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Keywords:

  • corrosion;
  • TIG welding;
  • zirconium

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References

Zirconium R60702 weldment was achieved by tungsten inert gas welding. Hot acid corrosion and electrochemical corrosion tests were performed on the specimens taken from the base metal (BM), the heat-affected zone (HAZ), and the weld zone (WZ) of the weldment in 75 wt% sulfuric acid. Metallographic examination and X-ray diffraction (XRD) analysis showed that the impurity generating in the weldment during welding was zirconium dioxide. Cumulative mass loss test demonstrated that the BM exhibited good resistance to hot acid corrosion, the HAZ less, and the WZ least. Scanning electron microscope images illustrated that the corrosion mechanism of the BM was pitting whereas that of the WZ was inter-granular corrosion. In contrast, pitting and inter-granular co-existed in the HAZ. Anodic polarization and electrochemical impedance spectroscopy tests showed similar result to that of the hot acid corrosion test. In the electrochemical corrosion condition, the WZ had worse corrosion resistance than the other two parts. Therefore, the WZ is the key area of corrosion-resistance protection of the Zr R60702 weldment.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References

Zirconium (Zr) and its alloys have excellent corrosion resistance and outstanding mechanical properties in working conditions of strong acid, alkali, and molten salts, therefore they are widely used in nuclear, chemical, and petrochemical industries [1, 2]. In the last two decades, Zr especially its alloys have been exploited as fuel cladding and coolant channels in water cooled power reactors in nuclear industry [3-6]. Thanks to the prosperity of the chemical industry, the utilization of Zr is increasingly wide and the corresponding research is rising up [7].

Zr R60702 (with a composition of 98.815 wt% Zr, 0.99 wt% Hf, 0.09 wt% Fe + Cr, 0.07 wt% O, 0.021 wt% C, 0.012 wt% N, and 0.002 wt% H) is mainly used for containers of concentrated sulfuric acid in the chemical industry. As Zr R60702 has a very high melting point (1845–1854 °C), Zr R60702 containers for sulfuric acid are usually fabricated by welding different parts or pieces [8]. In practice, tungsten inert gas (TIG) welding, owing to its stable arc in shielding gas, is usually applied for the manufacture of Zr R60702 containers [9]. As the weld zone (WZ) tends to corrode, being a potential risk in practical application, therefore it is exigent to study the corrosion mechanism of the Zr weldment.

In this paper, the corrosion behavior of TIG butt-welded Zr R60702 weldment was investigated in a 75% concentrated sulfuric acid at 100 °C and in the electrochemical condition. The electrochemical properties and the morphologies of different welding zones after corrosion, i.e., the base metal (BM), the heat-affected zone (HAZ), and the WZ, were investigated, and the corrosion mechanisms of the corresponding zones were revealed. The corrosion resistance of the BM is the best while that of the WZ is the worst. The corrosion mechanism of the BM is pitting whereas that of the WZ is inter-granular corrosion, while pitting and inter-granular corrosion coexist in the HAZ.

2 Experimental

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References

2.1 Materials and specimen preparation

Two pieces of Zr R60702 plates (3.0 × 20 × 20 mm) were degreased by using ultrasonication in acetone solution for 15 min. Manual TIG butt-welding was then carried out on ZX7-400STG III welding machine with ERZr2 filler wires of 2.4 mm in diameter. The process was performed in the shielding gas of 99.998% ultrapure argon. The chemical compositions of the BM and the filler wires are shown in Table 1, and the details of welding processing parameters are listed in Table 2.

Table 1. The chemical compositions of the base metal and the filler wires
 Chemical compositions (wt%)
CNHOFe + CrHfZr
Base metal0.0210.0120.0020.070.090.9998.815
Filler wires0.010.0040.0010.140.11.097.45
Table 2. The parameters of tungsten inert gas welding
PolaritySpeed (cm/min)Current (A)Prior gas supply (lag gas supply)/sGas flow (L/min)
Welding gunThe back sideDrag hood
DC straight polarity10120Prior gas supply 15–60162530
Lag gas supply 15–30

After the weldment was ground and became smooth, there appear three different zones, i.e., the BM, the HAZ, and the WZ, as shown in Fig. 1a. Experimental specimens for corrosion and electrochemical tests were cut from these three zones and their dimensions and positions are shown in Fig. 1b, denoted as Z1, Z2, Z3, Z4, Z5, and Z6, respectively. The six specimens were cut by using electric discharge machining and then polished until they showed bright metallic surfaces.

