Corrosion Behavior of Carbon Steel Rock Bolt in Simulated Yucca Mountain Ground Waters A. YILMAZ, D. CHANDRA, and R.B. REBAK Medium carbon steel (AISI 1040) was one of the candidate materials for rock bolts to reinforce the borehole liners and emplacement drifts of the high-level nuclear waste repository in Yucca Mountain. The corrosion performance of this structural steel was investigated by techniques such as linear polarization, electrochemical impedance spectroscopy (EIS), and laboratory immersion tests in simulated ground waters. The corrosion rates of the steel were measured for the temperatures in the range from 25 °C to 85 °C, for the ionic concentrations of 1 time (1), 10 times (10), and a hundred times (100) ground water concentration. The steel corroded uniformly at penetration rates of 35 to 200 m/year in the deaerated waters, and at 200 to 1000 m/year in the aerated waters. Increasing temperatures in the deaerated waters increased the corrosion rate of the steel. However, increasing ionic concentrations influenced the corrosion rate only slightly. In the aerated 1 and 10 waters, increasing temperatures increased the rates of the steel significantly. In the aerated 100 waters, the corrosion rate increased from 25 °C to 45 °C and decreased at higher temperatures (65 °C and up) due to the formation of oxide/hydroxide films and salt scales on the surface of the steel specimen. The steel suffered pitting corrosion in the both deaerated and aerated hot ground water environments after anodic polarization.
I. INTRODUCTION
YUCCA Mountain (Nevada) is being characterized as the site for geological disposal of commercial spent nuclear fuel and other forms of high-level nuclear waste. Yucca Mountain is located about 160 km northwest of Las Vegas on land owned by the federal government.[1] The overall strategy in isolating high-level nuclear waste is to make use of the natural barriers present in the host geologic site along with the construction of a series of engineered barriers. The current waste package design consists of two concentric metal containers. The outer container is made of a Ni-22Cr-13Mo-3W Alloy 22 (N06022), which is among the most corrosion resistant of all engineering materials. The purpose of this outer container is to provide protection against corrosion. The inner container is thicker and made of nuclear grade type 316L stainless steel (S31603). The intended purpose of the inner barrier is to provide shield for radiation and mechanical integrity. The engineering barrier design also includes a detached drip shield that would be emplaced above the waste package to deflect any falling water or rocks from the walls of the emplacement onto the containers. The proposed material for the drip shield is Titanium Grade 7 or UNS R52400.[2] Carbon steel was an earlier candidate for the container material for the oxidizing and dry Yucca Mountain environment.[3] This is no longer the case; however, the character-
A. YILMAZ, Materials Scientist, and R.B. REBAK, Corrosion Scientist, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Contact e-mail:
[email protected] D. CHANDRA, Professor, is with the Metallurgical and Materials Science and Engineering Department, University of Nevada-Reno, Reno, NV 89557. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
ization of the corrosion behavior of carbon steels in Yucca Mountain type environments is important since other components in the drifts may still be constructed using carbon steels. These components included rock bolts, tunnel ribs, borehole liners or mesh, and inverts for the pedestals or pallets. Since 2004, austenitic stainless steel is being considered for the rock bolts and the mesh instead of carbon steel. Carbon steel and low alloy steels are still candidate materials for the containers in other countries.[3,4,5] For example, in Japan and Europe, carbon steel is considered a viable material for the waste containers since it is planned to emplace them deep below the water line where the environment is predicted to have a reducing redox potential.[4,5] The corrosion behavior of carbon steels in natural waters and in soil has been extensively studied.[6] For example, it is well known that the corrosion resistance of carbon steels in natural waters is highly influenced by the pH and the degree of aeration of the water in contact with the steel.[6] The corrosion behavior of carbon steel in soils is a complex function of several variables including moisture content, amount of dissolved salts, acidity, and degree of aeration. Carbon steel is virtually inert in soils with low moisture content, low electrical conductivity, low acidity, and low dissolved salts.[6] In the last 20 years, many studies specifically determined the corrosion rate of carbon steels regarding their application in nuclear waste isolation. Until 1998, the United States’ design for the containers consisted of a thick external layer of carbon steel surrounding a thinner layer of a corrosion-resistant alloy. Therefore, many corrosion studies have been conducted in simulated Yucca Mountain ground waters. One of the most popular waters is designated J-13, which is the name of a well near the repository site. The composition of J-13 water is given in Table I. The corrosion rates of carbon steels, low alloy steels, and stainless steels were measured by weight loss at 100 °C in both J-13 water and steam by McCright and Weiss.[7] All the tested materials had lower corrosion rates in steam than in water, especially for the longer testing times. For example,
