Investigation of cobalt buildup behavior and suppression by zinc injection on stainless steel under HWC conditions using simultaneous continuous measurements of corrosion and cobalt buildup

Abstract

In boiling water reactor (BWR) plants, cobalt-60 (⁶⁰Co) is the main source of radiation exposure, and it builds up on oxide films of structural materials. The ⁶⁰Co buildup is caused by its incorporation into the oxide films. In the BWR plants using hydrogen water chemistry (HWC) to mitigate the oxidative environment, Zn injection has been applied to reduce the ⁶⁰Co incorporation. In this work, we studied the incorporation mechanism of ⁶⁰Co into the oxide films on type 316 stainless steel and the suppression mechanism of ⁶⁰Co incorporation. In order to discriminate between coprecipitation and adsorption of ⁶⁰Co incorporation under HWC conditions, we measured the corrosion amount of the base metal and the ⁶⁰Co buildup amount, using simultaneous continuous measurements for 500 h. The ⁶⁰Co incorporation increased with time both with and without Zn injections. We found that the time dependencies of ⁶⁰Co incorporation with and without Zn have one and two regions, respectively. In the initial stage for both, ⁶⁰Co was incorporated mainly by coprecipitation. After 100 h without Zn, ⁶⁰Co was incorporated by both coprecipitation and adsorption. These results mean that Zn suppressed both coprecipitation and adsorption of ⁶⁰Co.

Keywords:

• boiling water reactor
• cobalt 60
• hydrogen water chemistry
• stainless steel
• coprecipitation
• adsorption

1. Introduction

In boiling water reactor (BWR) plants, corrosion products are released from structural materials into the reactor water and transported to in-core regions. These corrosion products become radioactive species by neutron exposure. Then these species are redissolved as radioactive ions or released as radioactive crud into the reactor water. Most of the ions and crud redeposit on the fuel cladding surface. However, some of the ions and crud reach the surface of the primary loop re-circulation system (PLR) piping and other reactor components. The oxide films incorporating the radioactive species grow on the surface of the components, and so become a radiation source [1]. Since cobalt-60 (⁶⁰Co) is the main radioactive nuclide associated with this radiation source, some research groups have been developing countermeasure technologies for the ⁶⁰Co buildup on the surface of the components and have been studying the mechanisms of ⁶⁰Co incorporation into the oxide films [2–7].

In many aging BWR plants, hydrogen water chemistry (HWC) [8] and noble metal chemical addition [9] into reactor water are used to mitigate the oxidative environment of the plants to suppress stress corrosion cracking (SCC) of the structural materials. In these plants that apply such SCC mitigation technologies, zinc (Zn) injection is essentially applied to suppress ⁶⁰Co incorporation into the oxide films on the surface of the components which are made of stainless steel, e.g. the PLR piping [3]. In the corrosion of stainless steel, metal ions elute from the base metal and diffuse through the oxide films. The reported effect of Zn is stabilization of the oxide films to suppress the diffusion of metal ions. Therefore, Zn injection decreases the oxide film amount and lowers the ⁶⁰Co concentration in the oxide films formed on stainless steel [4,5].

According to previous reports [1,10], the ⁶⁰Co buildup consists of two processes: coprecipitation and adsorption. In coprecipitation, ⁶⁰Co and Fe ions coprecipitate to form oxide films. In adsorption, ⁶⁰Co is adsorbed on the formed oxide films. In the coprecipitation process, the ⁶⁰Co buildup amount is proportional to the corrosion amount of the base metal. In the adsorption process, the ⁶⁰Co buildup behavior is assumed to depend on the corrosion behavior of the base metal, but details are unclear.

In order to suppress the ⁶⁰Co incorporation by Zn injection more effectively, we studied the mechanisms of the ⁶⁰Co incorporation into the oxide films on type 316 stainless steel (316 SS) which is used for the PLR piping and of the ⁶⁰Co incorporation suppressed by Zn injection. In order to discriminate between coprecipitation and adsorption of ⁶⁰Co incorporation under the conditions simulating HWC, we measured the corrosion amount of the base metal and the ⁶⁰Co buildup amount, using simultaneous continuous measurements consisting of the potential drop method (PDM) and a NaI γ-ray detector, respectively. The formed oxide films were analyzed with a scanning electron microscope (SEM).

