M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 0 0 – 1 4 0 6

Electrochemical characterization of non-chromate surfacetreatments on AZ80 magnesium

Wen C. Saya, Chien Chon Chena,⁎, Sheng-Jen Hsiehb

aDepartment of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan 10643bDepartment of Engineering Technology and Department of Mechanical Engineering, Texas A and M University, College Station,TX 77843-3367, United States

A R T I C L E D A T A

⁎ Corresponding author.E-mail address: chentexas@gmail.com (C.C

1044-5803/$ – see front matter © 2008 Elsevidoi:10.1016/j.matchar.2007.12.007

A B S T R A C T

Article history:Received 2 August 2007Received in revised form13 September 2007Accepted 29 December 2007

Surface treatments were applied to AZ80 magnesium alloy (8%Al–92%Mg) within non-chromate environments to develop its anti-corrosion characteristics. The non-chromateprocess involved chemical conversion using a phosphate and hydrofluoric acid (HF) coating,followed by anodizing. The chemical conversion generated good adhesion between thesubstrates and the anodic film. The anodizing process formed a 30 μm-thick film on thesurface of the AZ80. Electrochemical properties were obtained using Pourbaix diagram,polarization, and EIS (Electrochemical impedance spectroscopy) analyses to characterizethe effects on the treated AZ80. Tafel polarization andmodels of equivalent circuits revealedthat the corrosion rate of anodized AZ80 was reduced from 14.44 to 0.28 mpy with passive-film resistance increasing from 428 to 54,200 Ω.

© 2008 Elsevier Inc. All rights reserved.

Keywords:AZ80 magnesium alloyNon-chromateAnodizingTafel polarizationAC impedance

1. Introduction

The use of AZ80 magnesium alloy for structural applications isgrowing rapidly [1–3]. Recently, AZ80 has been used in the 3Cindustries (Computer, Communication, and Consumption elec-tronics production) because of its high strength-to-weight ratios(density: 1.738 g/cm3), anti-electromagnetic field properties andhigh thermal conductivity (122W/mK). Nevertheless, due to itschemical reactivity (standard electrode potential: −2.363 V, SHE)and inferior corrosion resistance, the alloy has had limitedpractical applications [4]. Chromate films are most commonlyused for surface treatment of AZ80 alloy, but emerginglegislation prohibits the use of chromates in many countries.Alternate surface treatments are urgently needed [5–7]. Thefollowing types of protective coatings on AZ80 have beenpatented [8,9]: (1) oils and waxes for temporary protection,(2) chemical conversion for temporary protection or as a paintbase, (3) anodized coatings to improved paint base or wear

. Chen).

er Inc. All rights reserved

resistance, (4) paints and powder coatings to protect againstcorrosion or to preserve appearance, (5) metallic plating forappearance, surface conductivity, and protection against corro-sion. Among those patents, chemical conversion limits a stand-alone interior, while anodizing results in a porous coating thatneeds to be sealed. Anodizing using non-chromates haspotential to be the primary surface treatment for AZ80. Themost common patented anodizing electrolytes are alkalihydroxide and fluoride [5,6,9]. Anodizing generates an anti-corrosive oxide film on the surfaces after chemical conversionproviding a strongly adhesive film. However, very few articlesand patents to date have focused on the electrochemicalproperties of chemical and anodic methods when applied toAZ80 simultaneously.

In this study, electrochemical properties were observedfrom a Pourbaix diagram, polarization, and impedanceanalyses [10–15] to characterize the effects of non-chromatesurface treatments on AZ80.

