chloride-containing H2SO4 solutions

Accepted Manuscript Influence of Mn2+ on the performance of Pb-Ag anodes in fluoride/chloride-containing H2SO4 solutions

Xiaocong Zhong, Ruixiang Wang, Zhifeng Xu, Liangxing Jiang, Xiaojun Lv, Yanqing Lai, Jie Li PII: DOI: Reference:

S0304-386X(17)30282-7 doi:10.1016/j.hydromet.2017.10.014 HYDROM 4673

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

7 April 2017 18 August 2017 14 October 2017

Please cite this article as: Xiaocong Zhong, Ruixiang Wang, Zhifeng Xu, Liangxing Jiang, Xiaojun Lv, Yanqing Lai, Jie Li , Influence of Mn2+ on the performance of Pb-Ag anodes in fluoride/chloride-containing H2SO4 solutions. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Hydrom(2017), doi:10.1016/j.hydromet.2017.10.014

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ACCEPTED MANUSCRIPT Influence of Mn2+ on the performance of Pb-Ag anodes in fluoride/chloride-containing H2 SO4 solutions

Xiaocong Zhonga, Ruixiang Wanga, Zhifeng Xua, Liangxing Jiangb,*, Xiaojun Lvb, Yanqing Laib, Jie Lib

School of Metallurgical and Chemical Engineering, Jiangxi University of Science and Technology,

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a

Ganzhou 341000, Jiangxi, China.

School of Metallurgy and Environment, Central South University, Changsha 410083, Hunan, China .

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b

* Corresponding author. E-mail address: [email protected] (L. Jiang).

ACCEPTED MANUSCRIPT ABSTRACT In this study, surface morphology, inner structure and phase composition of anodic layers on Pb-Ag anode in fluoride/chloride-containing H2 SO4 solutions were investigated in the presence/absence of Mn 2+. Additionally, corrosion morphology of metallic substrate and anodic potential variation of Pb-Ag anodes were studied as well. The influence of Mn 2+ on the anodic layer property and corrosion behavior of Pb-Ag anodes in fluoride/chloride-containing H2 SO4 solutions was then clarified. The results revealed that in the presence of Mn 2+, anodic layer consisted of an external layer in MnO2 /PbO2 -PbSO4 /MnO2 structure and an inner layer adhering to the metallic substrate. Due to the

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coverage of an external layer, the corrosion of metallic substrate was relieved by the presence of Mn 2+ in fluoride/chloride-containing H2 SO4 solution. As was demonstrated, Mn 2+ decreased PbO2 concentration in the inner layer, especially in fluoride-containing solution. In the presence of Mn 2+, the

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anodic potential was higher than that in solutions without Mn2+ due to larger thickness and resistance of anodic layer. In addit ion, the anodic potential was characterized by oscillat ion in Mn 2+-containing

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solution, indicating low stability of anodic layer. Co mparatively, the anodic layer formed in the presence of Mn2+ and chloride ion was more stable than that in solutions containing Mn2+ and fluoride ion.

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Keywords: lead-based anode; anodic layer; corrosion behavior; inner structure; potential oscillation

ACCEPTED MANUSCRIPT 1. Introduction Electrowinning (EW) has been a preferred process for extract ing many non-ferrous metals, such as Zn, Cu, Mn and Co (Clancy et al., 2013). During the EW process, valuable metal is deposited on the cathode, and oxygen evolution proceeds as the main reaction on the anode (Ivanov et al., 2004). Most metal EW processes perform in H2 SO4 solutions, and a few anode materials are insoluble in H 2 SO4 solutions during polarization under high current density (~500 A m -2 ). This makes Pb-based alloys a primary candidate for this application (Zhang et al., 2010). Specifically, a b inary Pb -Ag alloy with Ag concentration typically between 0.25 ~ 1.0% (mass fraction) is the standard anode material currently applied in zinc EW (Felder et al., 2006). Ho wever, Pb -Ag anodes exhibit several drawbacks, such as