2. Experimental procedure

2.1. Immersion test

2.1.1. Apparatus and water chemistry conditions

A schematic drawing of the immersion test apparatus is shown in Figure 1.

Figure 1. Schematic drawing of immersion test apparatus.

In order to simulate HWC, as shown in Table 1, demineralized water with an electrical conductivity (EC) of <10 μS m⁻¹ at 298 K was stored in a reservoir tank, and H₂ and N₂ gases were bubbled into this water to keep their concentrations at the desired levels of under 5 μg kg⁻¹ for dissolved oxygen and 50 μg kg⁻¹ for dissolved hydrogen. ⁶⁰Co (activity: 6.7 Bq kg⁻¹), cobalt sulfate (cobalt ion concentration: 0.1 μg kg⁻¹) and zinc nitrate (zinc ion concentration: 0, 5 μg kg⁻¹) were added before reaching the test section. The tests carried out under the conditions of Zn = 0 μg kg⁻¹ and Zn = 5 μg kg⁻¹ are described as “without Zn” and “in the presence of Zn”, respectively. In the test section, simulated reactor conditions of high water temperature and pressure (553 K, 7.8 MPa) were used.

Table 1. Water chemistry conditions.

Components	Without Zn	In the presence of Zn
O2	<5 μg kg^−1
H2	50 μg kg^−1
Zn	0 μg kg^−1	5 μg kg^−1
Co	0.1 μg kg^−1
^60Co	6.7 Bq kg^−1

The PDM part and the NaI γ-ray detector (51B51/2, Scionix Holland) were installed in the test section of the apparatus to allow simultaneous and continuous measurements of the corrosion amount and the ⁶⁰Co buildup amount. The test pieces (T.P.s) part was also installed in the test section.

2.1.2. Continuous measurements of corrosion amount

The corrosion amount of 316 SS was measured by PDM, which can continuously evaluate the corrosion amount using electrical resistance measurements. Details of PDM have been described elsewhere [11]. In this work, “corrosion amount” means the decreased amount of the base metal due to oxidation and elution.

The relationship between the corrosion amount (Δr (μm)) and the change of resistance of the test wire is represented by the following equation: (1) $$\Delta r=r_0\left(1-\sqrt{\frac{R_0}{R_t}}\right)$$(1)

Here, r₀ (μm) is the initial radius of the test wire, R₀ (Ω) is the resistance of the test wire at 0 h and R_t (Ω) is the resistance of the test wire at t (h). Equation (1) shows that Δr can be obtained from the measurement of R_t.

A schematic drawing of the PDM part is shown in Figure 2. In order to measure the corrosion amount, 316 SS fine wire (ϕ50 ± 5 μm × 180 mm) was used as test wire. Lead wires of 316 SS (ϕ0.5 mm) were silver-soldered to both ends of the test wire. The connected lead wires were insulated with polytetrafluoroethylene (PTFE) heat-shrink tube to avoid contact with water. A 316 SS tube was polished with 600-grit emery paper and used as a test tube. The test wire and test tube were degreased in a 10-minute ultrasonication with acetone. The test wire was fixed coaxially in the test tube. The lead wires were extracted from the PDM part and connected to a DC source meter (2400, Keithley Inc.) and a low voltage meter (2812A, Keithley Inc.).

Figure 2. Schematic drawing of PDM part.

The resistance of the test wire was continuously measured by PDM under the conditions shown in Table 2. Resistivity of the test wire was calculated, supposing that the diameter of the test wire was 50 μm at 0 h. The resistance of the test wire measured by PDM was converted to the corrosion amount via Equation (1).

Table 2. Parameters of PDM system.

Applied current	1 mA
Resistivity of 316 SS	1.17 × 10^−2 Ω m
Collection time	0.5 s
Repetition number	50
Measurement interval	10 min

2.1.3. Continuous measurements of ⁶⁰Co buildup amount

The NaI γ-ray detector was located outside the test tube in the PDM part, and the γ-rays radiated from the ⁶⁰Co buildup on the inner surface of the test tube which had an energy of 1.3325 MeV were counted. The ⁶⁰Co counts were collected for 10 h intervals.

The T.P. was used as a reference to convert the counts, which were obtained by the NaI detector, to the ⁶⁰Co buildup amount. A schematic drawing of the T.P. part is shown in Figure 3. 316 SS T.P.s (surface area, 8 mm × 15 mm; thickness, 1.5 mm) were polished with 600-grit emery paper, and degreased with acetone, and installed in the T.P. part. The ⁶⁰Co buildup amounts on the T.P.s were measured with a Ge γ-ray detector (GEM-15190-P, Seiko EG&G Co. Ltd.) with a multichannel pulse height analyzer (MCA7600, Seiko EG&G Co. Ltd.).