U.S. GOVERNMENT WORK NOT PROTECTED BY U.S. COPYRIGHT
VOLUME 36A, MAY 2005—1097
Table I. Chemical Composition of the Carbon Steel Rock Bolt (Weight Percent): Balance Fe; Steel Type 1040 Element Percent
C
Mn
S
Si
Cu
Ni
Cr
Mo
P
0.44 1.57 0.31 0.27 0.19 0.06 0.08 0.03 0.013
after 30 weeks of testing, the corrosion rate of 1020 carbon steel in J-13 water at 100 °C was approximately 27 m/year in water and 13 m/year in steam. The corrosion rate of stainless steels in the same conditions was almost negligible, even for those with only 9 pct chromium such as 9Cr1Mo alloy steel. The effect of irradiation on the corrosion rate was also studied.[7] Under irradiated conditions, the corrosion rates were higher when the rocks were present in the tested environment. It was explained that the production of oxygen and hydrogen peroxide by irradiation might have increased the corrosion rate of the tested materials.[7] Concurrently, the corrosion rates by weight loss of carbon steel, cast iron, and alloy steel in air saturated J-13 water were measured in the temperature range of 50 °C to 100 °C for 1500 hours.[7] Results showed that for all the alloys, a maximum corrosion rate was reached in the 70 °C to 80 °C range and decreased at 100 °C. This could be related to the amount of dissolved oxygen in the tested electrolyte. At 100 °C, the corrosion rate of 1020 carbon steel, gray cast iron, and 2.25 Cr1Mo alloy steel was approximately 300 m/year; however, for 9Cr1Mo alloy steel, it was only 7 m/year.[7] Carbon steel 1020 was also investigated regarding the effect of water vapor or relative humidity (RH) on its corrosion behavior at 65 °C.[8] At low RH, the corrosion degradation of carbon steel was negligible and it was determined that an RH between 75 and 85 pct is necessary before 1020 carbon steel starts suffering any obvious corrosion damage.[8] The corrosion rate of 1016 low-carbon steel was also measured in concentrated J-13 type waters (Table I) using electrochemical techniques.[9] The corrosion rate was always below 10 m/year for all the tested water concentrations at the temperatures of 25 °C, 50 °C, 70 °C, and 90 °C. Only in dilute aerated water (10 J-13) was the corrosion rate of the carbon steel higher than 10 m/year. In 1998, Cragnolino et al. published a comprehensive review of the corrosion modes of carbon steels regarding nuclear waste disposal, evaluating the different degradation modes such as microbial influenced corrosion, general corrosion, localized corrosion, and environmentally assisted cracking (EAC).[10] They remarked that the corrosion rate of carbon steel only becomes important at a RH higher than 75 to 80 pct. This fact is attributed to capillary condensation of water in the pores of the solid corrosion products.[10] Cragnolino et al. also commented on other facts regarding the corrosion of carbon steel in ground waters. For example, the corrosion rate in aerated waters increases linearly with the temperature up to 75 °C and then decreases from 75 °C to 100 °C. At the higher temperatures and in the more concentrated solutions (and also at longer times), the corrosion rate may decrease due to the precipitation of CaCO3, which limits the transport of diffusional oxygen to the surface.[10] In a laboratory study, the corrosion behavior of A516 carbon steel was investigated at 25 °C, 65 °C, and 95 °C in solutions containing different levels of carbonate/bicarbonate and chloride ions.[11] Again, it was found that a couple of the 1098—VOLUME 36A, MAY 2005
