Handbook of Corrosion Engineering Episode 2 Part 10 pps

Pearson survey. The Pearson survey, named after its inventor, is used to locate coating defects in buried pipelines. Once these defects have been identified, the protection levels afforded by the CP system can be investigated at these critical locations in more detail. Methodology. An ac signal of around 1000 Hz is imposed onto the pipeline by means of a transmitter, which is connected to the pipeline and an earth spike, as shown in Fig. 11.25. Two survey operators make earth contact either through metal studded boots or aluminum poles. A distance of several meters typically separates the operators. Essentially, the signal measured by the receiver is the potential gradi- ent over the distance between the two operators. Defects are located by a change in the potential gradient, which translates into a change in signal intensity. As in the CIPS technique, the measurements are usually recorded by walking directly over the pipeline. As the front operator approaches a defect, increasing signal intensity is recorded. As the front person moves away from the defect, the signal intensity drops and later picks up again as the rear operator approaches the defect. The interpreta- tion of signals can obviously become confusing when several defects are located between the two operators. In this case, only one person walks directly over the pipeline, with the connecting leads at a right angle to the pipeline. In principle, a Pearson survey can be performed with an impressed cathodic protection system remaining energized. Sacrificial anodes Cathodic Protection 913 Buried pipeline Earth spike Test station Receiver Aluminum pole X ( ( ( Signal emitted at defect Coating defect Transmitter Figure 11.25 Pearson survey methodology (schematic). 0765162_Ch11_Roberge 9/1/99 6:37 Page 913 should be disconnected because the signal from these may otherwise mask actual coating defects. A three-person team is usually required to locate the pipeline, perform the survey measurements, place defect markers into the ground, and move the transmitters periodically. The operator carrying the receiver should be highly experienced, especially if the survey is based on audible signals and instrument sensitivity settings. Under these conditions, the results are completely dependent on this operator’s judgment. Advantages and limitations. By walking the entire length of the pipeline, an overall inspection of the right-of-way can be made together with the measurements. In principle, all significant defects and metal- lic conductors causing a potential gradient will be detected. There are no trailing wires and the impressed CP current does not have to be pulsed. The disadvantages are similar to those of CIPS because the entire pipeline has to be walked and contact established with ground. The technique is therefore unsuitable to roads, paved areas, rivers, and so forth. Fundamentally, no severity of corrosion damage is indicated and no direct measure of the performance of the CP system is obtained. The survey results can be very operator dependent, if no automated signal recording is performed. Direct current voltage gradient (DCVG) surveys. DCVG surveys are a more recent methodology to locate defects on coated buried pipelines and to make an assessment of their severity. The technique again relies on the fundamental effect of a potential gradient being estab- lished in the soil at coating defects under the application of CP cur- rent; in general, the greater the size of the defect, the greater the potential gradient. The DCVG data is intricately tied to the overall performance of a CP system, because it gives an indication of current flow and its direction in the soil. Methodology. The potential gradient is measured by an operator between two reference electrodes (usually of the saturated Cu/CuSO 4 type), separated by a distance of say half a meter. The appearance of these electrodes resembles a pair of cross-country ski poles (Fig. 11.26). A pulsed dc signal is imposed on the pipeline for DCVG mea- surements. The pulsed input signal minimizes interference from other current sources (other CP systems, electrified