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Volume 3, Paper 8


Corrosion Behaviour of Mo and Mo Containing and Mo Free Steels In Basic and Acidic Media

Z. Mısırlıoğlu, S.Uneri, 
University of Ankara, Department of Chemistry, Faculty of Science, Besevler Ankara , TURKEY

M. L. Aksu*
Gazi University Faculty of Education, Department of Chemistry, Besevler Ankara , TURKEY,

* The author to whom any correspondence should be addressed

Abstract

In this study the corrosion behaviour of Mo and Mo containing (AISI type 316) and Mo free (AISI type 304)�� steels were comparatively investigated. It was found that the passivation� of Mo starts at �550 mV (all potentials are reported with respect to SCE) in basic and �1400 mV in acidic media. The transpassive dissolution in both these media was found to take place at 400 mV. The addition of NaCl into the media� causes the evolution of oxygen at 1700 mV. The anodic dissolution potential of Mo containing steel 316 was found to be 100 mV more anodic than non-Mo containing 304 steel.� It was concluded that the passive layer of Mo consisted of Mo2O3 and MoO2. The metal undergoes to dissolution precipitation mechanism in the transpassive dissolution region.�

Introduction

The susceptibility of iron against corrosion makes it necessary to be used in the form of stainless steel. The corrosion resistance of steels is provided by a very thin oxide layer (passive film) formed upon them. Although this layer readily dissociates in various media, it quickly renews itself. The addition of elements such as Cr, Ni and Mo increases the corrosion resistance of steels to a great extent.

Tomashov et al [] observed that the elements such as Mo, Si, V and Re prevented the formation of pitting corrosion in steels. Lizlovs and Bond [] proved that the addition of Mo into high purity stainless steel decreased their critical current densities. Rockel [] on the other hand examined the effect of addition of Mo to Fe18-Cr steel and reported that the Mo had a significant effect in the anodic dissolution region but the effect of Mo in the transpassivation region was not so big. Sugimoto and Sawada [] investigated the effect of alloying elements in steels. They reported that there was no pitting corrosion after the addition of Mo till the transpassive dissolution started and the corrosion potential shifted to more anodic direction. The Mo content of the passive film showed a linear increase with the Mo content of the steel. Lu et al. [] investigated the behaviour of steel in a medium containing MoO42- and found that Mo present in the passive layer was both in 6+ (as [MoO(OH)2]2- and MoO42-) and 4+ (as MoO2) oxidation states. It was proposed that MoO42- increased the corrosion resistance of the passive film by two-pole action or rectification[].

It was observed that the formation of pitting is largely prevented in solutions containing 50 �100 mM molybdate ions�[]-[]. In acidic solutions molybdate ions forms polymolybdate as follows:

MoO42- + H+ MoO3(OH) -

MoO3(OH) - + 2H2O MoO(OH)5-

This ion then forms a polymeric structure forming oxy bridges

2MoO(OH)5- [(HO)4OMo-O-MoO(OH)4]2-+ H2O

which have an important �role in the prevention of pitting . The solubility products of the compounds formed by the oxy-ions of molybdate and metal ions are very small. This forms a protective barrier against pitting.

Nishumura and Kudo[] concluded that the addition of Mo decreases the number and/or the activity of the active sites upon the surface and increases the resistance of� AISI 304 and 316 steels against the corrosion . Hashimoto et al [] gave the average structure of the surface film upon Fe-30Cr-2Mo steel as Fe(II)0.22 Fe(III)0.37 Cr(IV)0.27 Cl0.02 O1.36 (OH)0.46 . 0.61H2O. According to Boucherit [] it was a FeMoO4 layer, which prevents the pitting corrosion. Goetz and Landolt [] investigated the corrosion behaviour of pure Cr and its alloys containing varying amounts of Mo and they observed that the active dissolution peak decreased by the addition of Mo into Cr. The addition of Mo above %5 resulted in the disappearance of dissolution peak. Mischler et al. [] stated that Fe-Cr alloy was subjected to pitting corrosion in acidic media while Fe-Cr-mo alloys displayed no such phenomenon. Olefjord [] reported that in acidic media there was a� formation of� a very thick layer of MoO3 but the protective ability of this layer was very low. Olefjord and Elfstr�m [] investigated the corrosion rates of 21.7Cr-13.3Ni-3.6Mo (steel 1) and 18.7Cr-11.7Ni-1.7Mo(steel 2) steel and pure Cr, Fe, Ni and Mo and determined the following order: iCr >iFe> iNi> isteel 1 >isteel 2>iMo� .Kodama and Ambrose [] concluded that molybdate ions caused the repassivation of Fe and prevented the formation of pitting. According to Stout� et al.[] molybdate ions form a protective chemisorbed or salt film. Yang et al. [] reported that the molybdate ions form a protective layer of FeMoO4 and repassivate the steel. Wanklyn [] on the other hand claimed that the solid state reaction forming MoO2 upon the surface hinders the local corrosion. Schneider et al. [] examined the role of Mo ion in Mo containing steels in acidic Cl- containing media and claimed that the decreasing effect of Mo upon pitting corrosion was due to the formation of highly insoluble molybdenum-chloride compounds.