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mailto:chentexas@gmail.comhttp://dx.doi.org/10.1016/j.matchar.2007.12.007

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2. Experimental Methods

2.1. Chemical Conversion and Anodization

Non-chromate processes, involving chemical conversion usinga phosphate and HF coating followed by anodizing, were usedfor coating specimens as shown in Fig. 1. Specimens ofmagnesium alloy (AZ80) plates of size 50 mm2×5 mm were cutand abraded using 1500-grit SiC papers and then pickled in 8%vol. H2SO4 solution at 25 °C for 25 s. The solution used for thechemical conversion was 95% vol. H3PO4 with 5% vol.HF. TheAZ80was converted at 160 °C for 5 s. The porous surfaces of theconverted specimens were then sealed in a 3% vol. HF solutionat 25 °C for 45 s and anodized in a 10-cmdiameter stainless steelcylinder. The specimens were placed in the cylinder (whichfunctioned as a cathode) and 200 V was applied for 40 min. Thespecimens were then anodized within a stirred electrolyte witha composition of KOH: 25 g/l, K2SiO3: 50ml/l, KF: 10 g/l with a pH7.5 at 8 °C. The anodized film, which had a composition ofmagnesium hydroxide (Mg(OH)2) was sealed by immersion in100 °C H2O(l) for 5 min to obtain a porous structure.

Fig. 1 –Flowchart of experimental process.

Fig. 2 –Schematic diagram of experimental setup.

2.2. Electrochemical Measurements

The electrochemical properties of the specimens were deter-mined using a classical three-electrode arrangement [16]. Fig. 2shows theconfiguration,which consists of a 3-cmdiameter glasscylinder fixed to the surface of a working electrode by an O-ringcovering an area of 7.1 cm2, a platinum sheet (2 cm×2 cm)counter electrode, and a saturated calomel reference electrode(SCE). The electrolyte was 0.9% NaCl solution. The polarization

Fig. 3 –Porous film on the sample surface after chemicalconversion treatment of AZ80.

Fig. 4 –SEM image of chemical conversion film on AZ80surface after sealing treatment.

Fig. 5 –Tafel curves comparison. Note that the Tafel curve ofAZ80 through chemical conversion was more noble incomparison with the original magnesium AZ80.

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curves were measured electrochemically using an EG&G Model273A Potentiostat/Galvanostat. Electrochemical impedancespectroscopy (EIS)datawereobtainedusingSolartronequipment(a 1286 electrochemical interface and a 1255 frequency responseanalyzer) [17],whichcovereda frequencyrangeof5×103 to106Hzwith10mVrmssignal amplitude. Theelectrochemical dataweresimulated and written in Microsoft Excel programs.

3. Results and Discussion

3.1. Chemical Conversion

Phosphoric acid is the primary constituent of the solution usedfor chemical conversion to generate phosphate conversioncoatings. The coatings provide lubrication and wear resistance,facilitating cold forming, short-term resistance against mildcorrosion, and adhesion in oxide-metal lamination applications[9]. The phosphate coating herein was used to increase theadhesion between AZ80 alloy and its anodic oxide film. Thesurface morphology of AZ80 appeared porous after conversion,as shown in Fig. 3. The porous layer was sealed immediatelyusing sealing solution at 100 °C. The poreswere sealed and theirSEMmorphologies are shown in Fig. 4. The converted AZ80 hadhigher nobility than the original specimen, which decreased itscorrosion rate from 14.44 (mpy) to 4.86 (mpy) with the opencircuit voltage range of −1.52 to −1.48 V (SCE), as shown in Fig. 5.

3.2. Anodization

The pH of the electrolyte and the amount of voltage applied arecrucial parameters in controlling AZ80 anodizing. A Pourbaixdiagram was constructed for water-immersed AZ80 beforeanodizing. Fig. 6 describes the relationship between over-potential of AZ80 and its thermodynamical stability based onNernst's equations [18]. TheMg+2 region is anactive region,withits SHE above −2.7 V and the pH of the solution has to be lessthan 11.47 at 25 °C.

Mg→Mgþ2 þ 2e−

Fig. 6 –Constructed Pourbaix diagram for the hydration ofmagnesium at constant temperature 25 °C.

ΔE1 ¼ 0:0295� log½Mgþ2�−2:363: ð1Þ

Further, Mg+2 reacts with H2O to form a thick oxide of Mg(OH)2, which covers the surface of the specimen and evapo-rates hydrogen from the solution.

