Rare Metals 2009,28(03),277-283

Improvement in corrosion resistance of magnesium coating with cerium treatment

Samia Ben Hassen Latifa Bousselmi Patrice Ber¬Ìot El Mustafa Rezrazi Ezzeddine Triki

Unit¨¦ de Recherche Corrosion & Protection des M¨¦talliques,ENIT,BP 37,Tunis-Belv¨¦d¨¨re,Tunisia

Institut UTINAM,CNRS UMR 6213,Universit¨¦ de Franche-Comt¨¦,16 route de Gray 25030 Besan¬Ìon Cedex,France

Laboratoire Traitement et Recyclage des Eaux,CERTE,Borj Cedria,Tunisia

×÷Õß¼ò½é£ºPatrice Ber¬Ìot E-mail: patrice.bercot@univ-fcomte.fr;

ÊÕ¸åÈÕÆÚ£º24 October 2008

Improvement in corrosion resistance of magnesium coating with cerium treatment

Abstract£º

Corrosion protection afforded by a magnesium coating treated in cerium salt solution on steel substrate was investigated using open circuit potential, polarization curves, and electrochemical impedance spectroscopy (EIS) in 0.005 M sodium chloride solution (NaCl). The morphology of the surface was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The cerium treated coating was obtained by immersion in CeCl3 solution. The results showed that the corrosion resistance of the treated magnesium coating was improved. The corrosion potential of the treated coating was found to be nobler than that of the untreated magnesium coating and the corrosion current decreased significantly. Impedance results showed that the cerium treatment increased corrosion protection. The improvement of anti-corrosion properties was attributed to the formation of cerium oxides and hydroxides that gave rise to a physical barrier effect.

Keyword£º

magnesium coating; cerium; corrosion protection; impedance spectroscopy;

Received£º 24 October 2008

1. Introduction

Magnesium coating has been used as sacrificial anodes for a variety of metals [ 1, 2, 3] .However,its protection was time-limited.This led to alternative methods in order to improve the anti-corrosion properties of magnesium coating Surface treatments were commonly applied in order to increase the corrosion resistance.

Recent researches,for more environmentally acceptable alternatives in surface modification processes,have corroborated that one of the most promising surface treatments is based on the use of rare earth elements,mainly cerium species.

Some researches demonstrated that treatments with aqueous solutions of rare earth salts inhibited the corrosion of aluminium alloys [ 4, 5, 6] ,steels [ 7, 8] ,galvanized steels [ 9, 10] ,and magnesium and its alloys [ 11, 12, 13] .

The effect of the counter ion is very important on the formation of cerium coating and even on the anti-corrosion properties of the coated systems.However,it depends greatly on the metal substrate [ 13, 14, 15] .Different cerium salts were tested in previous researches.Debal¨¤et al. [ 11] demonstrated that cerium conversion coating obtained from cerium chloride solution improved the corrosion resistance of AZ63 magnesium alloys.Rudd et al. [ 12] reported tha magnesium dissolution was reduced in a pH 8.5 buffer solution with a pre-treatment in Ce(NO₃)₃ solution.More recently,Montemor et al. [ 13] found that the pre-treatment of AZ31 Mg alloy in different cerium salts,nitrate,sulphate and chloride,produced a significant decrease of both anodic and cathodic currents in 0.005 M NaCl solution.The marked results were obtained with treatment in CeCl₃ solution.Corrosion inhibition by cerium salts was generally attributed to the formation and the precipitation of cerium oxides and/or hydroxides over cathodic sites on the surface [ 12] .These precipitates gave rise to a blocking effect.Two mechanisms were proposed in the literature to explain the formation of the cerium conversion layer.The first one proposed by Kobayashi et al. [ 15] argued that the oxidation of Ce(III)into ceria(Ce O₂)was the result of the increase in the local p H due to oxygen reduction reaction.The second theory was advocated by Mansfeld et al. [ 16] who postulated that during the formation of the conversion layer Ce(III)was oxidised into Ce(IV)which precipitated into CeO₂.

The aim of this work was to improve the corrosion resistance of an electroplated magnesium coating by means of treatment in cerium chloride solution Ce Cl₃.The corrosion resistance was studied using electrochemical and analytical investigations.