Molybdenum is used in wide range of fields ranging from catalysis to dyes and corrosion inhibitors. The literature related to Mo is so numerous :

Massing and Roth [] investigated the behaviour of Mo electrode in various media and determined that the molybdates are highly insoluble in a medium with low acidity. They also reported that Mo is resistant to HCl due to Mo5+ and Mo6+ oxides formed upon its surface.

Besson and Drautzburg [] also claimed that the surface of molybdenum is covered with coloured film of Mo6+ in both acidic and alkaline media. K�nig and G�hr [] determined three potential regions for Mo in acidic media. According to these workers there was hydrogen evolution in the first region with E <-1.0 V(SCE) , the formation of MoO2 with the passage of a small current in the second region ranging between �1.0 and 0.4 V(SCE) and the coverage of the surface with a layer of MoO3 with dissolution of MoO2 in the third region which ranged between 0.4 V to 4.0 V (SCE) with the passage of a considerable anodic current. Wikstrom and Nobe [] found that the capacitance of the first region was constant. They explained this phenomenon with the coverage of the surface by MoO2 in this region. Johnson et. al.[] found that the Mo surface was rapidly covered with Mo2O5 in both acidic and basic media which subsequently oxidised to MoO3. They claimed that MoO3 formed upon the surface is hydrolysed and dissolved as molybdate. Hull [] determined that there were three oxide films [Mo(OH)3,MoO2, MoO3] formed in basic and two oxide films (MoO2, MoO3) in acidic solutions

Sakashito and Sato [] investigated the ion selection behaviour of MoO42- films. They found that the MoO42- hydrated Fe(II) membrane acts as a cation selector� in solution containing one valent cations and anion selector in solution containing multi-valent cations. They claimed that the corrosion is hindered under cation selective membranes. They called the membranes having adsorbed multi-valent anions at one side and adsorbed cations at the other,� �bipolarised� membranes.

The aim of this study is to investigate the anodic dissolution and the corrosion behaviour of Mo and� the steels containing and not containing Mo in� acidic and basic media using cyclic voltammetry and comparing our findings with those given� literature. It is hoped that this work will contribute to our understanding of the effects of Mo on corrosion phenomena.

Experimental

There were four different electrodes used in this study: Fe (tempered at 1100°C with a diameter �of 0.3 mm and �a purity of 99.999%), AISI 304 ( with a diameter of 0.5 cm) , AISI 316 ( with a diameter of 0.8 cm) and Mo ( with a diameter of 1 mm). The Mo electrode with a purity of 99.99% was obtained from Johnson & Mathey Company. It was mechanically cleaned and embedded in a polyester resin. The cylinder shaped� steel electrodes (AISI 304 and 316) were bolted to brass rods, placed in Teflon and the gap between Teflon and the steel was filled with polyester resin. The nominal compositions of AISI 304 and 316 steels are as follows: 

Type of steel

C(%)

Si(%)

Mn(%)

P(%)

S(%)

Ni(%)

Cr(%)

Mo(%)

AISI 304

0.060

0.580

1.180

0.028

0.009

8.750

18.310

-------

AISI 316

0.054

0.670

1.360

0.030

0.005

11.160

17.210

2.210

The reference and counter electrodes were saturated calomel electrode (SCE) and a Pt plate with area of 1cm2. All the solutions were prepared with bi-distilled and de-ionised water and analytical grade reagents.

