silver-poly(pyrrole-co-3,4-ethylenedioxythiophene) dual layer coatings on low nickel stainless steel for anti-corrosion applications

values for the Ag-p(Py-co-EDOT)/Cu-HNT dual layer copolymer coated LN SS demonstrate its better corrosion protection ef ﬁciency than those of the other p(Py-co-EDOT), Ag-p(Py-co-EDOT) an[r]

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Original Article

Novel copper doped Halloysite Nano Tube/silver-poly(pyrrole-co-3,4-ethylenedioxythiophene) dual layer coatings on low nickel stainless steel for anti-corrosion applications

Palanisamy Karthikeyana, Saravanan Sathishkumarb, Kannaiyan Pandianc,

Liviu Mitud,**, Rangappan Rajavelb,*

aDepartment of Chemistry, Periyar University Constituent College of Arts and Science Idappadi, Salem 637 102, Tamilnadu, India bDepartment of Chemistry, Periyar University, Salem 636 011, Tamilnadu, India

cDepartment of Inorganic Chemistry, University of Madras, Chennai 600 025, India dFaculty of Science, University of Pitesti, Pitesti 110040, Romania

a r t i c l e i n f o

Article history:

Received 21 October 2017 Received in revised form 16 December 2017 Accepted 22 December 2017 Available online January 2018 Keywords:

Dual layer coatings Electrochemical studies Surface analysis Antibacterial activity Ion leachout test

a b s t r a c t

The increase of the diverse and complicated applications of stainless steel in allﬁelds of industry pro-duction and various research activities have induced immense efforts in research and fabrication to increase its efﬁciency and sophisticated to minimize its corrosion by using among others conducting polymer coatings The present work discusses the corrosion resistant behavior of stainless steel with copolymer and composite dual layer coatings The coated samples were analyzed by various analytical studies and the results are discussed The dual layer composite coating Ag-p(Py-co-EDOT) thus obtained was uniform in nature and highly adherent to the stainless steel surface, when compared to the monolayer coatings An antibacterial effect of coating and the coatings against marine and pathogenic bacteria have also been studied

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Stainless steels (SS) have been in progress and diverse utilization and applications for all kind of purposes in almost all branches of industry production, in various research and development envi-ronments in a varied form from technical and technological facil-ities to hygiene sanitary facilfacil-ities, household appliances and sophisticated medical instrumentations because of their mechan-ical, biocompatible, cost saving and anticorrosive properties SS are FeeCr-based alloys containing more than 12% Cr[1,2] While iron may corrode in an aqueous environment, chromium provides SS with the facility to form a protective or passiveﬁlm that resists corrosion But stainless steels might still be corroded in such particular environments containing sulphate or chloride ions[3,4] However, the microbiologically imparted corrosion also called biocorrosion is directly associated with the simple metal surface

[5e10] In recent years, conducting polymers have been used as shielding agents to prevent corrosion of metals [11,12] Several studies have shown that the corrosion of SS is successfully inhibited by coating them with polypyrrole (PPy) and their derivatives which can be synthesized by the electrochemical technique[13]

Poly(3,4-ethylendioxythiphene), generally denoted as PEDOT, is one of the most efﬁcient thiophene derivatives and is widely used as a corrosion inhibitor because of its attractive properties like high conductivity, a curious electroactivity and environmental stability [14,15] With these properties the PEDOT derivatives are used in an extended range of electronic and electrochemical applications In recent years, PEDOT has been combined with other typical con-ducting polymers with the hope to produce new copolymers There are many studies dealing with corrosion resistance properties of copolymers[16]such as co-o-toluidine), poly(pyrrole-co-N-methylpyrrole), poly(pyrrole-co-o-anisidine), poly(aniline-co-pyrrole) and poly(aniline-co-o-anisidine), prepared on various metal substrates [17,18] It was reported that these coatings can provide an effective protection of the substrates against corrosive species Generally, copolymers assemble through the electro-chemical polymerization from mixtures of the different monomers * Corresponding author

