C
The Creq and Nieq of the filler rod, ER 308L, are 20.78 and 11.45, respectively. As a result of solidification, welds can contain delta-ferrite content of 9% [28]. In the same way, the delta-ferrite content of ER 347 filler is also around 9%. It is likely that the higher values obtained in the experiment are a result of the different cooling rates of the welding process. Meanwhile, delta-ferrite can be formed in HAZs as a result of austenite to delta-ferrite transformations. Since the welding process involves a short heat cycle, the level of transformation is low [29].
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Figure 6. Delta-ferrite contents measured at welds (a) and HAZ (b) before and after exposure to molten salt at 600 °C for 1, 10, 100, and 300 h.
Figure 6. Delta-ferrite contents measured at welds (a) and HAZ (b) before and after exposure to molten salt at 600 °C for 1, 10, 100, and 300 h.
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A dendritic structure is evident in the austenite matrix of the AISI 321H weld produced by ER 347 filler. This is shown in Figure 1b. It is unusual to find such a structure in HAZs, rather than many carbide precipitates. This observation agrees with the low delta-ferrite content in HAZs [29]. It is well known for austenitic stainless steel welds with Creq to Nieq ratios exceeding 1.5 that primary ferrite and secondary austenite or FA mode solidification occurs [30]. Thus, the dendritic structure was once delta-ferrite. With prolonged exposure to molten salt at 600 °C, delta-ferrite decreases. According to Figure 1, the dendritic structure of AISI 321H welds has not been reduced after 300 h of exposure to molten salt. According to this, the decrease in delta-ferrite is due to other phases with lower magnetic susceptibility, such as austenite or sigma. As explained by Tavares et al., austenite and sigma precipitate through delta-ferrite to sigma and secondary austenite transformation, reducing steel’s magnetic properties because ferrite is ferromagnetic, while sigma and austenite are paramagnetic [31]. When exposed to molten salt for a prolonged period of time, delta-ferrite content decreases in all welds as shown in Figure 6a. Figure 6b shows the delta-ferrite contents at HAZs of 304, AISI 304H, and AISI 321H. At the HAZ, delta-ferrite contents of all steels have decreased trends as prolonged exposure time. Except for the first hour of exposure, AISI 321H HAZ show a small increase. This was due to distribution of the HAZ width resulting in a deviation of measurement.
Figure 7a–f shows SEM images of microstructure at welds and HAZs of AISI 304, AISI 304H, and AISI 321H. In the welds shown in Figure 7a,c,e, the sigma, which contains Fe, Cr, Ni and Mo, can be observed in the dendritic structure. The lower Ni level can be detected, indicating that primary ferrite solidified dendrites have been replaced by sigma. In Figure 7e, Nb was observed due to the application of ER 347 filler. Chen et al. found that Cr- and Mo-enriched intermetallic phases, sigma and chi, precipitated at the delta-ferrite/austenite boundary, and within delta-ferrite grains after aging at temperatures ranging from 875 to 900 °C [32]. In contrast, some short-range dendritic structures can be detected in HAZs as shown in Figure 8b,d,f. At those localized sites, delta-ferrite can be confirmed, but sigma can only be observed in the HAZs of AISI 304 and AISI 321H. The transformation of delta-ferrite to sigma and secondary austenite was also reported in other studies [32,33,34].