The electrodes were subjected to following mechanical, chemical and electrochemical pre-treatment prior to each experiment: The electrodes were mechanically polished with emery paper having various roughness in order to remove any oxide layer which may form upon the electrode surface. They were then thoroughly washed with acetone and bi-distilled water. They were finally placed into the cell�and  kept at -600, -1000 and �1500 mV for fifteen minutes each in order to eliminate anodic oxide films upon the surface. The solutions were purged with purified N2 throughout the experiments. The cyclic voltammograms were taken with Wenking Model VSC72 voltage scan generator with a scan rate of 1 mV/s .

Results and Discussion

Figure 1 shows that the highest dissolution peak for Fe electrode is observed in H2SO4, which decreases and shifts to more negative potentials in Na2SO4. There is no anodic dissolution peak observed in NaOH and the system goes directly to the transpassive region.

Figure 1 The cyclic voltammetric curves of Fe electrode in various media. 

Figure 2 shows the polarisation curves of 304 and 316 stainless steels in H2SO4. The anodic current observed around 1000 mV in 316 steel is due to the dissolution of Mo6+ species []. Same increase in current was also observed in 0.5 M Na2SO4 and� attributed to the dissolution of Mo6+ species as well (figure 3).

Figure 2 The cyclic voltammetric curves of 304 and 316 stainless steel� in 0.05 M H2SO4

Figure 3 The cyclic voltammetric curves of 304 and 316 stainless steel in 0.5 M Na2SO4

The polarisation curves of 304 and 316 steels in NaOH (figure 4 ) revealed that Mo containing 316 steel started to dissolve at� 100 mV more anodic than 304 steel. The increase in the base concentration increases the Mo dissolution

Figure 4 The cyclic voltammetric curves of 304 and 316 stainless steel in 0.5 M NaOH.

When the cyclic voltammetric curves of Mo in H2SO4 is examined (Figure 5) it is seen that the transpassive dissolution starts at �400 mV (SCE). The current passing at the transpassive region increases as the acid concentration is increased. When the current goes above 600 mV (SCE) a black layer formed upon the surface, which peels off with evolution of a gas. The fact that there is no cathodic current passing below 400 mV in dilute H2SO4 and Na2SO4 (Figure 6) was attributed to the reduction of MoO22-. In fact it was stated that the reduction of MoO22- ions takes place at potentials under 200 mV (SCE) [],[],[].

Figure 5 The cyclic voltammetric curves of Mo in 1 M H2SO4.

Figure 6  The cyclic voltammetric behaviour of Mo in 0.1 M Na2SO4

The effect of the Cl- ions does not have a significant effect upon the shape of the curves but the initial potential of the passivation� region changes a lot. It was seen that the passivation� region starts at �1200 mV in the neutral NaCl solution (figure 7) and at �550 mV in acidic solutions (Figure 8). The increase in the Cl- concentration causes the peaks IIa and IIa* to become much more distinctive and the oxygen evolution to take� place (the black region with wild oscillations in Figure 9).

Figure 7 The cyclic voltammetric curves of Mo in 0.1 M HCl and 0.3 M NaCl.

Figure 8 The cyclic voltammetric curves of Mo in 0.1 M HCl.

Figure 9 The cyclic voltammetric curves of Mo in 3M NaCl. The oxygen evolution �is clearly apparent.

The effect of number of scans carried out was displayed in Figure 8. There is a peak II1 around 600 mV in the first scan and the curve then reaches to the transpassive region. This peak appears with the same magnitude but at more anodic potentials in the second scan (II2).� There is only one peak in the second scan, which corresponds to the potential where there was the highest current density in the first scan (i.e. peak II3). It is clear that there is a formation of a less soluble layer upon the electrode after the first scan.

The appearance of peaks IIa and IIa and the disappearance of IIa in the second scan is indicative of a film with good covering properties. Therefore, there is formation of highly insoluble� Mo-O-Cl or Mo-Cl layers mentioned by Stout et al.[] . The increase of Cl- species causes the big oscillations resulting from O2 evolution (Figure 9)

Figure 10 The effect of addition of NaCl on the anodic behaviour of Mo in basic media.

The figures 10 and 11 show that the addition of NaCl to the reaction medium does not change the general appearance and the transpassive dissolution potentials of Mo. Therefore the dissolution phenomenon is not dependent very much� upon the pH of the media. All these potentials are in perfect compliance with those given in literature [],[], [].

