Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

Electrochemical Corrosion of Molybdenum Electrodes in Soda-Lime

Glass Containing Antimony

M.-Ch. Bölitz*, R. Völkl*, H. Traxler**, R. Holzknecht**, U. Glatzel*

* Metals and Alloys, University Bayreuth, 95440 Bayreuth, Germany

** PLANSEE SE, 6600 Reutte, Austria

Abstract

The electrochemical corrosion of Mo electrodes was investigated at 1435°C in an Sb-containing soda-

lime glass with the chemical components SiO2-Al2O3-Na2O-K2O-MgO-CaO-BaO-B2O3. For comparison

experiments were also carried out with Sb-free glass. Corrosion was only significantly observed in Sb-

containing soda-lime glass, suggesting that the polyvalent element Sb is the cause for the corrosion.

Analytical scanning and transmission electron microscopy reveal indeed the presence of pure Sb on the

surfaces and along grain boundaries of electrodes. This indicates a reduction of Sb cations to metallic

Sb which is accompanied by an oxidation of metallic Mo to Mon+ cations.

The frequency dependence and rate of corrosion has been determined for two electrode materials, pure

Mo and Mo-ZrO2, that were reacted with the antimony-containing soda-lime glass at frequencies of

50 Hz, 500 Hz, and 1000 Hz. Corrosion can already be observed in experiments without electrical

current, and is only electrolytically augmented in 50 Hz experiments. An additional electrolytic corrosion

could not be detected in the 500 Hz and 1000 Hz experiments. Glasses reacted with pure Mo electrodes

contain more Mo than glasses reacted with Mo-ZrO2 electrodes, which can be attributed to the

stabilization of grain boundaries due to ZrO2. The Mo concentration in glasses increases linearly with

time, indicating that the corrosion process is surface controlled.

Keywords

Soda-lime glass, corrosion, transmission electron microscopy; microstructure, electrodes, molybdenum

Introduction

High quality glasses are industrially melted in huge tank furnaces. The electric heating and melting of

glass is standard practice in the glass industry. The heating of electrodes provides an easy process

control and allows us to reach higher temperatures than in conventional gas-fired furnaces. The melt is

more homogeneous, improving the quality and transparency of the glass. Due to the strong

environmental consciousness that has emerged in recent years, and the tightening of environmental

laws, this method also has become an increasingly important option for environmental issues. A large

Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

number of glasses, in particular optical glasses, are melted with electrical resistance heating. Different

electrode materials are used, e.g. refractory metals, such as molybdenum, tungsten and their alloys, or

platinum, particularly PtRh- or PtIr-alloys.

Materials made of molybdenum are known for their excellent corrosion resistance in glass melts. Due to

the low price of molybdenum the high melting point of 2623 ° C and a mechanical stability up to

temperatures of over 1600 ° C it is often used as electrode material. Despite these excellent properties,

molybdenum is known to be strongly corroded, resulting in a contamination of the glass. Even if it does

not lead to total loss of electrodes, small amounts of molybdenum produce an undesired coloring of the

glass melt. For the industry the coloration of the melt and the loss of the metallic contact materials mean

a considerable economic damage.

Previous studies concentrated mostly on the electrochemical analysis by anodic and cathodic

polarization and the corrosion protection by means of molybdenum passivation [2-6]. This non-metallic

protective layer prevents corrosion of the base material. Very little work has been carried out with regard

to the microstructural characterization of electrode materials and corrosion products. A lack of

knowledge also exists regarding the kinetics of corrosion. There are few data on corrosion rates [7-8]

and the frequency dependence of rates is hardly investigated [1].

A great influence on the corrosion of molybdenum electrodes has the chemical composition of the melt.

Since molybdenum is not a noble metal, it easily oxidizes in the presence of almost all polyvalent

elements and complexes in the melt, e.g. Sb, Zn, As, Ni, Mn, Co, or Pb, which have a higher standard

potential than molybdenum [7, 9-11]. Studies have shown the high influence of antimony on the

corrosion of molybdenum electrodes. Yamamoto et al. [1] performed experiments in a slicate glass with

an antimony content of 0.0 to 0.34 wt%. The experiments were carried out at 50 Hz, 1435 ° C, and a

fixed current density. The investigations showed that the corrosion rate increases with increasing current

density and increasing antimony concentration. The corrosion rate increases linearly with the antimony

concentration, regardless of whether the experiments were performed with or without current flow.

Experiments with an antimony-free glass show no corrosion of molybdenum electrodes. Yamamoto et al.

[1] have suggested that the primary corrosion process consists in an oxidation of metallic molybdenum

and a simultaneous reduction of antimony cations. Thereby the applied alternating current enhances the

corrosion of the Mo electrodes due to reduction of antimony cations, and the formation of Mo cations.