most important parameters controlling the mode and rate of corrosion of carbon steel were pH and the ratio of chloride to total carbonate.[11] The rock bolts in Yucca Mountain have a diameter of 1 in. (2.54 cm) and are 5 feet long (1.5 m). The bolts are driven into the rock radially from the waste emplacement drifts and used to avoid drift collapse. That is, the bolts are exposed to the rock and its corresponding water (moisture) and salts. During their lifetime, the bolts may be exposed to wet-dry cycles and to a range of temperatures above and below the boiling point of water. Each particular bolt may experience different environmental conditions depending on its location around the cross section of the drift. It is also expected that the bolts will be mechanically loaded (stressed). As with any other field material, the bolts may experience several corrosion degradation mechanisms including general or uniform corrosion, localized corrosion (pitting corrosion and crevice corrosion), and environmentally assisted cracking (EAC) (stress corrosion cracking and hydrogen embrittlement). The study of EAC is outside the scope of this article; however, EAC of buried carbon steel has been extensively studied in relation to gas transmission pipelines. McCright and Weiss did not find cracking of 1020 carbon steel after exposure of welded bent beam specimens to J-13 water at 90 °C for 9000 hours.[7] The objective of the current work was to investigate the effect of temperature and solution concentration on the general and localized corrosion behavior of the bolt steel in the simulated ground water environments. Corrosion rates were measured using electrochemical, salt spray, and immersion (weight loss) tests. II. EXPERIMENTAL A. Specimens and Coupons Electrochemical specimens and weight loss coupons were prepared from 1 in. diameter steel bolts received from the repository site. The chemical composition of the steel is given in Table II. The designation of this steel is type AISI 1040 (UNS G10400). From the original bolt, 5 mm long cylindrical specimens of 1 cm2 cross-sectional area were prepared. Each specimen was mounted in epoxy resin with an electrical connection wire protruding from the back. The front of the cylinder was ground exposing a 1 cm2 circular surface area for testing. Then, the manual surface grinding was continued with successive finer SiC emery papers until a 600-grit finish. These prepared specimens were degreased with acetone and washed with tap water without rubbing the surface. Then, they were rinsed with alcohol and distilleddeionized water, and immediately immersed into the gas-purged preconditioned solutions. The specimens were reground and reused as necessary. Immersion test coupons were also cut from the bolts, avoiding heating them during the preparation, as required by the standards ASTM G 31 and G 85.[12] The coupon sizes were 40 17.5 0.5 mm for the ambient temperature tests, and 25 13 0.5 mm for the 75 °C tests. The coupons for the spray or fog tests were 120 mm long, 70 mm wide, and 0.5 mm thick. The total surface area was approximately 14.5 cm2 for ambient tests, 6.9 cm2 for 75 °C tests, and 170 cm2 for the salt spray tests. Surface grinding was done manually METALLURGICAL AND MATERIALS TRANSACTIONS A
Table II. Chemical Composition of Original YMW and Well J-13 Water in Milligrams per Liter (Parts per Million) Ions
Na
SiO2 (aq)
Ca2
K
Mg2
HCO 3
1 YMW J-13
61.3 51.0
70.5 66.4
101 14.0
8.0 4.9
17.0 2.1
200 120.0
Fig. 1—Arrangement of the cell elements used for the potentiodynamic polarization and EIS techniques for the corrosion rate determination of the carbon steel in the simulated YM ground waters.
Cl
SO42
F
NO3
pH
117 7.5
116 22.0
0.86 2.2
— 5.6
7.2 7.1
test cell and a proportional, integral and derivative type (PID) temperature controller maintained the desired temperatures in the accuracy of 0.1 °C. The overall construction required a minimum of glass blowing and met the specifications of ASTM standard G 5.[12] The immersion corrosion experiments were carried out in a 1 L glass flask with a lid containing four holes sealed on by O-rings. The cell also contained a purging tube, a condenser, and a thermometer. The temperature of the solution was maintained at 0.1 °C using a heating mantle with a temperature controller. The cell was half-filled with the testing solution (600 mL). The coupons were hanged from the lid at different heights in the cell: one totally immersed in the liquid, one partially immersed, and one above the level of the solution (in the vapor space). Aeration was carried out by purging air into the solution continuously at the rate of 150 mL/min. The cell design met the specifications of ASTM Standard G 31.[12] Additional immersion (weight loss) tests were carried using the spray or fog chamber described in ASTM B 117, using the dilute ground water as the testing electrolyte. These tests were conducted at the National Exposure Testing Facilities (Sylvania, OH). C. Solution Preparation