rail transit lines, telluric effects). This signal can be obtained with an interrupter on an existing rectifier or through a secondary current pulse superimposed on the existing “steady” CP current. The operator walking the pipeline observes the needle of a milli- voltmeter needle to identify defect locations. (More recently devel- 914 Chapter Eleven 0765162_Ch11_Roberge 9/1/99 6:37 Page 914 oped DCVG systems are digital and do not have a needle as such.) It is preferable for the operator to walk directly over the pipeline, but it is not strictly necessary. The presence of a defect is indicated by a increased needle deflection as the defect is approached, no nee- dle deflection when the operator is immediately above the defect, and a decreasing needle deflection as the operator walks away from the defect (Fig. 11.27). It is claimed that defects can be located with an accuracy of 0.1 to 0.2 m, which represents a major advantage in minimizing the work of subsequent digs when corrective action has to be taken. Cathodic Protection 915 Figure 11.26 DCVG measuring equipment. (Courtesy of CSIR North America Inc.) 0765162_Ch11_Roberge 9/1/99 6:37 Page 915 An additional feature of the DCVG technique is that defects can be assigned an approximate size factor. Sizing is most important for iden- tifying the most critical defects and prioritizing repairs. Leeds and Grapiglia 15 have provided details on the sizing procedure. An empiri- cally based rating based on the so-called %IR value has been adopted in general terms as follows: ■ 0 to 15%IR (“small”): No repair required usually. ■ 16 to 35%IR (“medium”): Repairs may be recommended. ■ 36 to 60%IR (“large”): Early repair is recommended. ■ 61 to 100%IR (“extra large”): Immediate repair is recommended. To establish a theoretical basis for the %IR value, the pipeline poten- tial measured relative to remote earth at a test post must be consid- ered. This potential (V t ) is made up of two components: 916 Chapter Eleven Increasing signal strength (when approaching defect) Needle deflection points toward defect Needle deflection points toward defect No needle deflection Buried pipeline Decreasing signal strength (when leaving defect) No signal (when directly above defect) X X Location of coating defect Equipotential lines Figure 11.27 DCVG methodology (schematic). 0765162_Ch11_Roberge 9/1/99 6:37 Page 916 V t ϭ V i ϩ V s where V i is the voltage across the pipe to soil interface and V s is the voltage between the soil surrounding the pipe and remote earth. The %IR value is defined as %IR ϭ Essentially the pipe-to-soil interface and the soil between the pipe and remote earth can be viewed as two resistors in series, with a potential difference across each of them. Although V i cannot be mea- sured easily in practice, V s can be measured relatively easily with the DCVG instrumentation (one reference electrode is initially placed at the defect epicenter, and the voltage change is then summed as the electrodes are moved away from the epicenter to remote earth). In practice, the V s value measured at a test post has to be extrapolated to a value at the defect location. Two test post readings bracketing the defect location and simple linear extrapolation are usually employed. For effective protection of the defect by the CP system, the V s /V t ratio should be small. The overall shift in pipeline potential due to the appli- cation of CP should be manifested by mainly shifting V i , not V s . Higher %IR values imply a lower level of cathodic protection. Because the DCVG technique can be used to determine the direction of current flow in the soil, a further defect severity ranking has been proposed. As indicated in Fig. 11.1, current will tend to flow to a defect under the protective influence of the CP system. Corrosion damage (anodic dissolution) at the defect has an opposite influence; it will tend to make current flow away from the defect. Using an adaptation of the DCVG technique, it has been claimed that it is possible to establish whether current flows to or from a defect, with the CP system switched ON and OFF in a pulsed cycle. Advantages and limitations. Fundamentally, the DCVG technique