This study aims at assessing the corrosion behavior and at identifying the corrosion products of

molybdenum and an molybdenum base alloy in soda-lime glass by means of microstructural

investigations using with scanning (SEM) and transmission electron microscopy (TEM).

Experimental

Figure 1 shows a schematic sketch of the corrosion furnace. The furnace is heated via two Super-

Kanthal heating elements. The crucible, filled up with about 650 g glass, is placed on top of a disc which

is continuously rotated during experiments. The furnace can be topped by two different caps. If the glass

is melted and the temperature is reached, the standard cap is replaced by a cap with water-cooled

electrode holders. This cap including the electrodes and a thermocouple is moved downward until the

working electrodes immerse completely into the melt. In our experiments, the heating and cooling rate

were set to 60 ° C per hour. The experiments were performed in a two-electrode arrangement in soda-

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Seminar 2013

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b)

Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

Figure 5: (a) Scanning transmission electron microscopy (STEM) image and (b) electron diffraction pattern of an Sb-inclusion in the glass and (c) Mo- and (d) Sb elemental mapping. The lamella is enriched with Mo.

To identify the lamellar phase EDX element distribution maps and spectra were acquired with TEM-EDX.

see Fig. 5 c) and d). The globular inclusions consist of pure Sb and the electron diffraction pattern

matches the structure of crystalline Sb (Fig. 5 b, and Table II).

Table II: The table shows the observed and calculated lattice spacings of pure antimony. The corresponding selected area electron diffraction pattern is shown in Figure 5 b). For the calculation of the interplanar spacings, lattice constants a = 430.5 pm and b = 112.5 pm were used for hexagonal Sb.

hkl Interplanar spacing (observed) [pm]

Interplanar spacing (calculated) [pm]

003 390 375

011 369 354

012 321 311

The lamella is composed of Mo and Sb. Quantification of the EDX spectra reveal the intermetallic phase

Mo3Sb7 which is known as the only intermediate phase in the Mo-Sb system [12]. According to the phase

diagram Mo3Sb7 is formed at a temperature below 800°C, thus it is generated by segregation during

quench at the end of the experiments.

a) b)

c)  d)

[100]

Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

Glass analyses taken by ICP-OES show clear differences in the molybdenum concentrations for the pure

Mo-electrodes and the Mo-ZrO2-electrodes. Microstructural observations suggest that pure molybdenum

is more resistant to corrosion than Mo-ZrO2-electrodes, because less antimony penetrated into the

molybdenum electrode, while antimony migrated well along grain boundaries into the Mo-ZrO2

electrodes.

However, ICP-OES analyses show that after 24 h in contact with Mo-ZrO2 only 600 ppm of molybdenum

were dissolved in the glass. The molybdenum content in experiments with pure Mo-electrodes is

1100 ppm and thus almost twice. This result is confirmed by experiments run for 72 h. The glass melted

with Mo-ZrO2–electrodes contains 2000 ppm molybdenum after 72 h, while glass in contact with pure

Mo-electrodes contains 3100 ppm molybdenum (Fig. 6). Thus the Mo concentration is reduced by

approximately 35% using Mo-ZrO2-electrodes. The increase of Mo contents in the glass at both

electrode materials generally shows that the corrosion proceeds at constant rates. Presumably, the

lower concentration of molybdenum in the glass melted with Mo-ZrO2-electrodes is due to the fact that

molybdenum is dissolved in metallic antimony that in turn is largely associated with grain boundaries in

the electrode surface.

Fig. 6: Mo concentration in soda-lime-reference glass without electrical current (160 ppm) and after experimental periods of 24 h and 72 h.

The linear increase of the Mo-content in the glass with time for both electrode materials shows that the

corrosion is a surface-controlled process, which takes place at a constant rate R [13]. Probably less

molybdenum is dissolved in experiments with ZrO2 stabilized Mo electrodes because a large fraction of

elemental Sb with the dissolved molybdenum remains on grain boundaries. The lower contamination of

glass when using Mo-ZrO2-electrodes agrees with experiments of Martinz et al. [8].

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Grain size

18

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n experimen

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s were carrie

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n the Mo co

n

Seminar 2013

ed out at dif

es with a co

f 3 cm to 1 c

all experime

act with the

l frequencie

Mo-electrode

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ow identical

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oncentration

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Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

frequency. The corrosion rates are a clear function of frequency. The Mo concentration is the highest at

50 Hz with 360 ppm (Fig. 8).

Figure 8: The red data points show the Mo contents in Sb-containing soda-lime glass for experiments without current, at 50 Hz, 500 Hz and 1000 Hz. The black points correspond to the Mo contents in antimony-free glass for experiments without current and at 50 Hz. The black horizontal line corresponds to the Mo concentration in the original glass. The figure shows that the frequency and the Sb content greatly affect the corrosion.