3.4. Evaluation of SIGMA Phase Using LSV

It is possible to detect a sigma phase in the microstructure, but its quantitative properties are difficult to evaluate. The LSV technique is introduced to analyze and evaluate sigma. Figure 8a–f represents the LSV measured results for AISI 304, AISI 304H, and AISI 321H samples. The analyzed weld zones are shown in Figure 8a–c, respectively. The dissolved phases have peaks around 0.16–0.17 VSCE that can be observed after exposure to molten salt within 10 h. This value is a small difference from 0.15 VAg/AgCl reported by Marcelo [18]. It is important to note that the differences in potentials can be attributed to a number of factors, including the variation in ambient temperature and the pH of the solution [18]. Measurements were conducted for the HAZs of AISI 304, AISI 304H, and AISI 321H samples, as shown in Figure 8d–f, but the results differ from those of the weld zones. There is little evidence of dissolved peaks at present, since the current densities are quite low. Consequently, delta-ferrite to sigma phase transformation is limited at HAZs due to the low level of delta-ferrite. The evaluation for sigma was performed by the calculation of charge density. These were the areas under the LSV curves of the dissolved peak of the weld zones. The results are shown in Figure 9. Within 10 h of exposure, charge densities increase drastically. This finding illustrates the drastic increase in the sigma phase. After 10 h of exposure, the charge densities increase gradually. AISI 304 and AISI 304H welds have slightly higher charge densities than AISI 321H welds after 100 h of exposure. Segregation may occur during multi-pass welding when ER 347 filler is used. In the austenitic phase field, elements such as Mo and Nb have limited solubility, which causes them to segregate preferentially in the liquid phase. As a result, Mo and Nb are depleted in dendritic cores [35]. Consequently, the transformation of delta-ferrite to sigma is deferred due to the depletion of Mo and Nb [35]. A correlation between the increase in DOS levels in the weld zones can be determined using the LSV curves for evaluating sigma. Because of low carbide precipitates, intergranular corrosion sensitization in weld zones is quite low. However, the transformation from delta-ferrite to sigma is detrimental to intergranular corrosion in the weld zones.

4. Conclusions

The IGC sensitization investigation of austenitic stainless steel welds produced by GTAW with ER 308L and ER 347 fillers was performed on AISI 304, AISI 304H, and AISI 321H. They were exposed to molten nitrate salt at 600 °C for 0, 1, 10, 100, and 300 h. The following conclusions can be drawn from the following:
(1)
Using the DL-EPR technique, DOS levels of IGC can be evaluated. It is found that DOS levels at HAZs and at weld zones have increasing trends with exposure time. DOS levels at HAZs reached 60% when the exposure time was 300 h. These DOS levels were extremely high compared to those exposed to air. Meanwhile, DOS levels at weld zones were quite low, not exceeding 15% at 300 h of exposure. According to the DL-EPR technique, the pitting potential was correlated with DOS levels. IGC of stainless steel welds in molten salt depends strongly on carbide precipitation.
(2)
Different mechanisms of IGC sensitization have been found in weld zones. By analyzing the delta-ferrite contents, they decreased after exposure to molten salt for a prolonged period of time, with an intermetallic phase, sigma, in the weld zones. On the other hand, a method based on the LSV technique was introduced to quantify the sigma phase. As delta-ferrite transforms into sigma during exposure, sigma increases. This increase in the sigma phase leads to an increase in the DOS to IGC in the weld.
It was determined that, at the HAZ, there is a risk of IGC, and recovery effects cannot be expected. There are other processes to mitigate this failure, such as solution annealing as a post-annealing process to reduce carbide formation. The key materials for these applications will be austenitic stainless steels and filler rods with low carbon contents.

Author Contributions

Conceptualization, N.K., T.S. and P.P.; Methodology, N.K., T.S., P.W., W.I. and P.P.; Formal analysis, N.K., P.W., W.I. and T.N.; Writing—original draft, N.K., T.S., P.W. and T.N.; Writing—review and editing, N.K., T.S., P.W., W.I., T.N. and P.P.; Funding acquisition, T.N.; Resources, N.K., T.S., T.N. and P.P.; Supervision, P.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research budget was allocated by National Science, Research and Innovation Fund (NSRF) and King Mongkut’s University of Technology North Bangkok (Project No. KMUTNB-FF-67-A-06).

Data Availability Statement

The raw/processed data required to reproduce these findings are available upon request from the corresponding author.

Acknowledgments

The authors would like to thank King Mongkut’s University of Technology North Bangkok Department of Materials and Production Technology Engineering for laboratory support and the Thai-French Innovation Institute for cooperation, laboratory support, and material support. Furthermore, the authors thank Siriporn Daopiset for helpful comments and discussion.

Conflicts of Interest

The authors declare no conflicts of interest.

