Corrosion behavior of TiO₂-treated type 304 stainless steels in high temperature water containing with hydrogen peroxide

ABSTRACT

Titanium dioxide (TiO₂) treatment coupled with ultraviolet irradiation was selected as a corrosion mitigation technique for Type 304 stainless steel (SS) in high-temperature pure water with the presence of hydrogen peroxide (H₂O₂). Type 304 SS specimens were pre-oxidized in oxygenated pure water at 288 °C and then coated with TiO₂ nanoparticles by hydrothermal deposition. Electrochemical polarization analyses were conducted to investigate the corrosion behavior of both TiO₂-treated and pre-oxidized specimens in 288 °C pure water with 300 ppb H₂O₂. Ultraviolet (UV) irradiation was then imposed upon the TiO₂-treated specimens to examine if there was any distinct photoelectric effect on the corrosion behavior of the treated samples. It was found that the electrochemical corrosion potentials of the TiO₂ treated specimens under UV irradiation were 10–20 mV lower than those without UV. In addition, the corrosion current densities of the treated specimens were also lower in the presence of UV radiation. Without UV radiation, however, no significant differences were observed between the TiO₂ treated and untreated specimens. These results indicate that the TiO₂ treatment in combination with UV radiation would reduce the corrosion rate of Type 304 SS in H₂O₂-rich, high-temperature pure water.

Keywords:

• boiling water reactor
• hydrogen peroxide
• electrochemical corrosion potential
• titanium dioxide
• UV irradiation

1. Introduction

Intergranular stress corrosion cracking (IGSCC) on stainless steel (SS) and nickel-base alloy components has been a major material degradation issue for decades for all boiling water reactors (BWRs) around the world. The IGSCC susceptibility of structural materials in BWRs has been well known to influence by the electrochemical corrosion potential (ECP), which is mainly determined by the concentrations of all redox species (i.e. H₂, O₂, and H₂O₂) in the reactor coolant. Utilities around the world have adopted the technology of hydrogen water chemistry (HWC) in BWRs for mitigating the problem of IGSCC in the structural components. The HWC technology is deemed effective when the ECP in the primary coolant circuit of a BWR at a specific feedwater hydrogen concentration ([H₂]_FW) is reduced to below −0.23 V_SHE that signifies immunity of structural components to IGSCC [1]. Due to the adverse effects of HWC in elevating the operational and shutdown dose rates in a BWR, novel technologies such as catalytic coatings [2,3] and inhibitive coatings [4,5] have been developed to enhance the effectiveness of HWC in lowering hydrogen consumption and expanding protection areas.

To further maximize the efficiency of coatings, a technique involving deliberate photoexcitation on coating materials has also been studied [6–9]. Among the potential coating materials, titanium dioxide (TiO₂) is an n-type semiconductor that can be excited by photon irradiation to generate photoelectric currents. TiO₂ has a characteristic to generate electron-hole pairs in its space charge layer under UV irradiation. The excited electrons in the conduction band are then transferred to the coated metal surface, leading to decreases in both the corrosion potential and the oxidation reaction rate of the metal.

For the two major oxidizing species (O₂ and H₂O₂) in the BWR coolant, the electrochemical characteristic of H₂O₂ on the metal surface is relatively more complicated than that of O₂. In theory, H₂O₂ could act not only as an oxidizing agent but also as a reducing one in high temperature water. Furthermore, H₂O₂ could easily decompose into O₂ and water at elevated temperature. As a result, it is difficult to quantitatively evaluate the corrosion mitigation effectiveness of TiO₂ treatment in H₂O₂ containing environments.

In the past several years, we have studied the impact of TiO₂ treatment on the corrosion behavior of Type 304 SS in high temperature water of various oxygen concentrations [10]. The aim of this study was to explore the electrochemical characteristics of H₂O₂ and TiO₂ treated Type 304 SS under UV irradiation in high-temperature pure water. TiO₂ was deposited by hydrothermal deposition treatment on 304 SS specimens at various temperatures. Electrochemical polarization analyses were carried out under a designated water chemistry condition of 300 ppb H₂O₂. Experimental results on variations in corrosion potential and corrosion current density of both untreated and TiO₂-treated specimens under UV irradiation are reported and discussed.