2. Experimental

The substrate was prepared from carbon steel whose chemical composition(wt.%)is as follows:C<0.07,Si<0.002,Mn 0.38,P 0.021,S 0.004,Cr<0.004,Cu 0.002,Al0.03,V<0.004.The carbon steel electrodes were embedded in resin leaving an exposed surface area of 0.29 cm².The exposed surfaces were polished up to 1000 SiC paper,then cleaned in distilled water and methanol.The surfaces were then washed with anhydrous acetone and dried under a stream of air.The magnesium electrodeposition was realised with the continuous current process as mentioned in previous work [ 1] .The electrodeposition bath was a Grignard reagent solution of methylmagnesium chloride in tetrahydrofuran obtained from Sigma-Aldrich as 3 M.The obtained magnesium coated sample was then immersed in a cerium chloride solution of 2¡Á10^-2 mol?L^-1 under gentle agitation for 2 min at room temperature.A 0.005 M sodium chloride solution(open to air)was used as the corrosion electrolyte.

The electrochemical cell consisted of a three-electrode pyrex glass cell.A saturated calomel electrode(SCE)was used as the reference electrode and platinum was used as the counter electrode.

The polarization curves were realized by means of a potentiostat-galvanostat Radiometer Copenhagen PGZ 402model,piloted by software Voltlab4.They were plotted after3 h of immersion to have the reproducibility.All potentials quoted were on the SCE scale.The electrochemical impedance spectroscopy measurements were carried out using a Parstat 2273 model with a ZSimpWin version 3.20 software.The measuring frequency ranged from 10² Hz down to 10^-2Hz and the signal amplitude was 10 m V.All the experiments were performed at the corrosion potential.Scanning electron microscopy(SEM)was carried using a JEOL type5600 associated to an energy dispersive spectroscopy(EDS)FONDIS model.X-ray diffraction(XRD)measurements were obtained by an ADVANCE BRUKER with a Bragg Brentano configuration,the sample was in horizontal position,and the speed depended on the time of acquisition.The wavelength was equal to 0.154056 nm.

3. Results and discussion

3.1. Coating characterization

The cerium conversion coating produces a pale yellow surface on the sample due to the presence of Ce(IV)species on the conversion layer as mentioned in previous work [ 12, 17] .

Fig.1(a)shows SEM observation of an untreated Mg coated steel specimen.The morphology is smooth with cracks.The Mg coated steel treated in cerium chloride solution has a quiet different morphology(Fig.1(b)):there is a formation of an additional layer of cerium on the magnesium coating.This cerium layer presents a ribbed morphology with a large crack network.The EDS analysis shows the same iron atomic fraction of both treated and untreated Mg coated specimens and a decrease in Mg atomic fraction from81%to 36.3%.This decrease is accompanied with an increase of the oxygen amount from 3.4%to 33.9%and of the chloride amount from 2%to 10.1%.The cerium atomic fraction is about 6.8%.It is possible to assume that the layer is constituted principally of magnesium-cerium oxideshydroxides.Furthermore,the reduction reactions increase the interfacial p H which leads Ce³⁺to undergo hydrolysis and cerium hydroxides/oxides to precipitate through the following reactions [ 20, 21] :

Ce(OH)₄ is known to be more stable than Ce(OH)₃;consequently Ce(III)will oxide into Ce(IV)when exposed to air [ 22] .This is in agreement with our EDS results showing tha the O/Ce ratio is approximately equal to 4 which leads to suppose the precipitation of Ce(OH)₄ as it was postulated in other work [ 18, 19] .

The Ce/Cl ratio is different from 1/3,so Cl is not related to Ce,but rather to Mg.Indeed,the chloride ion is well known as a Lewis base stronger than H₂O or OH^-and therefore an efficient complexing agent for the Mg²⁺ion.This may explain the presence of chloride in the EDS spectrum.

The X-ray diffraction patterns of the untreated and treated Mg coated specimens are shown in Fig.2.As for cerium treated sample(Fig.2(b)),it can be indicated that the main composition of the surface coating is Ce O₂.The pattern shows the presence of 3 peaks of ceria.The most importan peaks of CeO₂ are found at 2¦Èof 28.62¡ãand 47.20¡ã,corresponding to the diffraction from the(111)plan and from the(220)plan,respectively.