prior to immersion with successive finer SiC emery papers from 240- to 600-grit finish. The same preparation was also done for the spray (fog) tests (ASTM B 117).[12] B. Test Cells The electrochemical corrosion tests including polarization resistance (linear polarization), Rp, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) (ASTM G 106)[12] were conducted in a typical 1 L glass resin flask corrosion cell. A polytetrafluorethylene lid with many ports allowed the entire cell elements to be inserted into the solution at once. Figure 1 shows the setup of the electrochemical cell, which contained the working electrode (1 cm2), a platinum foil counter electrode (10 cm2), a Luggin capillary for the reference electrode, a gas sparger, and a thermocouple well. The Luggin capillary was connected to the saturated silver-silver chloride (Ag/AgCl) reference electrode (SSC) outside the cell via a salt bridge (saturated KCl). At ambient temperature, the SSC electrode is 199 mV more positive than the standard hydrogen electrode. The tip of the Luggin probe always maintained the same distance (1 mm) from the specimen surface to eliminate the Luggin probe distance effects. A sealed glass capillary was used as the thermowell for controlling the temperature of the test solutions. A fritted glass capillary was used for continuous aeration/deaeration of the solution throughout the experiments at the rate of 150 mL/min. A water-cooled condenser minimized the solution loss due to evaporation, while its outlet was connected to a gas trap for a better sealing against oxygen ingress. An electric mantle heater surrounded the METALLURGICAL AND MATERIALS TRANSACTIONS A
Table I shows the basic composition of the test solution, which was called Yucca Mountain water (YMW) or 1 ground water. This test solution slightly differs from the J-13 water used previously by other researchers (Table I). The test solution (1 ground water) was prepared by mixing appropriate chemicals with distilled deionized water initially kept at 35 °C. The 10 and 100 waters were prepared by increasing the amount of dissolved salts. That is, 10 and 100 were not prepared by evaporating 1 solution. A 10 solution was 10 times more concentrated than a 1 solution. If insoluble salts precipitated at the bottom of the flask, the solution was filtered and only the clear portion was used for testing. The initial pH of the solutions was approximately 7.2 for 1 and 10 ground waters and 8.2 for the 100 water. The more concentrated solution had a higher pH probably due to the hydrolysis of the oxyanions present in the solution. D. Test Procedures Electrochemical tests were conducted using a potentiostat controlled by a computer and a commercial software. The simulated solutions in the test cells were conditioned (deaerated or aerated) by the continuous purging of the corresponding gas (nitrogen/oxygen) for 2 hours, before immersing the specimen. Then, the freshly finished wet-ground specimens were immersed into the preconditioned solutions. The specimens were kept in the testing electrolytes for another 2 hours or until reaching a stable open circuit potential (corrosion potential, or Ecorr). The potentiodynamic scans were started at 300 mV below the corrosion potential and were increased at a constant rate of 0.166 mV/s until the current density VOLUME 36A, MAY 2005—1099
reached 100 mA/cm2. The potentiodynamic Rp or linear polarization method (ASTM G 59) was used to determine the Rp.[12,13] The Rp was calculated by selecting the linear portions of the polarization scans (E vs I) within 25 mV anodic and cathodic overpotentials. Then, corrosion rates were calculated from Rp values, according to Eq. [1]: CR(mm/year) k
EW # 1 r Rp
[1]
The conversion factor k in Eq. [1] is 3.27 106 mmg/ Acmyear, for a corrosion rate in a unit of m/year.[12] The Tafel slopes (a and c) were assumed to be 0.12 V/decade, the density of the steel was 7.85 g/cm3, and the equivalent weight was 27.90 g/mol (ASTM G 102).[12] Electrochemical impedance spectroscopy tests were conducted at Ecorr.[12,14,15] The Rp values were determined from the real axis intercepts of the depressed semicircles of the Nyquist plots.[14,15] The Rp values were then converted into corrosion rates according to Eq. [1]. Laboratory immersion corrosion testing of the rock bolt steel was carried out in 1 YMW at the temperatures of 25 °C and 75 °C, following the standard procedures of ASTM G 31.[12] Weight loss values for the ambient temperature tests were recorded approximately in weekly periods and those for the 75 °C tests roughly every 48 hours. For the 75 °C tests, the immersion was carried out under full immersion water line (half-immersion), and in the vapor moist environment; however, only the full immersion was conducted for the ambient temperature tests (25 °C). Aeration was maintained by purging air into the test solution continuously at a constant rate of 150 mL/min. The time for specimen cleaning and weight loss measurements did not exceed 5 minutes during the data taking processes. Constantly flowing cold water (14 °C) was introduced through the condenser, in order to minimize the solution loss due to evaporation. The testing time was 107 days at 25 °C and 500 hours at 75 °C. Standard ASTM G 85[12] procedures were applied for dilute or 1 ground water. The same exposure chamber described in ASTM B 117 (salt spray test) was used. The test temperature was 35 °C and the testing time was 1 week.