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high oxygen evolution over-potential and significant corrosion rate (Lai et al., 2010; Petrova et al., 1996). Substantial work has been conducted on improving electrochemical reactiv ity and corros ion

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resistance of Pb-based anodes, such as optimization of alloying elements (El-Sayed et al., 2015; Čekerevac et al., 2010), regulat ion of metallic structure (Osório et al., 2008; Osório et al., 2013), and

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modification by catalytic metal-o xide coatings (Ma et al., 2016; Mohammadi et al., 2015). Apart fro m phase composition and structure of Pb-based anodes, electrolyte impurit ies also play a significant role in the performance of Pb-based anodes, such as Mn2+, Ca2+, Mg 2+, Cl- and F- (Jamies et al., 2015),

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among which Mn 2+ attracts wide attention due to its crucial effects on zinc EW operation. Mn 2+ in electrolytes partly originates fro m zinc ore. Ho wever, most of Mn 2+ is deliberately introduced. In leaching process, manganese dioxide o r potassium permanganat e is added to oxidize

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iron impurities (Mohammadi et al., 2016). During zinc EW process, Mn 2+ is oxid ized to MnO2 . MnO2 particles will accu mulate in two areas: first as a scale adhering to the anodes, and second as a precipitate mud p roduct residing on the bottom of the cell (Mahon et al., 2014). It was reported that MnO2 layer acted as a physical barrier between electrode and electrolyte, provid ing further protection

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for the metallic substrate (Ivanov et al., 2000). In addition, Mn 2+, depending on varied concentrations, can have promoting/suppressive effects on the o xygen evolution reaction on the Pb -based anode (Tunnicliffe et al., 2012).

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Apart fro m Mn 2+, fluoride and chloride ions in electro lytes also attract more and more attention due to their increasing concentration. Fluoride and chloride ions in electrolytes mainly co me fro m zinc concentrate and secondary ZnO dust. Previous investigation demonstrated that both fluoride and chloride ions have significant influence on the anodic layer property and corrosion behavior

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(Ramachandran et al., 1980; Newnham R., 1992; Ramachandran et al., 1996; Ivanov et al., 2000; Mohammadi et al., 2011; Tunnicliffe et al., 2012; Yang et al., 2014; Zhong et al., 2015a, b). As demonstrated in our previous work (Zhong et al., 2015a, b), in the presence of fluoride ions, the anodic layer surface was in a scale-like structure, and detachment of the anodic layer accelerated. While in the presence of chloride ions, the anodic layer showed a cement structure, and the anodic layer surface became co mpact and flat. In addition, it was found that both fluoride and chlo ride ions inhib ited the growth of PbO2 , while pro moted the format ion of Pb O·PbSO4 and PbSO4 in the anodic layer. Furthermore, chloride and fluoride ions were proved to b e detrimental to the corrosion rate and o xygen evolution reactivity of Pb-based anodes. Although substantial work has reported on oxygen evolution behavior and corrosion behavior of Pb-based anodes in fluoride/chloride-containing electro lytes (Ramachandran et al., 1980; Newnham R., 1992; Ramachandran et al., 1996; Ivanov et al., 2000; Mohammad i et al., 2011; Tunnicliffe et al., 2012; Yang et al., 2014; Zhong et al., 2015a, b), the previous work d id not take account of the influence of Mn 2+. The performance of Pb-based anodes in H2 SO4 solutions containing both fluoride/chloride ions

ACCEPTED MANUSCRIPT and Mn2+ remains unclear. It has been known that Mn 2+, fluoride and chloride ions are all involved in the growth of anodic layer and exert important influence on the morphology, structure and phase composition of the anodic layer. Therefore, the aim of this research is to clarify the influence of Mn 2+ on the performance of Pb-Ag anodes in fluoride/chloride-containing electrolytes. Through comparing the anodic layer property, corros ion behavior and anodic potential variation of Pb -Ag anodes in the presence of Mn 2+ with those in the absence of Mn 2+, the ro le of Mn 2+ on the performance of Pb-Ag anodes in fluoride/chloride-containing electrolytes will be uncovered.