** Corresponding author

E-mail addresses:drrajavelpu@gmail.com,ktm7ro@yahoo.com(R Rajavel) Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2017.12.003

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and these are considered interesting materials with promising technological applications[19,20] The copolymers PEDOT with PPy and their derivatives etc., hereafter denoted as p(Py-ce- EDOT) have been prepared by the electrochemical CV technique[21,22] The copolymer was prepared on the low nickel stainless steel (LN SS) electrodes using monomer mixtures with various concentration ratios In comparison between the p(Py-co- EDOT) and the corre-sponding mono polymers, it was found that, the single layer coating of copolymers does not bring any useful advantage with respect to PEDOT and PPy It was explained that the double layer coating could establish an effective barrier that keeps up the metal substrate in a passive state during the immersion time[14,19] In recent times, conducting polymer/inorganic composites have attracted great attention due to their unique micro and nano structure, excellent physio-chemical and electro optical properties as well as a wide range of their potential usages in the battery cathodes and in the creation of nano-scopic assemblies inside the sensors and micro-electronics[23e26] It is believed that along with the development of materials science, the developed Halloysite Nano Tube (HNT) has several advantages over other materials[27e33] In this work, we present how all these deficiencies are successfully prevailed over by the coating with dual layeredfilms of Cu-HNT followed by the Ag-p(Py-co-EDOT) layer using the dual layer electrochemical deposi-tion technique The systems containing different layers with alter-nate PEDOT, PPy and copolymers have been synthesised and their electrochemical and electrical properties have been characterized In order to reach the environmental stability in the dual layer systems, Cu-HNT has been used as an inner layer From the results it is clear that, the electrochemical stability and the electroactivity of Ag-p(Py-co-EDOT)/Cu-HNT are much higher than those deter-mined forfilms of the copolymers and the single layer copolymer composite generated under the identical experimental conditions On the other hand, these dual layer systems represent a significant improvement in the corrosion resistance with respect to Ag-p(Py-co-EDOT)/Cu-HNT copolymer composite coatings on LN SS in M H2SO4medium

2 Experimental

All the chemicals, including 3,4-ethylendioxythiophene(Sigma), pyrrole(Sigma), copper sulphate, silver chloride, lithium perchlo-rate (LiClO4 Alfa aesar) and acetonitrile (Renkem) of analytical

grade were used without further puriﬁcation in all experiments All the aqueous solutions were prepared by using ultra pure water The experimental procedure can be described exactly the same as that reported in our pervious papers[4,24]

3 Results and discussion

3.1 PPy, PEDOT, p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOP)/Cu HNT dual layer composite coatings

Mono layer (PPy, PEDOT, p(Py-co-EDOT), Ag-p(Py-co-EDOT)& Cu-HNT), and dual layer (Ag-p(Py-co-EDOT/Cu-HNT) polymer composites were cathodically electrodeposited on LN SS using the oxidizing agent lithium perchlorate solution Cyclic voltammo-grams of the samples were recorded at a ﬁxed scan rate of 100 mV1 for 10 cycles in the potential range between 0.8 V andỵ2.0 V vs SCE and the results are shown inFig 1aef The anodic peak observed around 1.0 V vs SCE is attributed to the electro-chemical polymerisation of pyrrole as well as the oxidation of the PPy coating Separate peaks could not be detected due to the overlapping of oxidation peaks of PPy after theﬁrst cycle (Fig 1a) [34] With increasing cycle numbers, the intensity of the oxidation peak of pyrrole decreases, while a slight increase in the current density is observed at the end of each sequence The peak intensity is being results of pyrrole involved in the passivation process and reduce in current density continues in the repetitive cycles The slight increase in the current at the end of each sequence is attributed to the process of monomer oxidation occuring in a step by step approach which leads to the growth of a PPy layer with increasing thickness The CV of EDOT is shown inFig 1b The anodic peak observed at 0.8 V vs SCE is attributed to the electrochemical