2. Experimental

2.1. Specimen preparation

A commercial grade ASTM SUS304 SS plate was used in this study. Table 1 shows the chemical composition of this material as measured by a glow discharge spectrometer. For electrochemical polarization tests, the SS plate was machined into square disks with a dimension of 20 mm×20 mm×1 mm, and a small hole of 1 mm in diameter was drilled at one corner of each disk specimen. All specimens were then sealed inside a quartz tube in vacuum and thermally sensitized in a chamber furnace at 650 °C for 24 h. After the sensitization process, both sides of the specimens were mechanically polished by abrasive paper starting from 80 grit up to 1200 grit. Each specimen was then ultrasonically cleaned in acetone (CH₃COCH₃), dried, and rinsed in distilled water for 10 minutes. The degree of sensitization of a specimen was determined by the double-loop electrochemical potentiokinetic reactivation method, and all specimens were found heavily sensitized.

Table 1. The composition (wt%) of 304 SS used in this work.

Fe	C	Cr	Ni	Mn	Co	Si	Cu	Mo	P	S	N
Bal.	0.04	17.70	8.01	1.21	0.15	0.48	0.43	0.28	0.04	0.01	0.02

2.2. Pre-oxidation treatment

To produce stable oxide films on the specimens, a pre-oxidation treatment on each specimen was conducted in pure water under a high-temperature (288 °C) and high-pressure (8.3 MPa) condition in a circulation loop system. The coolant flow rate at the outlet of the loop (diameter of 1/4 inch) was measured at approximately 30 mL/min. A low conductivity of 0.06 μS/cm of the coolant was measured using a GLI conductivity meter. Figure 1 shows a conceptual schematic of the loop system. The specimens were exposed to the foregoing environment with a dissolved oxygen concentration ([O₂]_dis) of 300 ppb for 14 days. These specimens were designated as Prefilm ones. The surface morphologies of the specimens were examined by field emission scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) of JEOL JSM-6330F.

Figure 1. Schematic diagram of the high temperature water circulation loop.

2.3. TiO₂ treatment by hydrothermal deposition

TiO₂ (Anatase-type) particles with an average size of 38 nm in diameter were used in this study. These particles were deposited on the 304 SS specimens by hydrothermal deposition at different temperatures of 150 °C and 280 °C with a treating time of 96 h. The concentrations of TiO₂ were maintained at 100 and 10 ppm during the deposition treatment. The treated specimens were designated as N-TiX-Y, where X and Y are the deposition temperature and the TiO₂ concentration, respectively. The surface of the treated specimen was analyzed by SEM, and the TiO₂ coating was characterized by EDX and X-ray diffraction pattern analyses. The TiO₂ loading was estimated after the inductively coupled plasma mass spectrometry (ICP-MS) analysis. Figure 2 shows a schematic view of the TiO₂ deposition loop. Preparation conditions of all specimens used in this study were summarized in Table 2.

Figure 2. Schematic diagram of the hydrothermal deposition circulation loop for TiO₂ treatment.

Table 2. Preparation conditions of the specimens.

Specimens	Pre-oxidation condition	TiO2 treating condition
Prefilm	[O2]dis = 300 ppb,  2 weeks, 288 °C	None
N-Ti150-100	Same as above	38 nm, 100 ppm TiO2, 96 h, 150 °C
N-Ti150-10	Same as above	38 nm, 10 ppm TiO2, 96 h, 150 °C
N-Ti280-100	Same as above	38 nm, 100 ppm TiO2, 96 h, 280 ^oC
N-Ti280-10	Same as above	38 nm, 10 ppm TiO2, 96 h, 280 ^oC

2.4. ECP and polarization measurements

ECP measurements and potentiodynamic polarization analyses were conducted in a high-temperature pure water circulation loop, coupled with an autoclave that could accommodate the testing specimens. The water chemistry in the loop could be controlled by properly adjusting the injection rates of O₂, H₂, and Ar gases into the reservoir tank. The flow rate at the outlet of the autoclave was measured at approximately 30 mL/min. On the other hand, since H₂O₂ is unstable in high temperature water due to its easy decomposition into O₂ and water, bulk H₂O₂ solution preserved in a dark glass tank was directly injected into the autoclave through a Teflon-lined tube by a high pressure metering pump. The H₂O₂ concentration in the autoclave was determined by the mixing ratio of the injected H₂O₂ and the circulating coolant. Figure 3 shows the cross-sectional view of the specimens and electrodes setup in the autoclave for ECP measurements and polarization tests. A three-electrode system, including an Ag/AgCl reference electrode, a platinum counter electrode, and pre-oxidized or TiO₂ treated specimens as working electrodes, was used in this study. Heat-shrinkable PTFE tubes were used for electrical insulation between the connecting wires and the coolant. The electrodes were then mounted in a CONAX fitting secured at the top lid of the autoclave and inserted into the coolant region inside the autoclave. The Ag/AgCl electrode was installed in parallel with the platinum electrode and the specimens. The measured ECPs were converted to the standard hydrogen electrode (SHE) scale. The body of autoclave was deliberately machined with a sapphire-sealed hole, serving as a viewing window, through which the specimen could be irradiated by UV. This experiment used a 250–365 nm UV light source, which was sufficient to excite the anatase type TiO₂. The intensity of the UV light was set at 4500 mW/cm² with a wavelength of 365 nm, and the intensity of the UV light irradiating onto the metal surface was calculated to be 17 mW/cm².