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Figure 1. Schematic illustration of the weldment: (a) the digital image of the weldment and (b) the schematic image of the specimens

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2.2 Structural characterization

The microstructures of the specimens Z1, Z2, and Z3 before corrosion were characterized by using optical microscopy (OM, XJP-100). The crystalline phases in the surface of the specimens before corrosion tests were identified by XRD (Buker D8-ADVANCE) with the scanning speed of 5°/min, and radiation conditions of 30 kV and 50 mA.

2.3 Hot acid corrosion test

After ultrasonic cleaning and drying, the specimens of Z1, Z2, and Z3 were weighed by Sartorius BS210S balance (with the sensitivity up to 0.1 mg) before and after hot acid corrosion test in 75 wt% concentrated sulfuric acid solution for 100 h in an electro-thermostatic water bath with the temperature of 100 °C.

2.4 Surface and cross-sectional morphologies of corroded specimens

Surface and cross-sectional morphologies after corrosion tests were characterized by using scanning electron microscopy (SEM, JSM-6360LV).

2.5 Electrochemical test

After being cleaned and dried, specimen Z4, Z5, and Z6 were embedded in epoxy resin to construct the working electrodes, with a Pt foil as the counter electrode and a saturated calomel electrode as the reference electrode, a conventional three-electrode cell was employed for anodic polarization and electrochemical impedance spectroscopy (EIS) testing in concentrated sulfuric acid solution. The experiments were initiated after the steady-state open circuit potential (OCP) had been established and maintained for about 30 min. The Tafel curves were obtained in duplicate by sweeping the potential (0.01 V/s) 1.0 V cathodic and 1.0 V anodic relative to the OCP. EIS tests were performed with root mean square applied potential amplitude of 0.005 V at the OCP. The frequency range of 0.01–10 000 Hz was used for EIS experiments with a sampling rate of 10 points per decade.

3 Results and discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References

3.1 Microstructure of the weldment

Optical microscopy observations reveal that the microstructures of the three zones of the BM (Z1), the HAZ (Z2), and the WZ (Z3) are different, as shown in Fig. 2. In the BM (Fig. 2a), the matrix grains are strip-shaped and parallel to the specimen surface, indicating that recrystallization does not occur after rolling. However, both the microstructures of the HAZ (Fig. 2b) and the WZ (Fig. 2c) consist of dendrites, and the dendrites are much coarser in the WZ than those in the HAZ.

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Figure 2. Microstructure of the Zr R60702 weldment: (a) the BM, (b) the HAZ, and (c) the WZ

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3.2 X-ray diffraction analysis

XRD patterns taken from the surface of the specimens before hot acid corrosion are shown in Fig. 3. There are numerous sharp peaks of Zr, together with other weak peaks of ZrO2, indicating that there exist ZrO2 in the BM, the HAZ, and the WZ. Comparatively, the peaks of ZrO2 in the WZ are much sharper than those in the HAZ, implying that the amount of ZrO2 in the WZ is the most of the three zones. This can be ascribed to the different oxidization of Zr in the three zones with different temperatures during welding since Zr tends to absorb oxygen over 200 °C [10]. The temperature of the WZ is higher than 300 °C during welding, and there is a negative temperature gradient from the WZ to the BM, resulting in the extent of oxidation experiences a downward trend. When reacting with acids, Zr has a great tendency to decompose water with the evolution of hydrogen, and dissolves as Zr4+ in concentrated acid solutions.

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Figure 3. XRD pattern taken from the surface of the specimens before hot acid corrosion test

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According to reaction (2), these zirconic ions form stable ZrO2 film on the surface. It is generally accepted that the excellent corrosion resistance of Zr to acids is related to the crystal structure and stability of the ZrO2 film formed on the surface [11]. It has been reported that the impurity of ZrO2 during welding will cause defects of the oxide films and therefore decrease the corrosion resistance of the weldment [12].