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polymerization of EDOT as well as the oxidation of ClO4forming a

passive ﬁlm underneath the PEDOT coating [35] The electro-chemical deposition of the copolymer was done with similar pa-rameters but theﬁrst forward scan of homo polymers as a potent and irreversible anodic peak starts to appear at approximately 0.8 and 1.0 V as shown inFig 1c

This corresponds to the oxidation of the pyrrole and the EDOT monomers, respectively Going on the subsequent cycles, monomer oxidation occurs at a little lesser potentials due to the variation occurred with the substrate surface after theﬁrst cycle, i.e., the LN SS surface was covered with an electrochemically deposited layer of PPy and PEDOT

Also in the CV of the p(Py-co-EDOT) and the dual layer of Ag-p(Py-co-EDOT)/Cu-HNT coatings, similar anodic peaks are observed, since the difference between the oxidation potential of Ag-p(Py-co-EDOT) and Ag-p(Py-co-EDOT)/Cu-HNT is smaller (Fig 1d and e) Fig 1f shows the combinations of 1e2 cycles of Fig aee From the ﬁgures it can be believed that the subsequent oxidation is feasible with the radical cations formed and these could combine to yield a copolymer[36]

3.2 Spectral characterizations 3.2.1 FT-IR analysis

In order to conﬁrm whether the second layer was coated or not, FTIR spectra of the p(Py-co-EDOT) composite layer and the Ag-p(Py-co-EDOT)/Cu-HNT dual layerﬁlms were recorded (shown in Fig 2a-d) and compared to those of their respective copolymers and copolymer composite coatings Firstly, the copolymers and copolymer composite coatings were characterized In the spectrum of the p(Py-co-EDOT coating (seeFig 2a) the bands corresponding to the stretching modes of the (NeH), (CeN) and (C]C) bonds are observed at 636, 3386, 1129 and 1634 cm1, respectively[37] In the Fig 2a, the FT-IR spectrum of EDOT indicates that the peaks at 734, 793, 1146, 1298, 1494 and 1619 cm1originate from the stretching modes of the (CeS), (CeOeC) groups and from the vibration modes of the (C]C) and (CeC) bonds in the thiophene rings, respectively [38] The copolymer Ag-p(Py-co-EDOT) composite coating exhibits the corresponding peaks of both monomers EDOT and Py units Moreover, a characterized strong peak at 1122 cm1is assigned to the (ClO4-) ions that act as dopants originating from the supporting

electrolyte[4] The FTIR spectrum for the Cu-HNT coating on LN SS sample is shown inFig 2c, in which a strong absorption band of OeSieO appears at 1081 cm1 The presence of SieO at 482 cm1 and of the AleOH groups at 918 cm1in the backbone of Cu-HNT

are also conﬁrmed [30], whereas, the deformation vibration of the AleOeSi band observed at 545 cm1is in the Cu-HNT The

strong absorption peaks located at 3495 and 1671 cm1are due to the OeH stretching and the deformation of water, respectively The individual peaks located at 738 cm1represent the stretching mode of SieO[30]

3.2.2 X-Ray diffraction studies

The XRD patterns obtained for p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and dual layer coating (Ag-p(Py-co-EDOT)/Cu-HNT) are shown inFig S1 The diffraction patterns of the mono and dual layer coatings showed that the peaks obtained in the monolayer are shifted towards lower angles (Fig S1aef) when compared to that of the dual layer coating (Ag-p(Py-co-EDOT)/Cu-HNT) [39] In this regard, the main diffraction peaks of the dual layer obtained are shifted to the left side due to the dual layer coating of Ag-p(Py-co-EDOT)/Cu-HNT, which occurred by expansion and contraction in the polymer lattices These XRD results strongly point out the for-mation of a dual layer coating on the LN SS substrate[40] 3.3 Surface Characterizations