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Metals 14 01284 g001
Figure 1. The schematic weld bead shape using three-layer welding is shown in (a) and the optical microscope cross-section of the weld is shown in (b).
Figure 1. The schematic weld bead shape using three-layer welding is shown in (a) and the optical microscope cross-section of the weld is shown in (b).
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Metals 14 01284 g002
Figure 2. Cross-section images of AISI 321H welds exposed to molten nitrate salt at 600 °C for 300 h show intergranular corrosion at the HAZ in (a), dendritic structure of delta-ferrite in (b), detected Ti-C precipitates at HAZ in (c), Ti mapping in (d), and C mapping in (e).
Figure 2. Cross-section images of AISI 321H welds exposed to molten nitrate salt at 600 °C for 300 h show intergranular corrosion at the HAZ in (a), dendritic structure of delta-ferrite in (b), detected Ti-C precipitates at HAZ in (c), Ti mapping in (d), and C mapping in (e).
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Figure 7. SEM images of AISI 304, AISI 304H, and AISI 321H at welds (a,c,e) and HAZs (b,d,f) after exposure to molten salt at 600 °C for 300 h.
Figure 7. SEM images of AISI 304, AISI 304H, and AISI 321H at welds (a,c,e) and HAZs (b,d,f) after exposure to molten salt at 600 °C for 300 h.
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Metals 14 01284 g008
Figure 8. The LSV curves measured at room temperature at the weld zones (a–c) and HAZs (d–f) of AISI 304, AISI 304H, and AISI 321H samples after being exposed to molten salt at 600 °C for 0, 1, 10, 100, and 300 h.
Figure 8. The LSV curves measured at room temperature at the weld zones (a–c) and HAZs (d–f) of AISI 304, AISI 304H, and AISI 321H samples after being exposed to molten salt at 600 °C for 0, 1, 10, 100, and 300 h.
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Metals 14 01284 g009
Figure 9. The charge densities calculated from dissolved peaks of LSV curves measured at room temperature at the weld zones of AISI 304, AISI 304H, and AISI 321H samples after being exposed to molten salt at 600 °C for 0, 1, 10, 100, and 300 h.
Figure 9. The charge densities calculated from dissolved peaks of LSV curves measured at room temperature at the weld zones of AISI 304, AISI 304H, and AISI 321H samples after being exposed to molten salt at 600 °C for 0, 1, 10, 100, and 300 h.
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Table 1. Chemical composition of the filler rods.
Table 1. Chemical composition of the filler rods.
MaterialsNiCMoMnPSSiCrCuNbFe
ER 308L10.00.020.301.700.0110.0090.3220.00.21–Bal.
ER 3479.500.040.301.300.0250.0150.4019.50.100.40Bal.
Table 2. Welding parameters used in three-layer welding.
Table 2. Welding parameters used in three-layer welding.
Parent MetalFillerLayerCurrent (A)Potential (V)
AISI 304ER 308L110011.9
212012.8
310011.7
AISI 304HER 308L110011.0
212012.5
311012.7
AISI 321HER 347110011.9
212012.8
310011.7
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Kanjanaprayut, N.; Siripongsakul, T.; Wiman, P.; Issaard, W.; Nilsonthi, T.; Promdirek, P. Electrochemical Analysis on Intergranular Corrosion of Austenitic Stainless Steel Weld in Molten Nitrate Salt. Metals 2024, 14, 1284. https://doi.org/10.3390/met14111284

AMA Style

Kanjanaprayut N, Siripongsakul T, Wiman P, Issaard W, Nilsonthi T, Promdirek P. Electrochemical Analysis on Intergranular Corrosion of Austenitic Stainless Steel Weld in Molten Nitrate Salt. Metals. 2024; 14(11):1284. https://doi.org/10.3390/met14111284

Chicago/Turabian Style

Kanjanaprayut, Noparat, Thamrongsin Siripongsakul, Panya Wiman, Wannapha Issaard, Thanasak Nilsonthi, and Piyorose Promdirek. 2024. "Electrochemical Analysis on Intergranular Corrosion of Austenitic Stainless Steel Weld in Molten Nitrate Salt" Metals 14, no. 11: 1284. https://doi.org/10.3390/met14111284

APA Style

Kanjanaprayut, N., Siripongsakul, T., Wiman, P., Issaard, W., Nilsonthi, T., & Promdirek, P. (2024). Electrochemical Analysis on Intergranular Corrosion of Austenitic Stainless Steel Weld in Molten Nitrate Salt. Metals, 14(11), 1284. https://doi.org/10.3390/met14111284

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