Evaluation of self-healing properties of inhibitor loaded nanoclay-based anticor

Swpnil H.Adsul,K.R.C.Som Rju,B.V.Srd,Shirish H.Sonwne,R.Susri,∗

Abstract This study emphasizes on the evaluation and comparison of the anticorrosive properties of sol–gel coatings with and without inhibitor loaded nanocontainers.In this case,naturally available clay nanotubes(halloysite)were loaded with cationic corrosion inhibitors Ce3+/Zr4+.These nanocontainers were dispersed in hybrid organic–inorganic sol–gel matrix sol.Coating was applied on magnesium alloy AZ91D using the sols containing modifie and unmodifie nanocontainers employing the dip coating method and cured at 130°C for 1 h in air.Corrosion resistance of coated/uncoated substrates were analyzed using electrochemical impedance spectroscopy,potentiodynamic polarization and weight loss measurements after exposure to 3.5 wt%NaCl solution for varying time durations between 24 h to 120 h.Self-healing ability of coatings was evaluated by micro-Raman spectroscopy after 120 h exposure to 3.5 wt%NaCl solution.Coatings generated after dispersion of corrosion inhibitor loaded clay in hybrid sol–gel matrix have shown more promising corrosion resistance when compared to just the sol–gel matrix coatings,after prolonged exposure to corrosive environment.

Keywords:Self-healing coating;Halloysite nanoclay;Cationic corrosion inhibitors;Magnesium alloy AZ91D;Micro-Raman spectroscopy;Corrosion protection.

1.Introduction

Magnesium alloys are characterized by their low density(65%that of aluminum and 25%that of iron)and high specifi strength which make them valuable materials for components in automobile and aerospace applications[1,2].However,when compared to other metals and alloys,corrosion resistance of magnesium alloys is inadequate due to their high-chemical reactivities(standard electrode potential:-2.37 V)and this limits the range of applications for magnesium and its alloys.The galvanic couples formed between secondary phases and the matrix,as well as the damage of the passive thin fil of oxide are the main sources of localized corrosion in magnesium alloys[3,4].Hence,it is necessary to develop appropriate coating compositions/configuration for the corrosion protection of magnesium alloys.The conventional coating techniques such as electrochemical plating,conversion coatings,anodizing,chemical vapor deposition,laser cladding,gas phase deposition and flame/plasm spraying,etc.have their own limitations such as poor adhesion,less wear resistance,difficul to obtain uniform and pore-free layer and environmental concerns[2].

Fig.1.(a)FESEM image and(b)TEM image,of as-received HNTs.

Thin(0.1–10 μm)sol–gel coatings is an alternative,ecofriendly,attractive approach that has the potential to overcome the aforementioned limitations of other techniques.The thickness of coatings varies depend on the coating compositions.Sol–gel coatings exhibit a great potential to provide barrier for corrosive environment and a good adhesion between metal substrate and organic top coat[5–7].Hybrid organic–inorganic sol–gel coatings come with two advantages;firstl their ability to form crack-free thick coatings with low-curing temperature and secondly their flxibility to include additives for improving coating properties[8].However,only hybrid coatings cannot give prolonged corrosion protection in case of any mechanical damage to the coated surface.Some researchers have used sol–gel method along with other conventional methods such as anodizing,conversion coatings,etc.and sol–gel coatings modifie with doping agents,have achieved better corrosion protection for magnesium alloys[9–11].Hu et al.[12]observed that molybdate conversion coating followed by multilayered silica sol–gel coating on AZ91D substrate gave better protection as compared to single layered porous molybdate conversion coating.Ecofriendly coatings of Mg(8-hydroxyquinoline)2synthesized by chemical conversion treatment in 8-hydroxyquinoline sodium solution at room temperature have shown prolonged corrosion protection to AZ91D substrates[13].Composite coatings comprising of micro-arc oxidation(MAO)with titanium organic polymer as sealing agent were generated on AZ91D alloy.It showed that composite coating gave more effective corrosion resistance than only MAO coating,because of the latter having a porous structure[14].