3.3.1 Field emission scanning electron microscopic, mapping and elemental analysis

The morphology and structure of the copolymers, copolymer composite and dual layer coating strongly affect their properties Representative SEM micrographs, EDS images of the polymer, copolymer, composite and dual layer coated LN SS, respectively, are displayed inFig 3aef

The micrographs of the p(Py-co-EDOT) copolymer coating show a slight difference with the varying monomers feed ratio in the p(Py-co-EDOT) chain The morphology of the PPy coating on LN SS (Fig 3a) exhibits typical cauliﬂower-like grains with unequal void spaces in between them The morphology inFig 3b of EDOT is quite different from that of PPy For instance, globules are smaller, dispersed and the void spaces between the globules are uniform and smaller than those of PPy These structural differences help us to follow the incorporation of both monomers to the structure when they make a p(Py-co-EDOT) copolymer In Fig 3c, at the optimum monomer ratio (1:1) of the copolymer p(Py-co-EDOT), the nucleation and accumulation have been clearly observed in theﬁlm with a three-dimensional granular morphology [37,50] The Cu-HNT inFig 3e reveales a uniform arrangement of a rod like structure The Fig 3f also reveales a uniform and more compact coating with a well ordered three dimensional structure formation, colloids in nature These results are in good agreement with other SEM (in Fig 3d) results Thus, the Ag-p(Py-co-EDOT)/Cu-HNT coating on the substrate can act as an adherent barrier to reduce the rate of corrosion The adhesion and hardness of the dual layer coating of Ag-p(Py-co-EDOT)/Cu-HNT are much higher than those of their copolymer and composite counterparts[4]

3.3.2 Cross sectional SEM images

The FESEM image shown inFig S2presents the cross section of the Ag-p(Py-co-EDOT)/Cu-HNT dual layer coatings on LN SS The formation of the dual layer with a distinct rod and uniform morphology of Cu-HNT over the Ag-p(Py-co-EDOT) coated LN SS sample is illustrated in a cross-sectional SEM image and the thickness of the rod like coating is about ~1.77mm on the Cu-HNT/ Ag-p(Py-co-EDOT) coated LN SS specimen (seeFig S2) However, the top Ag-p(Py-co-EDOT) coating (thickness ~2.23mm) shows a three-dimensional granular network A continuous interface col-loids along the coating length is observed between the Cu-HNT layer and substrate Hence, the Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS surface may be more corrosion protective Fig FT-IR spectra of (a) p(Py-co-EDOT) copolymer, (b) Ag-p(Py-co-EDOT) composite

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because there is no leach out of any metal ions from the LN SS in an acidic environment

The EDS spectrum of the Ag-p(Py-co-EDOT)/Cu-HNT dual layer coating reveals the presence of C, O, S, N, Ag, Cu, Al, Si and Cl (see Fig S3j), indicating the existence of PPy, PEDOT, Ag and Cu-HNT in it Thus, the homogenously distributed PPy, PEDOT, Ag, Cu, Al, Si and Cl particles can be observed (seeFig S3aei) upon EDS mapping analysis[41]

3.3.3 Atomic force microscopic studies

The surface topography of the co-EDOT) and Ag-p(Py-co-EDOT)/Cu-HNT dual layer coatings on LN SS samples was studied using the Atomic Force Microscopy (AFM) andFig 4(a and b) shows the observed topographic images The AFM topographical image for the Ag-p(Py-co-EDOT) coating with optimum feed ratios of Ag (0.2 M) and monomers (1:1) on LN SS sample presented in Fig 4a clearly shows that the coating surface is continuous, more compact, dense and uniform in nature The Ag-p(Py-co-EDOT)/Cu-HNT (with 0.1Me2%) dual layer coated LN SS topography is pre-sented inFig 4b The topographical image of the dual layer coatings Fig 4b, reveals the uniform distribution and colloids in the struc-tural nature of the compact polymer layer The average root-mean-square roughness value for the Ag-p(Py-co-EDOT) compact layer

measured for 14 14mm2is of 3.5± 14.6mm, whereas for Ag-p(Py-co-EDOT)/Cu-HNT dual layer coatings measured for 25 25mm2, a value of 1.3 ± 0.16 mm was found Comparatively, a lesser average roughness value was observed for the dual layer coatings surface