III. RESULTS AND DISCUSSION A. Corrosion Behavior in Deaerated Ground Waters Using Polarization Bolt steel was potentiodynamically polarized in 1, 10, and 100 ground waters at 25 °C, 45 °C, 65 °C, and 85 °C. Figure 2 shows the anodic and cathodic polarizations of the steel in 1 ground water at 25 °C and 85 °C. The polarization curves show that the corrosion rate (corrosion current density) was higher at 85 °C (with lower corrosion potential) and lower at 25 °C (with higher corrosion potential). This common feature is generally observed with steel-ground water systems, suggesting that anodic dissolution of the steel was hindered more in the colder ground waters, giving rise to the cathodic reaction rates, which in turn raised the Ecorr. The cathodic branches of both curves were linear, that is, there was no limiting current by cathodic reactions within the applied potential range (Figure 2). The curves of the 1100—VOLUME 36A, MAY 2005
Fig. 2—Temperature effect on polarization of the carbon steel: potentiodynamic behavior in the deaerated simulated 1 ground waters at 25 °C and 85 °C.
intermediate test temperatures 45 °C and 65 °C were not plotted to avoid interference with the ones shown. However, the curves not shown had the same linear regions as similar slopes that were in between the cathodic lines of 25 °C and 85 °C. Thus, there was an increase in corrosion rate with increasing temperatures from 25 °C to 85 °C. Increasing temperatures increased the passive current density in the anodic region as well, as shown in Figure 2. The passive region shape changed and became a vertical line when temperature was raised to 85 °C, showing that the dissolved oxygen amount in the electrolyte was lower at this temperature. This also suggested that the surface film that caused passivity was relatively thin but more stable (no observed alteration in current) with better deaeration. The breakdown potential (BP) was decreased with increasing temperatures from 25 °C to 65 °C; however, it increased again at 85 °C. This effect may have been caused by the inhibition of chloride attack via an increase in the protectiveness of the films formed at 85 °C. This increase in BP at 85 °C was mostly observed with the dilute waters such as 1 and 10, which had relatively lower amounts of Cl ions. The protective film scales on the steel were of the form of FeCO3 and Fe(oxide/hydroxide), and were determined by X-ray photoelectron spectroscopy (XPS) in the concentrated ground waters.[16] Figure 3 shows that the corrosion currents of all test concentrations (1, 10, and 100) at room temperature (RT) were practically the same in the deaerated waters. By increasing the water concentration from 1 to 100, the corrosion current changed slightly at each test temperature. Figure 3 shows that the anodic region of the polarization is not affected by water concentration as opposed to the temperature effect, but in the cathodic region, the current density seemed to increase as the ionic concentration increased. Table III shows the corrosion current (Icorr) values and the corresponding rates determined by linear polarization technique for the deaerated environments from 25 °C to 85 °C. The corrosion rates were between 35 and 168 m/year for the various ionic METALLURGICAL AND MATERIALS TRANSACTIONS A
concentrations in the investigated temperature range. These values are higher than the ones reported by Lian and Jones for the low-carbon steel studied in J-13 well water.[9] The
Fig. 3—Concentration effect on polarization of the carbon steel: potentiodynamic behavior of various concentrations in the deaerated simulated ground waters at 25 °C.
J-13 well water given in Table II is more dilute than the YMW used in this work, especially considering the corrosion influencing anions such as Cl and SO42 . Besides, the medium carbon steel studied here has a higher carbon content than the steel studied by Lian and Jones, and therefore, would have more galvanic interactions between ferrite and relatively large cementite structures, leading to a more rapid dissolution. The obtained corrosion rate values (Table III) can still be considered low for this particular engineering application. As a complementary technique, EIS was conducted to confirm the trends of the corrosion rates determined by the linear polarization method. Experimental results showed that EIS had a better sensitivity to the corrosion rates for the electrolytes of low conductivity such as 1 and 10 ground waters. The results of EIS experiments for the deaerated waters conducted at Ecorr are shown as Nyquist plots in Figures 4 and 5. The results of EIS showed the same corrosion rate trends for the steel, as in the linear polarization tests. The Rp value for each environment is represented by the diameters of the depressed semicircles (Figures 4 and 5). The corrosion rate is inversely proportional to Rp; that is, the larger the Nyquist semicircle, the lower the corrosion rate. Figure 4 shows that as the temperature increases, the corrosion rate of the steel in deaerated 1 water decreases. This agrees well with the data shown in Table III obtained using the Rp method. Figure 5 shows the effect of the water concentration on the corrosion rate at ambient temperature. The corrosion rate at RT slightly decreased when concentration increased to 10, but increased for the 100 (Figure 5).