Figure 3. Schematic drawing of T.P. part.

Assuming that the ⁶⁰Co buildup amounts on the test tube used with the PDM and the reference T.P. agreed with each other, we calculated the amount of ⁶⁰Co buildup on the test tube using the following equation: (2) $${\Gamma }_{\mathrm{mea}}={\Gamma }_{500}\times \frac{{\mathrm{C}}_t-{\mathrm{C}}_{\mathrm{B}.\mathrm{G}.}}{{\mathrm{C}}_{500}-{\mathrm{C}}_{\mathrm{B}.\mathrm{G}.}}$$(2)

Here, Γ_mea (kBq m⁻²) is the ⁶⁰Co buildup on the inner surface of the test tube at t, Γ₅₀₀ (kBq m⁻²) is the ⁶⁰Co buildup on the surface of the T.P. at 500 h, and C_t and C₅₀₀ are the γ-ray counts at t and 500 h, respectively. C_B.G. is the γ-ray count from the background.

2.1.4. Control of ECP

Platinum (Pt) and 316 SS coils were mounted in the downstream region of the test section, as shown in Figure 3, to measure the electrochemical corrosion potential (ECP). The coils were connected to an electrometer (6517B, Keithley Inc.). In all measurements in this work, the water chemistry conditions were HWC; therefore, the Pt coil worked as a hydrogen electrode. The ECP of 316 SS was calculated every 10 minutes using the Pt potential as the reference which is −0.5 V vs. standard hydrogen electrode (SHE). ECP of 316 SS was in the range of −0.53 ± 0.02V vs. SHE during 500 h under both conditions with and without Zn.

2.2. Oxide film analysis

In order to study the effect of Zn on corrosion of base metal and on ⁶⁰Co incorporation into the oxide films, the T.P.s were analyzed. The oxide films formed on the T.P.s were observed with SEM (S-3000FB/H, Hitachi, Ltd.).

According to previous reports [4,5], a double-layer oxide film consisting of outer and inner layers is formed on the SS surface. The immersed T.P.s were processed as described below to determine the amounts of Zn, ⁶⁰Co and total amounts of the oxides in each layer. The outer layer could be dissolved in a reductive condition, and was therefore separated by cathodic electrolysis. Each T.P. was electrolyzed in an electrolyte solution of 5 wt% H₂SO₄ and 0.5 wt% hexamethylenetetramine with 1 kA m⁻² current. The inner layer could be dissolved in alternating oxidizing and reducing atmospheres, and was separated by alternately soaking in potassium permanganate solution (300 ppm KMnO₄ solution) and 10 wt% ammonium citrate solution at 368 K. The amounts of ⁶⁰Co in the separated oxide films were measured using the Ge γ-ray detector with a multichannel pulse height analyzer. The amounts of Zn in the separated oxide films were determined with an inductively coupled plasma atomic emission spectroscope (ICP-AES; P-4010, Hitachi Ltd.).

3. Results and discussion

3.1. Suppression of SS corrosion by Zn

Figure 4 shows the time dependencies of the corrosion amount of the test wire under the test conditions in Table 1. Figure 4(a) shows the entire immersion time and Figure 4(b) shows the initial 50 h with an expanded time scale. The corrosion amounts of the 316L SS under HWC conditions without Zn, which were reported by Ishida and Lister [11], are also plotted. Our results and those of the previous report agreed with each other within a margin of error of 10%. This indicated that the PDM measurements well monitored the corrosion amounts of 316 SS.

Figure 4. Comparison of corrosion amounts for (a) the entire immersion time and (b) initial 50 h.

The corrosion amount of the base metal increased with time in each condition for immersion times of less than 10 h. The corrosion amounts at 10 h without Zn and in the presence of Zn were 0.109 ± 0.007 and 0.102 ± 0.007 μm, respectively. This indicated that the corrosion amounts of these conditions were equivalent to each other during 10 h. On the other hand, the corrosion amount in the presence of Zn had a tendency to saturate after 10 h. The difference between the corrosion amounts without Zn and in the presence of Zn increased with time, and the corrosion amount at 500 h in the presence of Zn was about half of that without Zn. We concluded from these results that the suppression effect of Zn on the corrosion appeared with a delay time of 10 h and the effect increased with time.

