Electrochemical Studies on Silicate and Bicarbonate ... - Springer Link

Electrochemical Studies on Silicate and Bicarbonate Ions for Corrosion Inhibitors MICHAEL E. MOHORICH, JOSHUA LAMB, DHANESH CHANDRA, JAAK DAEMEN, and RAUL B. REBAK Several types of carbon and high-strength low-alloy (HSLA) steels are being considered for use in the underground reinforcement of the Yucca Mountain Nuclear Waste Repository. In this study, potentiodynamic polarization under reducing conditions was used to determine the corrosion rates (CRs) and passivity behavior of AISI 4340 steel using different combinations of sodium silicate (Na2SiO3) and sodium bicarbonate (NaHCO3), in both pure water (PW) and simulated seawater (SW, 3.5 pct NaCl). These experiments were carried out to examine the potential inhibiting properties of the silicate or bicarbonate ions on the surface of the steel. The addition of sodium silicate to solution reduced the observed CR at room temperature to 19 lm/y at 0.005 M concentration and 7 lm/y at 0.025 M concentration in PW. The addition of sodium bicarbonate increased the CR from 84 lm/y (C = 0.1 M) to 455 lm/y (C = 1 M). These same behaviors were also observed at higher temperatures. DOI: 10.1007/s11661-010-0234-2 The Minerals, Metals & Materials Society and ASM International 2010

I.

INTRODUCTION

THE AISI 4340 steel (UNS G43400) is an ultrahighstrength low-alloy steel that combines deep hardenability with high ductility, toughness, and strength, and is considered as the benchmark for ultrahigh-strength steels.[1] These properties make the steel useful for industrial applications where high strength in heavy sections is required.[1] Yucca Mountain, located on the southwest corner of the Nevada Test Site, in Nye County, Nevada, has been chosen as the potential site for spent nuclear fuel and high-level radioactive waste in the United States.[2] Currently, different types of steels are being considered for tunnel reinforcement applications inside the repository. These include stainless steels, low- and medium-carbon steels, and high-strength lowalloy (HSLA) steels. These materials may see significant amounts of corrosion over the lifetime of the repository. The inhibiting effect of silicate and bicarbonate ions on the surface of AISI 4340 steel was investigated in this study. We measured this steel’s electrochemical and corrosion (CR) behavior in aqueous solutions containing sodium silicate or sodium bicarbonate to determine if either of these chemicals could be used as a suitable corrosion inhibitor for HSLA-candidate steels proposed for use as rock bolts and I-beams inside the repository MICHAEL E. MOHORICH, Graduate Research Assistant, formerly with the Chemical and Metallurgical Engineering Department, University of Nevada, Reno, NV. JOSHUA LAMB, Postdoctoral Scholar and DHANESH CHANDRA, Professor, are with the Chemical and Metallurgical Engineering Department, University of Nevada. Contact e-mail: [email protected] JAAK DAEMEN, Professor, is with the Mining Engineering Department, University of Nevada, Reno, NV 89557. RAUL B. REBAK, Research Scientist, is with GE Global Research, Schenectady, NY 12309. Manuscript submitted September 1, 2009. Article published online June 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A

tunnels. A significant amount of research has been devoted on stress corrosion cracking of 4340 steel,[3–7] hydrogen embrittlement,[8–10] and hydrogen-induced cracking,[11–14] due to the material’s heavy use in the automotive and aerospace industries[1] and the need to understand the steel’s fracture mechanics and stress behavior under severe loading conditions.[1] However, rather limited studies have been conducted on 4340 steel to understand the corrosion science and relations to engineering applications. Myers and Saxer[15] studied the anodic polarization behavior of 4340 steel in sulfuric acid solutions, and Mansfeld et al.[16] published the effects of 4340 steel using various sodium-based salt solutions in tap water, deionized water, and a mixture of phosphonic/polyacrylic acids with fatty amine as inhibitors. Our current study builds on previous work published by Deodeshmukh;[17,18] however, his studies were limited to room temperature measurements using Rock Bolt Carbon Steel, a medium-carbon resulfurized steel, at a constant pH of 8.5. The electrolytes used in his studies may be used to predict the CRs of the other steels, if they should come in contact with solutions containing high concentrations of these ionic compounds. The results from this study are expected to help in the selection of steels with respect to their strength and corrosion resistance considerations. The electrochemical and general corrosion behavior of AISI 4340 steel in different multi-ionic solution environments is reported in this study. II.

EXPERIMENTAL

A. Materials 1. Chemical analysis The material used in the experiments was originally produced by Republic Engineered Products (Lorain, VOLUME 41A, OCTOBER 2010—2563

Table I.

Chemical Composition (Weight Percent) of AISI 4340 Steel

Element Iron (Fe) Carbon (C) Chromium (Cr) Manganese (Mn) Molybdenum (Mo) Nickel (Ni) Phosphorous (P) Sulfur (S) Silicon (Si)

Required Weight (Pct)[21]

Tested Weight (Pct)[20]

balance 0.38 to 0.43 0.70 to 0.90 0.60 to 0.80 0.20 to 0.30 1.65 to 2.00 0.035 max 0.040 max 0.15 to 0.30

balance 0.43 0.84 0.77 0.26 1.86 0.015 0.020 0.26

OH) as AISI E4340 steel (UNS G43406), with an austenitic grain size of seven.[19] It was purchased in 1- and 1/2-in. diameter air-melt round bar, in the coldfinished and normalized condition. The steel’s chemistry was independently verified by Laboratory Testing, Inc.[20] (Hatfield, PA) and found to meet the specifications for AISI 4340 steel.[21] Table I shows the composition of the steel. 2. Specimen preparation For the electrochemical experiments, samples were cut and machined into 1/4-thick specimens from 12-in. sections of 1/2-diameter rounds, giving a surface area of ~1.2 cm2. An exposed portion of insulated, 2-mmdiameter copper wire was soldered on one side of the specimen, and the sample was then cast in an epoxy resin consisting of a 5:1 ratio of Buehler EPOXICURE* *EPOXICURE is a trademark of Buehler, Lake Bluff, IL.