is particularly suited to complex CP systems in areas with a relatively high density of buried structures. These are generally the most diffi- cult survey conditions. The DCVG equipment is relatively simple and involves no trailing wires. Although a severity level can be identified for coating defects, the rating system is empirical and does not provide quantitative kinetic corrosion information. The survey team’s rate of progress is dependent on the number of coating defects present. Terrain restrictions are similar to the CIPS technique. However, it may be possible to place the electrode tips in asphalt or concrete sur- face cracks or in between the gaps of paving stones. V s ᎏ V t Cathodic Protection 917 0765162_Ch11_Roberge 9/1/99 6:37 Page 917 Corrosion coupons. Corrosion coupons connected to cathodically pro- tected structures are finding increasing application for performance monitoring of the CP system. Essentially these coupons, installed uncoated, represent a defect simulation on the pipeline under con- trolled conditions. These coupons can be connected to the pipeline via a test post outlet, facilitating a number of measurements such as potential and current flow. A publication describing an extensive coupon development and mon- itoring program on the Trans Alaska Pipeline System 16 serves as an excellent case study. This coupon monitoring program was designed to assess the adequacy of the CP system under conditions where tech- niques involving CP current interruption on the pipeline were imprac- tical. Although the coupon monitoring methodology is based on relatively simple principles, significant development efforts and atten- tion to detail are typically required in practice, as this case study amply illustrates. Methodology. Perhaps the most important consideration in the installation of corrosion coupons is that a coupon must be representa- tive of the actual pipeline surface and defect. The exact metallurgical detail and surface finish as found on the actual pipeline are therefore required on the coupon. The influence of corrosion product buildup may also be important. Furthermore the environmental conditions of the coupon and the pipe should also be matched (temperature, soil con- ditions, soil compaction, oxygen concentration, etc.). Current shielding effects on the bonded coupon should be avoided. Several measurements can be made after a coupon-type corrosion sensor has been attached to a cathodically protected pipeline. 17 ON potentials measured on the coupon are in principle more accurate than those measured on a buried pipe, if a suitable reference electrode is installed in close proximity to the coupon. The potentials recorded with a coupon sensor may still contain a significant IR drop error, but this error is lower than that of surface ON potential measurements. Instant- OFF potentials can be measured conveniently by interrupting the coupon bond wire at a test post. Similarly, longer-term depolarization measurements can be performed on the coupon without depolarizing the entire buried structure. Measurement of current flow to or from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sen- sors provide an option for in situ corrosion rate measurements as an alternative to weight loss coupons. The surface area of the coupon used for monitoring is an important variable. Both the current density and the potential of the coupon are 918 Chapter Eleven 0765162_Ch11_Roberge 9/1/99 6:37 Page 918 dependent on the area. In turn, these two parameters have a direct relation to the kinetics of corrosion reactions. Advantages and limitations. A number of important corrosion parame- ters can be conveniently monitored under controlled conditions, with- out any adjustments to the energized CP system of the structure. The measurement principles are relatively simple. It is difficult (virtually impossible) to guarantee that the coupon will be completely represen- tative of an actual defect on a buried structure. The measurements are limited to specific locations. The coupon sensors have to be extremely robust and relatively simple devices to perform satisfactorily under field conditions. References 1. Ashworth, V., The Theory of Cathodic Protection and Its Relation to the Electrochemical Theory of Corrosion, in Ashworth, V., and Booker, C. J. L. (eds.), Cathodic Protection, Chichester, U.K., Ellis Horwood, 1986. 