2. Experimental

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2.1. Electrolytes

Several electrolytes in different chemical co mpositions were used to simulate industrial

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electrolytes. To simplify the description of electrolytes used in this research, connotations were adopted to distinguish these electrolytes. BE referred to basic electrolyte (160 g L-1 H2 SO4 ). Chemical

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compositions of other electrolytes were listed in Table 1. Mn, F and Cl were added in the form of MnSO4 ·H2 O, NaF and NaCl.

Table 1 Chemical compositions of different electrolytes (c(H2SO4) = 160 g L-1 )/ g L-1 Cl

Mn

BE

0

0

0

BEF

0.1

0

0

BECl

0

0.5

0

Connotation

F

Cl

Mn

BEMn

0

0

4

BEFMn

0.1

0

4

BEClMn

0

0.5

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F

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Connotation

-

4 -

Note： Concentration of F presented was the total concentration of different fluorine species (F , HF, HF 2 ).

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2.2. Preparation of electrodes

Pb-Ag anodes were made of industrial Pb -Ag (0.9%, mass fraction) sheets. Pb-Ag sheets were initially wire-cut into samples in a size of 10 mm × 10 mm × 7 mm. The samp les were then connected

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to a plastic-isolated copper wire and cast in denture base resin with a working area o f 1.0 cm2 . Electrochemical measurements were carried out in a three-electrode system using an electrochemical workstation (1470 E, Solart ron Analytical). A platinu m plate with an area of 4 cm2 and Hg/Hg 2 SO4 /sat. K2 SO4 were used as counter and reference electrodes respectively. All potentials shown in this study

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were relative to this reference electrode. The temperature of electro lytes was controlled at 35 ± 0.5 C through a thermostat. Pb-Ag anodes were ground to 1500 grit using a SiC abrasive paper, and washed by deionized water before each electrochemical measurement. 2.3. Measurements

In order to obtain stable anodic layers on Pb-Ag anodes, galvanostatic electrolysis of Pb-Ag anodes was performed in d ifferent electrolytes at a current density of 500 A m-2 for 72 hours. During the galvanostatic electrolysis, anodic potentials were recorded by an electrochemical workstation (Solartron, 1470E). A fter the galvanostatic electrolysis, Pb -Ag anodes were taken out, washed by deionized water and dried for 8 hours at 80°C. Next , the surface morphology of anodic layers was observed via scanning electron microscopy (SEM, MIRA 3). In terms of other samp les for parallel test, after being washed and dried as previously mentioned, ano dic layers were packed and preserved by casting with resin. The packed Pb-Ag anodes were abraded and polished to expose their cross sections, and the cross section mo rphology was further observed via SEM as well. An X-ray diffracto meter

ACCEPTED MANUSCRIPT (D/ max 2500, Rigaku) with Cu Kα as the radiation source was used to identify the phase composition of anodic layers. In addition, after SEM/XRD measurements, the anodic layers on the surface of Pb -Ag anodes were removed by immersing in boiling solutions (100 g L-1 NaOH and 20 g L-1 g lucose). Afterwards the corrosion morphology of metallic substrates was observed via SEM.