and Buehler Epoxide Hardener, respectively. After curing, the specimen’s surface was polished using wet Buehler Carbimet 240 and 600 grit SiC emery papers and then cleaned with tap water followed by deionized/ distilled water. 3. Solution preparation All solutions were prepared using deionized-distilled water and ACS reagent-grade salts. Bicarbonate was added in the form of NaHCO3, and silicate was added in the form of Na2SiO3Æ9H2O. Simulated seawater (SW, 3.5 pct NaCl) solution was prepared by adding 35 g of NaCl to 965 mL of distilled water, prior to addition of any other salts. 4. Electrochemical tests All electrochemical experiments were conducted in accordance with ASTM standard G 5-94.[22] Experiments were conducted in a typical 1 L Pyrex (Ace Glass, Vineland, NJ) glass reaction vessel covered with a polytetrafluoroethylene lid. This lid has several ports to contain the working electrode, counter electrode (platinum), gas dispersion tube, inlet and outlet for the gas, Luggin probe containing the reference electrode, and gas trap. A large (~5 cm2) platinum sheet sealed to a glass capillary was used as a counter electrode to provide 2564—VOLUME 41A, OCTOBER 2010

good conductivity in the electrolyte. The reference electrode used was a saturated silver/silver chloride (Ag/AgCl) electrode, containing 4 M potassium chloride saturated with silver chloride. This type of electrode has a potential of 199 mV more positive than the standard hydrogen electrode. Continuously purged nitrogen gas in the sealed cell maintained constant pressure above the solution. A fritted glass capillary was used for continuous deaeration of the solution throughout the experiment at the rate of 100 mL/min using a flow meter. A heating mantle surrounded the test cell, and an auto-tune PID temperature controller (Ace Glass Econo Temperature Controller) maintained the temperature of the solution. Prior to specimen immersion, the solution was conditioned with nitrogen for at least 30 minutes, or until the electrolyte’s temperature was within ±1 K of the targeted temperature. The specimen was freshly polished, attached to the Luggin probe apparatus, and inserted into the cell. The specimen was then allowed to reach a steady-state open-circuit potential, prior to the start of the experiment. Potentiodynamic tests were carried out at 298 K (25 C), 318 K (45 C), 338 K (65 C), and 353 K (80 C) with a scan rate of 0.2 mV/s and were conducted using a commercially available Gamry (Warminster, PA) potentiostat connected to a computer. CRs were calculated using applicable ASTM standards.[23,24] Tafel slopes (ba and bc) were held constant at 0.12 V per decade each.[25,26] The density (q) of the steel was taken as 7.87 g/cm3, and the equivalent weight (EW) was determined to be 27.95 g/mol. The polarization resistance for each experimental run was calculated using the lowest current density value of the potentiodynamic scan as the Ecorr. This Ecorr value ±10 mV was fitted to its corresponding current density values and plotted in a Microsoft Excel spreadsheet. A leastsquares linear regression fit was performed to calculate the slope. This slope was used as the polarization resistance (Rp) to determine the corrosion current (Icorr) and CR. The CRs were calculated using the following relationships: B¼

ba bc 2:303ðba þ bc Þ

B Icorr ¼ 106 Rp CR ¼ K1

Icorr EW q

½1

½2

½3

where B is the Stern–Geary coefficient, Rp is in ohm cm2, and K1 = 3.27 9 10–3 mmÆg/lAÆcmÆy. Further, the effectiveness of corrosion inhibition was evaluated using the corrosion current. This is typically expressed as the inhibition efficiency percentage (IE pct), as determined by Eq. [2.4]:[27] IE pct ¼

Iuninhibited Iinhibited 100 Iuninhibited

½4

METALLURGICAL AND MATERIALS TRANSACTIONS A

where Iuninhibited is the corrosion current without inhibitor, and Iinhibited is the corrosion current with inhibitor.

III.

RESULTS AND DISCUSSION

A. Effect of Temperature and Ionic Concentration on Passivity and CRs Temperature is an important variable in controlling the rate of corrosion. As the temperature is increased in the system, less activation energy is required to produce an electrochemical reaction. Results show that, in general, the CRs increased with increasing temperature. In general, critical current density (Icrit) increases as temperature increased for most of the solutions, with the exceptions of the silicates in saltwater, the solution of [0.005M SiO32– + 0.1M HCO3–] in PW, and the [silicates + 0.5M HCO3–], both in PW and SW. Likewise, the passive current density (Ipass) increased for most of the solutions tested. Only the bicarbonates in PW, the silicates in simulated SW, and the 0.1M HCO3– in SW showed a decrease in the passive current density. The 1M HCO3– solutions showed neither an increase nor a decrease in Ipass. However, most of the solutions tested showed a decreasing breakdown potential (Ebp). The exceptions were the bicarbonates, [silicates + 0.5M HCO3–] and [silicates + 0.1M HCO3–], all in simulated SW. Additionally, some scans showed the formation of secondary oxygen peaks with secondary passive regions, such as the bicarbonates in PW, as well as the [silicates + 0.1M HCO3–] in simulated SW. In this section, we present the results from potentiodynamic scans with the addition of silicate ions and bicarbonates individually. 1. Effect of silicate ions on passivity and CRs The effects of the addition of silicate ions at increasing silicate concentrations on the polarization and passivity behavior of 4340 steel in PW and simulated SW are presented. We used ionic concentrations of 0.005M, 0.01M, and 0.025M Na2SiO3 solutions both in PW and

