Characterization of the Corrosion Layers Electrochemically Formed on the Lead–Silver/H[sub 2]SO[sub 4]+Mn(II) Interface

Abstract

The corrosion layers electrochemically formed on the lead–silver/ normalH2SO4+Mn(II) interface were studied through open-circuit potential monitoring, cyclic voltammetry, and chronoamperometry and were characterized by cyclic voltammetry, X-ray diffraction, and scanning electron microscope coupled to an energy dispersive spectrometer to analyze microregions. Mn(II) oxidation reactions take place in the range of 1<E<2.2V vs standard hydrogen electrode. When an oxidation potential pulse of 1.96 V is applied on a lead–silver electrode, different species are formed as the electrolysis time proceeds. This modification can be divided into four stages: at 0<t<5min , Pb3normalO4 is formed on the metallic lead; this compound favors the formation of α-MnO2 . At t=5min , α-MnO2 completely covers the surface, generating high current densities. At 5<t<60min , the anodic layer loses its electroactive properties as it grows and MnO4− and δ-MnO2 , which are less electroactive than α-MnO2 , are formed. At t60min , the formation of δ-MnO2 predominates, transforming the crystalline surface into an amorphous and nonelectroactive one. Furthermore, the anodic layer is broken, liberating Pb2+ and exposing metallic lead, which is oxidized to PbO2 . The Pb2+ released forms PbMn8normalO16 on the electrode surface and PbSO4 in the bulk.

Characterization of anodic deposits formed on Pb–Ag electrodes during electrolysis in mimic zinc electrowinning solutions with different concentrations of Mn(II)

“…We may thereby conclude that after an hour of electrolysis, electrocatalytic properties of MnO 2 anodic layers have diminished and tend to be equal in both scan directions. On comparing the charges obtained, the ACZn12Mn system exhibits higher values for O2 process, which may be related to simultaneous reactions occurring in this system, already described by several authors (Chen and Dutrizac, 2011;Liu et al, 2011;Masatsugu, 2011;Recéndiz et al, 2008Recéndiz et al, , 2009Recéndiz et al, , 2010Schmachtel et al, 2008;Tjandrawan and Nicol, 2013;Zhang and Hua, 2009). Fig.…”

“…It is important to say that this catalytic effect is observed in the first 10 min of oxidation, but after 1 h it tends to inhibit itself. On other hand, α-MnO 2 was identified by XRD studies and reported by Recéndiz et al (2009) in tests carried out at 180 g/L H 2 SO 4 and Mn(II) without Zn 2+ ions. Despite this evidence, XRD studies are necessary in order to confirm the presence of α-MnO 2 .…”

“…In the case of manganese, the Pourbaix diagram, shows that OCP values are inside the zone in which the predominant species is Mn(II) (Pourbaix, 1974). Thus, Pb 2+ forms a layer of PbO, PbO 2 , PbSO 4 , PbO 2 which covers the electrode surface, and the Mn 2+ forms different oxides, such as a very catalytic α-MnO 2 formed by reactions 4 and 5 (Recéndiz et al, 2009):…”

“…Chen and Dutrizac (2011) have determined that the main anodic growth in grooved Pb-Ag anodes (0.75%) corresponds to MnO 2 and gypsum; there are also hydrated and amorphous manganese oxides, probably formed as a result of oxide evolution with the time of electrolysis, and PbSO 4 and SrSO 4 particles as well as other impurities. After studying an electrolyte composed of only Mn(II) and H 2 SO 4 for 2 h, Recéndiz et al (2009) report that the manganese dioxide layer changes its properties during electrolysis. In the case of electrolytic systems in the presence of zinc, as reported by Zhang and Hua (2009) who have studied 150 g/L H 2 SO 4 and 55 g/L Zn system at different Mn(II) concentrations (0-10 g/L) during 2 h, these authors concluded that the presence of Mn(II) ions catalyzes the reaction of oxygen evolution up to concentrations of 10 g/L.…”

Electrochemical Reactions on Metal-Matrix Composite Anodes for Metal Electrowinning

“…In the forward scan direction, the surface is covered mostly with PbO and PbSO 4 , which are non-conductive phases, whereas in the reverse direction the surface is covered by the semi-conductive PbO 2 phase facilitating the OER. 49 The difference in current densities in the forward and reverse scan directions was more pronounced when the MnO 2 fraction was increased in the anodes. At 1.9 V Ag/AgCl , for example, this difference for the Pb-20MnO 2 is three times that of the Pb-5MnO 2 .…”