3.2. Electrochemical characterization

3.2.1. Open circuit potential

Open circuit potential curves shown in Fig.3 were obtained during immersion in 0.005 M Na Cl solution.The open circuit potential evolution of both treated and untreated Mg coated steel is quite similar to that of pure magnesium [ 1] .The potentials rise then reach stable value after 2 h of immersion.

Fig.1.SEM images(a,b),EDS spectra(c,d),and atomic fraction of elements in the materials(e,f)of untreated Mg coating(a,c e)and treated Mg coating(b,d,f).

Fig.2.XRD patterns of magnesium deposit untreated(a)and treated(b)in cerium solution.

The potential of untreated Mg coated steel follows a rising variation to reach finally a value about-810 m V vs.SCE.The potential of cerium treated Mg coated steel shows a slow rise during the first 2 h of immersion.The potential then reaches a steady value of around-550 m V vs.SCE.These two obtained values are enclosed between that of pure Mg(-1600 mV vs.SCE) [ 1] and that of steel(-493 mV vs.SCE).According to these results,the corrosion potential of the Mg coated specimen treated in cerium chloride solution is nobler than that of the untreated Mg coated specimen.

Fig.3.Open circuit potential curves of untreated Mg coating(¡ð)and treated Mg coating(¡ö)in 0.005 M NaCl solution.The steel substrate curve(¡ø)was plotted for comparison.

3.2.2. Anodic behaviour

Fig.4 shows the anodic polarization curves obtained in0.005 M Na Cl solution.The corrosion current densities of both untreated and treated Mg coated samples decrease with respect to that of the steel substrate.

The cerium treated specimen has the lowest corrosion current density and the corresponding potentials are shifted towards more anodic values with respect to the untreated Mg coated steel.In comparison with the steel substrate,the anodic curves of both untreated and treated Mg coated specimens present a plateau indicating effective corrosion protection.

In the first part of the anodic curve of the untreated Mg coated system,only magnesium oxidation occurs.When the potential attains close value to the steel corrosion potential(E_corr(steel)),dissolution of the steel substrate begins,the current density increases,and the anodic curve of the Mg coated system fits well with the steel anodic curve for E>0m V vs.SCE [ 1, 2] .

Fig.4.Anodic polarization curves of untreated Mg coating(¡ð)and treated Mg coating(¡ö)in 0.005 M NaCl solution.The steel substrate curve(¡ø)was plotted for comparison.

The Mg coated steel treated in cerium chloride solution has a passivity plateau larger than that of the untreated Mg coated system even when E>E_corr(steel).The corresponding current density values decrease.It is only starting from E>122 m V vs.SCE that the steel substrate corrosion begins.Table 1 lists the curve-fitted results from the potentiodynamic polarizations experiments.The pitting potential(E_pit)of treated Mg coated steel increases to a more noble level with respect to the untreated sample.The difference between the corrosion potential and the pitting potential can be used to evaluate the pitting corrosion resistance of the materials [ 23] .The mathematics equation can be expressed by

The values of?E are also presented in Table 1.?E values of the untreated Mg coated system and the treated Mg coated system are about 547 and 663 m V,respectively,so the pitting corrosion resistance increases when the cerium treatment is applied.The corrosion current density determined by Tafel method decreases by three orders of magnitude for the untreated Mg coated system and by four orders of magnitude for the cerium treated one with respect to the steel sample.

3.2.3. Cathodic behaviour

The cathodic polarization curves obtained in 0.005 M Na Cl are reported in Fig.5.As for coated specimens,it seems that the plateau due to the diffusion controlled oxygen reduction is poorly defined.This may suggest that the cathodic reaction is controlled by the water reduction or the occurrence of an additional cathodic reaction simultaneously with the oxygen reduction.This behaviour can be attributed to the formation of¡°aqua-complexes¡±that behave as a porous barrier against the oxygen diffusion via the interface [ 22] .The curve corresponding to the cerium treated specimen shows a considerable ennoblement in the corrosion potential with respect to the untreated Mg coated system as already presented in Fig.3.The slope of the cerium treated electrode is found to be low.This behaviour suggests slightly faster reduction kinetics for the untreated Mg coated electrode [ 12] .