simulated SW. Four different temperatures, 298 K (–25 C), 318 K (45 C), 338 K (65 C), and 353 K (80 C), were used to evaluate the results. The passivity obtained by using silicate ions (0.025M Na2SiO3) in PW between 298 K (25 C) to 353 K (80 C) is shown in Figure 1(a). The maximum potential is ~0.8 VSCC at 298 K (25 C). It can be seen from these scans that the passivity of 4340 steel in sodium silicate is very high, as are the breakdown potentials (Ebp) and pitting potentials (Epit). When compared to Rock Bolt Carbon Steel, the breakdown potential is approximately 1100 mV higher.[17] This passivity is due to a ‘‘self-healing’’ thin film of silica, which has an amorphous, ‘‘honeycomb-like’’ structure.[28,29] Putilova et al.[28] describe this film as a coagulation of metal hydroxides and silicic acids that adsorb onto the surface of the metal. Their data on similarly treated pipelines showed a two-layer surface film formed of metallic corrosion products on the upper layer and a composite-adsorption compound on the lower layer. The variation of current density and potentials using simulated SW with addition of silicate ions are different from the PW and are shown in Figure 1(b). The passivation improved using 0.025M Na2SiO3 solution at 298 K (25 C), but the breakdown potentials were much lower for temperatures in the range of 318 K (45 C) to 353 K (80 C). The polarization behavior of 4340 steel in PW and simulated SW (3.5 pct NaCl solution) with three different silicate concentrations (0.005M, 0.01M, and 0.025M) obtained at 298 K (25 C) is shown in Figure 2. The polarization behavior is about the same for all three ionic concentrations shown in Figure 2(a) with PW. However, there is an overall reduction in both passivity and potential as compared to observations with PW in Figure 2(b) with simulated SW. They also show an increasing passive current density prior to their breakdown potentials. This is due to the increased conductivity and electronegativity of the chloride ions in solution. The breakdown potential for the 0.025M SiO32–, although with a low value of ~0.1 VSSC (as compared to ~0.8 VSSC for PW), was higher than the 0.01M and 0.005M SiO32– additions.

Fig. 1—Potentiodynamic scans showing the effects of temperature: 0.025M Na2SiO3 in (a) PW and (b) 3.5 pct NaCl solution. METALLURGICAL AND MATERIALS TRANSACTIONS A

VOLUME 41A, OCTOBER 2010—2565

Fig. 2—Effect of silicate ions on passivity at 298 K (25 C) in (a) PW and (b) 3.5 pct NaCl solution.

However, the Ecorr and Icorr for both the 0.005M and 0.025M SiO32– additions were lower than 0.01M SiO32– additions; the average CRs are listed in Tables II and III and shown in Figure 3. Polarization curves taken at 353 K (80 C) show similar behavior, but are not shown here. The CRs for the samples in silicate ion solutions in 3.5 pct NaCl are shown in Tables III and IV. The CRs between 298 K (25 C) and 318 K (45 C) are low, below 30 lm/y, using both the PW and simulated SW. As the temperature is increased from 318 K (45 C) to 338 K (65 C), the CRs for SW increase more significantly than those for PW (Figure 3). The only peculiarity occurs when the temperatures are raised from 338 K (65 C) to 353 K (80 C), where all the ionic concentrations for SW show a significant decrease in CRs and the samples using PW show a gradual increase, although below ~40 lm/y. The adsorption of the inhibitor can also be evaluated using the Langmuir isotherm[30] by plotting C/IE as a function of C, where C is the inhibitor concentration and IE is the fractional inhibitor efficiency. However, the slope was not close enough to unity to consider using the Langmuir isotherms. Also, for most of the temperatures tested, the CR decreases as the silicate concentration is increased. Putilova et al.[28] also observed that by adding sodium silicate to naturally aerated water, the CR of steel decreased by nearly a factor of 5 using 0.1 pct Na2SiO3, until the corrosion process nearly stopped at 0.4 pct Na2SiO3. However, at higher temperatures and low silicate concentrations, the electrochemical passivation becomes unstable, generating electrochemical noise or an incomplete curve. This is due to precipitation of the silicate ions out of solution at these high temperatures. 2. Effect of bicarbonate ions on passivity and CRs Looking at the temperature effects using the sample with PW (0.5M HCO3–), we note that the breakdown potential decreases as the temperature is increased from 298 K (25 C) to 353 K (80 C). This is unlike the silicate additive, which did not show any significant decrease (Figure 4(a)) with PW. In addition, there are transpassive regions observed at 298 K (25 C) and 318 K (45 C) (Figures 4(b) and (c)). The polarization 2566—VOLUME 41A, OCTOBER 2010

