The Preparation and Corrosion Performance of Self-Assembled Monolayersof Stearic Acid and MgO Layer on Pure Magnesium

Liying Qiao1,2,3,+, Yong Wang1,2, Weilang Wang1, Marta Mohedano3,Caihua Gong1 and Jiacheng Gao1

1College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China2National Engineering Center for Magnesium Alloy, Chongqing University, Chongqing 400044, China3Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Geesthacht 21502, Germany

Self-assembled monolayers (SAMs) of stearic acid (SA) film were developed on pure magnesium with heat-pretreatment in order toimprove both the corrosion resistance and the bioactivity of pure magnesium. A new method using a high concentration of SA solution(10¹1mol/L) is proposed to reduce the process time of the conventional SAMs methods. The effect of ultrasonic cleaning treatment on SAMswas investigated. Contact angle measurements were employed to evaluate the hydrophobic properties of the SAMs. The corrosion resistance andthe bioactivity of the specimens were evaluated under biological conditions (simulated body fluid, 37°C) using potentiodynamic polarizationmethod and long term immersion test. Surface morphology, composition and structure analysis of samples were characterized by the scanningelectron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and attenuated total reflectance fourier transform infrared spectroscopy(ATR-FTIR) after immersion test. The results showed the corrosion resistance of pure Mg was improved by stearic acid-SAMs and the MgOlayer made by heat-pretreatment, the SAMs made in high concentration (10¹1mol/L) of stearic acid solution just for 1.5 h showed betterproperties than conventional SAMs. Furthermore, ultrasonic cleaning treatment led to the formation of more dense and orderly SAMs andimproved the anticorrosion property of SAMs prepared in high concentration of SA solution. Moreover, SAMs-Mg had better bioactivity thanuntreated magnesium. [doi:10.2320/matertrans.M2014117]

(Received April 2, 2014; Accepted May 20, 2014; Published July 11, 2014)

Keywords: self-assembled monolayers, magnesium and its alloys, stearic acid, corrosion resistance, biomaterials

1. Introduction

In recent years, the use of magnesium and its alloys asdegradable implant biomaterials has caused great interestbecause of their excellent biocompatibility and similarmechanical properties to human bones.1­3)

However, the high reactivity and poor corrosion resistanceof Mg and its alloys in chloride-containing biologicalenvironment have limited their practical applications inorthopedics and more efforts are needed to control theircorrosion process in order to keep their mechanical integritybefore the tissue has healed sufficiently.4­6) Besides, althoughthe biocompatibility of magnesium alloys is good, theirbioactivity needs to be improved for promoting the earlytissue healing.7­9)

For these problems, surface modification/coatings issupposed to be one of the most effective approaches of bothcontrolling magnesium degradation and endowing them withother functional properties including bioactivity. Extensiveinvestigations have been focused on this field.7­12) Amongthem, SAMs is a promising way to improve the anticorrosionand the bioactivity of magnesium in one process.

It is well known that one of the characteristics of thebioactivity of a metallic implant for bone reconstruction is itsapatite-forming ability in simulated body fluid, and it hasbeen demonstrated that SAMs are able to induce thedeposition of apatite on the metal substrate.13­17) In thesupersaturated Ca-P solution, the end-group of SAMs reactswith inorganic ion, which induces the heterogeneousnucleation of hydroxyapatite and the apatite can grow upsubsequently. SAMs used in this field are mainly thiols,silanes, carboxylic acids and organic-phosphate,13­18) and

researches on metal are basically limited to the matrix of Auand Ti.

Regarding to the corrosion protection, most of the studiesabout aliphatic acid-SAMs have been done on Ag,19,20)