3. Results and discussion 3.1. Surface morphology of anodic layers Previous studies proved that Mn2+, fluoride and chloride ions were all involved in the formation of

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anodic layers. Therefore, the influence of Mn 2+ on the mo rphology of anodic layers should be clarified. Fig. 1 showed the surface morphology of anodic layers after 72-hour galvanostatic polarization in BEF (BECl) and BEFMn (BEClMn) solutions. In the presence of fluoride ions (Fig. 1(a)), the anodic layer

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was in a scale-like structure. Due to the weak adhesion of scales to the anodic layer bu lk, a large number of scales detached from the anodic layer, exposing a porous and rough surface. In BEFMn

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solution (Fig. 1(b)), the anodic layer was sharply divided into two areas. One area presented a smooth and compact surface, which main ly consisted of MnO2 (Yu et al., 2002). The other area showed porous and rough morphology similar to that in BEF solution. Apparently, the porous area was covered with a

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compact MnO2 layer as well during electrolysis. However, o wing to the evaporation of H 2 O during SEM samp le preparation process (8-hour drying at 80°C), the compact MnO2 layer failed to co mbine with the porous area and detached. Due to the coverage of a co mpact layer, the scales underneath were

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much thinner and better adhered to the anodic layer bulk compared with that in BEF solution. In the presence of chloride ions (Fig. 1(c)), the anodic layer showed a cemen t structure, similar to the morphology of dried slurry. Though there were some tiny holes on the surface, the compactness was much better than that in BEF solution. In BEClMn solution (Fig. 1(d)), the anodic layer was

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separated into two areas as well; however, the compact area showed a cluster surface, wh ich was much rougher than that in BEFMn solution. It was reported that the cluster structure was mainly co mposed of

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MnO2 as well (Yu et al., 2002). Similar to the morphology shown in Fig. 1(c), the underneath layer in BEClMn solution also showed a cement structure, but the compactness was slightly enhanced. As demonstrated above, in the presence of Mn 2+, anodic layer was divided into two areas with different structures. The external part had a co mpact surface which mainly consisted of MnO2 . However,

the

co mpact

surface

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morphology

in

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and

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chloride-containing solutions. On the other hand, the underneath part presented morphology similar to that in solutions without Mn 2+ (BEF/BECl).

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Fig. 1. Surface mo rphology of anodic layers formed through 72 -hour galvanostatic electrolysis in different solutions: (a) BEF; (b) BEFMn; (c) BECl; (d) BEClMn.

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3.2. Inner structure of anodic layers

As a physical barrier between metallic substrate and electrolyte, the inner structure of anodic

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layers plays a crucial role in the performance of Pb-Ag anodes. The cross section morphology of Pb-Ag anodes after 72-hour electro lysis was shown in Fig. 2. The black area on the top indicated the insulate resin, while the white area at the bottom ind icated the metallic substrate. In this wh ite area appeared some black dots, which were SiC particles embedded in the substrate during sample preparat ion

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process. The gray area between the black and white areas indicated the anodic layer. In BEF solution (Fig. 2(a)), the anodic layer surface was porous and irregular. This could be explained by the scale-like structure shown in Fig. 1(a). The pores and crevices shown on the surface

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suggested the weak contact between scales and the anodic layer bulk. The area near the metallic substrate was much more co mpact than the surface even though some tiny pores appeared in this area. In BEFMn solution (Fig. 2(b, c)), the anodic layer was much thicker than that in BEF solution. Notably, the anodic layer consisted of two layers, namely, an external layer (Fig. 2(b)) and an inner layer (Fig.

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2(c)). The external layer was thick and in a laminated structure. This layer corresponded to the compact area shown in Fig. 1(b) and Fig. 1(d). The co mpact area was composed of MnO2 , and the porous area enwrapped by MnO2 main ly consisted of PbO2 and PbSO4 (Yu et al., 2002). Therefo re, the structure of external layer could be described as MnO2 /PbO2 -PbSO4 /MnO2 . During the d rying process, the external layer was part ly separated fro m the anode, as shown in Fig. 1(b, d). In contrast, the inner layer was firmly adhered to the metallic substrate. As shown in Fig. 2(c), some crevices and holes existed in the surface area of inner layer, which could also be explained by the scale structure in fluoride-containing solution.