3.4 Mechanical characterizations 3.4.1 Adhesion strength

The adhesion strength of the as-developed coatings over LN SS specimens was examined using the ASTM F 1044-05 adhesion test method Values of the adhesion strength are shown inFig 5a and Table Here, the adhesion strength of the p(Py-co-EDOT) and Ag-p(Py-co-EDOT), the Cu-HNT and the Ag-p(Py-co-EDOT)/Cu-HNT dual layer coatings (with optimized 0.2 M Ag and 1:1 copolymer) on LN SS specimens, respectively, was evaluated The adhesive strength of the Ag-p(Py-co-EDOT)/Cu-HNT (with 0.2 M Ag and 1:1 of copolymer) dual layer coated LN SS (14.5± 0.7 MPa) is foundgreater than those of the p(Py-co-EDOT) and Ag-p(Py-co-EDOT) composite coatings (1:1 copolymer and 0.2 M Ag), which are 13.6± 1.0 MPa and 14.1± 0.8, respectively

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This adhesion strength of the as developed Ag-p(Py-co-EDOT)/Cu-HNT dual layer coatings will make them appropriate for anticor-rosion applications under acidic conditions

3.4.2 Hardness

The results of the Vickers micro hardness test on various poly-mer, copolypoly-mer, composite and dual layer coatings are presented in Fig 5b andTable The copolymer (1:1) and composite (Ag-p(Py-co-EDOT)) and Cu-HNT layer coated substrates exhibited higher hardness values of 98.8± 6.0, 123.2 ± 5.7 and 290.6 ± 6.1, respec-tively The Hv value for the Ag-p(Py-co-EDOT)/Cu-HNT dual layer

coated LN SS sample is observed as high as 325.6± 5.8 Hv, which is still higher than those of the co-polymer (1:1) and composites (Ag-p(Py-co-EDOT) and Cu-HNT layer coated specimens This is due to the inseparable and intact bonding strength between the copolymer and the composite coatings of the as-developed dual layer coating 3.5 Electrochemical characterizations

3.5.1 Potentiodynamic polarisation studies

The corrosion resistance of the uncoated LN SS, of p(Py-co-EDOT), Ag-p(Py-co-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS in a sulphuric acid solution was assessed as a result of potentiodynamic polarization studies The corresponding polarization parameters, such as the corrosion po-tential (Ecorr) with the corrosion current (Icorr), were obtained from

the respective polarization plot (seeFig 6) The Ecorrand Icorrvalues

for the uncoated LN SS specimen are found to be 0.482 and 2.818 103mA∙cm2vs SCE respectively, while the polarization

curve recorded for the p(Py-co-EDOT) coated LN SS specimen yield Ecorrand Icorrvalues of0.433 mV, 0.562 103mA∙cm2vs SCE,

respectively

The results show that the Ag-p(Py-co-EDOT) coated LN SS specimen has slightly higher Ecorrand Icorrvalues than the uncoated

and PPy coated LN SS specimen The polarisation curve of the dual layer Ag-p(Py-co-EDOT)/Cu-HNT coated LN SS specimen shows an Ecorrand Icorr0.360 mV, 0.073 103mA∙cm2vs SCE,

respec-tively The Ecorrand Icorrvalues of the p(Py-co-EDOT),

Ag-p(Py-co-EDOT) and Cu-HNT coated LN SS specimen are found lesser than those of the Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS and they are given inTable The Ecorrand Icorrvalues of the dual layer