Table III. Corrosion Rates of Rock Bolt Steel in Ground Waters Using the Rp Technique Temperature (°C)
ICorr (A/cm2)
1 1 1 1 10 10 10 10 100 100 100 100
25 45 65 85 25 45 65 85 25 45 65 85
3.8 106 6.0 106 1.0 105 1.3 105 3.0 106 5.5 106 1.0 105 1.4 105 4.5 106 6.0 106 1.0 105 1.5 105
1 1 1 1 10 10 10 10 100 100 100 100 100 100 100
25 45 65 85 25 45 65 85 25 35 45 55 65 75 85
1.1 105 2.8 105 5.6 105 1.0 104 1.2 105 3.5 105 6.6 105 9.4 105 2.0 105 3.9 105 4.5 105 3.3 105 2.3 105 1.8 105 1.7 105
Water Concentration (Times)
METALLURGICAL AND MATERIALS TRANSACTIONS A
Corrosion Rate (mpy) Deaerated Conditions 1.7 2.7 4.7 6.3 1.4 2.3 4.6 6.4 2.0 2.7 4.5 6.7 Aerated Conditions 4.9 12.6 24.8 45.0 5.4 16.0 30.0 42.0 9.0 18.0 20.0 15.0 10.3 7.9 7.6
Corrosion Rate (m/year) 45 70 120 160 35 58 116 162 51 68 113 168 124 320 630 1134 136 396 750 1058 226 446 510 378 261 199 193 VOLUME 36A, MAY 2005—1101
Fig. 4—Temperature effect on Rp of the carbon steel: Nyquist plots from EIS at Ecorr in the deaerated 1 ground waters of various temperatures. Polarization resistance determined by the radius of the Nyquist plot semicircle is inversely proportional to the corrosion rate.
Fig. 5—Solution concentration effect on Rp of the carbon steel: Nyquist plots from EIS at Ecorr, for various concentrations of deaerated YMWs at 25 °C. Polarization resistance determined by the radius of the Nyquist plot semicircle is inversely proportional to the corrosion rate.
This confirmed the rate trend obtained by the polarization technique given in Table III. Figure 6 shows the corrosion rates of steel calculated from the Rp experiments in 1, 10, and 100 deaerated ground water. The corrosion rate increased approximately 3 times as the temperature increased from 25 °C to 85 °C. At 25 °C, the corrosion rate was the lowest for the most dilute solution; however, as the temperature increased to 45 °C and higher, the corrosion rate was practically the same for all deaerated 1102—VOLUME 36A, MAY 2005
Fig. 6—Temperature effect on corrosion rates of MCS rock bolt in simulated deaerated concentrated YMWs by Rp technique.
concentrated ground waters. The apparent activation energy for the corrosion rates illustrated in Figure 6 is 18 kJ/mol. This value of activation energy is typical for mild corrosion processes. Pitting corrosion of the polarized steel specimens was observed in the entire tested temperature range in the deaerated waters. The passive films that formed on the specimens were colorless and brittle, as generally associated with the dielectric types. Figure 8 shows the corrosion pits formed on a steel specimen tested in 100 solution at 85 °C. The pit formations on the steel shown in Figure 8 are nearly round in shape and distributed uniformly over the surface of the specimens, which could be attributed to a homogeneous film of a uniform thickness. This behavior may suggest that the dissolution of the carbon steel in the environment might be fairly uniform, if it was not subject to any condition that promotes localized corrosion. Therefore, the measured corrosion rate values would also represent the penetration rates. Visual examinations (Figure 8) showed that the carbon steel in the YM aqueous environments was prone to localized corrosion such as pitting corrosion after anodic polarization in the studied temperature range. B. Corrosion Behavior in Aerated Waters Using Polarization In the concentrated aerated water at 65 °C and 85 °C, the steel specimens started to show evidence of corrosion during the 2 h conditioning time, that is, even before the actual polarization scans started. The specimens did not suffer any corrosion during the conditioning period in the dilute solutions at 25 °C and 45 °C. Figure 7 shows the corrosion rates for the tested steel as a function of temperature and electrolyte concentration both for aerated and deaerated solutions. The corrosion rates were significantly higher for the aerated solutions than for the METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 9—Protective film formation and pitting over the MCS rock bolt in the aerated 100 YM ground waters at 85 °C after anodic polarization (magnification 50 times).