Figure 11 The effect of addition of NaCl upon the anodic behaviour of Mo in acidic media.

The behaviour of Mo electrode in basic solutions is displayed in Figure 12. In alkaline solution the transpassive dissolution starts at �200 mV. The low valent oxide formed at cathodic potentials converts into high valent one as the potential is taken to more anodic values. as the NaOH concentration is increased it is seen that the peak Ia increases and IIa and IIa* decrease. There is a linear relation between the current density of peak Ia and NaOH concentration which indicates that the reaction is controlled by the diffusion of OH- ions . The dissolution takes place as HMoO4- in acidic and as MoO42- in basic media [],[]. The peaks IIIg, IIg and Ig observed in the reverse scan become more apparent as the NaOH concentration is increased. The increase in NaOH concentration causes the anodic dissolution dominate to film formation. In concentrated NaOH solution the transpassive dissolution potential is seen to shift to more cathodic values. This potential which is �160 mV in 0.1 M NaOH� shifts to �450 mV in 4M NaOH solution. In concentrated alkaline solution the pores of the films get larger and Mo dissolves as MoO42- through those pores []. All the phenomena in the anodic region take place through dissolution- precipitation mechanism.

Figure 12 The effect of NaOH concentration upon the anodic behaviour of� Mo.

The effect of addition of NaCl to NaOH solution is displayed in Figure 13. The hydrogen evolution region obtained in these media corresponds to IIIa peak in pure NaOH solutions. The increase in NaOH concentration causes the disappearance of IIIa peak. peak Ia* is only observed in solutions with the addition of 3 M NaCl and 5 M NaCl. This peak is related to the formation of Mo5+ species since MoO(OH)3 or Mo2O5.3H2O do not dissolve in alkaline solution.

Figure 13 The effect of addition of NaCl upon the anodic behaviour of Mo in� 0,1 M NaOH with added NaCl.

In all the solutions containing H2SO4, HCl, NaCl, Na2SO4 and NaOH there is a rapid dissolution starting from 400 mV. The basic film protecting the surface in the passive region is MoO2, which converts into less protective Mo6+ film in the transpassive region. There is both a dissolution of Mo as Mo6+ ions and the partial coverage of the surface with less protective layer of MoO3 at the same time till the potential of peak IIa is reached in accordance to the dissolution-precipitation reaction [] ,[].

Figure 14 shows that the excessive instability in current corresponds to the O2 evolution region in the media having high NaCl concentration. This phenomenon is not observed at low NaCl concentrations. It is also seen that the increase in NaCl concentration decreases the cathodic limiting current density at 2400 mV. This indicates that the film becomes much more protective.� Lu and Clayton [] claimed that there occurs Mo2O3, MoO2 and MoO(OH)2 in the passive region ranged between �180 mV and 600mV. In the transpassive region Mo is dissolved as HMoO4-. In other words there is the formation of Mo2O3, MoO2 and MoO(OH)2 in the passive region ranging up to 600 mV and the formation of Mo2O5 and MoO3 above this potential. In this region the metal undergoes dissolution �precipitation as outlined above. The MoO3 layer formed as a result of this mechanism gives peak IIa as it gets thicker. The metal also dissolves as HMoO4- up to peak IIa� (Figure 13 ). The sharp decrease in the current density after peak IIa shows that the surface is covered with highly insoluble Mo-Cl and Mo-O-Cl type compounds [], [].

Figure 14 The effect of NaCl concentration upon the anodic behaviour of Mo in 0.1 M HCl.

Povey and Metcalfe [] argued that the passive region is mainly consisted of Mo2O3 and Mo(OH)3 type oxides . They proved that there was a formation of MoO2, Mo3+ and Mo4+ at the potential where transpassivation starts and the amount of Mo6+ increased above this potential

Hull [] on� the other hand claimed that the first wave is related to the formation of Mo2O3 or Mo(OH)3 , the second wave is due to MoO2 and the third wave results from the formation of Mo2O5 or non stoichiometric oxides .

This study revealed that Mo stays passive in a large potential range in basic media starting from �1200 mV. Since IIa peak in acidic media is due to the formation of Mo6+ species , the peak Ia observed in basic media must be due to the conversion of MoO2 into MoO(OH)2. Habazaki et al (1992) [] showed that there was a dissolution peak of Mo between �400 and �200 mV. The absence of this peak in our study can be attributed to the fact that the cathodic current (H2 evolution) is much bigger than the anodic dissolution. The peak given by Habazaki [] corresponds to peak Ia in basic media.

