Galvanic Corrosion between Alloy 690 and Magnetite in Alkaline Aqueous Solutions

The galvanic corrosion behavior of Alloy 690 coupled with magnetite has been investigated in an alkaline solution at 30 °C and 60 °C using a potentiodynamic polarization method and a zero resistance ammeter. The positive current values were recorded in the galvanic couple and the corrosion potential of Alloy 690 was relatively lower. These results indicate that Alloy 690 behaves as the anode of the pair. The galvanic coupling between Alloy 690 and magnetite increased the corrosion rate of Alloy 690. The temperature increase led to an increase in the extent of galvanic effect and a decrease in the stability of passive film. Galvanic effect between Alloy 690 and magnetite is proposed as an additional factor accelerating the corrosion rate of Alloy 690 steam generator tubing in secondary water.


Introduction
Nickel-based Alloy 600 has been widely used as a steam generator (SG) heat transfer tubing material in pressurized water reactors (PWRs).However, Alloy 600 tubing experienced various types of corrosion damage, such as stress corrosion cracking, intergranular attack, pitting corrosion, etc.Consequently, Alloy 600 has been changed to Alloy 690 with a higher Cr content of about 30% in
Magnetite particles released from the surface of carbon steel piping are transported into an SG, accumulated on the top of tube sheet and deposited on the outer surface of SG tubes.These corrosion products reduce the efficiency of the SG by deterioration of the heat transfer and accelerate the corrosion of SG tubes [10][11][12].Magnetite deposited on the Ni-based alloy tube in operating SGs has a porous structure [13,14].In addition, the magnetite adhering to SG tubes distorts eddy current signals from the tubes during in-service inspection [15].
The surface of Alloy 690 tube is in electrical contact with the porous magnetite deposits.A need exists to know about the galvanic corrosion behavior between Alloy 690 and magnetite in secondary water of PWRs.
To elucidate the galvanic corrosion behaviour between Alloy 690 and magnetite, it is necessary to simulate magnetite deposited on the surface of Alloy 690 in a secondary circuit system.An electrodeposition technique is a simple method to produce a pure magnetite layer on Alloy 690 substrate.Magnetite is produced by a chemical reaction following the oxidation of Fe(II) [16][17][18] or the reduction of Fe(III) [19,20] at the electrode surface.Recently, Kothari et al. [21] have shown that magnetite is easily produced by electrodeposition on a stainless steel substrate in Fe(III)-triethanolamine (TEA) solution at 60-80 °C.Goujon et al. [22] characterized magnetite films electrodeposited on the Ni-based alloy and established the optimal temperature and agitation condition for electrodeposition growth of magnetite films in the Fe(III)-TEA solution.
In this study, the galvanic corrosion behavior of Alloy 690 coupled with magnetite was investigated in alkaline aqueous solutions.A magnetite layer was electrodeposited on the surface of Alloy 690 substrate from a Fe(III)-TEA electrolyte.Then, the electrochemical corrosion parameters of Alloy 690 and magnetite were measured under simulated wet layup conditions through electrochemical techniques.

Material Preparation
Alloy 690 was melted in a high frequency vacuum induction furnace, and hot-rolled in a temperature range of 1150-1250 °C.The plates were cold-rolled with a total area reduction of about 70%.The cold-worked samples were annealed at 1100 °C for 5 min followed by water quenching, and then heat-treated at 715 °C for 10 h in a vacuum furnace (Carbolite, Hope valley, UK).Specimens for electrodeposition and electrochemical tests were cut into a dimension of 10 × 5 × 1 mm (length by width by thickness).The specimens were ground to #1000 grit using silicon carbide papers, and then ultrasonically cleaned in acetone and ethanol for 5 min.The chemical composition of Alloy 690 is given in Table 1.