2. Peabody, A. W., Control of Pipeline Corrosion, Houston, Tex., NACE International, 1967. 3. Eliassen, S., and Holstad-Pettersen, N., Fabrication and Installation of Anodes for Deep Water Pipelines Cathodic Protection, Materials Performance, 36(6):20–23 (1997). 4. Sydberger, T., Edwards, J. D., and Tiller, I. B., Conservatism in Cathodic Protection Designs, Materials Performance, 36(2):27–32 (1997). 5. Shreir, L. L., and Hayfield, P. C. S., Impressed Current Anodes, in Ashworth, V., and Booker, C. J. L. (eds.) Cathodic Protection, Chichester, U.K., Ellis Horwood, 1986. 6. Shreir, L. L., Jarman, R. A., and Burstein, G. T. (eds.), Corrosion, vol. 2, 3d ed., Oxford, Butterworth Heinemann, 1994. 7. Beavers, J. A., and Thompson, N. G., Corrosion Beneath Disbonded Pipeline Coatings, Materials Performance, 36(4):13–19, (1997). 8. Jack, T. R., Wilmott, M. J., and Sutherby, R. L., Indicator Minerals Formed During External Corrosion of Line Pipe, Materials Performance, 34(11):19–22 (1995). 9. Kirkpatrick, E. L., Basic Concepts of Induced AC Voltages on Pipelines, Materials Performance, 34(7):14–18 (1995). 10. Allen, M. D., and Ames, D. W., Interaction and Stray Current Effects on Buried Pipelines: Six Case Histories, in Ashworth, V., and Booker, C. J. L. (eds.), Cathodic Protection Chicester, U.K., Ellis Horwood, 1986, pp. 327–343. 11. NACE International and Institute of Corrosion, Cathode Protection Monitoring for Buried Pipelines, pub. no. CEA 54276, Houston, Tex, NACE International, 1988. 12. Goloby, M. V., Cathodic Protection on the Information Superhighway, Materials Performance, 34(7):19–21 (1995). 13. Pawson, R. L., Close Interval Potential Surveys—Planning, Execution, Results, Materials Performance, 37(2):16–21 (1998). 14. NACE International, Specialized Surveys for Buried Pipelines, pub. no. 54277, Houston, Tex, NACE International, 1990. 15. Leeds, J. M., and Grapiglia, J., The DC Voltage-Gradient Method for Accurate Delineation of Coating Defects on Buried Pipelines, Corrosion Prevention and Control,42(4):77–86 (1995). 16. Stears, C. D., Moghissi, O. C., and Bone, III, L., Use of Coupons to Monitor Cathodic Protection of an Underground Pipeline, Materials Performance, 37(2):23–31 (1998). 17. Turnipseed, S. P., and Nekoksa, G., Potential Measurement on Cathodically Protected Structures Using an Integrated Salt Bridge and Steel Ring Coupon, Materials Performance, 35(6):21–25 (1996). Cathodic Protection 919 0765162_Ch11_Roberge 9/1/99 6:37 Page 919 921 Anodic Protection 12.1 Introduction 921 12.2 Passivity of Metals 923 12.3 Equipment Required for Anodic Protection 927 12.3.1 Cathode 929 12.3.2 Reference electrode 929 12.3.3 Potential control and power supply 930 12.4 Design Concerns 930 12.5 Applications 932 12.6 Practical Example: Anodic Protection in the Pulp and Paper Industry 933 References 938 12.1 Introduction In contrast to cathodic protection, anodic protection is relatively new. Edeleanu first demonstrated the feasibility of anodic protection in 1954 and tested it on small-scale stainless steel boilers used for sulfuric acid solutions. This was probably the first industrial application, although other experimental work had been carried out elsewhere. 1 This tech- nique was developed using electrode kinetics principles and is some- what difficult to describe without introducing advanced concepts of electrochemical theory. Simply, anodic protection is based on the for- mation of a protective film on metals by externally applied anodic cur- rents. Anodic protection possesses unique advantages. For example, the applied current is usually equal to the corrosion rate of the pro- tected system. Thus, anodic protection not only protects but also offers a direct means for monitoring the corrosion rate of a system. As an Chapter 12 0765162_Ch12_Roberge 9/1/99 6:40 Page 921 enthusiast and famous corrosion engineer claimed, “anodic protection can be classed as one of the most significant advances in the entire his- tory of corrosion science.” 