In order to study the suppression effect of Zn on the corrosion, we analyzed the compositions of the oxide films. According to [4,5], the oxide films formed on the SS surface have a double-layer structure comprising outer and inner layers. The outer layer is mainly magnetite (Fe₃O₄), and the inner layer is mainly chromite (FeCr₂O₄). We separated the oxide films into outer and inner layers, as mentioned in Section 2.2. Total amounts of the oxides and the amounts of Zn which were incorporated in each layer are shown in Table 3. The amounts of Zn in the outer and inner layers formed in the presence of Zn were 41 and 49 mg m⁻², respectively. We considered that Zn ions in the water environment had been incorporated into the inner layer which is mainly FeCr₂O₄, and the inner layer had changed to the thermodynamically stable ZnCr₂O₄. Haginuma et al. [4] proposed a similar effect by Zn injection on the SS corrosion under HWC conditions.

Table 3. Total amounts of metals and oxygen, and amounts of Zn and ⁶⁰Co in each separated layer.

	Outer layer			Inner layer
	Total	Zn	^60Co	Total	Zn	^60Co
Conditions	(μg m^−2)	(μg m^−2)	(MBq m^−2)	(μg m^−2)	(μg m^−2)	(MBq m^−2)
Without Zn	1321	0	0.19	1343	0	0.39
In the presence of Zn	615	41	0.16	944	49	0.03

To confirm the influence of the corrosion suppression of Zn on the oxide films, SEM observations were made. We compared the SEM images of T.P.s (Figure 5) and the film amounts (Table 3) without Zn and in the presence of Zn. Maximum diameters of the polyhedron particles in the outer layer without Zn and in the presence of Zn were 1.2 and 0.4 μm, respectively. The amounts of the outer layers without Zn and in the presence of Zn were 1321 and 615 mg m⁻², respectively. As regards the outer layer, the film amount was decreased and the maximum diameter of the particles became small when Zn was present. We considered that the inner layer stabilized by Zn had suppressed the dissolution of metal ions from the base metal; therefore, the number of polyhedron particles for the outer layer was lowered and the size of them became smaller. On the other hand, the thicknesses of the inner layers without Zn and in the presence of Zn were 0.3 and 0.2 μm, respectively. The amounts of the inner layers without Zn and in the presence of Zn were 1343 and 944 mg m⁻², respectively. We considered that the inner layer stabilized by Zn had suppressed the oxidation of the base metal; therefore, the amount of the inner layer was lowered and the inner layer became thinner. These results indicated that Zn decreased the amounts of the oxide films in which ⁶⁰Co is incorporated as mentioned in [4,5].

Figure 5. SEM images of surface and cross section of test pieces after 500-h immersion.

From the above results, we considered the delay of the suppression effect of Zn on the corrosion of 316 SS as follows. The oxide films incorporated Zn from the environment at the time of oxide formation and afterwards. Therefore, more ZnCr₂O₄ was formed and that stabilized the oxide films [4]. There was a lead time of 10 h before the suppression effect by the stabilized film appeared. We considered that the growth rate of the oxide films had been fast, and the oxide films had grown before incorporation of enough Zn to be stabilized in the initial 10 h.

3.2. Suppression of  ⁶⁰Co buildup by Zn

Figure 6(a) shows the entire immersion time dependencies of ⁶⁰Co buildup amount without Zn and in the presence of Zn, and Figure 6(b) shows the initial 50 h with an expanded time scale. Here, experimental error in the ⁶⁰Co buildup measurements was within ±5 kBq m⁻².

Figure 6. Comparison of ⁶⁰Co buildup amounts for (a) the entire immersion time and (b) initial 50 h.

The ⁶⁰Co buildup amount increased with time in each condition for immersion times of less than 10 h. The ⁶⁰Co buildup amounts at 10 h without Zn and in the presence of Zn were 15 ± 5 and 11 ± 5 kBq m⁻², respectively. This indicated that the corrosion amounts for these conditions were equivalent to each other at 10 h. On the other hand, the ⁶⁰Co buildup amount in the presence of Zn showed a tendency to saturate after 10 h. The difference between the ⁶⁰Co buildup amounts without Zn and in the presence of Zn increased with time, and the ⁶⁰Co buildup amount at 500 h in the presence of Zn was about 1/3 of that without Zn. From these results, we concluded that the suppression effect of Zn on the ⁶⁰Co buildup increased with time.