Figure 3. Arrangement of the specimen and other electrodes in the autoclave for ECP and polarization tests.

The concentration of H₂O₂ was controlled by using a metering pump to inject a proper amount of H₂O₂ directly into the autoclave and was maintained at 300 ppb. A Gamry PC4/300 electrochemical analyzer was adopted to perform the polarization analyses. The ECP for each specimen under the designated water chemistry condition was obtained prior to each polarization test. The potential increment or decrement was set at 30 mV for every 10 minutes for the polarization analyses. The cathodic polarization range was 0.3 V below the corrosion potential (E_corr), and the anodic polarization range was 0.3 V above the E_corr. The positive-feedback loops in the electrochemical analyzer were activated to compensate for the ohmic resistance experienced by the testing system.

3. Results and discussion

3.1. Morphologies and properties of the surface oxides and TiO₂ coatings

The surface morphology of the Prefilm specimen after the pre-oxidation treatment is shown in Figure 4. The oxide particles were uniformly distributed on the surface of the specimen, and the oxide particle size ranged from 0.5 to 1 µm in diameter. It was also observed that the surface oxides exhibited a two-layer structure. EDX analyses revealed that the O/Fe ratios in the outer layer and the inner layer of the oxides were different, as shown in Figure 4.

Figure 4. SEM micrographs and EDX results of the Prefilm specimen.

Figure 5 shows the SEM micrographs and EDX mapping results of the four TiO₂-treated 304 SS specimens, namely, N-Ti150-100, N-Ti150-10, N-Ti280-100, and N-Ti280-10. The SEM images show that the TiO₂ nanoparticles were deposited on the oxide surfaces in an uneven and discontinuous manner. The TiO₂ coatings could be either thick or relatively thin, as characterized by the Ti signal mapping. In spite of uneven deposition and discontinuous distribution, the TiO₂ loadings on specimens N-Ti280-100 and N-Ti280-10 were found higher than those on specimens N-Ti150-100 and N-Ti150-10, as verified by ICP-MS loading estimations. The data are summarized in Table 3. The TiO₂ loadings on the specimens increased with both increasing treating temperature and increasing TiO₂ concentration.

Figure 5. SEM micrographs and EDX results of TiO₂-treated specimens.

Table 3. TiO₂ loadings on the TiO₂-treated specimens.

Specimen	TiO2 (μg/cm^2)
N-Ti150-10	0.487
N-Ti150-100	0.575
N-Ti280-10	0.867
N-Ti280-100	0.906

3.2. Electrochemical polarization analyses

Figure 6 shows ECP variations on the specimens exposed to 288 °C pure water at the designated concentration ([H₂O₂]) of 300 ppb before and after UV irradiation. Upon UV irradiation, the ECPs of N-Ti150-100 and N-Ti280-100 decreased promptly by around 12 to 15 mV and returned to their original values when the UV light was off. The ECP decreases on N-Ti150-10 and N-Ti280-10 upon UV irradiation were about 5 mV and 7 mV, respectively. The difference between the ECPs without and with UV was greater on the specimen treated with a higher TiO₂ concentration at a higher temperature.

Figure 6. ECP variations of the TiO₂-treated specimens in 288 °C pure water at 300 ppb H₂O₂ under interrupted UV irradiation.

It should be noted that prior to each single polarization test the specimen was immersed in the test environment for a period of time for the ECP to stabilize. Figure 7 shows the polarization curves of the untreated and TiO₂-treated specimens with and without UV irradiation in the high temperature water containing 300 ppb H₂O₂. The Tafel extrapolation method was used to calculate the i_corr. A small linear Tafel region on each of the cathodic polarization curve could be observed starting from 50 mV below the E_corr, which is also the second data point below E_corr. The linearity was not maintained over a large range of the scanned potential, and only from second data to the fourth data points were used to construct the Tafel line. The intersection of the Tafel line and the E_corr was the i_corr. The ECP and corrosion current density (i_corr) data are summarized in Table 4. In the absence of the UV light, there was no significant difference between the untreated and treated specimens in terms of anodic current density. Slight decreases in the cathodic current densities of the four treated specimens were observed. Among them, a comparatively more significant decrease in cathodic current density on the N-Ti280-100 specimen was recorded, and the i_corr of this specimen was 23.3 μA/cm², which was much lower than that of the Prefilm specimen at 37.2 μA/cm². This outcome was attributed to the higher Ti loading and thus, the greater inhibiting efficiency on the cathodic reaction. In general, the i_corrs and the ECPs of all TiO₂-treated 304 SS specimens were reduced in pure water with 300 ppb H₂O₂ in the absence of UV radiation, and the specimens treated at a higher TiO₂ concentration resulted in more effective reductions in i_corr and ECP.

