Materials Performance

AUG 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

Issue link: http://mp.epubxp.com/i/852556

Contents of this Issue

Navigation

Page 39 of 92

37 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 displacement of ~400 mV toward more pos- itive potentials. These features are like those repor t ed for sulf ide min eral s. 6-7 Therefore, for a potential range between –600 to 300 mV vs. SHE, a similar kinetic expression of the Equation (2) model can be assumed. For this case, the analogue elec- trochemical parameters are denoted with an additional subscript "m" (Table 2). The larger i ml values, in comparison to those of i l , suggest the occurrence of addi- tional reduction reactions whose CDs are superimposed on the actual i O 2 current. Probably, these additional reactions would correspond to the reduction of oxidized products formed under atmospheric condi- tions. 6 Despite the unknown kinetic fea- tures of these additional reduction reac- tions, we adopt th e use of th e L e vich equation as an approximation. From data listed in Table 4, Equation (5) for i ml is obtained: v = × i 1.06 ml 1/2 (5) with a linear regression coefficient of r 2 = 0.7. This poor regression is probably the main cause of th e low reproducibility observed for mineral polarization curves. Values of i ml ω –1/2 for various sulfide miner- TABLE 4. ELECTROCHEMICAL KINETIC PARAMETERS FOR MINERAL CONCENTRATE IN SEAWATER AT VARYING ROTATION RATES Rotation Rate Oxygen Reduction Hydrogen Reduction Mineral Oxidation ω rpm b mO2 A/m 2 t mO2 mV/dec i ml A/m 2 b mH2 A/m 2 t mH2 mV/dec b mFe A/m 2 t mFe mV/dec E mcorr mV vs. SHE i mcorr A/m 2 0 0.195 1,350 0.235 –0.025 228 0.039 95.2 51.6 0.14 100 2.58 622 6.26 –0.0017 140 0.47 161 86.5 1.6 200 4.44 576 6.01 –0.0067 165 0.37 150 117 2.2 400 3.74 303 10.4 –0.0056 136 2.0 2,090 57.8 2.1 600 2.69 606 8.61 –0.0044 152 0.28 127 98.6 1.7 800 9.95 470 7.07 –0.23 303 0.24 124 144 3.6 1,000 4.44 853 8.81 –0.017 189 0.20 121 134 2.6 1,200 5.63 886 13.4 –0.0022 139 0.25 132 144 3.3 1,400 10.8 743 17.7 –0.0004 121 0.65 161 134 5.6 1,600 13.6 640 13.5 –0.0022 155 1.0 207 149 5.8 1,800 16.2 698 14.1 –0.024 200 1.7 284 152 6.7 2,100 14.3 562 14.5 –0.23 329 0.47 162 157 5.6 2,400 17.1 501 12.7 –0.18 311 0.65 172 165 5.9 als, including galena, pyrrhotite, pyrite, chalcopyrite, and arsenopyrite in perchlo- r i c a c i d ( H C l O 4 ) a n d s o d i u m b o ra t e (Na 2 B 4 O 7 ) solutions, have been reported in the range between 1.06 and 1.2. 8 The high PE values for the hydrogen evolution reaction indicate a polarization curve shift. This shift is corroborated by the significantly larger absolute b mH2 values in comparison with those of b H2 and is visually demonstrated in Figure 1. This is probably a distortion generated from a superim- posed mineral reduction current in the cathodic branch. Galvanic Corrosion Parameters from Polarization Curve Measurement Using the Mixed Potential Theory Under coupled conditions (at a galvanic potential E g ) the galvanic CD i g can be determined using Equation (6): = × = i –(A / A ) i i g m x m (6) where i equals i Fe + i O 2 + i H 2 , i m equals i mO + i mO 2 + i mH 2 , and A m and A s are the metal and mineral surface areas, respectively. From Figure 1, note that the i H 2 and i mO values at potential in Equation (6) can be neglected. Considering A m equals A s and the electrical solution resistance is negligi- ble, the galvanic corrosion CD (i g ) and gal- vanic potential (E g ) are found from the intersection point between the cathodic and anodic branches for the concentrate and steel, respectively 9 (Figure 1). These i g values range between 3.0 to 13 A/m 2 and can be expressed as a function of ω as in Equation (7): v = × + i 1.0 6 i g 1/2 mH 2 (7) In the E g range from –220 to –170 mV vs. SHE, Equation (7) should be valid. From the PE listed in Table 2, the error interval for the predicted i g values can be estimated as ± 25%. This variability may be attributed to the mineral particle attachment to the wax graphite surface in the WRDE electrode. Galvanic Corrosion Parameters from ZR A Measurements Figure 2 shows E g and i g time evolution values measured from a CS-mineral couple at various mineral/steel surface ratios (M/S) in seawater during a 30-min interval. While i g approached relatively stable val- ues, E g values asymptotically decreased with time from a range between –260 and Galvanic Interaction Between Carbon Steel and Copper Concentrate

Articles in this issue

Archives of this issue

view archives of Materials Performance - AUG 2017