Polarization curves alloy deposition

Figure 12. Comparison of hypothetical polarization curves for deposition of metals onto pure metal phases (/a and i b) with those that should be obtained for the deposition on a eutectic-type alloy, according to Eq. (58) it— the total current density line.

The electrodeposition of an alloy requires, by definition, the codeposition of two or more metals. In other words, their ions must be present in an electrolyte that provides a cathode film, where the individual deposition potentials can be made to be close or even the same. Figure ll.l depicts typical polarization curves, that is, deposition... 

Figure 11.1. Polarization curves for the deposition of alloys. (From Science and Technology of Surface Coating, a NATO Advanced Study Institute publication, 1974, with permission from Academic Press.)...

It must be understood that in a case such as that illustrated in Figure 11.1, the plating bath is being depleted of metal B ions more quickly than of metal A ions. To keep matters under control (i.e., maintain uniform deposition conditions), metal ions must be replenished in direct proportion to their rates of dep>osition dictated by the specific alloy. It is clear, therefore, that ideally, the polarization curves of the competent metals being codeposited should be identical. It is next to impossible to reahze this condition in practice. 

The individual polarization curves for the metals are often modified as a result of interactions resulting from codeposition. If the alloy deposition occurs at low polarization, the nobler metal will be deposited preferentially (Cu in Example 11.1). All factors, however, that increase polarization during electrodeposition, such as high current density, low temperature, and quiescent solution—factors that increase concentration polarization—will favor the deposition of the less noble metal (Zn in Example 11.1). 

Fig. 7.2 Polarization curves for the electrodeposition of more noble metal (A) and less noble metal (B) /l(A) diffusion limiting current density for the electrodeposition of metal (A), M(B) current density for the electrodeposition of metal (B), /d(all) current density for the electro-deposition of alloy (Reprinted from Ref. [5] with kind permission from Springer)...

It was found that for the Fe(III) salts electrolytes the current efficiency was very low, 1-2% (the polarization curves for powder deposition (ytot) and for hydrogen evolution (yn) practically overlapped), and it was necessary to deposit powders at least for 2 h to obtain the amount of powder that could be used for the morphology and composition analysis (SEM, EDS). In the case of Fe(II) salt electrolytes, current efficiency at the potentials more negative than the second inflection point ( in Fig. 5.21) varied between 8% and 15% depending on the Ni/Fe ratio, as shown in Fig. 5.22. The average values for the diffusion limiting current densities for alloy powder electrodeposition were ype-Ni = —0.26 A cm for the ratio 1/3 and /pe-Ni = —0.49 A cm for the ratio 9/1. 

The polarization curves are presented in Fig. 5.43. As can be seen, the polarization curves characterized by two inflection points (Fig. 5.43a), as in all previous cases, were obtained. It is important to note that the potential of the beginning of alloy deposition (A) becomes more negative with the increase of molybdate ions concentration (with the decrease of Ni/Mo ratio), as it could be expected, since the potential of the Mo deposition is much more negative than that of Ni [116]. At the same time, a deposition of Mo can only take place in the presence of Ni (induced codeposition [116]). Taking into account that the concentration of Ni ions was constant, it is quite reasonable that the value of current density of the inflection point B does not change with changing Ni/Mo ions concentration ratio (being about —1.2 A cm ). 

Figure 10 shows potentiod5mamic polarization curves measured in DI water [135,137]. The plated Co-8%P material was considered for use as the magnetic alloy in thin-frlm disks when they were first developed. Like pure Co, it is not very corrosion resistant and does not readily passivate. The corrosion potential of two different sputter-deposited carbon thin films is seen to be about 600 mV higher than that of plated CoP. Although the nature of C thin films can change drastically as a function of deposition conditions, the two C films sustain reasonably laige cathodic... 

Potentiodynamic polarization curves of the bare Mg alloy (curve a) and the Al-coated Mg samples deposited in the 53 m/o (curve b) and 60 m/o (curve c) ionic liquids, respectively. 

The Electrochemical impedance spectroscopy (EIS) results for the Mg alloy without and with surface Al coated from the 53 m/o and the 60 m/o ionic liquid, respectively, are depicted in Fig. 14.10. For bare Mg alloy, the polarization resistance was about 470 Qcm. A substantial increase in the polarization resistance, as evidenced by an enlarged diameter of the semicircle of the Nyquist plot, can be obtained for Mg alloy if it is electroplated with Al. For those with surface Al electrodeposited at -0.2 V from the 53 m/o and the 60 m/o ionic liquid, the polarization resistance in 3.5 wt% NaCl solution are 3000 and 5200 Qcm, respectively. The results were consistent with those revealed in the polarization curves demonstrated in Fig. 14.8. The improved polarization resistance of AZ91D Mg alloy with Al coating from ionic liquid is clearly demonstrated. However, the passivity or the polarization resistance of the Al-coated Mg alloy depends on the deposition conditions. The Al film formed in more acidic AICI3-EMIC and at a lower deposition rate renders a better passivation behavior. 

Comparison of the potentiodynamic polarization curves measured in 3.5 wt% NaCI solution curve a, AZ91D alloy curve b, Al deposit formed in AICI3-EMIC at -0.2V (vs. Al) curve c, Al-Zn codeposited at -0.2 V (vs. Al) in AICI3-EMIC + 1 wt% ZnCl2. 

Polarization curves of coated and bare Mg-Zn-Ca alloy in simulated body fluid (a) micro-arc oxidation coating and rod-like nano-HA deposition, (b) micro-arc oxidation coating and (c) bare alloy (Gao etai, 2011). 

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