Mgþ2 þ 2H2O→MgðOHÞ2 þ 2Hþ

pH ¼ 0:5 � ð16:95−log½Mgþ2�Þ: ð2Þ

When the over-potential is below −2.7 V (SHE), a thick layeris also formed; however, this oxide is less protective to AZ80.

Mgþ 2H2O→MgðOHÞ2 þ 2Hþ þ 2e−

ΔE2 ¼ −1:862−0:059pH: ð3Þ

At 25 °C, the formation enthalpy of Mg(OH)2 is −142,580 cal./gmol,which is less than that ofMgO (−136,136 cal./gmol). SinceMg(OH)2 is more thermodynamically stable than MgO in the pre-sence of water; the oxide can be easily hydrated according toreaction (4).

MgO þ H2O→MgðOHÞ2: ð4Þ

The diagram indicates that increasing the applied voltageand reducing the solution pH is essential for AZ80 anodizing.

Fig. 7 –SEM image of AZ80 surface after anodizing and sealing.

Fig. 9 –Curve comparison of open circuit potential vs. time forAZ80, chemical conversion, and anodizing.

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Regarding the composition of the anodizing solution,insufficient alkaline hydroxide increases the decompositionvoltage and produces a rough film, such that the anodizedvoltage does not reach the desired level. In contrast, insufficientfluoride degrades the coating, causing sparks to be generated.Kobayashi [6] found that insufficient silicatewill not formahighdensity oxide on the metal surface; rather, it will precipitatewith other constituents of the solution. Fig. 7 shows a goodquality dense anodic film covered the AZ80 surface with anodicelectrolyte of potassium silicate 50 ml/l, potassium hydroxide25 g/l and fluoride 10 g/l at pH 7.5. A sealed glassy oxide filmcomposed mainly of forsterite (2MgO·SiO2) formed on thesurface of the specimens. The thickness of the anodic film, asshown in Fig. 8, was approximately 30 μm.

Fig. 8 –SEM examination of cross-section of AZ80 anodiclayer (about 30 μm).

3.3. Electrochemical Characteristics

The effects of the converted and anodized AZ80 were electro-chemicallymeasured in the 0.9%NaCl solution. The curve of theanodized specimen rose to a higher potential of −0.45 V (SCE),whereas the original and converted AZ80 remained at the lowpotential of −1.53 V (SCE), as illustrated in Fig. 9. The substrate,along with the converted layer retained numerous pore-generating structures that caused anions (such as Cl− and OH−)to react with AZ80 easily and lower the open circuit potential. Incontrast, the dense layer of (Mg(OH)2) on the surface of theanodized AZ80 resulted in an insulated barrier and a higherpotential. Fig. 10 shows a plot of the potentiodynamic polariza-tion curves and their Tafel approximation. A shift of the Tafelline toward theupper left of thediagramindicatesan increase inthe resistance to corrosion and the corrosion current density,and a decrease in corrosion behavior. The anodized specimen

Fig. 10 –Comparison of Tafel curves of AZ80, chemicalconversion, and anodizing.

Fig. 12 –ComparisonofAZ80 andanodizingNyquist diagrams.

Table 1 – Electrochemical results from Tafel polarizationand EIS testing

Results AZ8Ooriginal

Chemicalconversion

Anodizing

Tafel polarization resultsCorrosion voltage

(Ecorr, V, SCE)−1.52 −1.48 −0.460

Corrosion current density(icorr, μA/cm2)

7.62 2.35 0.37

Polarization resistance(Rp, Ω)

229.2 359.4 103837

Corrosion rate (C.R., mpy) 14.44 4.86 0.28

EIS resultsSolution resistance (Rso, Ω) 72 77 75Charge transfer resistance

(Rct, Ω)98 105 402

Double layer capacitance(Cdl, nF)

0.53 0.62 1.3

Passive capacitance (Cpf, μF) 50 32 0.56Passive resistance (Rpf, Ω) 428 829 54,200

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revealed the highest nobility with a corrosion current density of0.37 μA/cm2, which was much lower than that of the originalAZ80 (7.62 μA/cm2). The respective corrosion rates, 0.28mpyand14.44mpy, are listed, alongwith other corrosiondata, inTable 1.Polarization resistance can be used to estimate the corrosionresistance at electrochemical equilibrium.