2 Anodic protection can decrease corrosion rate substantially. Table 12.1 lists the corrosion rates of austenitic stainless steel in sulfuric acid solu- tions containing chloride ions with and without anodic protection. Examination of the table shows that anodic protection causes a 100,000- fold decrease in corrosive attack in some systems. The primary advan- tages of anodic protection are its applicability in extremely corrosive environments and its low current requirements. 2 Table 12.2 lists several systems where anodic protection has been applied successfully. Anodic protection has been most extensively applied to protect equip- ment used to store and handle sulfuric acid. Sales of anodically pro- tected heat exchangers used to cool H 2 SO 4 manufacturing plants have represented one of the more successful ventures for this technology. 922 Chapter Twelve TABLE 12.1 Anodic Protection of S30400 Stainless Steel Exposed to an Aerated Sulfuric Acid Environment at 30°C with and without Protection at 0.500 V vs. SCE Corrosion rate, ␮m и y -1 Acid concentration, M NaCl, M Unprotected Protected 0.5 10 Ϫ5 360 0.64 0.5 10 Ϫ3 74 1.1 0.5 10 Ϫ1 81 5.1 510 Ϫ5 49,000 0.41 510 Ϫ3 29,000 1.0 510 Ϫ1 2,000 5.3 TABLE 12.2 Current Requirements for Anodic Protection Current density To passivate, To maintain, H 2 SO Temperature, °C Alloy mAиcm Ϫ2 ␮Aиcm Ϫ2 1 M 24 S31600 2.3 12 15% 24 S30400 0.42 72 30% 24 S30400 0.54 24 45% 65 S30400 180 890 67% 24 S30400 5.1 3.9 67% 24 S31600 0.51 0.10 67% 24 N08020 0.43 0.9 93% 24 Mild steel 0.28 23 99.9% (oleum) 24 Mild steel 4.7 12 H 3 PO 4 75% 24 Mild steel 41 20,000 115% 82 S30400 3.2 ϫ 10 Ϫ5 1.5 ϫ 10 Ϫ4 NaOH 20% 24 S30400 4.7 10 0765162_Ch12_Roberge 9/1/99 6:40 Page 922 Among the parameters that are particularly affected by sensitization are i p and i cc , as defined in Fig. 12.1. In this example, the ability to sus- tain passivity increases as the current density to maintain passivity (i p ) decreases and as the total film resistance increases, as indicated from measurements obtained with different metals exposed to 67% sulfuric acid (Table 12.3). The lower or more reducing the potential at which a passive metal becomes active, the greater the stability of pas- sivity. The depassivation potential corresponding to the passive-active transition, called the Flade potential, can differ appreciably from E pp measured by going through the active-passive process of the same sys- tem. This technical distinction is important for the control aspect of anodic protection where E pp is the potential to traverse to obtain pas- sivation, and the Flade potential is the potential to avoid traversing back into active corrosion. Passivity can also be readily produced in the absence of an externally applied passivating potential by using oxidants to control the redox potential of the environment. Very few metals will passivate in nonoxi- dizing acids or environments, when the redox potential is more cathodic than the potential at which hydrogen can be produced. A good example of that behavior is titanium and some of its alloys, which can be readily passivated by most acids, whereas mild steel requires a strong oxidizing 924 Chapter Twelve Potential Log (Current density) E corr (corrosion potential) i p (passive current) Oxygen evolution i cc (critical current) active E pp (passivation potential) transpassive passive Figure 12.1 Hypothetical polarization diagram for a passivable system with active, pas- sive, and transpassive regions. 0765162_Ch12_Roberge 9/1/99 6:40 Page 924 [...]... Acceleration: (m/s2)/X 0.01 7.716E-08 0.3048 8.47E-05 2. 35E-08 cm/s2 m/h2 ft/s2 ft/min2 ft/h2 Acceleration, angular: (rad/s2)/X 2. 78E-04 7.72E ϩ 08 1.74E-03 rad/min2 rad/h2 rev/min2 939 07651 62_ AppA_Roberge 940 9/1/99 6: 42 Page 940 Appendix A Area: m2/X 1.0E-04 1.0E- 12 0.0 929 6.452E-04 0.8361 4,047 2. 59E ϩ 06 cm2 ␮m2 ft2 in2 yd2 acre mi2 Current: A/X 10. 0 3.3356E -10 abampere statampere Density: (kg/m3)/X 100 0.0... mass: (kgиm2)/X 1.0E-07 0.0 421 4 1.355 2. 93E-04 0.11 gиcm2 lbmиft2 lbfиftиs2 lbmиin2 lbfиin/s Momentum: (kgиm/s)/X 1.0E-05 0.1383 2. 