Fig. 7—Temperature effect on corrosion rates of MCS rock bolt in aerated and deaerated concentrated YMWs by Rp technique.
Fig. 8—Protective film formation and pitting over the MCS rock bolt in the deaerated 100 YMWs at 85 °C after anodic polarization (magnification 50 times).
deaerated solutions. In aerated 1 and 10 ground waters, the corrosion rate of steel increased as the temperature increased (Figure 7). The corrosion rates ranged from 120 m/year to 1000 m/year within the studied temperature range. In aerated 100 water, the corrosion rate increased from 25 °C to 45 °C and then decreased from 55 °C to 85 °C (Figure 7). The decrease in the corrosion rate above 45 °C for highly concentrated waters could be a consequence of the development of a semiprotective oxide and salt scale on the surface of the coupon. The presence of CaCO3 was shown by an XPS study on the carbon steel in similar ionic environments.[16] This phenomenon of decreasing corrosion rates in oxygenated solutions with increasing temperatures was also reported by other researchers.[10] Figure 7 shows that in 1 and 10 water, the dependence (slope) of the corrosion rates with the temperature METALLURGICAL AND MATERIALS TRANSACTIONS A
was higher in the aerated solutions as compared to the deaerated solutions. The activation energy for the corrosion processes in the aerated environments was determined as 30 kJ/mol, which is approximately 1.5 times higher than that of the deaerated environments. More active corrosion processes tend to yield higher activation energies. The dependence between corrosion rate and temperature for the steel was different in the 100 ground water than in the 1 and 10 concentrations. In the 100 water, the polarization scans showed an increase in corrosion rates from 25 °C to 45 °C, and then a decrease after 45 °C through 65 °C, reaching the lowest value of corrosion rate at 85 °C. The tests for the 100 aerated water in Figure 7 were repeated several times with intermediate temperature sets, and the maximum corrosion rate value was always found to be at 45 °C. Observed precipitation and pitting on the specimens at 65 °C and 85 °C confirmed the formation of a film on the surface, which caused reduction of the general corrosion rates in the aerated hot waters. The lower oxygen solubility at the higher temperatures may have also been a contributing factor in the decrease of corrosion rates. The fact that carbon steel has a higher corrosion rate at intermediate temperatures has been previously reported.[7,10] Figure 9 shows that white precipitates and thick films were formed on the surface of the specimens in aerated conditions, while thinner films formed in the deaerated waters (Figure 8). The grinding marks in Figure 8 are visible, suggesting a transparent thin film, as compared to the absence of grinding marks in Figure 9. Figure 9 also shows the formation of corrosion pits in the aerated hot waters. The diagonally aligned inclusions in the given micrograph are the sites where the deterioration has just started on the steel (innermost, elliptically shaped dark portions of the pit). Results show that the tested steel was prone to pitting and other localized corrosion attacks at the inclusions under the tested conditions. The light gray section is the precipitation layer over the metal, and the dark gray is the corroded steel (Figure 9). The white spots accumulated around the pits are the salt islands. The calculated corrosion currents and penetration rates for the steel in aerated ground waters are also given in Table III. VOLUME 36A, MAY 2005—1103
C. Corrosion Behavior by Immersion Tests Figure 10 shows the mass measurements of the steel coupon immersed in the 1 ground water solution at RT. The mass loss is linear as a function of time, that is, the steel is corroding in active conditions at a certain rate. The calculated corrosion rate from Figure 10 after 107 days exposure was 45 m/year. This rate is comparable with the results of electrochemical studies for the deaerated 1 water (Table III and Figure 6). Figure 11 shows the normalized mass measurements for three coupon specimens exposed to the vapor,
the solution line, and the solution. The tests were carried out in aerated 1 water at 75 °C for 500 hours. The results show that for the fully immersed coupons, the corrosion rate was uniform in time for the entire testing period. However, in the vapor and waterline position, there was an induction period of approximately 100 hours before the corrosion rates became larger and linear in time. The calculated corrosion rates of the steel coupons were approximately 1000 m/ year for the water line, 510 m/year for the vapor phase, and 200 m/year for the full immersion. The rate obtained for full immersion is comparable to the value obtained electrochemically (Table IV and Figure 7). In the immersion tests, the corrosion rate was the lowest for full immersion and the highest in the water line, showing the importance of oxygen availability as a promoter of corrosion rate. In the water line, the coupons were also exposed to oxygen concentration cell effects, which clearly differentiated anodic and cathodic regions in the coupon and thus accelerated the corrosion process.[16,17,18] The attack on the coupons at the water line was always more severe than that on the rest of the area. D. Corrosion Behavior by Ground Water Spray Tests