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Fig. 2. Cross-section morphology of Pb-Ag anodes after 72-hour galvanostatic electrolysis in fluoride-containing solutions: (a) BEF; (b, c) BEFMn.

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In BECl solution (Fig. 3(a)), the anodic layer surface was much flatter and more co mpact than that in BEF solution, in accordance with the surface morphology shown in Fig. 1. Interestingly, the

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thickness of anodic layer was about 10 μm in BECl solution, much thinner than that in BEF solution. In BEClMn solution (Fig. 3(b, c)), the anodic layer also consisted of two layers. As shown in Fig. 3(b),

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the external layer showed a structure similar to that in BEFMn solution. However, the PbO 2 -PbSO4 area enwrapped by MnO2 was smaller in BEClMn solution than that in BEFMn solution. Fig. 3(c) displayed the inner morphology of anodic layer in BEClMn solution. The surface of inner layer was

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flatter and much more co mpact than that in BECl solution. Notably, the thickness of inn er layer in BEClMn solution was much thicker than that in solutions without Mn 2+. It was inferred that the coverage of external layer was of benefit to the growth of inner structure in chloride-containing solution.

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It could be inferred fro m above that Mn 2+ had similar effect on the structure of anodic layers in fluoride/chloride-containing electrolytes. In the p resence of Mn 2+, anodic layer consisted of two layers, namely, an external layer in MnO2 /PbO2 -PbSO4 /MnO2 structure, and an inner layer that firmly adhered

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to the metallic substrate. With the coverage of external layer, the inner layer had a flatter and more compact surface, and its thickness was increased in BEClMn solution. Co mparat ively, in a chloride-containing solution, the amount of PbO2 -PbSO4 area enwrapped by MnO2 was much smaller,

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and the inner layer was more compact than that in a fluoride-containing solution.

Fig. 3. Cross-section morphology of Pb-Ag anodes after 72-hour galvanostatic electrolysis in chloride-containing solutions: (a) BECl; (b, c) BEClMn. 3.3. Corrosion morphology of metallic substrates Corrosion resistance is one of crucial factors to evaluate Pb -based anodes. To investigate the influence of Mn 2+ on the performance of Pb-Ag anodes in fluoride/chloride -containing electrolytes, it is essential to uncover the effect of Mn 2+ on the corrosion behavior of Pb-Ag anodes. Fig. 4 displayed the corrosion morphology of metallic substrates in BEF (BECl) and BEFMn (BEClMn) solutions after removing anodic layers through chemical d issolutio n methods. In BEF solution (Fig. 4(a)), there were large numbers of corrosion pits and holes on the substrate. Tiny corrosion pits and holes at the bottom

ACCEPTED MANUSCRIPT of these large corrosion pits revealed the growth and developing pattern of corrosion. In BEFMn solution (Fig. 4(b )), even though there existed some corrosion holes on the substrate, several areas of the substrate remained flat, ind icating that the corrosion was remarkably relieved by the p resence of Mn 2+. In BECl solution (Fig. 4(c)), there were continuous corrosion pits on the substrate, the number of which was much s maller than that in BEF solution. In addit ion, the corrosion depth was smaller than that in BEF solution. Therefore, it could be concluded that fluoride ions were more detrimental to the corrosion of metallic substrate than chloride ions. This could be explained by the scale-like surface structure and less compact inner structure of the anodic layer in fluoride-containing solution. These

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features increased the contact possibility between electrolyte and metallic substrate, and further deteriorated the corrosion of Pb-Ag anode. In addition, the more aggressive nature of fluoride ions

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compared with chloride ions could be another reason for severer corrosion of Pb-Ag anode in fluoride-containing solution.

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In BEClMn solution (Fig. 4(d)), it was apparent that the presence of Mn 2+ largely relieved the corrosion of Pb-Ag anode as well in ch loride-containing solution. The metallic substrate was flat and compact with a small nu mber of corrosion pits, indicating that the corrosion process of Pb-Ag anode in

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BEClMn solution was remarkably inhibited by the presence of Mn 2+.