Number of approaches have been implemented to attain effective corrosion protection on magnesium alloys[15,16].However,prolonged corrosion protection requires sustained release of corrosion inhibiting additive materials,which can be achieved either by introducing corrosion inhibiting materials directly into the coating matrix or by encapsulating them into suitable micro/nanocontainers[17–20].Previous studies[21,22]have shown that the use of rare earth elements like cerium as cationic corrosion inhibitors has been effective in improving the corrosion resistance.Use of micro/nanocontainers for encapsulating corrosion inhibiting materials is considered to be an effective way to provide sustained release of inhibitor to obtain self-healing ability in the coating.Naturally occurring halloysite nanoclay(HNT)has been considered as most promising way of encapsulating corrosion inhibiting materials.It comes with the advantages such as non-toxic,cheap,easily available and can accommodate large amount of inhibitors[23–26].Previous studies[27]reveal that only benzotriazole was used as corrosion inhibitor loaded into halloysite nanotubes for corrosion protection of magnesium alloys.Joshi et al.[28]confirme that corrosion inhibitor has got loaded inside the lumen of HNTs by applying vacuum evacuation.However,there are no reports on magnesium alloys with cationic corrosion inhibitors such as Ce3+and Zr4+loaded into nanocontainers other than our earlier work[29].

Hence,the main objective of present work was to study the effect of addition of halloysite clay nanotubes loaded with cationic corrosion inhibitors Ce3+and Zr4+into hybrid organic–inorganic matrix sol upon exposure to 3.5 wt%NaCl solution.

2.Materials and methods

2.1.Substrate

Magnesium alloy,AZ91D coupons of dimensions 2.5 cm×2 cm and with chemical composition in wt%Al-9.14;Zn-0.86;Mn-0.30;Cu-0.09;Si-0.08;Fe-0.01;Ni-0.01 and rest Mg,were used as the substrates.All substrates were mechanically ground successively to 1000 grit with silicon carbide(SiC)papers followed by repeated degreasing with acetone for 30 min and were finall dried in air.

2.2.Sol synthesis and coating deposition

Fig.2.(a)EDS analysis of as-received HNTs,(b)EDS analysis of loaded HNTs and(c)pore volume vs.pore diameter for as-received and loaded HNTs.

Halloysite nanotubes(HNTs)with lumen diameter of～10 nm were etched in 1 M H2SO4(SD Fine Chemicals,India,98%)for 3 days at 60°C,followed by washing with deionized water and drying,in order to get enlarged lumen diameter[24].Corrosion inhibitors,cerium nitrate hexahydrate(Loba Chemie,India,99.9%)and zirconium n-propoxide(Gelest Inc.,USA,70%in propanol)were taken in molar ratio of 1:23,and added to the etched HNTs.The mixture was then dried in vacuum desiccator.The organic–inorganic hybrid matrix(abbreviated hereafter as MAT)sol was prepared by hydrolysis of 3-Glycidoxypropyltrimethoxysilane(GPTMS,Gelest Inc.,USA,98%)with tetraethoxysilane in molar ratio of 3:5:1 with 0.1 N HCl as catalyst.The ends of HNTs were stoppered with polymeric microcapsules of ureaformaldehyde that were synthesized using the procedure as described elsewhere[29].The self-healing(abbreviated hereafter as SH)sol was synthesized by dispersing 2 wt%of inhibitors loaded HNTs stoppered with polymeric microcapsules into the hybrid matrix sol.For evaluating the effect of HNTs,as-received HNTs(without inhibitors’loading)were also dispersed in the matrix sol to obtain a modifie sol,abbreviated hereafter as clay matrix(CM)sol.

Coatings were generated on AZ91D substrates by dip coating technique at a withdrawal speed of 1 mm/s followed by curing at 130°C for 1 h in hot air oven.The coating thickness was measured using a non-destructive coating thickness gauge(PosiTector®6000 supplied by DeFelsko Corporation,USA)and was found to be in the range of 3–5 μm.

The adhesion of MAT sol,CM sol and SH sol coatings on AZ91D substrates was investigated by cross-hatch cutter as per ASTM D3359-17 test procedure.The surfaces were scribed in the form of 1 mm2grid lines.A pressure sensitive adhesive tape was applied over the grid and pulled off rapidly at an angle close to 180°.The samples were inspected for any removal of the coating using optical microscope(Olympus BX51M).

2.3.Transmission electron microscopy,scanning electron microscopy and BET pore volume analysis

Transmission electron microscopy(TEM,Tecnai 200 G2 FEI,Netherlands)was used to confir the lumen diameter and length of the nanotubes.The length of nanotubes was also ascertained using scanning electron microscope,Hitachi model-S3400 N.BET surface area and pore volume analysis was carried out using a Micromeritics ASAP 2020 surface area and porosimetry analyzer.