3.3 Hot acid corrosion test of the weldment

Figure 4 shows the curves of the cumulative mass loss as function of hot acid erosion time of the BM, the HAZ, and the WZ. Noticeable mass loss is not detected for the BM and the HAZ in the initial 20 h, while the mass loss of the WZ reaches about 5 mg over the same period, implicating that the ZrO2 film on the surface of the BM and the HAZ are much stronger than that on the surface of the WZ. After 20 h of testing the increment of the cumulative mass loss of the HAZ become much larger than that of the BM, whereas the WZ experiences a considerable cumulative mass loss, which means that the HAZ loses mass faster than the BM, and the WZ loses its mass the fastest. After 100 h of hot acid corrosion, the cumulative mass losses of the BM, the HAZ, and the WZ are 8.2, 20.4, and 29.0 mg, respectively. The cumulative mass loss of the BM is about ⅓ and ¼ of that of the HAZ and the WZ, respectively, indicating that the BM possesses better hot acid corrosion resistance than that of the HAZ and the WZ.

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Figure 4. Cumulative mass loss as function of erosion time of the specimens

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SEM observations on the surface morphologies of the specimens after hot acid corrosion tests for 100 h in 75 wt% concentrated sulfuric acid solution reveal that the BM specimen surface is rather smooth (Fig. 5a), while those of the WZ and the HAZ are very rough (Fig. 5b and c). Close-up view further displays that apart from some pits the surface of the BM is still in perfect condition (Fig. 5d); however, metal is deformed severely in the HAZ and the WZ (Fig. 5e and f), and there exist cracks and large potholes in the WZ (Fig. 5f).

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Figure 5. SEM micrographs of the surface of the specimens after hot acid corrosion test for 100 h in 75 wt% concentrated sulfuric acid solution: (a, d) the BM, (b, e) the HAZ, and (c, f) the WZ

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Microstructural observations have also been conducted on the cross-section of the specimens after hot acid corrosion for 100 h in 75 wt% concentrated sulfuric acid solution. As can be seen in Fig. 6a, SEM micrograph taken from the corroded specimen of the BM, the surface layer of corrosion product is very thin and there is no obvious transition zone formed between the surface layer and the matrix. Some little pits near the corroded surface of the BM can be observed. For the specimens of corroded HAZ and WZ, inter-granular and inter-dendrite corrosion can be observed in the areas under the surface layer, as shown in Fig. 6b and c, SEM micrograph taken from the corroded specimens of the HAZ and the WZ, respectively. It can also been seen that there are a few large potholes as well as some small pits in Fig. 6b, indicating that its corrosion type is the coexistence of pitting and inter-granular corrosion. By contrast, the surface of the WZ contains a large number of grooves, as shown in Fig. 6c, implying that its main corrosion type is inter-granular corrosion. These SEM photographs show that among the three zones of the Zr R60702 weldment the WZ is attacked most severely, the HAZ less and the BM least.

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Figure 6. SEM micrographs of the cross-section of the specimens after hot acid corrosion test for 100 h in 75 wt% concentrated sulfuric acid solution: (a) the BM, (b) the HAZ, and (c) the WZ

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3.4 Anodic polarization testing

The anodic polarization curve between voltages and currents of the electrode can be exploited to measure the corrosion resistance of a given material [13].

Anodic polarization tests for the BM, the HAZ and the WZ were carried out in 75 wt% concentrated sulfuric acid solutions at room temperature. The corresponding polarization curves (Fig. 7) reveal that the BM has the best corrosion resistance characteristic with a more noble corrosion potential and a more negative corrosion current (−0.112 V, 1.122E-5A), followed by the HAZ (−0.222 V, 2.482E-5A) and then the WZ (−0.228 V, 2.190E-4A) [14].

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Figure 7. Typical anodic polarization curves for the specimens in 75 wt% sulfuric acid at room temperature. Scanning rate: 0.01 mV/s

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3.5 Electrochemical impedance spectroscopy analysis

The electrochemical impedance spectra of the BM, the HAZ and the WZ in concentrated sulfuric acid solution are shown in Fig. 8. As can be seen from Fig. 8a, the curve of the BM is semi-circle, implying that its reaction is controlled by electrochemical procedure. In the Nyquist plots of the HAZ and the WZ, an incomplete semi-circle at high frequencies is followed by a linear region in lower frequencies, suggesting that their reactions are controlled by the diffusion of electroactive species from or to the surface. Moreover, the radius of the curve of the BM is the largest, indicating that its corrosion resistance is the best. Likewise, the corrosion resistance of the HAZ is worse, and the WZ worst. According to reference [15], Bode representations of the spectra are adopted to discern the impedance features at both high and low frequencies. Furthermore, it can help us to choose a proper equivalent circuit to simulate the electrochemical corrosion process.