The above results indicated that the presence of Mn2+ would relieve the corrosion of Pb-Ag anodes in fluoride/chloride-containing solutions. Especially, in BEClMn solution, the corrosion of metallic substrate was remarkably relieved compared with that in BECl solution. The positive effect of

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Mn 2+ on inhibiting corrosion could be attributed to the formation of external part of anodic layer. The external layer was in MnO2 /PbO2 -Pb SO4 /MnO2 structure, which p rovided extra protection for the metallic substrate fro m the attack o f electrolyte. In chloride-containing electrolyte, the p resence of

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Mn 2+ significantly increased the thickness of inner layer of anodic layer, which further decreased the

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contact possibility between electrolyte and metallic substrate.

Fig. 4. Metallic substrate corrosion morphology of Pb-Ag anodes after 72-hour galvanostatic electrolysis in different solutions: (a) BEF; (b) BEFMn; (c) BECl; (d) BEClMn. 3.4. XRD patterns of anodic layers It was suggested that Mn2+ was involved in the growth of anodic layer, resulting in the deposition

ACCEPTED MANUSCRIPT of MnO2 on the anodic layer. Apart fro m Mn O2 , it also changed the phase composition of anodic layer. Therefore, XRD tests were conducted to analyze the composition of anodic layer. Notably, as for anodic layer fo rmed in Mn 2+-containing solution, its external layer was separated from the anode during the drying process, so the phase composition information mainly concerned its inner layer. As shown in Fig. 5, in BEF, BECl, BEFMn and BEClMn solutions, the main phase composition of anodic layers was the same, namely Pb O2 and PbSO4 . After co mparing the XRD patterns in BE and BEF solutions, it was found that the intensity of PbO 2 peak was lower in BEF solution, indicating that the presence of fluoride ions inhibited the growth of PbO2 . In BEFMn solution, the intensity of PbO2 was much lower than that in BEMn and BEF solutions, indicating that the PbO 2 concentration in its

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inner layer was very low. Similarly, the presence of chloride ions also decreased the concentration of PbO2 . In BEClMn solution, the intensity of PbO2 was slightly lower than that in BECl and BEMn

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solutions as well. However, the Pb O2 concentration in BEClMn solution was much larger than that in BEFMn solution. Therefore, it could be inferred that the pres ence of Mn 2+ had smaller impact on the

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composition of anodic layers in chloride-containing solutions than in fluoride-containing solutions.

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Fig. 5. XRD patterns of anodic layers on Pb-Ag anodes formed through 72-hour galvanostatic electrolysis in different electrolytes. 3.5. Anodic potentials

Anodic potential is a crucial factor to evaluate the energy consumption of EW process. Besides, it also provides information on growth process and stability of anodic layer (Clancy et al., 2015). It was proved earlier in this study that in the presence of Mn 2+, the anodic layer consisted of an external layer and an inner layer. The external layer was in the fo rm of MnO 2 /PbO2 -PbSO4 /MnO2 . Due to the weak contact between MnO2 and PbO2 -PbSO4 , and contact between the external layer and the inner layer, the anodic layer was relat ively unstable. When the surface state or structure of the anodic layer changed, the anodic potential varied accordingly. Therefore, the anodic potential variation of Pb -Ag anodes was monitored during polarization to analyze the growth process of anodic layers and characterize the stability of anodic layers. Fig. 6 showed the anodic potential variat ion of Pb-Ag anodes in H2 SO4 solutions with Mn 2+ and