Ag-p(Py-co-EDOT)/Cu-HNT coated LN SS specimen illustrate a maximum shift in the noble direction when compared to the case of the uncoated, p(Py-co-EDOT), Ag-p(Py-co-EDOT) and Cu-HNT coated LN SS specimens

Fig AFM images of (a) Ag-p(Py-co-EDOT and (b) Ag-p(Py-co-EDOT)/Cu-HNT coat-ings on LN SS

Fig (a) Adhesion strength of p(Py-co-EDOT), p(Py-co-EDOT), Cu-HNT and p(Py-co-EDOT)/Cu-HNT polymer coating on LN SS; (b) Hardness values of p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT polymer coating on LN SS

Table

Hardness and adhesion strength value of p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-Cu-HNT polymer coating on LN SS

Sample Adhesion (MPa) Hardness (Hv)

p(Py-co-EDOT 13.6± 1.0 98.8± 6.0

Ag-p(Py-co-EDOT) 14.1± 0.8 123.3± 5.7

Cu-HNT 14.8± 0.8 290.6± 6.1

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This shift in the Ecorrand Icorrvalues toward the noble direction

indicates that the dual layer Ag-p(Py-co-EDOT)/Cu-HNT coating has a high corrosion protection potential in M H2SO4 It could be

noticed that the Cu-HNT coating between the LN SS surface and Ag-p(Py-co-EDOT)/Cu-HNT coating acts as an anti-corrosion barrier, thereby preventing the decline of the metal ions into the electrolyte solution

3.5.2 Electrochemical impedance spectroscopic studies

The impedance analysis provides the most constructive infor-mation on both resistive and capacitive behavior of all the speci-mens in the M H2SO4solution The impedance spectra obtained

for the uncoated, p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS specimens were ﬁtted using equivalent circuits as shown inFig S4aandFig S4b The ﬁtting equivalent circuit model, represented as Rs(R1Cdl) (R2Cdl1)

and shown inFig S4a, consists of two combinations of a resistor and a capacitor in series with the solution resistance and was used to obtain the spectra for the mono and copolymer coated LN SS specimens In this equivalent circuit, Rs represents the solution

resistance which corresponds to the ohmic resistance of the system whereas R1and Cdlrepresent the resistance and the capacitance of

the uncoated LN SS, respectively R2and Cdl1represent the

resis-tance and the capaciresis-tance of the mono and copolymer, respectively Fig S4bshows the equivalent circuit used toﬁt the spectrum ob-tained for dual layer coated LN SS specimen consisting of the three combinations of resistors and capacitors in series with the solution resistance, represented as Rs(R1Cdl) (R2Cdl1) (R3Cdl2), where, R3and

Cdl2representing the resistance and capacitance for the

Ag-p(Py-co-EDOT) layer on Cu-HNT coated LN SS, respectively This in-dicates the presence of two time constants, which corresponds to the inner Cu-HNT layer and the top Ag-p(Py-co-EDOT) layer

The Nyquist plots obtained for the uncoated, p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT dual

layer coated LN SS specimens in M H2SO4 solution under the

condition of an open circuit potential are shown inFig S4 The impedance parameters were calculated afterﬁtting the spectra to the equivalent circuits (Fig S4aandFig S4b) as discussed earlier For the uncoated LN SS specimen, the polarisation resistance (Rp) is

shown to be 78U∙cm2 The R

pvalue obtained for the

p(Py-co-EDOT) and Ag-p(Py-co-p(Py-co-EDOT) coated LN SS specimen is found to be greater as 843 and 964U∙cm2than that of the uncoated LN SS,

respectively The Cu-HNT coated LN SS has higher Rp value

(1262U∙cm2) due to the effective protection of LN SS As seen from

Fig 7a, the Rpvalue obtained for the mono layer coated LN SS is

lesser than that of dual layer coated LN SS specimen owing to its void space in between them The Rp value is found to be