2.4.Weight loss measurements

Weight loss measurements were carried out on uncoated/coated AZ91D substrates as per ASTM G31[30].Uncoated and coated substrates of AZ91D were exposed to 3.5 wt%NaCl solution for 24 h,72 h and 120 h.Substrates were washed with deionized water and ethanol followed by immersion in chromic acid(200 g/l)for 2 min to remove corrosion products from the surface.The substrates were again washed with water and ethanol to remove traces of chromic acid.The substrates were weighed before and after exposure to NaCl solution and corrosion rate was obtained from the weight loss values.The consistency of measurements was checked by measuring the weight loss for three samples each for uncoated and coated substrates.

Fig.3.Adhesion test micrographs of MAT sol coated(a)and(b),CM sol coated(c)and(d)and SH sol coated(e)and(f)substrates before putting on tape and after removal of tape.

2.5.Micro-Raman spectroscopic analysis

Micro-Raman spectroscopic analysis was carried out for bare and SH sol coated substrates in the scribed area before and after 120 h exposure to 3.5 wt%NaCl solution.The data were acquired using Horiba Jobin Yvon-Lab Ram HR-800 Raman spectrometer with Argon ion laser of 514 nm as the light source over the scan range of 100–1000 cm-1.

2.6.Electrochemical tests

Potentiodynamic polarization and electrochemical impedance spectroscopy measurements were carried out using Electrochemical Analyzer(CH instruments Model 600E series).A three-electrode system comprising of saturated calomel electrode (SCE)as reference electrode,platinum electrode as counter electrode and the coated substrate as working electrode with exposure area of 1 cm2was used.The electrochemical impedance spectroscopy(EIS)was carried out over a frequency range of 1 MHz to 0.01 Hz using AC signal of 10 mV amplitude.The potentiodynamic polarization and open circuit potential measurements were carried out on AZ91D substrates after different time exposures in 3.5 wt%NaCl solution.The potentiodynamic polarization measurements were carried out at scan rate of 0.8 mV/s.The surface morphology of uncoated and coated substrates was observed using SEM after 120 h exposure to 3.5 wt%NaCl solution.

Fig.4.Comparison of corrosion rates of uncoated and coated substrates after different time of exposure in h.

Fig.5.Equivalent electric circuits used to fiEIS data for(a)bare and(b)coated substrates.

3.Results and discussion

3.1.Transmission electron microscopy,scanning electron microscopy and BET pore volume analysis

The morphology of as-received HNTs observed through FESEM and TEM is shown in Fig.1(a)and(b).As-received HNTs have shown a tubular structure with lumen diameter of～10 nm,with lengths varying between 0.1 and 0.5 μm.

Elemental analysis of as-received HNTs confirme the composition to be alumino silicate as shown in Fig.2(a)whereas elemental analysis of loaded HNTs shown in Fig.2(b)confirme the presence of corrosion inhibitors.The confirmatio of loading of corrosion inhibitors in the lumen of HNTs was confirme by using BET pore volume and surface area analysis.Pore volume as a function of pore diameter shown in Fig.2(c)confirme that in case of inhibitor loaded HNTs pore volume has substantially reduced by 80%.Literature studies[23–28]have also revealed that the corrosion inhibitor has got loaded in to the lumen of HNTs using vacuum evacuation.

Fig.6.Nyquist plots for bare and coated AZ91D substrates after(a)24 h,(b)72 h and(c)120 h exposure to 3.5 wt%NaCl solution.

Table 1 Tafel fittin parameters after fittin of potentiodynamic polarization measurements on bare and coated AZ91D substrates for different times of exposure in 3.5 wt%NaCl solution.

Fig.7.Comparison of charge transfer resistance of uncoated and coated substrates obtained from EIS fitte data.

3.2.Adhesion test

Fig.3 shows images obtained using optical microscope for coated substrates after carrying out adhesion test using crosshatch cutter.The MAT sol coatings on AZ91(Fig.3(a)and(b))have shown very smooth edges of cut after the tape was peeled off.The adhesion of the coatings was adjudged as rank 5B,which represented best adhesion property according to ASTM D3359[32].Similar case was observed with SH sol coatings(Fig.3(e)and(f)),where none of the edges of coating were detached and adhesion was ranked as 5B.However,CM sol coatings(Fig.3(c)and(d))had up to 35%of the removed coated area after the tape was removed and the adhesion of these coatings was ranked as 2B.