ACCEPTED MANUSCRIPT fluoride ions. As shown in Fig. 6(a), in BE solution, the anodic potential descended rapidly in the beginning. When the polarization lasted about 5 hours, the anodic potential leveled off, indicat ing that the anodic layer reached a relat ively steady state. In BEF solution, the anodic potential variation was similar to that in BE solution. At the end of 72-hour polarization, the anodic potential of Pb-Ag anode in BEF solution was about 10 mV h igher than that in BE solution. In BEMn and BEFMn solutions, the anodic potential was relatively low in the beginning, which could be attributed to the depolarizat ion effect caused by the oxidation of Mn 2+. Afterwards the anodic potential ascended rapidly until the polarization lasted more than 12 hours. Notably, in the presence of Mn 2+, the anodic potential was characterized by oscillat ion, which indicated the instability of anodic layer. At the end of polarization,

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the anodic potential in the presence of Mn 2+ was much higher than that in BE and BEF solutions. Fig. 6(b) showed the influence of Mn 2+ on the anodic potential variat ion of Pb-Ag anodes in

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H2 SO4 solutions with fluoride ions. As Mn 2+ concentration increased, the potential oscillat ion amp litude increased remarkably, indicat ing decreasing stability of anodic layer. Interestingly, in the

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presence of Mn2+, potential steps appeared on the potential-t ime curves. These potential steps were related with the detachment of anodic layer, especially the external layer. After the detachment, the thickness of anodic layer decreased, which largely decreased the resistance of anodic layer. Therefore,

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the potential instantly stepped down. Afterwards the anodic potential ascended slowly due to the

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recovery of anodic layer.

Fig. 6. Anodic potential variation of Pb-Ag anodes during 72-hour galvanostatic electrolysis in 160 g L-1 H2 SO4 solutions with fluoride ions and Mn 2+.

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Fig. 7 presented the anodic potential variation of Pb-Ag anodes in H2 SO4 solutions with Mn 2+ and chloride ions. In BECl solution, the anodic potential variation was similar to that in BE solution. At the end of 72-hour polarization, anodic potential was about 30 mV lower than that in BE solution. The reason for lower anodic potential in BECl solution was offered in our previous work (Zhong et al., 2015b). In chloride ion-containing solution, the presence of Mn 2+ increased the average anodic potential and caused the oscillation as well. In BEClMn solution, several spikes appeared on the potential-t ime curves. Co mpared with the potential steps in BEFMn solution, these spikes in BEClMn solution showed smaller oscillation amplitude. In BEClMn solution, the anodic potential suddenly dropped; however, it could recover quickly. This suggested that these s pikes could be caused by the detachment of a fraction of anodic layer, rather than the detachment on a large scale. Therefore, in H2 SO4 solutions with Mn 2+, the presence of fluoride ions was relat ively mo re detrimental than chloride ions in the aspect of stability of anodic layer. This could be explained by the anodic layer structure, as shown in Fig. 2 and Fig. 3. As demonstrated above, the external part of anodic layer in BEFMn and BEClMn solutions presented MnO2 /PbO2 -PbSO4 /MnO2 structure. During the galvanostatic electrolysis,

ACCEPTED MANUSCRIPT once PbO2 -PbSO4 was in contact with electrolytes, the passivation of this area occurred. Furthermore, the electronic contact between MnO2 and PbO2 -PbSO4 became weak, and the adhesion strength between MnO2 and PbO2 -PbSO4 greatly descended. Consequently, the anodic layer detached fro m the anodic layer bulk. As shown in Fig. 2 and Fig. 3, the amount of PbO 2 -PbSO4 in BEClMn solution was much s maller than that in BEFMn solution. The contact area between MnO 2 and PbO2 -PbSO4 was smaller than that in BEFMn solution. This would benefit the stability of external part of anodic layer. Consequently, it was spikes rather than potential steps that appeared on the potential-time curves in BEClMn solution. Fig. 7(b) presented the influence of Mn 2+ concentration on the anodic potential variat ion of Pb-Ag

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anodes in H2 SO4 solutions with chloride ions. As Mn2+ concentration increased, the number of potential spikes increased, so did the oscillat ion amp litude. At the end of 72-hour galvanostatic, the anodic