In order to study the suppression effect of Zn on the ⁶⁰Co buildup, we analyzed the amounts of ⁶⁰Co which were incorporated into the outer and inner layers. The ⁶⁰Co amounts in each layer are shown in Table 3. Focusing on the cases without Zn, the ⁶⁰Co amounts in the outer and inner layers were 0.19 and 0.39 MBq m⁻², respectively. ⁶⁰Co existed mainly in the inner layer. On the other hand, focusing on the cases in the presence of Zn, ⁶⁰Co amounts in the outer and inner layers were 0.16 and 0.03 MBq m⁻², respectively. ⁶⁰Co existed mainly in the outer layer. From these results, we concluded that Zn decreased the ⁶⁰Co amount in the inner layer to 1/10. In other words, Zn suppressed the ⁶⁰Co incorporation into the inner layer.

3.3. Relationship between ⁶⁰Co buildup and corrosion

The reported processes of ⁶⁰Co incorporation are coprecipitation [1,10] and adsorption [10]. We studied the contribution of both processes to the ⁶⁰Co buildup.

In the coprecipitation process, the oxide films incorporate ⁶⁰Co during the oxide formation, and the concentration of ⁶⁰Co in the oxide depends on the concentration of ⁶⁰Co in the environment. In this work, assuming that the production of oxide is proportional to corrosion, the concentration of ⁶⁰Co in the environment is constant; therefore, the ⁶⁰Co buildup by coprecipitation is proportional to the corrosion amount. The ⁶⁰Co buildup by coprecipitation is expressed as (3) $${\Gamma }_{\mathrm{cop}}=\delta ·\Delta r$$(3)

Here, Γ_cop (kBq m⁻²) is the ⁶⁰Co buildup amount by coprecipitation, δ (kBq m⁻³) is the deposition rate coefficient and Δr is the corrosion amount measured by PDM. We considered that the ⁶⁰Co buildup had been caused mainly by the coprecipitation in the initial stage for active corrosion, and δ was determined by fitting to the measured ⁶⁰Co buildup amount and the measured corrosion amount in the range of 0–50 h using Equation (3). On the other hand, the ⁶⁰Co buildup by the adsorption is expressed by Equation (4) and is the difference between the total ⁶⁰Co buildup amount and the ⁶⁰Co buildup amount by coprecipitation. (4) $${\Gamma }_{\mathrm{ads}}={\Gamma }_{\mathrm{mea}}-{\Gamma }_{\mathrm{cop}}$$(4)

Here, Γ_ads (kBq m⁻²) is the ⁶⁰Co buildup amount by adsorption and Γ_mea (kBq m⁻²) is the total ⁶⁰Co buildup amount measured.

The time dependencies of the ⁶⁰Co buildup amount without Zn, measured (shown in Figure 6) and calculated by Equations (3) and (4), are shown in Figure 7. Here, δ (determined by fitting using Equation (3)) is 5.0 × 10⁸ kBq m⁻³, Δr is the measured corrosion amount without Zn (shown in Figure 4). There were two different time dependencies of the ⁶⁰Co buildup amount without Zn, one for 0–100 h immersion times and the other for 100–500 h immersion times. In the range of 0–100 h, the measured amount agreed with the amount calculated using Equation (3). In other words, the ⁶⁰Co buildup amount was proportional to the corrosion amount. In the range of 100–500 h, the difference between the measured amount and the amount calculated using Equation (3) increased with time, and the measured amount was 20% greater than the amount calculated from Equation (3) at 500 h. We explained this as follows. ⁶⁰Co built up mainly by coprecipitation in the range of 0–100 h, and by both coprecipitation and adsorption processes in the range of 100–500 h. From the time dependency of Equation (4), the ⁶⁰Co buildup amount by adsorption was negligibly small in the range of 0–100 h, but increased with time in the range of 100–500 h. From the results, we concluded that the ⁶⁰Co buildup by adsorption had a delay time of about 100 h.

Figure 7. Comparison of measured ⁶⁰Co buildup amounts and amounts calculated using the equations (immersion without Zn).