Bode and Nyquist diagrams were obtained using electro-chemical impedance spectroscopy (EIS) testing. Fig. 11 showsthat the Bode diagram of anodized AZ80 has higher impedancethan the original and converted specimens. EIS represents theway in which charges are transferred or impeded betweenvarious phases, which may be studied by the status andbehavior of interfaces among conducting phases [19]. Fig. 12presents Nyquist diagrams of semicircles of anodized andoriginal AZ80 that correspond to their double layers and thepassive films. The semicircle of original AZ80 is much smallerthan that of the anodized film. The reactions across interfacesmight thus be explained with reference to the equivalentcircuits shown in Fig. 13(a). The resistance of the solution, andthe resistance and capacitance of the double layer between thetesting solution and the AZ80 substrate, are denoted as Rso, Rct,

Fig. 11 –Comparison of Bode diagrams of AZ80, chemicalconversion, and anodizing.

and Cdl, respectively. Other components of the resistor and thecapacitor are often considered to be associated with theinterfaces, resistance of passive film (Rpf), and capacitance ofpassive film (Cpf). A porous or non-condensed oxidation layershould establish anequivalent circuit as shown inFig. 13(b). Theanodic film with dense oxide that coated the AZ80 could berepresented using an equivalent circuit as shown in Fig. 13(c).

The Appendix A contains the equations (from a simulationprogramwritten in Microsoft Excel) associated with the circuit-models inFig. 13(a), (b) and (c). Table1 shows thatanodizedAZ80had thehighest valuesof charge transfer resistance (Rct) of 402Ωand double layer capacitance (Cdl) of 1.3 nF, which suggests thatthe bulk Cl− and OH− ions had relative difficulty going throughthedouble layer outside of the anodized film. The excess anionsand Mg+2 cations retained inside the double layer resulted in ahigh value of Cdl. The passive capacitance (Cpf) would be

Fig. 13 –Equivalent circuits: (a) without passive film on thesubstrate surface; (b) with a porous or non-dense passive filmon the substrate surface, and (c) with a dense passive film onthe substrate surface.

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inversely proportional to its resistance; that is, a lower Cpfestimate suggests that fewer ions remain between the interfaceof the dense oxide film and the substrate. The passive-filmresistance (Rpf) is an important parameter for evaluating thecorrosion behavior of specimens. The larger surface areas ofporous oxide in the converted and original specimens resultedfrommore ions remaining on the interface, and consequently, alower Rpf. Table 1 shows Rpf values of anodized, converted, andoriginal AZ80 of 54,200, 829 and 428 Ω, respectively. Moreover,the lower 0.56μFpassive capacitanceof theanodizedspecimensexplains why the adhesive strength between its active film andsubstrate would be the strongest.

4. Conclusions

The anti-corrosion characteristics of AZ80 magnesium alloywere improved in non-chromate processes involving chemi-cal conversion with anodizing. Anodizing generated a 30 μmthick layer as a result of chemical conversion providing anadhesive film on the interfaces. The anodizing film, whichhad a SEM morphology of a scale pattern exhibited a slowcorrosion rate of 0.28 mpy. Anodizing results in a higherresistance and a decrease in capacitance values of passivefilm. ThecorrespondingTafelpolarizationcurvesandsimulatedequivalent circuits showed evidence of fewer ions remaining inthe film,which explainswhy the adhesive strength between theactive film and substrate would be very strong.