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Figure 8. The electrochemical impedance spectra of the specimens: (a) Nyquist plots and (b) Bode plots

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The magnitude of the impedance at low frequencies can be interpreted as the inverse of the oxidation rate of the zirconium/oxide/electrolyte system, which can be interpreted as a decrease of the oxidation rate with the increase of the oxide layer. The corresponding equivalent circuits are shown in Fig. 9 and the result of simulation is demonstrated in Table 3.

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Figure 9. The equivalent circuits: (a) BM, (b) HAZ and WZ

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Table 3. The data of main electrical components belonging to equivalent circuits in Fig. 9
 BMHAZWZ
Rs (Ω cm2)0.603280.86353111.7
CPE1-T (μF)0.00041086.0109E−64.3592E−6
CPE1-P (μF)0.961820.84750.662
R1 (Ω cm2)24,52611,2417432
R2 (Ω cm2)0.0144790.000735570.00029064
CPE2-T (μF)4.4534E−50.922490.23712
CPE2-P (μF)0.196562.1871E122.3016E10

Here Rs is the resistance of the solution, R1 and CPE1 represent the resistance and capacitance of the outer layer, respectively, R2 and CPE2 refer to the resistance and capacitance of the inner layer. The impedance of the constant phase capacitance can be represented as: Z(CPE) = 1/[Y0·(jω)·n], where inline image is the imaginary unit, ω is the angular frequency, Y0 is the length of captance and admittance, and n is the dispersion coefficient.

In this study, R1, the charge transfer resistance, is used to describe the corrosion speed. Specifically, R1 refers to the traveling speed of the electron on the interface between the electrode and the electrolyte. The mobility of the electron is better when R1 is smaller, which means that the metal is easier to erode. Therefore, R1 can be used to measure the corrosion resistance of metals [16]. As shown in Table 3, the values of R1 of the BM, the HAZ, and the WZ are 24 526, 11 241, and 7432 Ω cm2, respectively, indicating that the corrosion resistance of the BM is the best, that of the HAZ is the worse and that of the WZ is the worst, being in agreement with the results shown in Fig. 7.

3.6 Analysis on corrosion mechanism

Hot acid corrosion and electrochemical corrosion tests show that the BM possesses good corrosion resistance while the corrosion resistance of the WZ is poor, which resulted from their different corrosion mechanism.

Generally, pitting occurs on the surface of self-passivating metals. As Zr R60702 can produce stable passivating film at ambient temperature, active-passive corrosion cells will emerge from the area where the passivating film has been destroyed, which drives corrosion from the surface to the interior of the metal in the BM, shown in Figs. 5d and 6a.

Inter-granular corrosion will take place as the destruction emerging alongside the grain boundaries when metals are treated in a specific corrosive environment. As for Zr R60702, Zr is an active metal that tends to absorb oxygen, hydrogen, and some other light elements, especially when the temperature is over 200 °C, it can capture a large amount of oxygen. Zr oxide is prone to generate when the amount of oxygen concentration is over 0.3% [17]. During welding, Zr oxide will inevitably appear as there is a slight amount of oxygen in the filler wires and the inadequate protection of argon, leading to the electrochemical inhomogeneity between grains and grain boundaries. Therefore, the corrosive current density of grain boundaries is much higher than that of grains, leading to inter-granular corrosion. From the morphology of inter-granular corrosion of Zr R60702 in the WZ (Figs. 5f and 6c), it can be seen clearly that corrosion takes place alongside grain boundaries and cracks are obvious in some fields that are severely eroded.

4 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References

Zr R60702 weldment was achieved via TIG welding and the corrosion behaviors of the BM, the HAZ, and the WZ in 75 wt% sulfuric acid at 100 °C and in the electrochemical corrosion condition were investigated. It is found that the BM exhibits better corrosion resistance than the WZ and the HAZ. Metallographic examination and XRD analysis illustrate that ZrO2 generating in the weldment accelerates the corrosive rate. Because of the largest content of oxygen that will cause the most defects in the passivating film, the WZ tends to erose intensively, which means that the WZ is the key area of corrosion-resistance protection of the Zr R60702 weldment. Moreover, the SEM analysis reveals that pitting occurs from the surface of the BM, causing several shallow holes dispersed on the exterior of the BM. On the surface of the WZ, inter-granular corrosion initiates at the grain boundaries, and then propagates into the easily damaged phases, resulting in damage of the whole surface of the WZ. By contrast, the surface of the HAZ reveals pitting and inter-granular corrosion.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References