30E-03 gиcm/s lbmиft/s lbmиft/min Momentum, angular: (kgиm2/s)/X 1.0E-07 0.0 421 5 7.02E-04 gиcm2/s lbmиft2/s lbmиft2/min Momentum flow rate: (kgиm/s2)/X 1.0E-05 0.1383 3.84E-05 gиcm/s2 lbmиft/s2 lbmиft/min2 Power: W/X 4.187 4187 1.0E-07 1.356 0 .29 3 105 5 745.8 0.0 421 4 0.1130... 16. 02 119.8 27 ,700 2. 289E-3 g/cm3 lbm/ft3 lbm/gal lbm/in3 grain/ft3 Diffusion coefficient: (m2/s)/X 1.0E-04 2. 78E-04 0.0 929 2. 58E-05 cm2/s m2/h ft2/s ft2/h Electrical capacitance: F/X 1 1 1.0E ϩ 09 1.113E- 12 3 .28 A2иs4/kgиm2 Aиs/V abfarad statfarad V/ft Electric charge: C/X 1 10 3.336E -10 Aиs abcoulomb statcoulomb Electrical conductance: S/X 1 ⍀Ϫ1 Electric field strength: (V/m)/X 1 100 1.0E-08 29 9.8... 07651 62_ AppA_Roberge 944 9/1/99 6: 42 Page 944 Appendix A Pressure, stress: Pa/X 0.1 1 9.8067 1.0E ϩ 05 1.0133E ϩ 05 1.489 47.88 6894 1.38E ϩ 07 24 9.1 29 89 133.3 3386 dyn/cm2 N/m2 kg(f)/m2 bar std atm pd1/ft2 lbf/ft2 lbf/in2 (psi) ton(f)/in2 in H2O ft H2O torr, mmHg inHg Resistance: ⍀/X 1 1 1.0E-09 8.988E ϩ 11 kgиm2/A2иs3 V/A abohm statohm Specific energy: (J/kg)/X 1 4187 4.187E ϩ 06 2. 99 23 26 5.92E ϩ... 1.0E-09 8.988E ϩ 11 kgиm2/A2иs2 Vиs/A abhenry stathenry Length: m/X 0.01 1.0E-06 1.0E -10 0.3048 0. 025 4 0.9144 1609.3 cm ␮m Å ft in yd mi Magnetic flux: Wb/X 1 1 kgиm2/Aиs2 Vиs Mass: kg/X 1.0E-03 0.4536 6.48E-05 0 .28 35 907 .2 14.59 g lbm grain oz (avdp) ton (U.S.) slug Mass per area: (kg/m2)/X 10 4.883 703.0 3.5E-04 g/cm2 lbm/ft2 lbm/in2 ton/mi2 07651 62_ AppA_Roberge 9/1/99 6: 42 Page 943 SI Units Conversion... ior of mild steel exposed to Kraft liquors 300 Potential (V vs SSE) 20 0 100 0 -100 -20 0 -300 -3 -2. 5 -2 -1.5 -1 -0.5 0 0.5 1 -2 Log current density (mA cm ) Figure 12. 9 Theoretical polarization curve illustrating the astable behavior of mild steel exposed to Kraft liquors 07651 62_ Ch 12_ Roberge 9/1/99 6:40 Page 937 Anodic Protection 937 300 -1 0. 02 mV s 20 0 -1 Potential (V vs SSE) 1 mV s 100 0 -100 -20 0... Energy per area: (J/m2)/X 41,868 4.187E ϩ 07 0.001 14.60 11,360 2. 89E ϩ 07 3.87E ϩ 07 cal/cm2 kcal/cm2 erg/cm2 ftиlbf/ft2 Btu/ft2 hpиh/ft2 kWh/ft2 Flow rate, mass: (kg/s)/X 1.0E-03 2. 78E-04 0.4536 7.56E-03 1 .26 E-04 g/s kg/h lbm/s lbm/min lbm/h Flow rate, mass/force: (kg/Nиs)/X 9.869E-05 1.339E-08 g/cm2иatmиs lbm/ft2иatmиh Flow rate, mass/volume: (kg/m3иs)/X 100 0 16.67 0 .27 78 16. 02 0 .26 7 4.45E-03 g/cm3иs... lbm/ft3иh Flow rate, volume: (m3/s)/X 1.0E-06 0. 028 32 1.639E-05 4.72E-04 7.87E-06 3.785E-03 6.308E-05 1.051E-06 cm3/s ft3/s (cfs) in3/s ft3/min (cfm) ft3/h (cfh) gal/s gal/min (gpm) gal/h (gph) Flux, mass: (kg/m2иs)/X 10 1.667E-05 2. 78E-07 4.883 0.0814 1.356E-03 g/cm2иs g/m2иmin g/m2иh lbm/ft2иs lbm/ft2иmin lbm/ft2иh 941 07651 62_ AppA_Roberge 9 42 9/1/99 6: 42 Page 9 42 Appendix A Force: N/X 1.0E-05 1 9.8067 9.807E-03... 9/1/99 6:40 Page 928 20 00 Potential (mV vs SHE) 1500 100 0 60°C 22 °C 500 0 -500 -100 0 -2. 00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2. 00 2. 50 -2 Log Current density (mA cm ) Figure 12. 3 Forward and backward potentiostatic anodic polarization curves for mild steel in 10% sulfuric acid at 22 and 60°C Hastelloy cathode Hg/HgSO4 reference electrode Power supply Sulfuric acid Figure 12. 4 Schematic of an anodic... 20 0 100 0 -100 -20 0 -300 -3 -2. 5 -2 -1.5 -1 -0.5 0 0.5 1 -2 Log current density (mA cm ) Figure 12. 7 Theoretical polarization curve illustrating the bistable behavior of mild steel exposed to Kraft liquors 07651 62_ Ch 12_ Roberge 936 9/1/99 6:40 Page 936 Chapter Twelve 300 Potential (V vs SSE) 20 0 100 0 -100 -20 0 -300 -3 -2. 5 -2 -1.5 -1 -0.5 0 0.5 1 -2 Log current density (mA cm ) Figure 12. 8 Theoretical . maintain, H 2 SO Temperature, °C Alloy mAиcm 2 ␮Aиcm 2 1 M 24 S31600 2. 3 12 15% 24 S30400 0. 42 72 30% 24 S30400 0.54 24 45% 65 S30400 180 890 67% 24 S30400 5.1 3.9 67% 24 S31600 0.51 0 .10 67% 24 N08 020 . 0.43 0.9 93% 24 Mild steel 0 .28 23 99.9% (oleum) 24 Mild steel 4.7 12 H 3 PO 4 75% 24 Mild steel 41 20 ,000 115% 82 S30400 3 .2 ϫ 10 Ϫ5 1.5 ϫ 10 Ϫ4 NaOH 20 % 24 S30400 4.7 10 07651 62_ Ch 12_ Roberge 9/1/99. ϩ0.06 27 0.0 12 0.041 ϩ0. 02 ϩ0.05 10 0.0013 0.011 ϩ0.04 ϩ0.08 1 1.0 5.0 Ϫ0. 32 Ϫ0 .20 0 1.5 8.0 Ϫ0.30 Ϫ0 .20 07651 62_ Ch 12_ Roberge 9/1/99 6:40 Page 926 -100 0 -500 0 500 100 0 1500 20 00 22 ° C 60 ° C -2. 00