curves at different temperatures, using PW, exhibited similar behavior, except that the breakdown potential was the same, ~1 VSSC at 298 K (25 C); thus, there is a benefit of increased breakdown potential as compared to no HCO3– addition. In a similar manner, we show that the polarization curves for the 4340 steel sample with bicarbonate additions using simulated SW from 298 K (25 C) to 353 K (80 C) show a complex behavior with transpassive regions (Figure 5(a)). The breakdown potential for this is very low, approximately –0.2 VSSC, as compared to the one with PW at +1 VSSC. The effects of ion concentration of HCO3– at 298 K (25 C) and 318 K (45 C) are shown in Figures 5(b) and (c) for 298 K (25 C) and 318 K (45 C). It can be seen that the breakdown potential is quite low, with a maximum at 1M HCO3– of –0.2 VSSC. However, for bicarbonates, both Icorr and Ipass increase with the increase in molar concentration, noting that these increases are more pronounced at lower temperatures as compared to the higher temperatures. Passivity was not observed for 0.1M HCO3– until 338 K (65 C) and for 0.5M HCO3– until 318 K (45 C). Also, this passive region produced an oxidation peak at a potential of approximately –300 mVSSC at 338 K (65 C). Passivity was apparent in all 1M HCO3– solutions for all temperatures tested. Higher temperatures produced unstable passive films for 0.5M and 1M HCO3– solutions. The corrosion current, Icorr, increased with increasing bicarbonate concentration, regardless of temperature. In both environments, the critical potential Ep increased with increasing HCO3– concentration during the activeto-passive transition. This result was observed by Dong et al.[31] while testing the effect of anodic polarization with increasing amounts of sodium bicarbonate at 298 K (25 C) on low-carbon steel using a saturated calomel electrode (SCE) at a constant pH of 8.33. They attributed this result to accelerated dissolution and passivation of iron in the steel from the increase in bicarbonate concentration, due to the formation of the soluble [32] The experimental results observed Fe(CO3)2– 2 complex. here, as well as in Section III.3, are in close agreement with those presented by Dong et al.[31] A plot showing the CRs made using Tafel curves show a steady increase with METALLURGICAL AND MATERIALS TRANSACTIONS A

Table II.

CR Data for 4340 Steel in Deaerated, Silicate, and Bicarbonate Solutions in PW

Temperature K (C) NaHCO3

NaHCO3 + Na2SiO3

Na2SiO3

Concentration (M = moles/L)

Ecorr (VSSC*)

Icorr (lA/cm2)

Corrosion Rate** (lm/y)

CR** (mpy)

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M

HCO3– HCO3– HCO3– HCO3–

–0.761 –0.72 –0.769 –0.794

7.7 11.4 12.1 14.4

84 124 150 153

3.3 4.9 5.9 6

298 318 338 353 298 318 338 353

(25) (45) (65) (80) (25) (45) (65) (80)

0.5M HCO3– 0.5M HCO3– 0.5M HCO3– 0.5M HCO3– 1M HCO3– 1M HCO3– 1M HCO3– 1M HCO3–

–0.749 –0.744 –0.791 –0.792 –0.753 –0.76 –0.79 –0.805

25.7 42 55.3 57.4 39.5 90 145.6 147.7

273 502 592 608 455 969 1525 1965

10.8 19.8 23.3 23.9 17.9 38.1 60 77.4

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M

HCO3– + 0.005M HCO3– + 0.005M HCO3– + 0.005M HCO3– + 0.005M

–0.739 –0.735 –0.744 –0.782

4.6 6.2 4.7 6.9

53 75 53 90

2.1 2.9 2.1 3.5

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M


–0.758 –0.752 –0.779 –0.789

3.8 5.5 5.2 6.9

43 64 66 81

1.7 2.5 2.6 3.2

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M


SiO32– SiO32– SiO32– SiO32–

–0.787 –0.784 –0.819 –0.795

2.6 4.8 9 8

30 56 105 97

1.2 2.2 4.1 3.8

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M



–0.739 –0.745 –0.763 –0.76

12.7 19.9 35.2 21.1

128 234 384 300

5.1 9.2 15.1 11.8

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M


–0.734 –0.734 –0.769 –0.774

11.1 23.5 22.7 22.1

144 250 266 276

5.7 9.8 10.5 10.9

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M


–0.74 –0.741 –0.755 –0.763

14.9 21.5 17.1 29.4

177 253 215 337

7 9.9 8.5 13.2

298 318 338 353

(25) (45) (65) (80)

0.005M 0.005M 0.005M 0.005M

–0.817 –0.735 –0.8 –0.795

1.5 1.4 1.8 3.1

19 18 23 35

0.7 0.7 0.9 1.4

298 318 338 353

(25) (45) (65) (80)

0.01M 0.01M 0.01M 0.01M

–0.812 –0.72 –0.853 –0.851

0.5 1.1 1.6 4.1

5 12 20 52

0.2 0.5 0.8 2.1

298 318 338 353

(25) (45) (65) (80)

0.025M 0.025M 0.025M 0.025M

–0.797 –0.781 –0.803 –0.839

0.5 0.6 1.7 1.9

7 7 20 22

0.3 0.3 0.8 0.9

Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3

Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3



SiO32– SiO32– SiO32– SiO32– SiO32– SiO32– SiO32– SiO32–

*SCC = silver-silver chloride (Ag-AgCl) reference electrode. **Average value (2 £ n £ 3).

the temperatures of both PW and SW, indicating that the CRs are not affected much by bicarbonate additions, but breakdown potentials are better than without any addition of bicarbonate. It can be noted that the CR is ~500 lm/y (maximum) at 298 K (25 C), which is about METALLURGICAL AND MATERIALS TRANSACTIONS A

25 times greater than that observed with Na2SiO3 [~20 lm/y (maximum)] at 298 K (25 C). Similarly at 318 K (45 C), 338 K (65 C), and 353 K (80 C) (Figure 6), the CR is between 20 to 30 times greater than the counterparts of silicate additions (Figure 3). VOLUME 41A, OCTOBER 2010—2567

Table III.