Fig. 10—Mass loss profile of fully immersed MCS rock bolt in the 1 YM ground water at 25 °C
Figure 12 shows the normalized average mass loss of the tested coupons exposed for one week to the 1 ground water spray test. This mass loss translated as an average corrosion rate of 800 m/year. One to one comparison of corrosion rate values using other techniques is not possible since all the testing temperatures were different. However, the corrosion rate from the spray test is on the same order of magnitude as the corrosion rates from the linear polarization in aerated 1 water at 65 °C and the corrosion rate from the coupons exposed to the water line in the immersion tests in aerated 1 water at 75 °C. After the spray test, the coupons showed partly corroded sections while some parts were unattacked and shiny (Figure 13). The steel suffered a nonuni-
Fig. 11—Lab immersion corrosion testing in 1 YM ground water at 75 °C with three different coupon positions such as total immersion, partial immersion, and vapor exposure (in the humid part above the water line).
Fig. 12—Mass loss of MCS rock bolt in YM spray environment at 35 °C.
1104—VOLUME 36A, MAY 2005
METALLURGICAL AND MATERIALS TRANSACTIONS A
4. Both in the deaerated and aerated waters, the changes in water concentration from 1 to 10 did not influence the corrosion rates significantly at fixed temperatures. 5. In the 100 aerated water, the corrosion rate increased from 25 °C to 45 °C and then decreased at higher temperatures up to 85 °C due to formation of protective films. 6. The bolt steel was susceptible to localized corrosion pitting under anodic polarizations. 7. The corrosion rate was measured using two electrochemical techniques and two weight loss techniques. All the techniques showed that the values of corrosion rate were comparable.
ACKNOWLEDGMENTS
Fig. 13—Simulated 1 ground water spray test coupons, before (left) and after (right) the exposure tests at 35 °C.
form corrosion in the spraying ground water tests because of the formation of large domains of anodic and cathodic reaction sites. These results suggest that the oxygen concentration cell effect[17,18] controlled the corrosion processes in the spray tests. This work shows that the corrosion rate of rock bolt steel (Table I) was low in the tested conditions and generally below 1000 m/year (40 mpy) even in fully aerated conditions. Under moist conditions (vapor), the corrosion rate was 200 m/year (8 mpy). These results are comparable to the corrosion rates obtained by previous researchers in other types of carbon steels intended for other applications in the repository.[7–11] The corrosion rates calculated here are extreme bounding conditions since the testing times were short and the availability of ground water was not limited for the study. In the actual repository conditions, the presence of water will be limited, and therefore, the degradation of the rock bolt steel is expected to be much slower. Even if a thin film of water is formed on the rock bolt, this will soon be saturated with corrosion products and the dissolution process of the steel is expected to decrease and even stop. IV. CONCLUSIONS 1. The general corrosion rate of the rock bolt steel increased with increasing temperatures for the deaerated and the more dilute (1 and 10) aerated simulated ground waters. 2. The steel had a lower corrosion rate in the deaerated simulated ground water than in the aerated waters. 3. The corrosion rates in the deaerated waters were between 35 m/year and 200 m/year in the temperature range from 25 °C to 85 °C. In the aerated, more dilute waters, the corrosion rates varied between 124 and 1100 m/year in the temperature range from 25 °C to 85 °C.
METALLURGICAL AND MATERIALS TRANSACTIONS A
This work was performed at the University of Nevada, Reno, and only partially supported under the auspices of the United States Department of Energy by the University of California, Lawrence Livermore National Laboratory, under Contract No. W-7405-Eng-48. This work is supported by the Yucca Mountain Project, LLNL, which is part of the Office of Civilian Radioactive Waste Management (OCRWM-DOE).
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