3.3.Weight loss experiments

Weight loss measurements provide more realistic information on the stability of coatings and corrosion resistance of test substrates.In this case,corrosion rate is calculated for bare and coated AZ91D substrates in 3.5 wt%after 24 h,72 h and 120 h.The following formula was used for calculating corrosion rate in millimeter per year(mmpy)[30]:

wherewis weight loss in grams(g),tis exposure time in hours(h),Ais exposed area in cm2andρis density of the alloy in g/cm3.

Fig.4 depicts the corrosion rate of bare,MAT sol coated,CM sol coated and SH sol coated substrates after 24 h,72 h and 120 h of exposure to 3.5 wt%NaCl solution.It can be observed that,MAT sol and SH sol coated substrates shown least corrosion rate up to 24 h of exposure indicating the barrier property of coating.However,this barrier property gets depleted after further exposure to corrosive medium and thereby showing increase in corrosion rate.Whereas in case of bare substrates,formation of thin oxide layer on the surface prevents the corrosive attack till 72 h of exposure.After 120 h,it can be discerned that corrosion rate is least for SH sol coated substrates,clearly indicating that there is an effect of the addition of inhibitor loaded clay nanotubes into the matrix sol unlike CM sol coated specimen where very high-corrosion rate was observed.

3.4.Electrochemical impedance spectroscopy(EIS)and potentiodynamic polarization

EIS studies were carried out on bare and coated AZ91D substrates after 24 h,72 h and 120 h exposure to 3.5 wt%NaCl solution.Suitable electrical equivalent circuit was selected for measuring and analyzing impedance of uncoated and coated substrates as shown in Fig.5.

Here,the charge transfer resistance(Rct)is in parallel with electrical double layer capacitance(CPEedl)that is in series with coating resistance(Rcoat).Ccoatcorresponds to coating capacitance.Here,constant phase element is preferred over pure capacitor as the Nyquist plots are deviating from ideal behavior.The value of pseudo capacitance is calculated from following expression:

whereCis pseudo capacitance in F/cm2;Q0is constant phase element in S-secn/cm2;nis frequency factor andRis resistance inΩ.

The Nyquist plots for bare and coated AZ91D after different exposure times to 3.5 wt%NaCl solution are shown in Fig.6.After 24 h of exposure,SH sol coated substrates exhibited highest impedance values in low-frequency region,indicating higher corrosion resistance for SH coated substrates.However,the corrosion resistance of SH sol coated substrates reduced after 72 h of exposure which may have occurred due to loss of barrier property of coatings.Further exposure of SH coated substrates depicted higher impedance values due to formation of passive layer of cerium and zirconium oxides after release of corrosion inhibitors from HNTs.This can be ascertained from Fig.7 showing the charge transfer resistance of uncoated and coated substrates as a function of different exposure times obtained from EIS fittin data.

Fig.8.SEM images of bare(a)and(b),MAT sol coated(c)and(d),CM sol coated(e)and(f)and SH sol coated(g)and(h)AZ91D substrates before and after 120 h of exposure to 3.5 wt%NaCl solution.

MAT sol coated substrates exhibited some resistance to corrosion similar to SH sol coated substrates,but it lasts only until the barrier properties of coatings remains intact.Among the coated substrates,CM coatings exhibit the least corrosion resistance.The reason for this can be attributed to the fact that the HNTs do not have the favorable aspect ratio for dispersion in the matrix sol.A similar investigation by Huttunen-Saarivirta et al.who dispersed HNTs in an epoxy matrix and studied the corrosion resistance of coatings generated from the modifie epoxy formulation observed that HNTs were distributed randomly on the coating surface and hence,not completely embedded in the coating matrix.This resulted in a less homogeneous coating morphology[31].Thus,CM sol coated substrates have shown the least impedance values due to porous nature of coatings and coatings were also seen to have a tendency to crack,as seen from the SEM images shown in Fig.8(f),which allows corrosive medium to diffuse through.The poor adhesion strength of coatings is another factor that affected the corrosion resistance of CM sol coatings as specifie earlier in Section 3.2.Though according to the SEM analysis,the SH coatings also exhibit cracks due to the presence of clay particles,the corrosion resistance is not affected due to release of corrosion inhibitors from the clay nanotubes in the vicinity of the cracks.