Deposition of Magnetite Films
Magnetite films were electrodeposited on Alloy 690 substrates in Fe2(SO4)3 solution complexed with TEA.The detailed concentrations in the deposition bath were 0.04 M Fe2(SO4)3, 0.1 M TEA, and 2 M NaOH.The solution was prepared by adding Fe2(SO4)3 solution to a solution of NaOH and TEA.The electrochemical deposition was carried out in a three electrode cell using a PAR273 potentiostat (Ametek, Berwyn, PA, USA) with Power suite software (Ametek, Berwyn, PA, USA).A saturated calomel electrode (SCE) and a platinum wire were used as a reference and counter electrode, respectively.The SCE was connected via a Luggin capillary whose tip was very close to the surface of the working electrode.The Luggin was cooled through a water cooled spherical condenser so that the temperature of the SCE was maintained at 25 °C.All electrochemical potentials were given in terms of the SCE at the test temperatures of 30 °C, 60 °C, and 80 °C.Magnetite was electrodeposited in the prepared deposition solution at an applied potential of −1.05 VSCE for 3600 s at 80 °C.
After electrodeposition, the magnetite films were analyzed using scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA) and X-ray diffraction (XRD) with a D/Max-2500 X-ray diffractometer (Rigaku, Tokyo, Japan).The magnetite films were also milled by a focused ion beam (FIB) toward the vertical direction of the magnetite surface using a QUANTA 3D FEG FIB-SEM (FEI Company, Hillsboro, OR, USA).Then, the morphology and thickness of the magnetite were analyzed.To calculate the surface area ratio of magnetite/Alloy 690, the surface area of magnetite and Alloy 690 was measured using an Olympus OLS3000 non-contact confocal optical profiler (Olympus, Tokyo, Japan).

Electrochemical Tests
Two kinds of electrochemical tests were performed: the zero resistance ammetry (ZRA) test and the potentiodynamic polarization test.All test solutions were prepared immediately before each test from high purity demineralized water with the resistivity above 16 MΩ• cm.The pH of the solutions at 25 °C was adjusted to 9.5 by adding ethanolamine (ETA), which is an organic chemical agent used to control the pH of secondary water in PWRs.The pH of the solution at 60 °C was 8.61, which was calculated using the MULTEQ software (EPRI, Palo Alto, CA, USA).Solutions were deaerated by continuously purging with high-purity (99.98%) nitrogen gas at a rate of 600 cm 3 /min during the tests.All tests were conducted at 30 °C and 60 °C.These test environments were designed to simulate wet layup conditions of SGs in PWRs.
The ZRA tests were performed using a Reference 600 Potentiostat/galvanostat/ZRA (Gamry, Warminster, PA, USA) embedded with Gamry framework system.An SCE was used as a reference electrode.Alloy 690 specimen was connected to the working electrode and the magnetite specimen was connected to the counter electrode.After the open circuit potential (OCP) was stabilized, the specimens were electrically connected though the ZRA.The variation of galvanic potential (Ecouple) and galvanic current density (icouple) of Alloy 690 coupled with magnetite was measured during 3600 s.
The potentiodynamic polarization tests were carried out using a PAR273 potentiostat (Ametek, Berwyn, PA, USA) with Power suite software (Ametek, Berwyn, PA, USA).An SCE and a platinum wire were used as a reference and counter electrode, respectively.After the OCP was stabilized, a polarization scan was started from the OCP to the cathodic or anodic direction with a scan rate of 1 mV/s.Therefore, each cathodic and anodic polarization curve was obtained from a newly prepared sample.The corrosion rates of Alloy 690 and magnetite were determined by means of the Tafel extrapolation method.The Ecouple and the icouple of the pair were calculated from the potentiodynamic polarization curves based on the mixed potential theory.All electrochemical tests were conducted at least three times on separate specimens to verify reproducibility.Good reproducibility was confirmed.The electrodeposition of magnetite film in a Fe(III)-TEA solution can be simplified by two steps.The first step is that the Fe(III)-TEA solution is electrochemically reduced to Fe 2+ and TEA.The second step is that the electrochemically produced Fe 2+ reacts chemically with Fe(III)-TEA solution to produce magnetite film.The proposed mechanism is expressed in the following reactions (1) and (2) [21][22][23][24].Fe(TEA) 3+ + e − → Fe 2+ + TEA (1)