CR Data for 4340 Steel in Deaerated, Silicate, and Bicarbonate Solutions in Simulated SW (3.5 Pct NaCl Solution) Temperature K (C)

NaHCO3

NaHCO3 + Na2SiO3

Na2SiO3

Concentration (M = moles/L)

Ecorr (VSSC*)

Icorr (lA/cm2)

Corrosion Rate** (lm/y)

Corrosion Rate** (mpy)

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M


–0.755 –0.745 –0.757 –0.768

11 15.2 17.4 10.7

124 179 179 139

4.9 7.1 7.1 5.5

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M


–0.732 –0.763 –0.785 –0.771

26.7 38.1 58.1 77.3

289 398 687 878

11.4 15.7 27.1 34.6

298 318 338 353

(25) (45) (65) (80)

1M 1M 1M 1M

–0.746 –0.753 –0.791 –0.791

29.1 73.4 129.6 182.1

335 961 1580 2062

13.2 37.8 62.2 81.2

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M


–0.725 –0.725 –0.766 –0.781

5 6.7 8 7.2

54 78 95 85

2.1 3.1 3.7 3.3

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M


–0.743 –0.765 –0.776 –0.795

4.8 6 8.7 7.9

52 85 107 92

2.1 3.3 4.2 3.6

298 318 338 353

(25) (45) (65) (80)

0.1M 0.1M 0.1M 0.1M



–0.787 –0.79 –0.792 –0.798

1.8 4.3 9.6 9.2

19 49 111 102

0.7 1.9 4.4 4

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M



–0.72 –0.734 –0.763 –0.763

22.2 32.9 35.2 35.1

248 389 412 427

9.7 15.3 16.2 16.8

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M


–0.727 –0.725 –0.748 –0.764

19 18.6 32.8 28.3

213 261 372 300

8.4 10.3 14.7 11.8

298 318 338 353

(25) (45) (65) (80)

0.5M 0.5M 0.5M 0.5M


–0.738 –0.739 –0.753 –0.765

14.6 26.9 32.1 23.2

171 293 363 277

6.7 11.5 14.3 10.9

298 318 338 353

(25) (45) (65) (80)

0.005M 0.005M 0.005M 0.005M

–0.789 –0.835 –0.823 –0.827

0.5 2.4 3.6 5.8

7 28 42 61

0.3 1.1 1.6 2.4

298 318 338 353

(25) (45) (65) (80)

0.01M 0.01M 0.01M 0.01M

–0.839 –0.837 –0.826 –0.814

0.4 1.7 5.2 3.9

5 20 68 42

0.2 0.8 2.7 1.7

298 318 338 353

(25) (45) (65) (80)

0.025M 0.025M 0.025M 0.025M

–0.793 –0.831 –0.831 –0.838

0.6 1.5 5.1 3

7 17 53 34

0.3 0.7 2.1 1.3


Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3

Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3 Na2SiO3



SiO32– SiO32– SiO32– SiO32– SiO32– SiO32– SiO32– SiO32–

*SCC = silver-silver chloride (Ag-AgCl) reference electrode. **Average value (2 £ n £ 3).

3. Effect of silicate and bicarbonate ions on passivation In light of the experiments performed with silicate and bicarbonate additions to pure and simulated SW, we found different beneficial effects that indicated mixtures 2568—VOLUME 41A, OCTOBER 2010

of these additives would enhance the electrochemical behavior of the 4340 steel. Polarization curves were obtained in mixtures from a series [0.005M SiO3– + 0.5M HCO3–], [0.01M SiO3– + 0.5M HCO3–], METALLURGICAL AND MATERIALS TRANSACTIONS A

[0.025M SiO3– + 0.5M HCO3–], as well as [0.005M SiO3– + 0.1M HCO3–] and [0.01M SiO3– + 0.1M HCO3–] in PW and simulated SW. The temperature effects of increasing concentration and temperature on passivity are shown in Figure 7 for 4340 steel using [0.025M SiO32– + 0.1M HCO3–] electrolyte in PW between 298 K (25 C) and 353 K (80 C). This figure shows an increase in Icrit, Ebp, and Ipass as the temperature increases. The passivation region is up to ~1 VSSC with these mixed solutions, as compared to ~0.8 VSSC (Na2SiO3) in Figure 2(a) and ~0.8 VSSC (NaHCO3) in Figure 4(b). In the studies of stainless steels, Jones[33] suggested that this phenomenon occurs under severe conditions of higher acidity and temperature, which decreases the passive potential range and increases current densities and CRs at all potentials. We observed similar effects for 4340 steel using the [0.025M Na2SiO3 + 0.1M NaHCO3] mixed in PW. This effect could also be due to a combination of FeCO3 and silicic acid (H2SiO3) during the active-to-passive transitions. Silicic acid is itself a weak acid,[34] and the formation of FeCO3 is thermodynamically favorable.[35] The interaction of Na2SiO3 with the Fe substrate increases the activation energy required for transition

from the active-to-passive regime, but FeCO3 forms in the presence of HCO3– at potentials more anodic than the active-to-passive transition,[17] thus creating a primary passivation potential (Epp) that increases with temperature. We noted that Fujita et al.[29] also observed similar characteristics. Thus, less energy is required for the formation of the passive layer and of FeCO3 from the previous chemical reaction. This, in turn, requires less energy for the thin film(s) to break down at high potentials, which show a decreasing Ebp as temperature is increased. The combined effects of these silicate and bicarbonate additives showed high passivity for the HSLA steel samples. Potentiodynamic polarization scans for 4340 steel in PW showing the effects of different concentrations of silicate and bicarbonate ions at 298 K (25 C) are shown in Figure 8. These curves show that the amount of passivity obtained for 4340 steel is significant when various concentrations of silicates or bicarbonates are added in solution. However, when no ions are present in solution, no passivity is obtained. When in the presence of bicarbonates, passivity is due to the formation of FeCO3 by a dissolution-precipitation mechanism originally proposed by Ogundele and White,[35] using the following reactions: Fe ! Fe2þ þ 2e