Figure 15 compares the polarisation behaviour of Mo and 316 steel in 0.05 M H2SO4 + 3 M NaCl. It is seen that the passive region of 316 steel corresponds to the anodic dissolution Mo. The O2 evolution region in Mo on the other hand is the region where 316 steel goes into transpassive dissolution.

Figure 15 The cyclic voltammetric behaviour of� Mo in 0.05 M H2SO4, Mo in� 0.05 M H2SO4+ 3.00 M NaCl  and AISI 316 steel 0.05 M H2SO4+ 3.00 M NaCl.

The transpassive dissolution takes place at 0.9 V in both 304 and 316 steels. The active dissolution potential of Cr is around �800 mV. The passivation� potential of Mo on the other hand is �50 mV, which shows a decrease in the active dissolution potential of steel. Cr and Mo oxides together protect the surface up to the transpassivation� potential of Mo. Since the transpassive dissolution potential of both steels is the same (0.9 V) the main protection must be provided by Cr2O3. The selective dissolution of Fe in the transpassive region makes the protective layer rich in Cr and the steel behaves as high Cr steel which are highly resistant against the corrosion. The main function of Mo in the protection of steels is that the formation of MoO2 and MoO(OH)2 covering the defects upon the surface. According to Schneider et al. there is a formation of Mo-Cl salts at the bottom of the pits which are highly insoluble and cannot be washed away [].

The results of this study are the following:

  1. The passivation� region of the Mo electrode, subjected to cathodic pretreatment at-1500 mV, starts at �550 mV. The transpassive dissolution takes place at 400 mV. The transpassive dissolution region in both H2SO4 and Na2SO4 is the same.
  2. In HCl solutions passivation� starts at �550 mV and the transpassive dissolution takes place at potentials higher than 400 mV. The transpassive dissolution manifests itself as peaks IIa and IIa�. The general shape of the curves obtained in NaCl solutions is the same but the initiation of passivation is shifted to �1100 mV.
  3. The behaviour of Mo in NaOH solutions however is different than those obtained in acidic media. In alkaline media there occurs peaks Ia and IIIa in addition to peak IIa observed in acidic solutions. The peaks IIa and IIIa merge and the peak Ia decreases as the concentration of NaOH is increased.
  4. The addition of NaCl in all the media investigated caused the increase in peak IIa and decrease in peak Ia. The increase in the concentration of NaCl added resulted in O2 evolution around 1700 mV with manifestation of wild oscillation in current. The magnitude of current observed in the second passivation� region at 2300 mV changed in the order of 0.1 M HCl < 0.1 M HCl + 3.0 M NaCl < 0.1 M NaOH + 3.0 M NaCl < 0.05 M H2SO4 + 3.0 M NaCl << 3.0 M NaCl. This indicates that the precipitation of 6+ valent Mo-Cl, Mo-O-Cl or Mo-OH-Cl at peak IIa had a protective effect upon the surface.
  5. The passivation� in the passive region is the result of very thin Mo2O3 and MoO2. the transpassive dissolution starts at peak Ia with the formation of MoO(OH)2 . The peak IIa� observed after the addition of NaCl is due to the formation of Mo2O5. The increase in current between IIa� and IIa is due to the dissolution of the film as HMoO4- in the acidic and MoO42- in the basic media and the coverage of the surface with MoO3 film which has poor protective qualities. The peak IIa is the result of the coverage of the surface with MoO3 film.
  6. When the corrosion behaviour of AISI 304 and 316 steels are compared it is observed that 316 steel dissolves more than 304 steel at the transpassive dissolution potential of� -900 mV. this is due to rapid dissolution of Mo in H2SO4 solutions. The selective dissolution of Fe in steels makes the surface rich in Mo, which dissolves as HMoO4- or MoO42-. these ions act as inhibitor and repair the damaged oxide film upon the surface. The adsorption of MoO42- ions on to the surface forms a double pole membrane upon it, which prevents Cl- ions reaching the surface. Also the formation of highly insoluble O-Cl, Mo-OH-Cl or Mo-O-Cl compounds formed at the bottom of the pits increase the protective effect of the oxide film.

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