In the experiment without electrical current, corrosion can already be observed due to the strong base

character of molybdenum (Mo concentration is about 160 ppm). An additional electrolytic corrosion is

detected at 50 Hz, increasing the Mo concentration to about 360 ppm. At higher frequencies of 500 Hz

and 1000 Hz the Mo concentrations in glasses are however indistinguishable within error limits from the

concentration in an experiment without current. To avoid an undesired electrolytic corrosion it should be

sufficient to perform melting experiments at a frequency of 500 Hz and higher.

Pure Mo Electrodes in Antimony-Free Soda-Lime Glass

The results of experiments with antimony containing soda-lime glass have shown that antimony is the

polyvalent ion responsible for the strong corrosion of molybdenum. The redox reaction between

antimony and molybdenum causes a deposition of elemental antimony and dissolution of Mo cations in

the melt. To study the corrosion in the absence of antimony, two further experiments were performed

with an antimony-free soda-lime glass. Both experiments were carried out for 24 h in contact with pure

Mo electrodes. The experiments were run without current and at a frequency of 50 Hz.

The corrosion without electrical current is very low, such that the electrode surface remains smooth

(Fig. 9). The Mo content in the glass corresponds to the concentration of the original glass. There are

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o particles d

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olybdenum

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with anothe

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18

that are rem

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mall Mo part

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8th Plansee S

moved from

ored.

ode after 24 h

ticles are tra

less colored

n to the orig

in the melt t

th other gla

ment distributiohe melt consist

b

M

Seminar 2013

the surface

ours in contac

ansformed

d than antim

ginal glass (

taking place

ass compon

on map of the t of pure moly

b)

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e and incorp

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from the su

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e. The Mo-p

ents (Fig. 1

molybdenum-bdenum.

porated into

mony-free gla

urface into t

ining glass.

parently, the

particles wh

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M 26

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Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

Discussion

The results of this study demonstrate that the underlying mechanism of the corrosion of pure Mo and Mo

base alloys is a redox reaction between metallic Mo and polyvalent cations (in this case Sb cations) in

the silicate melt. Metallic Mo is oxidized and dissolved in the silicate melt, while Sb cations are reduced

to Sb metal, which in turn can dissolve additional metallic Mo.

Our investigations concentrated on a frequency of 50 Hz, since electrode corrosion was strongest at this

frequency. Experiments with Mo und Mo-ZrO2 electrodes in soda-lime glass reveal the deposition of

elemental antimony on the electrode surface. In principle Mon+ ions are generated on the anode side,

while at the cathodic side Sb3+ or Mon+ cations can be reduced to metal. The latter depends on the

frequency. At 50 Hz there is sufficient time for the diffusion of Sb3+ ions, such that Sb3+ cations

accelerate to the electrode surface where they can be reduced. At 500 Hz the duration of a half-cycle is

too short, in order to diffuse Sb3+ to the electrode surface or to remove Mon+ cations far enough from it,

such that they deposit again.

Another important observation is that metallic antimony penetrates deeper into Mo-ZrO2 electrodes than

into pure Mo electrodes. We attribute this phenomenon to the observed reduction in grain size and

thereby an increase in internal grain boundaries in Mo-ZrO2 electrodes. In contrast, pure Mo electrodes

show a grain coarsening and thus a reduction in internal grain boundaries, which inhibit the penetration

of metallic Sb deeper into the electrodes. Despite this observation Mo-ZrO2 shows significantly better

corrosion resistance and lower molybdenum contents in the glass than pure Mo electrodes. The reason

is that the finely dispersed ZrO2 stabilizes the grain boundaries such that complete Mo grains can not

detach from the electrode surface as observed for pure Mo electrodes.

Time-resolved ICP-OES analyses (Fig. 6) have demonstrated that the Mo concentrations in soda-lime

glasses in contact with Mo and Mo-ZrO2 electrodes increases linear in time. The slope of the regression

line in Figure 6 allows to determine the dissolution rate R according to following equation [13]:

C(Mo) = C0(Mo) + R t (1)

with C and C0 the Mo concentrations in reacted soda-lime glasses and in the original glass and t the

time. A normalization to the surface area A finally yields the rate constant k:

k = R A-1 (2)

Table III summarizes the calculated corrosion rates and rate constants for Mo and Mo-ZrO2 electrodes.

The data shows that Mo-ZrO2 electrodes dissolve 35% slower than Mo electrodes.

Table III: Corrosion rates and rate constants of Mo and Mo-ZrO2 electrode (r = 0.9 cm) in antimony containing soda-lime glass for a current density of 1 A/cm2, frequency of 50 Hz and a temperature of 1430 ° C. The values are based on 1 kg glass.

Electrode material Corrosion rate R [mg h-1]

Rate constant [g m-2 h-1]

Mo 40,0 43,9

Mo-ZrO2 25,9 28,5

Bölitz, Völkl, Traxler et al. 18th Plansee Seminar 2013 RM 26

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