Fe,21­23) Cu,20) and Al19,20) and their oxides in early days.Recently, a few studies have been reported about theapplication of SAMs to improve the corrosion properties ofMg alloys.24­28) Most of these researches are focused oncarboxylic and phosphonic acids SAMs, especially aliphaticacid-SAMs25­28) as they are environmental friendly and canbe assembled on the metallic oxide directly, unlike thethiolate-SAMs which just have strong bonding with noblemetal. Ishizaki26) prepared alkanoic and phosphonic acidsderived SAMs on AZ31 magnesium alloy by vapor-phasemethod, and Szillies27) studied the adsorption and orientationof carboxylic and phosphonic acids SAMs with differentaliphatic chain lengths. These two papers reported phos-phonic acids SAMs show better anticorrosion than carboxylicacids SAMs. Ng28) prepared the stearic acid film having goodbonding force on hydroxide magnesium formed by hydro-thermal treatment. Salman25) built oleic and stearic acidSAMs with various solvents on AZ31 Mg alloy, and SAMsof oleic acid in ethanol solution obtained the best anti-corrosion property. Actually, there are just few studies aboutthese thin monolayers on magnesium at present, moreover,the research of improving both their anticorrosive andbioactive properties are not yet optimized. In this paper, thestearic acid (SA) with 18 carbons was selected to form SAMson surface of pure Mg because of the better properties of thelong carbon chain SAMs.25) And as the conventional SAMsmethods always treat samples in low concentration (10¹3,10¹4mol/L) organic solution for tens of hours, a new methodusing high concentration of 10¹1mol/L SA solution isproposed to reduce the process time. And the interesting+Corresponding author, E-mail: qiaoly@cqu.edu.cn

Materials Transactions, Vol. 55, No. 8 (2014) pp. 1337 to 1343©2014 The Japan Institute of Metals and Materials

effects of ultrasonic cleaning treatment on SAMs were alsoinvestigated. The effectiveness of this method is evaluated interms of corrosion behavior and bioactivity.

2. Experiments

2.1 Preparation of stearic acid SAMs on Mg substrate4N-magnesium (99.99% pure) samples with size of

20m © 10m © 5mm were polished with SiC paper upto1000 grit, and ultrasonically cleaned for 10min in ethanoland distilled water respectively. The samples were heat-treated at 773K for 10 h in air to form an MgO layer on theirsurface, So that the substrate can be combined with themolecule of SA.

Three different solutions containing 5 © 10¹1, 5 © 10¹3

and 5 © 10¹4mol/L SA/ethanol were used to form SAMsfor different immersion time (1.5 h, 24 h, 48 h). Theseparameters were selected according to other studies of theauthor.29)

2.2 Effect of ultrasonic cleaningAfter the SAMs treatment, specimens were ultrasonically

cleaned in pure ethanol for 2min to study the effect ofultrasonic cleaning (UC).

2.3 Contact angle measurementThe water contact angles of samples with SAMs forming

time 1.5 h, 5 h, 10 h, 15 h, 20 h, 24 h, 48 h were measured inair to analyze the film formation process using a ContactAngle Meter (Future Digital Sicetific Ascia Inc) with 5parallel samples.

2.4 Potentiodynamic scan testsThe potentiodynamic scan tests were carried out using

a LKII98B electrochemical workstation (Tianjin Lanlikechemical and electron high technology Co., LTD.) innaturally aerated simulated body fluid solution (SBF,Table 1) at 37°C. A typical three electrode system was used,which consists of platinum working as a counter electrode,saturated calomel electrode (SCE) as a reference electrodeand samples (exposed area < 1 cm2) as a working electrode.The potential was scanned at a rate of 1mV/s.

2.5 Immersing in SBFSAMs-samples which showed the best corrosion resistance

prepared in each concentration of SA solutions were soakedin SBF at 37 « 0.2°C for 30 days, with the SBF volume tosample area ratio of 1mL/10mm2. The pH variation of SBFwas monitored by pH meter (PHS-3C, Bei Jing weixinyiaoScience and Technology Development Co., LTD.). SEM andEDS (Amray-X2000, 20 kV, XL30-TMP, 20 kV) were used toobserve surface composition and morphology of the samples,and the sample immersed in SBF for 30 days was examinedby ATR-FTIR (Perkin-Elmer, Spectrum GX) to compare withthe samples before immersion.

3. Results and Discussion

3.1 Contact angleThe results that contact angles (Fig. 1, Fig. 2) showed a

significantly increase after the SAMs treatment for all thespecimens might be related to a hydrophobic SA-SAMsformed on the Mg substrate coated with MgO film. Thehighest value of the contact angle was obtained on the SAMsmade in 5 © 10¹1mol/L SA/ethanol solution for 1.5 h withultrasonic cleaning treatment, which indicates that underthese parameters the SAMs have a good coverage on thesurface of Mg.