½5

Fe þ HCO3 ! FeCO3 þ Hþ þ 2e

½6

The formation of FeCO3 on low-carbon steel was confirmed by Dong et al.[31] using X-ray diffraction (XRD) in the presence of 0.5 M and 1M HCO3– concentrations at 298 K (25 C). For 1M HCO3–, FeCO3 was observed over a potential range of –750 to –550 mV (SCE).[31] A passive film may also be created using a combination of Eq. [6] and the reaction[36] Fe þ 2H2 O ! FeðOHÞ2 þ 2Hþ þ 2e

Fig. 3—Comparisons of average CRs of 4340 steel, deaerated with Na2SiO3 in PW and simulated SW.

½7

According to Cheng et al., Eq. [6] is dependent on HCO3– concentration and pH and is favored by high HCO3– concentrations.[36] Therefore, for high concentrations of bicarbonate above approximately –0.2V,

Table IV. Inhibitor Efficiency of Sodium Silicate in 3.5 Pct NaCl Silicate Concentration (mol/L) 0.005

0.01

0.025

Temperature K (C) 298 318 338 353 298 318 338 353 298 318 338 353

(25) (45) (65) (80) (25) (45) (65) (80) (25) (45) (65) (80)


Icorr (Uninhibited)

Icorr (Inhibited)

Efficiency (Pct)

2 4.6 8 12 2 4.6 8 12 2 4.6 8 12

0.5 2.4 3.6 5.8 0.4 1.7 5.2 3.9 0.6 1.5 5.1 3

75 48 55 52 80 63 35 68 70 67 36 75


Fig. 4—Potentiodynamic scans showing the effects of temperature: (a) 0.5M NaHCO3 in PW and effect of bicarbonate ions on passivity in PW: (b) at 298 K (25 C) and (c) at 318 K (45 C).

Fig. 5—(a) Potentiodynamic scans showing the effects of temperature: 0.5M NaHCO3 in simulated SW, (b) effect of bicarbonate ions on 4340 steel in 3.5 pct NaCl solution at 318 K (45 C) in PW, and (c) passivity in 3.5 pct NaCl solution at 318 K (45 C).

Fig. 6—Comparisons of average CRs of 4340 steel, deaerated with NaHCO3 in PW and simulated SW.

Fe2+ oxidizes to ferric oxide according to the following reaction: 4Fe2þ þ 3O2 ! 2Fe2 O3

½8

These electrochemical reactions result in the passivation of the steel.[36] The products from Eqs. [6] and [7]—FeCO3 and Fe(OH)2—could combine to form complex iron-hydroxycarbonates on the surface of the steel. In fact, compounds such as Fe6(OH)12CO3 and Fe2(OH)2CO3 2570—VOLUME 41A, OCTOBER 2010

Fig. 7—Representative potentiodynamic scans of [0.025M SiO32– + 0.1M HCO3–] in PW showing the effect of increasing acid concentration and temperature on passivity.

were detected by Dong et al. as corrosion products in 0.1M HCO3– solution at 298 K (25 C) under potentiostatic polarization of low-carbon steel using XRD and Fourier transform–infrared (FT-IR) spectra analyses over a potential range of –750 to –650 mV (SCE).[31] However, these compounds would not be responsible for the passivation of 4340 steel at the same bicarbonate concentration. Instead, according to Figure 6, curve B, these compounds would be observed over the activeto-passive transition, even after taking into account the METALLURGICAL AND MATERIALS TRANSACTIONS A

1.2 H

0.8

Potential (VSSC )

E

D

0.4

G B

F

0

C A

-0.4 -0.8 -1.2 1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

Current Density (A/cm2)

Fig. 8—Potentiodynamic polarization curves of 4340 steel at 298 K (25 C) in PW: (a) no additional ions, (b) 0.1M HCO3–, (c) 0.5M HCO3–, (d) 0.01M SiO32–, (e) 0.01M SiO32– + 0.1M HCO3–, (f) 0.01M SiO32– + 0.5M HCO3–, (g) 0.025M SIO32–, and (h) 0.025M SiO32– + 0.5M HCO3–.

0.2

G

Potential (VSSC)

0

C F

H

-0.2

B

-0.4

E -0.6

D

A

-0.8 -1 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 Current Density (A/cm2)

Fig. 9—Potentiodynamic polarization curves of 4340 steel at 298 K (25 C) in 3.5 pct NaCl solution with: (a) no additional ions, (b) 0.1M HCO3–, (c) 0.5M HCO3–, (d) 0.01M SiO32–, (e) 0.01M SiO32– + 0.1M HCO3–, (f) 0.01M SiO32– + 0.5M HCO3–, (g) 0.025M SiO32–, and (h) 0.025M SiO32– + 0.5M HCO3–.