Electrodeposition of Magnetite
Therefore, the first reduction wave observed between −0.95 and −1.20 VSCE can be attributed to a one electron exchange reaction corresponding to the above reaction (1) [23].On the contrary, the second reduction wave between −1.20 and −1.30VSCE seems to be due to a two-electron process corresponding to the reduction of Fe(II) to Fe [22][23][24].In this work, the magnetite film was Figure 2 shows the SEM images of the magnetite electrodeposited on the Alloy 690 substrate.The surface of magnetite has a dense and highly faceted morphology, which is homogeneous on the entire surface of the deposit (Figure 2a). Figure 2b presents the FIB-SEM image of cross sectional electrodeposited magnetite film on the Alloy 690 substrate.The thickness of the magnetite film ranged from 0.8 to 1.5 μm, with an average of 1.2 μm.No defects, such as holes or cracks, could be observed at the interface between the Alloy 690 substrate and magnetite, confirming that magnetite film was tightly bonded to Alloy 690 substrate.Figure 3 shows the XRD analysis of the magnetite electrodeposited on the Alloy 690 substrate.The magnetite film is all crystalline, and all peaks correspond to magnetite film.prepared all specimens with the same nominal dimension.In a microscopic manner, however, magnetite specimens have a larger real surface area participating in a corrosion process.Therefore, it should be noted that all electrochemical data in this paper were measured under the condition that the area ratio of magnetite to Alloy 690 was 1.3.

Electrochemical Behavior of Alloy 690 and Magnetite
Figure 5 shows the corrosion potential (Ecorr) and the galvanic potential (Ecouple) of Alloy 690 and magnetite in the alkaline solution with pH 9.5 at 30 °C and 60 °C as a function of time.The Ecorr of the magnetite was nobler than that of Alloy 690, whereas the Ecouple of magnetite/Alloy 690 couple is between the Ecorr of the magnetite and Alloy 690.Accordingly, magnetite will behave as the cathode of the galvanic couple, while Alloy 690 will be the anode and is expected to undergo excessive corrosion.In addition, as the temperature increased from 30 °C to 60 °C, the Ecorr of both Alloy 690 and magnetite and Ecouple decreased.
Figure 6 shows the galvanic current density (icouple) of Alloy 690 coupled with magnetite in the alkaline solution with pH 9.5 at 30 °C and 60 °C using ZRA.In the Alloy 690/magnetite couple, the icouple of Alloy 690 was the positive value corresponding to the oxidation reaction, indicating that Alloy 690 specimen was the anode of the couple.The icouple of the Alloy 690 specimen is high initially and decreases considerably with time to an extremely low current density.This decrease in icouple with time can be attributed to the formation of passive film on the surface of Alloy 690.The icouple tends to diminish as the metal passivates by oxide film growth during the initial stage and to stabilize from the final stage [25].The icouple increased when the temperature increased from 30 °C to 60 °C.Especially, during the formation of passive film at 60 °C, the current density transients were observed from the curve of icouple (Figure 6b).These kind of transients was related to the nucleation of metastable pits, which were immediately repassivated [26][27][28].These ZRA results indicate that the increase in temperature leads to an increase in the extent of galvanic effect and a decrease in the stability of passive film.Figure 7 shows the potentiodynamic polarization curves of Alloy 690 and magnetite in the alkaline solution with pH 9.5 at 30 °C and 60 °C.The corrosion current density (icorr) of Alloy 690 and magnetite was calculated by means of the Tafel extrapolation method.The galvanic potential (Ecouple) and the galvanic current density (icouple) of the pair were also determined using the mixed potential theory.These electrochemical corrosion parameters are summarized in Table 2.The Ecorr of magnetite was higher than that of Alloy 690.The icorr of Alloy 690 was lower than that of magnetite.In addition, as the temperature increased from 30 °C to 60 °C, the icorr of both Alloy 690 and magnetite and icouple increased.
When magnetite and Alloy 690 are electrically coupled, the corrosion rate of coupled Alloy 690 at 60 °C will increase by about 2.6 times than that of non-coupled Alloy 690 due to the shift in its potential in the positive direction.In cathodic reaction, the reductive dissolution of magnetite and formation of dissolved ferrous or ferrous species will occur according to Equation (3) as well as the hydrogen evolution reaction [29].Jung et al. [29] showed that the reductive dissolution of magnetite occurred in the alkalized reducing condition.Fe3O4 + 2H2O + 2H + + 2e − → 3Fe(OH)2 The icouple increased when the temperature was changed from 30 °C to 60 °C.This result means that temperature promotes the galvanic corrosion rate of coupled Alloy 690.It is because higher temperature can promote the ion transport and decrease the pH and then accelerate the corrosion.These results calculated from the mixed potential theory show a very similar tendency to the results obtained from the ZRA tests.In general, an increase in temperature is expected within the magnetite deposited on the outer surface of Alloy 690 tubing due to the deterioration of heat transfer efficiency from the primary to secondary side.Increasing the temperature leads to an increase in the solubility of impurities until the pressurized water approaches the super-saturation, forming a corrosive environment in the porous magnetite deposits.Especially, the aggressive ionic species such as chloride, sulfate and sodium have been reported to accelerate the corrosion degradation of nickel-based SG tubing [30].Magnetite deposited on Alloy 690 tubes in operating SG has a porous structure [14,15].Therefore, the surface of Alloy 690 tubes is in galvanic contact with the porous magnetite deposits.In addition, the surface area ratio of the cathode to anode on the outer surface of steam generator tubes covered with the porous magnetite is much larger than the area ratio of this experimental condition.Consequently, the corrosion rate of Alloy 690 tubes in operating steam generators is accelerated.This study demonstrates that the corrosion rate of Alloy 690 increases by a galvanic couple with magnetite.Figure 8 shows the schematic of mechanism for the galvanic effect on the corrosion rate of Alloy 690 tubing coupled with magnetite.The corrosion rate of Alloy 690 SG tubing is additionally accelerated by a galvanic effect as well as by the concentration effect of aggressive chemical impurities.