Appendix A. Mathematical calculation of equivalentcircuits for Nyquist diagram analysis

(a) Without passive film on the substrate surface

Z ¼ Rso þ ZC;dlWRct� �

Z ¼ Rso þ Rct � ZC;dlRct þ ZC;dl¼ Rso þ

Rct � 1jxCdlRct þ 1jxCdl

¼ Rso þRct

jxCdl1þ jxRctCdl

jxCdl

¼ Rso þ Rct1þ jxRctCdlð Þ¼ Rct

1þ jxRctCdlð Þ� 1� jxRctCdlð Þ

1� jxRctCdlð Þ

¼ Rso þ Rct � jxR2ctCdl

1þ xRctCdlð Þ2¼ Rso þ Rct

1þ xRctCdlð Þ2" #

� j xR2ctCdl

1þ xRctCdlð Þ2" #

ZRE ¼ Rso þ Rct1þ xRctCdlð Þ2

" #ZIM ¼ xR

2ctCdl

1þ xRctCdlð Þ2" #

:

(b) With a porous or non-dense passive film on the substrate

Rso þ CdlW Rct þ CpfWRpf� �� �� �

Z ¼ Rso þRpfRctZC;dl þ ZC;pfZC;dlRct þ RpfZC;pfZC;dl

Rpf þ Cpf þ Cdl

¼

RpfRct1

jxCdl

þ Rct1jxCpf

þ 1jxCdl

þ Rpf1jxCpf

þ 1jxCdl

Rpf þ1

jxCdlþ 1jxCpf

¼ Rso þRpf þ Rct þ jxCpfRpfRct

1� x2CpfCdlRpfRct þ jx CpfRpf þ CdlRpf þ CdlRct� �:

Let A=CpfCdlRpfRct; B=CpfRpf+CdlRpf+CdlRct; C=Rpf+Rct;D=CpfRpfRct

Z ¼ Rso þ Cþ jxD1� x2Að Þ þ jxB

¼ Rso þCþ jxDð Þ½ 1� x2A� �� jxBÞ

1� x2Að Þ2þx2B2

¼ Rso þ C� x2AC� jxBCþ jxD� jx3ADþ x2BD

1� x2Að Þ2þx2B2

¼ Rso þ Cþ BD�ACð Þx2

1þ B2 � 2Að Þx2 þA2x4 þ jD� BCð Þx�ADx3

1þ B2 � 2Að Þx2 þA2x4

ZRE ¼ Rs þ Cþ BD� ACð Þx2

1þ B2 � 2Að Þx2 þ A2x4 ZIM ¼D� BCð Þx�ADx3

1þ B2 � 2Að Þx2 þ A2x4:

(c) With a compact passive film on the substrate

Z ¼ Rso þ ZC;dlWRct� �þ ZC;pfWRpf� �

Z ¼ Rso þ Rct � ZC;dlRct þ ZC;dlþ Rpf � ZC;pfRpf þ ZC;pf

¼Rsoþ Rct1þ xRctCdlð Þ2

" #� j xR

2ctCdl

1þ xRctCdlð Þ2" #( )

þ Rpf1þ xRpfCpf

� �2" #

� jxR2pfCpf

1þ xRpfCpf� �2

" #( )

¼Rsoþ Rct1þ xRctCdlð Þ2

" #þ Rpf

1þ xRpfCpf� �2

" #( )� j xR

2ctCdl

1þ xRctCdlð Þ2" #

þxR2pfCpf

1þ xRpfCpf� �2

" #( )

ZRE ¼ Rso þ Rct1þ xRctCdlð Þ2

" #þ Rpf

1þ xRpfCpf� �2

" #( )

ZIM ¼ xR2ctCdl

1þ xRctCdlð Þ2" #

þxR2pfCpf

1þ xRpfCpf� �2

" #:

1406 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 1 4 0 0 – 1 4 0 6

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Electrochemical characterization of non-chromate surface treatments on AZ80 magnesiumIntroductionExperimental MethodsChemical Conversion and AnodizationElectrochemical Measurements

Results and DiscussionChemical ConversionAnodizationElectrochemical Characteristics

ConclusionsMathematical calculation of equivalent circuits for Nyquist diagram analysisReferences