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potential also increased as the Mn 2+ concentration increased, wh ich might be ascribed to the increasing thickness and resistance of anodic layer. The larger thickness of anodic layer in electrolyte with higher Mn 2+ concentration also increased the possibility of detachment of anodic layer, wh ich could exp lained

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the larger oscillation frequency and larger number of potential spikes as Mn 2+ concentration increased. As mentioned above, the presence of Mn 2+ presented similar effect on the anodic potential

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variation of Pb-Ag anodes in fluoride/chloride-containing electrolytes. In general, the anodic potential in the presence of Mn 2+ was higher than that in the absence of Mn 2+ due to the larger thickness and resistance of anodic layer in Mn 2+-containing solution. Additionally, in the presence of Mn 2+, anodic

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potential curves were characterized by oscillat ion. Co mparatively, the stability of anodic layer in fluoride-containing electrolyte was lower than that in chloride-containing electrolyte. Though the presence of Mn 2+ relieved the corrosion of metallic substrates in fluoride/chloride-containing solutions, Mn 2+ of high concentration deteriorated the detachment of anodic layers, wh ich might incur pollution

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controlled at a proper level.

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of cathode products. Therefore, during zinc electro winning, the concentration of Mn 2+ should be

Fig. 7. Anodic potential variation of Pb-Ag anodes during 72-hour galvanostatic electrolysis in 160 g L-1 H2 SO4 solutions with chloride ions and Mn 2+.

4. Conclusions In this study, surface morphology, inner structure, phase composition of anodic layers, corrosion morphology and anodic potential of Pb-Ag anodes in H2 SO4 solutions containing fluoride/chloride ions and Mn 2+were investigated and compared with those in solutions without Mn 2+. The influence of Mn 2+ on anodic layer property and corrosion behavior of Pb-Ag anodes was evaluated. The main conclusions were as follows:

ACCEPTED MANUSCRIPT (1) In the presence of Mn 2+, anodic layer was divided into two layers. The external layer showed MnO2 /PbO2 -PbSO4 /MnO2 structure, wh ile the inner layer adhered to the metallic substrate and presented a flatter and more compact surface than that in solutions without Mn 2+. (2) The presence of Mn 2+ would relieve the corrosion of metallic substrate in electro lyte with fluoride/chloride ions. This positive effect could be attributed to the formation of an external anodic layer. The external layer provided ext ra protection fo r the metallic substrate through decreasing contact possibility between electro lyte and metallic substrate. In chloride-containing solution, Mn 2+ also relieved the corrosion of Pb-Ag anode due to the larger thickness of inner layer. (3) The main phase composition of inner layer was PbO 2 and PbSO4 . The presence of Mn2+

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decreased the concentration of PbO2 in the inner layer in fluoride/chloride-containing solution. Relatively, Mn 2+ showed smaller influence on the PbO2 concentration of the inner layer in

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chloride-containing electrolyte than that in fluoride-containing electrolyte.

(4) The anodic potential in the presence of Mn2+ was higher than that in the absence of Mn2+ due to the larger thickness and resistance of anodic layer in a Mn 2+-containing solution. In addition, the

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anodic potential was characterized by oscillation in the presence of M n 2+, and the stability of anodic layer in fluoride-containing electrolyte was comparat ively lo wer than that in a chloride -containing

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electrolyte.

Acknowledgements

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This research was supported by National Natural Science Foundation of China (No.51164011, No.51374240, No. 51564021 and No. 51704130) and Hunan Provincial Natural Science Foundation,

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China (No. 13JJ1003).

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ACCEPTED MANUSCRIPT Highlights Anodic layer consists of external and inner layers in the presence of Mn2+.

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Mn2+ relieves corrosion of Pb-Ag anode in the presence of fluoride/chloride.

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Mn2+ increases the thickness of inner layer in chloride containing solutions.

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Anodic potential shows oscillation characteristic in the presence of Mn2+.

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