  ÏÂÔØԭͼ

Table 1.Fitted results of polarization curves of untreated and treated Mg coated steel systems in 0.005 M NaCl solution

3.3. Electrochemical impedance spectroscopy(EIS)

To get more information about the corrosion behaviour occurring on the samples,impedance data were recorded after 24 h of immersion in 0.005 M Na Cl solution.Nyquist and Bode plots of steel,untreated and treated Mg coated specimens are shown in Fig.6.

The feature of the uncoated steel curve was a slightly depressed semicircle [ 1, 24] .The equivalent circuit of the steel was a simple Randles circuit R_s(Q_dlR_ct)where R_s is the solution resistance,Q_dl is the constant phase element(CPE)which is related to the non homogeneity of the steel surface,and R_ct is the charge transfer resistance.The impedance of CPE is given by the following equation:

in which Y₀ is the CPE constant;f is the frequency,Hz;j=-1;and the exponent n=¦Á/(¦Ð/2),¦Ábeing the phase angle of the CPE,radians.

Fig.5.Cathodic polarization curves of untreated Mg coating(¡ð)and treated Mg coating(¡ö)in 0.005 M NaCl solution.The steel substrate curve(¡ø)was plotted for comparison.

Fig.6.EIS plots of untreated Mg coating(¡ð)and treated Mg coating(¡ö)after 24 h of immersion in 0.005 M NaCl solution.The steel substrate curve(¡ø)was plotted for comparison(symbol:test;¡ªfitting).(a)Complex plane plot;(b)Bode phase representation;(c)Bode modulus representation.

10^-5 S?cm^-2?sⁿ,and 0.74,respectively.

The impedance diagrams have the same feature for both untreated and treated Mg coated systems and present one capacitive loop.It is indicated that the total impedance of the cerium treated electrode is higher than that of the untreated one.To compare these two specimens,impedance diagrams were fitted using the equivalent circuit(EC)of Fig.7.The suggested EC fits well with the physical process.The two time constants of the EC are well separated(4 orders of magnitude).

In the equivalent circuit,R_s is the solution resistance,Q_f is the constant phase element(CPE),R_f is the film resistance,R_ct is the charge transfer resistance,and C_dl is the double layer capacitance.

Table 2 presents the values of the electric circuit parameters obtained from the best fit to the impedance diagrams of the untreated and treated Mg coated specimens when tested

Fig.7.Equivalent circuit used to fit impedance spectra in Fig.6.

in 0.005 M sodium chloride solution after 24 h of immersion The n values are situated between 0.6 and 0.8 revealing the heterogeneity of the surface.It seems that the film resistance R_f of the cerium treated specimen is two orders of magnitude higher than that of the untreated specimen.The C_dl values are between 10 and 13?F?cm^-2,these values suggest that the reason of this capacitance could reasonably be attributed to the double layer with the passive film.

  ÏÂÔØԭͼ

Table 2.Values of the electric circuit elements obtained from the best fit to the impedance diagrams in 0.005 M sodium chloride solution after 24 h of immersion

The charge transfer resistance R_ct of the cerium treated system increases by a factor of 21 with respect to the untreated specimen and by a factor of 115 or more with respect to steel.This behaviour is consistent with the lower passive current density and the ennoblement of the pitting corrosion potential as observed in Fig.4.This behaviour is explained by the cerium barrier effect due to the formation of cerium oxides/hydroxides as mentioned by reactions(1)-(3).

4. Conclusion

The formation of a cerium conversion layer on magnesium coated steel by treatment in CeCl₃ solution was studied Results showed a significant improvement of the anti-corrosion properties of the magnesium coating.The SEM observations showed the formation of a cerium layer with a ribbed morphology mainly composed of cerium oxides and/or hydroxides.The XRD analysis confirmed the presence of ceria(Ce O₂)in the cerium conversion layer.The conversion layer led to a considerable ennoblement in the corrosion potential.Both anodic and cathodic corrosion current densities were reduced remarkably.The passive plateau was considerably enlarged and the pitting corrosion potential was shifted towards a more noble value.EIS results demonstrated that the corrosion resistance of the cerium treated Mg coating was increased at least for a short period of time.

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