1639U∙cm2for Ag-p(Py-co-EDOT) coated on Cu-HNT coated LN SS,

which shows greater Rpthan that of the individual coatings of

p(Py-co-EDOT), Ag-p(Py-co-EDOT) and Cu-HNT on LN SS samples The increment of the Rpvalue for this dual layer coating is due to

the effective barrier of the mono layer coated with an underneath passive ﬁlm followed by the Ag-p(Py-co-EDOT) layer From the above results, it could be well ascertained that the dual layer of Ag-p(Py-co-EDOT)/Cu-HNT on LN SS could yield the corrosion resistive coating EIS technique provides information on both resistive and capacitive behaviors of all the specimens in the sulphuric acid medium and the values are given inTable The studies were carried out based on the open circuit potential (Fig 7a)

The Bode and phase plots for the uncoated, p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS samples in M H2SO4solution at an OCP condition are

shown inFig 7b and c

An analysis of these impedance measurement results reveal higher values of (Rp) andjZj for the LN SS owing to the

electro-chemically synthesized p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-Cu-HNT dual layer coatings, indi-cating the corrosion protection efﬁciency of the mono and dual layer coating The RpandjZj values are ranked in the order as

p(Py-co-EDOT)< Ag-p(Py-co-EDOT) < Cu-HNT < Ag-p(Py-co-EDOT)/Cu-HNT, respectively These results reveal that the higher RpandjZj

values for the Ag-p(Py-co-EDOT)/Cu-HNT dual layer copolymer coated LN SS demonstrate its better corrosion protection efﬁciency than those of the other p(Py-co-EDOT), Ag-p(Py-co-EDOT) and Cu-HNT coatings due to the more compact and also the effective barrier nature of the coating which is further conﬁrmed by FESEM (Fig 3f) Also, the higher RpandjZj values of the Ag-p(Py-co-EDOT)/Cu-HNT

dual layer composite coating on LN SS are possible owing to an area effect where the Ag-p(Py-co-EDOT)/Cu-HNT dual layer composite coating is hindering the access of the aggressive electrolyte to the metal surface [4,37] From the above results, it could be well ascertained that Ag-p(Py-co-EDOT)/Cu-HNT dual layer co-polymer coating on LN SS is superior in corrosion protection

3.6 ICP-AES analysis

Fig 8shows the level of the metallic ion released from the un-coated LN SS, p(Py-co-EDOT), p(Py-co-EDOT) Cu-HNT and Ag-Fig Potentiodynamic polarisation curves for uncoated, p(Py-co-EDOT),

Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated on LN SS in a M H2SO4solution

Table

Electrochemical parameters of the uncoated, p(Py-co-EDOT), Ag-p(Py-co-EDOT), Cu-HNT and Ag-p(Py-co-EDOT)/Cu-HNT dual layer coating on LN SS in M H2SO4solution

Sample Ecorr[mV] Icorr[mA∙cm2] Rp[Um2] jZj [Um2] PE [%]

Uncoated 0.482 2.818 103 78 115 e

p(Py-co- EDOT) 0.433 0.562 103 843 1015 80.05

Ag-p(Py -co- EDOT) 0.417 0.433 103 964 1138 84.63

Cu-HNT 0.401 0.265 103 1262 1416 90.59

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p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS samples, respec-tively in H2SO4solution in the range of 165e1000 nm The ICP-AES

investigation made on the uncoated LN SS sample which was subjected to the impressed potential of 455 mV in the H2SO4

so-lution shows a major amount of leached metallic ions, such as Mn, Fe, Cr and Ni The p(Py-co-EDOT), Ag-p(Py-co-EDOT) and Cu-HNT coated LN SS sample in the H2SO4solution shows that the leach