Conclusions
The galvanic corrosion behavior of Alloy 690 coupled with magnetite in alkaline aqueous solutions was predicted by the mixed potential theory and verified by ZRA measurements.The potential differences between Alloy 690 and magnetite and the sign of the galvanic current of the couple indicated that Alloy 690 was the anode of the galvanic pair.Galvanic coupling increased the corrosion rate of Alloy 690 due to the shift in its potential to the anodic direction.The extent of galvanic effect between Alloy 690 and magnetite increased with increasing temperature from 30 °C to 60 °C.Therefore, the galvanic effect between Alloy 690 and magnetite should be considered as an additional factor accelerating the corrosion rate of Alloy 690 SG tubing in secondary water.

Figure 1
Figure 1 shows a linear sweep voltammogram for Alloy 690 substrate in the electrodeposition solution with Fe(III)-TEA at 80 °C.The potential was scanned from the OCP of −0.8 VSCE down to −1.3 VSCE at a scan rate of 40 mV/s.The electrochemical reduction of Fe(III)-TEA was observed at negative potentials below approximately −1.00 VSCE.The first reduction wave of this linear sweep was measured approximately between −1.00 and −1.20 VSCE.The second reduction wave was also observed in the potential range from −1.20 to −1.30 VSCE.
vs. SCE)electrodeposited on Alloy 690 substrate in the Fe(III)-TEA solution at the potential of −1.05 VSCE (first reduction wave region) for 3600 s.

Figure 2 .
Figure 2. Magnetite film electrodeposited on the Alloy 690 substrate; (a) surface of magnetite film and (b) cross section of magnetite film.

Figure 4
Figure4shows the surface morphologies of Alloy 690 and electrodeposited magnetite specimen through the confocal optical profiler.The deposited magnetite shows a rougher surface than the Alloy 690 sample.The measured surface area ratio of magnetite to Alloy 690 was about 1.3.Originally, we

Figure 5 .Figure 6 .
Figure 5. Corrosion potential and galvanic potential of Alloy 690 and electrodeposited magnetite in the alkaline solution with pH 9.5 at (a) 30 °C and (b) 60 °C.

Figure 7 .
Figure 7. Potentiodynamic polarization curves of Alloy 690 and electrodeposited magnetite in the alkaline solution with pH 9.5 at (a) 30 °C and (b) 60 °C.

Figure 8 .
Figure 8. Schematic of the galvanic corrosion between Alloy 690 tubing and magnetite.

Table 2 .
Electrochemical corrosion parameters of Alloy 690 and magnetite obtained from potentiodynamic polarization curves in the alkaline solution with pH 9.5.