difference in voltage potential (+0.19 mV) between SSC and SCE.[33] It is clear from Figure 8 that Icorr and Ipass are higher in both bicarbonate and bicarbonate-silicate concentrations than silicate concentrations alone in PW. As temperature is increased for pure silicate solutions, more current is required to sustain a corrosion-resistant film on the steel’s surface. At 298 K (25 C), the passive behavior for 0.1M HCO3– shows a large oxidation peak before minimal current density resumes, compared to 0.5M HCO3–, which shows a smaller oxidation peak prior to resuming passivity. This type of behavior is not seen at higher temperatures. This indicates that if bicarbonates are used solely to create a passive film on the surface of the steel at room temperature, passivation will require much higher current densities or a very limited potential range to apply a corrosion-resistant film. These problems can be alleviated if higher temperatures are used, as there is only one oxidation peak beyond the active-to-passive transition. Also, the passive-potential range is increased METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 10—CRs of 4340 steel in 3.5 pct NaCl solution with (a) 0.5M HCO3–, (b) 0.01M SiO32– + 0.5M HCO3–, (c) 0.1M HCO3–, (d) 0.01M SiO32– + 0.1M HCO3–, (e) no additional ions, (f) 0.01M SiO32–, (g) 0.025M SiO32–, and (h) 0.025M SiO32– + 0.5M HCO3–.

to more than 1000 mV, an observation made by Cheng and Luo[37] with ASTM A516-70 carbon steel. However, if sodium silicate is added, the passive current density (Ipass) is further minimized and the applied potential increased, due to suppression of any additional oxidation peaks by the silicate ions. At 298 K (25 C), a solution of [0.01M Na2SiO3 + 0.1M NaHCO3] produced a passive current density of ~1.4 to1.5 lA/cm2 over a potential range of ~80 to 870 mVSSC. The effects of silicate and bicarbonate ions on the polarization of 4340 steel in simulated SW (3.5 pct NaCl) solution are shown in Figure 9. The addition of sodium chloride into the solution decreases the passivity and the overall potential range of the electrochemical potentiodynamic scans. Also, the pitting potential (Epit) decreased as well. However, unlike the potentiodynamic scans without 3.5 pct NaCl, the Ecorr for 0.01M SiO32– in 3.5 pct NaCl was lower than the rest of the curves, including the 3.5 pct NaCl curve at 298 K (25 C). This was true for higher temperatures as well. On the other hand, the potentiodynamic scans using simulated SW only (i.e., no additional ions) showed that the Ecorr migrated to a more positive value as temperature was increased, while the Ecorr’s of the bicarbonate-based solutions stayed near the same potential. The CRs of 4340 steel in 3.5 pct NaCl solution with (A) 0.5M HCO3–, (B) 0.01M SiO32– + 0.5M HCO3–, (C) 0.1M HCO3–, (D) 0.01M SiO32– + 0.1M HCO3–, (E) No additional ions, and (F) 0.01M SiO32– are shown in Figure 10. It can be seen that high concentrations of bicarbonates, even when mixed with silicates, give relatively high CRs as opposed to solutions that only contain silicates. A solution of simulated SW gives a surprisingly higher CR at 353 K (80 C) than does 0.01M SiO32– or [0.01M SiO32– + 0.1M HCO3–], indicating that the addition of silicates or low concentrations of bicarbonates are competing with chloride ions for electrons during the cathodic-to-anodic transition, limiting the amount of current that can be impressed on the surface of the sample and reducing the rate of oxidation with the base metal. VOLUME 41A, OCTOBER 2010—2571

Fig. 11—Comparisons of average CRs of 4340 steel, deaerated with Na2SiO3 + 0.5M NaHCO3 in PW and simulated SW.

Fig. 12—Comparisons of average CRs of 4340 steel, deaerated with Na2SiO3 + 0.1M NaHCO3 in PW and simulated SW.

Potentiodynamic polarization was used to determine the CRs and electrochemical passivity behavior of AISI 4340 steel under deaerated (reducing) conditions. Deaeration was used as a baseline to determine the most stable passive regions for 4340 steel in silicate or bicarbonate solutions. Potentiodynamic tests were run at four different temperatures, 298 K (25 C), 318 K (45 C), 338 K (65 C), and 353 K (80 C), in 24 different solution concentrations containing sodium silicate,

sodium bicarbonate, or sodium chloride. Tables II and III show CR data for 4340 steel in silicate and bicarbonate solutions, in pure (deionized/distilled) water and simulated SW (3.5 pct NaCl) solutions. Representative summaries of these CRs, their inhibitor efficiencies, and their corresponding activation energies are shown in Tables IV and V and Figures 10 through 13. It is important to note that these graphs show the CR vs chemical composition. Therefore, at a constant temperature, the CR could be increasing or decreasing as the amount of chemical constituent in solution increases or decreases. In addition, experimental CRs for 4340 steel in deaerated 3.5 pct sodium chloride solution using potentiodynamic polarization are presented in Table VI, which was not reported in the literature. Table VII shows comparative electrochemical corrosion data collected by Deodeshmukh et al.[18] in 3.5 pct NaCl, with and without silicate and bicarbonate ions for additional comparison. Their results were similar to those observed here, with the exception of the corrosion current in 0.01M SiO32– solution, where the observed Icorr in this work was lower by a factor of 10. This may be due to the difference in materials tested. The activation energy for 4340 steel in deaerated 3.5 pct sodium chloride solution was determined to be 28 kJ/mol. The CR for 4340 steel in pure, deaerated water was determined to be ~51 lm/y at 298 K (25 C). Values for temperatures above 298 K (25 C) were not determined. This is because obtaining CRs in lowconductivity water is a challenge,[38] and monitoring CRs in low-conductivity water has only been done using sophisticated electrochemical techniques, LPR (linear polarization resistance), EIS, and crevice corrosion current, as well as direct measurement techniques, such as weight loss and highly sensitive electrical resistance galvanic probes. Currently, these are the only known techniques for measuring CRs and associated behavior of carbon steels in low-conductivity water. In summary, the CR of 4340 steel is significantly reduced when sodium silicate is added to solution, as compared to sodium bicarbonate. Silicates produce an amorphous, almost gelatinous material that coats the surface of the metal, limiting oxidation reactions. In