The contact angles plotted as a function of assembly timeis shown in Fig. 2. The assembly time had an obvious effecton the contact angles on SAMs made in 5 © 10¹3 and5 © 10¹4mol/L SA solution in the first 20 and 24 hrespectively, and the effect got less after that, while in thoselow concentration of SA solution, around 20 h of assemblytime of ASMs was needed. However, the effect on SAMsmade in 5 © 10¹1mol/L SA were relatively smaller,assembly time which was longer than 1.5 h won’t improvethe hydrophobicity of SAMs. We speculate that the film

Table 1 Concentration of the ions in SBF.

Mm/L Na+ K+ Mg2+ Ca2+ Cl¹ HCO3¹ HPO4

2¹ SO42¹

SBF 142.0 5.0 1.5 2.5 147.8 4.2 1.0 0.5

Fig. 1 Static water contact angles of different treated samples.

L. Qiao et al.1338

prepared in 5 © 10¹1mol/L SA gets thicker but less orderly,because the chain-alkyl, the end-group of SA, is hydro-phobic, and the more orderly and densely SA moleculearrange, the more hydrophobic SAMs are.

The ultrasonic cleaning treatment increased the contactangles on SAMs made in 10¹1mol/L SA, while the contactangles on SAMs made in 10¹3 and 10¹4mol/L SA solutiondecreased after ultrasonic cleaning, as shown in Fig. 3. Thefact that the contact angles on SAMs increased afterultrasonic cleaning implies that the monolayer are moredensely and orderly packed. The chemical reaction betweenhead-group of molecule and substrate causes molecules tooccupy active sites of substrate in the process of forming theSAMs. It can be inferred that the ultrasonic cleaningtreatment can remove the molecules connecting with thesubstrate by physical absorption, then the other moleculescan take active site more sufficiently, and more orderly fillup the vacancy of monolayer, which enhances the degreeof coverage and the order of SAMs, and improves thehydrophobicity of SAMs made in 10¹1mol/L SA solution.This SAMS formed in high concentration SA solution carriedenough SA molecules to occupy active sites of substrate andreassembled the SAMs vacancy after physical absorbedmolecules were removed. However, for SAMs made in lowconcentration solution, such as 10¹3mol/L SA solution, wespeculate that as there was an approximate balance betweenthe molecules being removed and reassembled in the SAMs,no obvious difference was made in the hydrophobicity ofSAMs. On the other hand, for SAMs made in 10¹4mol/L SAsolution, as there were no enough SA molecules to fill up the

vacancy after physical absorbed molecules were removed, thehydrophobicity was degraded consequently by the ultrasoniccleaning.

3.2 Potentiodynamic scan measurementThe results of polarization curve are exhibited in Table 2.

The corrosion current density of samples decreased after theoxide layer and SAMs formed on pure Mg. From the resultsof Table 2, the corrosion current density of heat-treated Mgwas nearly 1/2 of untreated Mg, owing to the fact that athickness of 15 µm dense MgO layer30) was formed (Fig. 4).And the corrosion resistance of pure Mg was furtherimproved by the SAMs. As assembly time of SAMsincreased, the current density of samples treated in5 © 10¹3 and 5 © 10¹4mol/L SA solution decreased, whilethe current density of samples treated in 5 © 10¹1mol/L SAsolution increased. The ultrasonic cleaning treatment hasreverse effect on the current density of samples made in highand low concentration of SA solution, which is similar to itseffect on contact angles; it reduced corrosion current densityof samples treated in 10¹1mol/L SA, however, increasedcorrosion current density of samples treated in 10¹3 and10¹4mol/L solution. The sample treated in 5 © 10¹1mol/Lfor 1.5 h with ultrasonic cleaning presented the lowest currentdensity, i.e., 17.2 µA/cm2, which approximately equaled to1/6 of the current density of untreated-Mg (111.9 µA/cm2),and showed the best corrosion resistance. From the polar-ization curves of samples (Fig. 5), the self-corrosion potentialof SAMs-Mg rose indicating that SAMs increase the anodicpolarization and restrain the anodic corrosion process.

Fig. 2 Water contact angles of SAMs as a function of assembly time.Fig. 3 The maximal water contact angles of SAMs prepared in different

concentration of SA and after ultrasonic cleaning.