In the same way as the case without Zn, we made comparison of the ⁶⁰Co buildup amounts measured and calculated using the equation of coprecipitation. The time dependencies of the ⁶⁰Co buildup amount in the presence of Zn, measured (shown in Figure 6) and calculated using Equations (3) and (4), are shown in Figure 8. Here, δ (determined by fitting using Equation (3)) is 3.7 × 10⁸ kBq m⁻³ and Δr is the corrosion amount measured in the presence of Zn (shown in Figure 4). In the presence of Zn, the measured amount agreed with the amount calculated by Equation (3) during the entire 500 h. In other words, the ⁶⁰Co buildup amount was proportional to the corrosion amount. From the results in the presence of Zn, we considered that ⁶⁰Co had built up mainly by coprecipitation because of the suppression effect of Zn on the ⁶⁰Co adsorption.

Figure 8. Comparison of measured ⁶⁰Co buildup amounts and amounts calculated using the equations (immersion in the presence of Zn).

From the above results, we were able to summarize the mechanisms of the ⁶⁰Co incorporation into the oxide films on 316 SS and of the ⁶⁰Co incorporation suppressed by Zn. We consider that the process of ⁶⁰Co incorporation into the oxide films on 316 SS under the conditions without Zn consists of two regions: (1) in the range of 0–100 h and (2) in the range of 100–500 h. In the range of 0–100 h, ⁶⁰Co is incorporated mainly into the oxide films by the coprecipitation process. In the range of 100–500 h, ⁶⁰Co is incorporated into the oxide films by both the coprecipitation and adsorption processes. In other words, the oxide films are formed on the surface of the base metal as a result of the corrosion of the base metal (>0.4 μm), and consequently the oxide films adsorb ⁶⁰Co. In the presence of Zn, similar to the conditions without Zn, ⁶⁰Co is incorporated mainly into the oxide films by the coprecipitation process in the range of 0–100 h. However, as mentioned in Section 3.1, Zn suppresses the formation of the oxide films which incorporate ⁶⁰Co. As a result, in comparison with the case without Zn, the ⁶⁰Co buildup amount at 100 h is reduced to about half of that when Zn is present. In the range of 100–500 h, adsorption of ⁶⁰Co on the oxide films is suppressed by Zn, but ⁶⁰Co is incorporated mainly into the oxide films by coprecipitation as in the range of 0–100 h. Therefore, in comparison with the case without Zn, the ⁶⁰Co buildup amount is reduced to about 1/3 by Zn.

4. Conclusions

We investigated the mechanisms of the suppression of ⁶⁰Co incorporation into oxide films on 316 SS surfaces by Zn under the conditions simulating HWC. In order to discriminate between coprecipitation and adsorption of ⁶⁰Co incorporation, we measured the corrosion amount of the base metal and the ⁶⁰Co buildup amount, using simultaneous continuous measurements. The conclusions are summarized as follows:

1. From the time tendencies of the corrosion amount, the suppression effect of Zn on corrosion appeared after immersion for 10 h and increased with time under the conditions of this work.

2. Under the immersion test condition without Zn, the time dependency of ⁶⁰Co incorporation had two regions. In the range of 0–100 h, ⁶⁰Co was incorporated mainly by coprecipitation. In the range of 100–500 h, ⁶⁰Co was incorporated by both coprecipitation and adsorption.

3. In the presence of Zn, the time dependency of ⁶⁰Co incorporation had only one region. Due to the suppression effect of Zn on the ⁶⁰Co adsorption, ⁶⁰Co was incorporated mainly into the oxide films by coprecipitation. In addition, Zn suppressed the corrosion of the base metal, and then reduced the ⁶⁰Co buildup caused by coprecipitation. In other words, Zn reduced the ⁶⁰Co buildup by suppressing both coprecipitation and adsorption, and the suppression effect increased with time.

Abbreviations

316L SS	=	Type 316L stainless steel
316 SS	=	Type 316 stainless steel
BWR	=	boiling water reactor
DH	=	dissolved hydrogen
DO	=	dissolved oxygen
ECP	=	electrochemical corrosion potential
HWC	=	hydrogen water chemistry
ICP-AES	=	inductively coupled plasma atomic emission spectroscope
PDM	=	potential drop method
PLR	=	primary loop recirculation system
PTFE	=	polytetrafluoroethylene
SEM	=	scanning electron microscope
SCC	=	stress corrosion cracking
T.P.	=	test piece
SHE	=	standard hydrogen electrode