out of metallic ions (Mn, Fe, Cr, Ni, Ag, Cu and Al) is noticeably

reduced compared to that of the uncoated LN SS The release of metal ions from Ag-p(Py-co-EDOT)/Cu-HNT dual layer coated LN SS is reduced more significantly for other coatings, which is due to the as-formed the Ag-p(Py-co-EDOT)/Cu-HNT in its underneath that protects the LN SS Thus, the Ag-p(Py-co-EDOT)/Cu-HNT coating plays a dual role in forming the passivefilm in its underneath and to shield the breakdown of the passivefilm from the harsh envi-ronment which in turn offers a better acidic nature corrosion pro-tection performance of LN SS with a longer life time in industrial applications

3.7 Antibacterial study

Microbiologically influenced corrosion (MIC) is directly associ-ated with the unwanted migration of bacteria on a metal surface and bacteria (biofilm) on stainless steels suffering from the local-ized corrosion in marine bacteria [6,9] Now, in order to further reduce the growth of (biofilm) on metal surface, the Cu and Ag polymer composite was coated on the metal surface The coated polymer material was found to have higher antibacterial activity against the marine bacteria (Pseudoalteromonas sp and Desulfato-bacter sp) as compared to the normal Desulfato-bacteria (Escherichia coli and Staphylococcus aureus), showing the high antibacterial inhibition values The above results reveal the more protective nature of the dual layer coating in the marine environment The results are depicted inFig 9a and b

4 Conclusion

The Ag-p(Py-co-EDOT)/Cu-HNT dual layer coatings on LN SS were successfully fabricated by the cathodic electrodeposition of the Cu-HNT layer followed by Ag-p(Py-co-EDOT) Surface morphological and cross sectional FE-SEM results of the dual layer coatings on LN SS show the small granular PEDOT layer of thickness ~1.77mm followed by the three-dimensional granular Ag-p(Py-co-EDOT) layer of thickness ~2.23mm We have analyzed the dual layer coatings using potentiodynamic polarisation study and found that the dual layer coating signiﬁcantly enhanced the corrosion pro-tection of LN SS, which is conﬁrmed by the Ecorrand Icorrvalues

showing a maximum shift in the noble direction Also, the highest Rpvalue obtained through the EIS study complemented these

re-sults The copolymer and composite are adherent, with a protection efﬁciency of 80.05, 84.63 and 90.59% towards LN SS corrosion in M H2SO4medium Whereas on coating with Ag-p(Py-co-EDOT),

the dual layer Ag-p(Py-co-EDOT)/Cu-HNT coating exhibits a pro-tection efﬁciency of 97.41% The antibacterial activity against Fig (a) Nyquist, (b) Bode and (c) phase plots for the uncoated, p(Py-co-EDOT),

Ag-p(Py-co-EDOT) Cu-HNT and Ag-Ag-p(Py-co-EDOT)/Cu-HNT dual layer coating on LN SS in M H2SO4solution

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bacteria Pseudoalteromonas sp and Desulfatobacter sp for the dual layer Ag-p(Py-co-EDOT)/Cu-HNT reveals the highest inhibition ef-ﬁciency among all other coatings against marine bacteria The leach out analysis for the two types of coatings showed only a trace level leach out of metal ions for the dual layer coating, conﬁrming its defence protective properties against the attack of corrosive ions The Ag-p(Py-co-EDOT)/Cu-HNT dual-layer coating can thus act as a better corrosion resistant coating for improved and prolonged performance The dual layer coating also acts as a good barrier towards LN SS corrosion in M H2SO4medium

Acknowledgements

P Karthikeyan acknowledges the financial support received from Periyar University in the form of University Research Fellowship (URF) and Dr Santhini Elango, Senior Scientific Officer, COE-Medical Textiles, The South India Textile Research Association (SITRA) Coimbatore for recording the FESEM and Biological Studies Appendix A Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.jsamd.2017.12.003

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