The results of potentiodynamic polarization analysis for uncoated and coated substrates after immersion in 3.5 wt%NaCl for 24 h,72 h and 120 h,shown in Fig.9 showed similar behavior as seen from results of EIS studies.The corrosion currents and corrosion potentials for different exposure times in 3.5 wt%NaCl solution are shown in Table 1.The corrosion potentials were calculated using Tafel extrapolation method.However,corrosion potential tends to reduce after 72 h of exposure in case of SH sol coated substrates because of loss of barrier property of coatings and bare substrates found to be showing more positive corrosion potential because of formation of thin fil of oxides as shown in Fig.9(b).Fig.9(c)shows polarization data after 120 h of exposure to 3.5 wt%NaCl solution.The corrosion current for SH sol-based coatings is least after 120 h of exposure,due to self-healing properties obtained from inhibitor loaded nanotubes.This confirm that inhibitor loaded nanoclay-based coatings are capable to provide prolonged corrosion protection.These results are further corroborated with micro-Raman spectroscopic analysis.

3.5.Micro-Raman spectroscopic analysis

Micro-Raman spectroscopic analysis was carried out on bare and SH sol coated AZ91D substrates before and after 120 h exposure to 3.5 wt%NaCl solution in the scribed area.The spectra of uncoated and coated substrates before and after exposure are as shown in Fig.10.In case of bare substrates,the spectrum shows broad peaks at 254 cm-1and 446 cm-1before exposure to 3.5 wt%NaCl solution.These peaks correspond to MgO and Mg(OH)2on scribed area.The spectrum also shows peaks at 633 cm-1;752 cm-1due to Al2O3and 380 cm-1due to ZnO on the scribed area(Fig.10(a)).Broad nature of peak indicated that very thin layer of oxides has formed on the substrates.However,after 120 h exposure to 3.5 wt%NaCl solution,sharp and high-intensity peak was observed at 254 cm-1on scribed area which indicated that the crystalline phase of oxides has formed(Fig.10(b))[33].

Fig.9.Potentiodynamic polarization data for uncoated and coated substrates after 24 h,(b)72 h and(c)120 h exposure to 3.5 wt%NaCl solution.

Fig.10.Micro-Raman spectroscopic analysis of bare AZ91D(a)before exposure;(b)after exposure and coated AZ91D(c)before exposure;(d)after exposure to 3.5 wt%NaCl solution for 120 h.

SH coated substrates in scribed area(Fig.10(c))have shown the appearance of MgO,Mg(OH)2at 254 cm-1and ZnO at 393 cm-1before exposure to NaCl solution.After 120 h of exposure(Fig.10(d)),sharp peaks at 233 cm-1and 560 cm-1correspond to ZrO2;peak at 277 cm-1corresponds to MgO,whereas peak at 449 cm-1corresponds to CeO2in scribed area[34,35].This indicates that inhibitor has got released into the scribed area further confirmin the self-healing action.

In our earlier investigations,scanning vibrating electrode technique(SVET)was used to confir the self-healing properties of the HNT-based self-healing coating[29].For the firs time,micro-Raman spectroscopic studies have been carried out on self-healing sol coated substrates,after scribing and exposing to corrosive medium.The phase composition on the scribe after exposure to NaCl solution could be unequivocally confirme by micro-Raman spectroscopy to be CeO2and ZrO2.It could be concluded that in addition to using scanning electrochemical techniques,micro-Raman spectroscopy can also be employed to confir the self-healing properties of the coatings.

4.Conclusion

Halloysite nanotubes loaded with cationic inhibitors Ce3+/Zr4+dispersed in hybrid matrix sol provide prolonged corrosion protection due to the controlled release of inhibitors,when exposed to corrosive environment.Hybrid matrix sol coatings were also found to be effective to provide barrier properties and retained this property until the coatings remain unblemished for certain duration of exposure,as confirme from electrochemical measurements.HNTs without loading of any corrosion inhibitors were found to be affecting the barrier properties of hybrid matrix sol,thereby deteriorating the corrosion resistance.The anticorrosive and self-healing properties of SH sol coated substrates could be confirme with EDS analysis after weight loss measurements and micro-Raman spectroscopic analysis.The use of cationic corrosion inhibitors loaded into naturally occurring halloysite clays can be used as a self-healing material as an additive to any hybrid sol–gel matrix as well as to paints to improve their corrosion protection properties.

Acknowledgements

The authors would like to thank Director,ARCI,Hyderabad for the constant encouragement and support throughout the entire duration of work.The authors would like to thank G.V.R.Reddy for the SEM-EDS analysis.Swapnil H.Adsul and R.Subasri would like to acknowledge the financia support from SERB,DST for the funding provided through grant number SB/S3/ME/007/2014.