Table 2 Extrapolation results of potentiodynamic polarization curves of samples.

UC UC UC

Samples10¹1

48 h10¹1

24 h10¹1

1.5 h10¹1

1.5 h10¹3

48 h10¹3

24 h10¹3

1.5 h10¹3

48 h10¹4

48 h10¹4

24 h10¹4

1.5 h10¹4

48 hHeat-treated

Untreated

Corrosionpotential (V)

1.20 1.15 1.20 1.15 1.20 1.25 1.20 1.15 1.23 1.25 1.30 1.20 1.20 1.34

Current density(µA/cm2)

60.4 38.0 35.3 17.2 34.9 36.0 56.2 46.7 46.0 51.7 56.7 58.3 64.4 111.9

UC-indicated samples with ultrasonic cleaning treatment.

The Preparation and Corrosion Performance of Self-Assembled Monolayers of Stearic Acid and MgO Layer on Pure Magnesium 1339

Corrosion inhibition efficiency of SAMs can be calculatedby following formula which can be used to evaluate the effectof corrosion resistance of SAMs:31)

© ¼ ðIcorr0 � IcorrÞ=Icorr0

Icorr and Icorr0 indicate the current density of samples with andwithout SAMs.

Corrosion inhibition efficiency of samples treated in5 © 10¹1mol/L solution for 1.5 h with ultrasonic cleaningwas 84.6%, corresponding to the lowest current density.

In our experiments, SA-SAMs absorb well on Mgsubstrate. On one hand, this absorption changes the structureof electric double layer and increases the active energy ofcorrosion. On the other hand, chain-alkyl group of SAMs isarranged orderly, forming a hydrophobic protective layer,hindering the spread of substances related to corrosionreactions, which reduce the corrosion speed of magnesium.

3.3 Immersing in SBF3.3.1 The pH value of SBF

Figure 6 shows the evolution of pH as a function ofimmersion time in SBF at 37°C. All the specimens showedan increase of the pH with time, which is associated with the

cathodic reaction in the corrosion process because of theformation of OH­, and high pH values suggest highcorrosion rate of the specimens.

In concordance with the electrochemical results, the heat-treatment obviously decreased the corrosion rate of pure Mgand SAMs further reduced the corrosion rate. The specimenstreated for 1.5 h in 5 © 10¹1mol/L solution with ultrasoniccleaning showed the lest value after 7 days of immersion,which indicates the best anti-corrosion performance, betterthan that of SAMs-Mg treated with low SA concentration.While, the highest value of the pH is found for the untreatedmagnesium.3.3.2 Surface morphology

Figure 7 illustrates the surface morphology of samplesafter immersing different time in SBF at 37°C. Afterimmersing in SBF for 3 days, on the surface of untreated-Mg (Fig. 7(a1)), corrosion pits with different diameters andshallow reticular cracks were distributed, and white particleswere non-uniformly scattered. Compared with the untreated-Mg, reticular cracks that formed by heat pretreatment andsome white hemispheres were regularly distributed on the

Fig. 4 SEM micrographs and EDS results of surface (a) and cross-sections (b) of heat-treated Mg.

Fig. 5 Polarization curves of (1) untreated-Mg (2) heat-treated-Mg(3) SAMs-Mg (UC, 10¹1 1.5 h) in SBF solution.

Fig. 6 pH value of SBF as a function of immersing time.

L. Qiao et al.1340

surface of SAMs-Mg (which were treated in 10¹1mol/L SAsolution for 1.5 h with ultrasonic cleaning), and pits were notfound yet (Fig. 7(b1)). According to Fig. 7(a3), 30 days later,wide cracks and deep pits formed on the surface of untreated-Mg, however, the surface of SAMs-Mg was covered withwhite precipitates, and there were no deep pits and widecorrosion cracks on it. In a word, SAMs-Mg showed moreuniform and slower corrosion performance (Fig. 7(b3)).3.3.3 Composition analysis

Table 3 shows the surface composition of samples afterimmersed in SBF for 30 days. The elements detected on thesurface were the same as all the specimens with and withoutSAMs. However, their amount depended on the treatment.

