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Fig. 1. Nyquist plots of phosphated magnet at OCP after 1 h immersion in 0.1 mol L”' Na;SOq, at @=0 (MM) and w=1000 rpm (O). In order to evaluate the effect of electrode rotation in the presence of tungstate ions on the magnet’s electrochemical behaviour, tests were also performed at 1000 rpm for the phosphated alloy in 0.1 mol L’!' Na,SOy, solution with 0.05 mol L! Na WO, (data not shown). At static conditions, E.or reaches a stable potential of -0.33 V whereas under electrode rotation (1000 rpm), FE... stabilized at —0.38 V. Under elec- trode rotation, R19 mu, and the phase angle increases, and this is probably due to the enhanced flux of dissolved oxygen to the

Figure 1 Nyquist plots of phosphated magnet at OCP after 1 h immersion in 0.1 mol L”' Na;SOq, at @=0 (MM) and w=1000 rpm (O). In order to evaluate the effect of electrode rotation in the presence of tungstate ions on the magnet’s electrochemical behaviour, tests were also performed at 1000 rpm for the phosphated alloy in 0.1 mol L’!' Na,SOy, solution with 0.05 mol L! Na WO, (data not shown). At static conditions, E.or reaches a stable potential of -0.33 V whereas under electrode rotation (1000 rpm), FE... stabilized at —0.38 V. Under elec- trode rotation, R19 mu, and the phase angle increases, and this is probably due to the enhanced flux of dissolved oxygen to the

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Chemical composition of commercial Nd—Fe—B magnets Table 1
Chemical composition of commercial Nd—Fe—B magnets Table 1
Fig. 3. Cyclic voltammograms for phosphated NdFeB electrode in 0.10 mol L! Na,SO, and x mol L”' NayWO, solution, from — 1.0 to +1.2 Vat v=0.020 Vs ! at w=0. (a) x=0.0; (b) x=0.01; (c) x=0.03; (d) x=0.05; (e) x=0.075 and (f) x=0.10.
Fig. 3. Cyclic voltammograms for phosphated NdFeB electrode in 0.10 mol L! Na,SO, and x mol L”' NayWO, solution, from — 1.0 to +1.2 Vat v=0.020 Vs ! at w=0. (a) x=0.0; (b) x=0.01; (c) x=0.03; (d) x=0.05; (e) x=0.075 and (f) x=0.10.
Corrosion potential (E.or) and resistance (Rio mz) of the untreated and phosphated NdFeB at static (w=0 rpm) and dynamic (w=1000 rpm) conditions after 1 h of immersion in 0.1 mol L”' Na»SOz solution
Corrosion potential (E.or) and resistance (Rio mz) of the untreated and phosphated NdFeB at static (w=0 rpm) and dynamic (w=1000 rpm) conditions after 1 h of immersion in 0.1 mol L”' Na»SOz solution
Fig. 2. Cyclic voltammograms for untreated NdFeB electrode in 0.10 mol L”! Na SO, and x mol L7' Na, WO, solution from — 1.0 to + 1.2 Vat v=0.020 Vs ! at w=0: (a) x=0.0; (b) x=0.01; (c) x=0.03; (d) x=0.05; (e) x=0.075 and (f) x=0.10.
Fig. 2. Cyclic voltammograms for untreated NdFeB electrode in 0.10 mol L”! Na SO, and x mol L7' Na, WO, solution from — 1.0 to + 1.2 Vat v=0.020 Vs ! at w=0: (a) x=0.0; (b) x=0.01; (c) x=0.03; (d) x=0.05; (e) x=0.075 and (f) x=0.10.
Fig. 4. Nyquist plots for phosphated NdFeB electrodes at OCP after 1 h of immersion in 0.10 mol L”' NaSO, and x mol L~! Na;WO, solution, at w =0. (A) x=0.01; (OC) x=0.03; (MI) x=0.05.
Fig. 4. Nyquist plots for phosphated NdFeB electrodes at OCP after 1 h of immersion in 0.10 mol L”' NaSO, and x mol L~! Na;WO, solution, at w =0. (A) x=0.01; (OC) x=0.03; (MI) x=0.05.
Fig. 5. Nyquist plots for phosphated NdFeB electrode at OCP in 0.10 mol L”! Na>SO, solution containing 0.10 mol L~! Na;WOu,, at w=0 after 48 (x), 164 (WH) and 212 (O) h of immersion. Based on the results presented in the previous section, tests were also performed by immersion of the phosphated magnet in tungstate containing solution to evaluate the incorporation of these anions into the conversion coating. Two sodium tungstate (Na2WO,) concentrations were tested, 0.05 mol L! and 0.1 mol L'. No beneficial effects were detected by the treatment in the 0.05 mol L ' NayWO, solution (data not shown). Thus, the 0.1 mol L~' Na:WO, solution was used for incorporation of tungstate. For this purpose, two surface treatments were
Fig. 5. Nyquist plots for phosphated NdFeB electrode at OCP in 0.10 mol L”! Na>SO, solution containing 0.10 mol L~! Na;WOu,, at w=0 after 48 (x), 164 (WH) and 212 (O) h of immersion. Based on the results presented in the previous section, tests were also performed by immersion of the phosphated magnet in tungstate containing solution to evaluate the incorporation of these anions into the conversion coating. Two sodium tungstate (Na2WO,) concentrations were tested, 0.05 mol L! and 0.1 mol L'. No beneficial effects were detected by the treatment in the 0.05 mol L ' NayWO, solution (data not shown). Thus, the 0.1 mol L~' Na:WO, solution was used for incorporation of tungstate. For this purpose, two surface treatments were
Fig. 6. Nyquist plots for NdFeB magnet at OCP after 1 h immersion in 0.10 mol L~' Na,SO4 Magnet (A) untreated, (Ml) phosphated, (O) phosphated+ treatment 1, (@) phosphated+treatment 2.
Fig. 6. Nyquist plots for NdFeB magnet at OCP after 1 h immersion in 0.10 mol L~' Na,SO4 Magnet (A) untreated, (Ml) phosphated, (O) phosphated+ treatment 1, (@) phosphated+treatment 2.
Equivalent circuit elements obtained from fitting experimental data to the equivalent circuit proposed to simulate untreated and phosphated magnet surfaces and shown in Fig. 6
Equivalent circuit elements obtained from fitting experimental data to the equivalent circuit proposed to simulate untreated and phosphated magnet surfaces and shown in Fig. 6
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Materials EngineeringCondensed Matter PhysicsSurface CoatingsElectrochemical Impedance SpectroscopySurface and Coatings TechnologyRotating Disc ElectrodeOpen Circuit Potential
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