Table V. Test Solutions Used and the Corresponding Index for Figure 13, Showing Selective Activation Energy Values Activation Energy (kJ/mol) Set Set 1 (S1) Set 2 (S2) Set 3 (S3) Set 4 (S4)

Solution Concentration M (moles/L) 2–

0.005M SiO3 0.01M SiO32– 0.025M SiO32– 0.1M HCO3– 0.5M HCO3– 1M HCO3– 0.1M HCO3– + 0.005M SiO32– 0.1M HCO3– + 0.01M SiO32– 0.1M HCO3– + 0.025M SiO32– 0.5M HCO3– + 0.005M SiO32– 0.5M HCO3– + 0.01M SiO32– 0.5M HCO3– + 0.025M SiO32–

2572—VOLUME 41A, OCTOBER 2010

Composition

PW

Simulated SW

1 2 3 4 5 6 7 8 9 10 11 12

— 27.5 — — — 19.0 — — 25.9 23.0 — —

— — — — 21.4 20.5 11.9 14.9 39.9 — — 15.9


silicate concentrations are increased, the CR decreases. However, when bicarbonates are kept at a relatively high concentration and the silicate concentration is increased, the CR increases. Addition of chlorides (in the form of simulated SW) to mixtures of silicates and bicarbonates in solution increased the CR when compared to no addition of chlorides.

IV.

Fig. 13—Overall CRs of 4340 steel in silicate-bicarbonate solutions. Top (a): PW; bottom (b): simulated SW. The CRs for 0.5M and 1M sodium bicarbonate at higher temperatures are not shown.

Table VI. CR Data for AISI 4340 Steel in Deaerated, 3.5 Pct NaCl Solution Temperature K (C)

Ecorr (VSSC)

Icorr (lA/cm2)

CR (lm/y)

CR (mpy)

298 318 338 353

–0.743 –0.717 –0.704 –0.694

2.0 4.6 8.0 12.0

24 51 92 139

1.0 2.0 3.6 5.5

(25) (45) (65) (80)

contrast, bicarbonates adhere to the surface of the metal as a porous film, which increases the ionic transfer of iron into solution. This produces significantly higher CRs, both with and without chlorides. Solutions containing mixtures of silicates, bicarbonates, or chlorides showed CRs with mixed results. When bicarbonates are kept at a relatively low concentration in solution, and Table VII.

CONCLUSIONS

1. Increasing the ionic concentration of HCO3– in PW from 0.1M to 1M leads to an increase in the CR of 84 to 455 lm/y (4.4 times) at room temperature. Simulated SW showed a similar increase of 124 to 335 lm/y (1.7 times) at room temperature. Higher temperatures revealed similar behavior, an increase in CR with temperature up to a maximum of 2062 lm/y in 1M concentration at 353 K (80 C). 2. Increasing the concentration of SiO32– decreased the CR from 19 lm/y at 0.005M to 7 lm/y at 0.025M (0.6 times) at room temperature in distilled water, effectively reducing the CR by roughly half. This trend was also seen at higher temperatures as well as in simulated SW. This decrease is due to a ‘‘selfhealing’’ thin film of silica, which passivates the steel. However, at higher temperatures and low silicate concentrations, the electrochemical passivation becomes unstable due to precipitation of the silicate ions out of solution. Addition of chlorides in solution decreases the passive-potential range of the steel. 3. The addition of SiO32– to 0.1M HCO3– reduced the CR with increasing ionic concentration of SiO32–, going from 53 to 30 lm/y when the SiO32– concentration was increased from 0.005M to 0.025M. However, at the increased concentration of 0.5M HCO3–, the addition of SiO32– actually increased the CR, changing from 128 to 177 lm/y at room temperature. 4. The addition of small concentrations of silicates and varying concentrations of bicarbonates in solution changes the electrochemical behavior during potentiodynamic polarization. When silicates are present in low concentrations of bicarbonates, less current is required to obtain passivity and CRs are lower. As the concentration of bicarbonate is increased, more current is required to obtain passivity, which increases Icorr and the CR. Additions of chloride ions reduce the overall potential range of the polarization curve and increase the likelihood of pitting by making the pitting potential (Epit) more electronegative.

Electrochemical Corrosion Data from Deodeshmukh et al.[18] (All Potentials vs Ag/AgCl)

Electrolyte: 3.5 pct NaCl with

Ecorr (mV)

Icorr (lA/cm2)

bc (mV/Decade)

Epit (mV)

Ip (lA/cm2)

No additional ions 0.01M SiO32– 0.5M HCO3– 0.5M HCO3– + 0.01M SiO32–

–854.2 –771.3 –762.5 –747

2.57 4.46 15.19 11.71

128.9 133.3 77.45 84.05

403.7 364.3 253.8 236.3

1.4 2.6 25.18 8.94



ACKNOWLEDGMENTS This work was performed at the University of Nevada, Reno, and funded by the United States Department of Energy, Office of Civilian Radioactive Waste Management, under Contract No. DE-FC2804RW12232.

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