The element contents of weight percentage of P, Ca weremuch higher and Ca/P ratio was closer to HA on SAMs-Mgthan those of untreated-Mg, especially within the first 15days. Element content of P, Ca increased to 15 and 24%(mass%) respectively and Mg decreased to 3% on SAMs-Mgafter the samples were immersed in SBF for 15 days, whichindicates that the P-Ca deposition-layer has formed onsurface of SAMs-Mg, and the coverings were also clear fromFig. 3(b2). Therefore, SAMs-Mg show better bioactivity thanuntreated-Mg during immersing in SBF.3.3.4 ATR-FTIR

The ATR- FTIR spectrums of SAMs-Mg before and afterbeing immersed in SBF for 30 days are showed in Fig. 8.

Fig. 7 Surface morphologies of (a) untreated-Mg (b) SAMs-Mg (UC, 10¹1 1.5 h) after immersed in SBF for 3d (a1,b1), 15d (a2,b2) and 30d(a3,b3).

The Preparation and Corrosion Performance of Self-Assembled Monolayers of Stearic Acid and MgO Layer on Pure Magnesium 1341

Compared with the spectrum of sample before immersion, forthe sample after immersion test, the anisomerous absorptionpeaks of PO4

3¹ at 1141 and 990 cm¹1 were attributed to theCa-P deposition, and some characteristic absorption peaks ofSA-SAMs still existed after immersed for 30d, such as theC-H vibration peaks of SA at 3014 and 721 cm¹1 and thecarboxylic acid (C=O) vibration peaks of SA at 1695 cm¹1,which showed that SA-SAMs still on the samples. For allspectrums, the C-O vibration peaks at 1557, 1445 cm¹1

(SAM-Mg) and 1457, 1576 cm¹1 (SAM-Mg(SBF30d)) andO-H vibration peaks took great change compared with SA,18)

which indicates that SA-SAMs assemble on the substrate byionic links between ­COO¹ and Mg2+, and then hydrophobicalkyl (­CH3) is the end group of SA-SAMs.

Our immersion test indicates that SAMs-Mg have betterbioactivity than untreated-Mg. While some researches15,32)

report that the Ca-P deposition can be induced by the groupof ­COOH rather than ­CH3. How can SA-SAMs improvethe bioactivity of the samples? It can be speculated that SA-SAMs enhance the corrosion resistance of Mg, thereforehinder the release of Mg2+, consequently reduce the negativeeffect of Mg2+ on the deposition of Ca-P and improve thebioactivity of the samples. Since some reports suggest thatthe influence of Mg2+ on Ca-P depends on the ratio of Ca/

Mg,33­35) and Mg2+ is beneficial for Ca-P nucleation at lowconcentration,36) but blocks the growing up of Ca-P at highconcentration.37)

4. Conclusions

(1) Hydrophobic SAMs of stearic acid were successfullyprepared on pure magnesium covering with MgO, andthis SAMs of stearic acid and MgO layer effectivelyimproved the corrosion resistance of pure Mg.

(2) The preparation method of SAMs by dipping samplesin high concentration (10¹1mol/L) of stearic acidsolution just for 1.5 h and with ultrasonic cleaningtreatment showed better properties than conventionalSAMs method, and the anti-corrosion efficiency is84.6%.

(3) The ultrasonic cleaning treatment led to the formationof a more densely and orderly SAMs.

(4) All samples with SAMs showed a faster deposition ofCa-P layer with similar Ca/P ratio to HA comparedwith the untreated Mg, and they presented betterbioactivity.

Acknowledgements

This work has been supported by National Natural ScienceFoundation of China (51101174), National Key TechnologiesR&D Program (2011BAE22B04) and the Natural ScienceFoundation of Chongqing Science & Technology Commis-sion (cstc2011jjA50012).

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Table 3 Composition of SAMs-Mg and untreated-Mg after immersed in SBF.

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30dSAMs-Mg 16.43 43.22 1.01 1.37 13.97 0.50 23.52 1.68

Untreated-Mg 21.22 40.86 0.89 1.9 13.91 21.21 1.52

Fig. 8 ATR-FTIR spectrum of the SAMs-Mg sample after immersed inSBF for 30 days and before immersion.

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The Preparation and Corrosion Performance of Self-Assembled Monolayers of Stearic Acid and MgO Layer on Pure Magnesium 1343