Figure 7. Polarization curves of the Prefilm and TiO₂-treated specimens in 288 °C water at 300 ppb [H₂O₂].

Table 4. ECPs and i_corrs of Prefilm and TiO₂-treated specimens at 300 ppb H₂O₂.

	ECP (mVSHE)	ΔE (mV)	βc (mV/decade)	icorr (µA/cm^2)	icorr,/icorr,metal (%)
Prefilm	184		200	25.3
N-Ti150-10	182	−2	236	23.8	94%
N-Ti150-10_UV	174	−10	213	17.7	70%
	ECP (mVSHE)	ΔE (mV)	βc (mV/decade)	icorr (µA/cm^2)	icorr,/icorr,metal (%)
Prefilm	174		200	36.9
N-Ti280-10	173	−1	244	35.2	95%
N-Ti280-10_UV	162	−12	221	24.4	66%
	ECP (mVSHE)	ΔE (mV)	βc (mV/decade)	icorr (µA/cm^2)	icorr,/icorr,metal (%)
Prefilm	199		207	26.3
N-Ti150-100	191	−8	192	20.0	76%
N-Ti150-100_UV	180	−19	233	13.7	52%
	ECP (mVSHE)	ΔE (mV)	βc (mV/decade)	icorr (µA/cm^2)	icorr,/icorr,metal (%)
Prefilm	184		200	37.2
N-Ti280-100	177	−7	208	23.3	63%
N-Ti280-100_UV	162	−22	257	7.7	21%

In the presence of UV radiation, photo-catalytic reactions occurred on the surfaces of all TiO₂-treated 304 SS specimens and generated anodic currents to reduce the ECPs of the treated specimens. For the N-Ti150-10 and N-Ti280-10 specimens, slightly lower ECPs by 10 and 12 mV than the corresponding Prefilm specimens were observed under UV irradiation at 300 ppb H₂O₂. The ECP of N-Ti-150-100 with UV was lower than that of Prefilm by 19 mV. In the meantime, the ECP of N-Ti280-100 with UV was 162 mV_SHE, which was lower than that of Prefilm by 22 mV. The results were consistent with what the theory predicted in that the n-type TiO₂ coating would generate excess electrons under UV irradiation, leading to a lower potential during the polarization analysis. Accordingly, the corresponding i_corrs of the four treated specimens were also reduced. Most distinctly, the i_corr of N-Ti280-100 with UV was 7.7 μA/cm², significantly lower than that of Prefilm at 37.2 μA/cm². The polarization data under UV irradiation for all specimens are also summarized in Table 4. The outcome clearly showed that the degree of reduction in i_corr of the TiO₂-treated specimen could be as high as 80% in 288 °C water with 300 ppb H₂O₂, signifying the effectiveness of TiO₂ coatings coupled with UV on corrosion mitigation of treated Type 304 SS.

3. Conclusions

The technique of TiO₂ treatment coupled with UV irradiation could be adopted as a corrosion mitigation measure for Type 304 SS in high-temperature pure water in the presence of H₂O₂. TiO₂ coatings could be obtained on pre-oxidized Type 304 SS specimens by hydrothermal deposition. A higher TiO₂ loading could be achieved at a deposition condition of a higher TiO₂ concentration and a higher temperature. In high-temperature pure water with 300 ppb H₂O₂, slight decreases in ECP and i_corr were observed on TiO₂-treated specimens even in the absence of UV irradiation. In the presence of UV irradiation, the photo-catalytic reaction on the surface of the treated 304 SS specimen would further enhance the corrosion mitigation efficiency of TiO₂ coatings, and the degree of reduction in i_corr of the TiO₂-treated specimen could be as high as 80%. Although the TiO₂ treatment in combination with UV radiation could not reduce the ECP below the E_crit (−0.23 V_SHE), the corrosion rate of TiO₂-treated specimens under UV irradiation would be effectively reduced in H₂O₂-rich, high-temperature pure water.

Acknowledgments

The authors wish to express sincere gratitude to Taiwan Power Company for the financial support [contract number TPC-056-98-00023]. We also acknowledge the technical information supported from Ms Ching Chang of Taiwan Power Company.

Disclosure statement

No potential conflict of interest was reported by the authors.