An intelligent coating based on pH-sensitive hybrid hydrogel for corrosion protection of mild steel

Journal Pre-proofs An Intelligent Coating Based on pH-Sensitive Hybrid Hydrogel for Corrosion Protection of Mild Steel Jiaxin Wen, Jinglei Lei, Jinlong Chen, Jianjun Gou, Ying Li, Lingjie Li PII: DOI: Reference:

S1385-8947(19)33157-2 https://doi.org/10.1016/j.cej.2019.123742 CEJ 123742

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

11 March 2019 15 November 2019 6 December 2019

Please cite this article as: J. Wen, J. Lei, J. Chen, J. Gou, Y. Li, L. Li, An Intelligent Coating Based on pH-Sensitive Hybrid Hydrogel for Corrosion Protection of Mild Steel, Chemical Engineering Journal (2019), doi: https://doi.org/ 10.1016/j.cej.2019.123742

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An Intelligent Coating Based on pH-Sensitive Hybrid Hydrogel for

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Corrosion Protection of Mild Steel

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Jiaxin Wena,b, Jinglei Leia, Jinlong Chena, Jianjun Goua, Ying Lib, Lingjie Lia*

4

aSchool

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400044, China

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of Chemistry and Chemical Engineering, Chongqing University, Chongqing

bSchool

of Chemical and Pharmaceutical Engineering, Chongqing Industry Polytechnic

College, Chongqing 401120, China

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Corresponding author at: School of Chemistry and Chemical Engineering, Chongqing

*

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University, China. Tel.: +86 15023661557.

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E-mail address: [email protected]

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Abstract In the present study, we developed an intelligent coating for the corrosion

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protection of mild steel. A novel hybrid hydrogel of BTA@PHVA/PEI was

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synthesized through free radical polymerization. The morphology and composition of

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BTA@PHVA/PEI were characterized by SEM, FTIR and TGA/DSC. The as-prepared

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BTA@PHVA/PEI presents high thermal stability and the weight percentage of the

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loaded BTA is about 10.12 wt%. The self-releasing property of BTA@PHVA/PEI was

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evaluated by UV-vis spectroscopy, which indicates that the releasing rate rises with

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increasing the external environment pH values. Furthermore, an intelligent coating

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was fabricated on mild steel by using an alkyd primer doped with various contents of

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BTA@PHVA/PEI. The corrosion protection of the intelligent coating was evaluated

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by electrochemical measurements and neutral/acid salt spray tests. The results

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demonstrate that the anti-corrosion capability of the intelligent coating increases with

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the content of BTA@PHVA/PEI doped into the coating. The corrosion of mild steel

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was prohibited by forming a layer of inhibitor-adsorptive film from the released BTA

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inhibitors, which was detected on the steel surface after 30 days immersion in the

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corrosive medium by EDS and FTIR. The as-prepared intelligent coating in this work

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shows remarkable corrosion protection for mild steel, which also sheds light on

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corrosion protection of other metals and alloys.

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Keywords: Intelligent coating; pH-sensitive hybrid hydrogel; Self-releasing

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inhibitor; Corrosion protection; Mild steel

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1. Introduction Mild steel is among the most widely utilized engineering materials [1] such as in

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machinery equipment, transportation, chemical processing, pipelines, mining and

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construction [2, 3]. However, the corrosion of mild steel significantly deteriorates its

38

strength, security, and appearance, resulting in catastrophic failure or other serious

39

consequences [4, 5]. Therefore, developing effective, economical, and eco-friendly

40

techniques for prohibiting the corrosion of mild steel is a critical design decision.

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Nowadays, coatings are commonly applied on steel to provide a dense barrier against

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the corrosive environment to protect steel surfaces from aggressive substances attack

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[4, 6-8]. There are varieties of corrosion protective coatings which can be roughly

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divided into metallic, inorganic and organic [9]. Organic coatings, such as paint, have

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been widely used in metal corrosion protection [10]. However, unsatisfactory

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durability reduces their value in industry application. When the paints have slight

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defects and the steel surface is exposed to the corrosive environments, the paints will

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fail to prevent the corrosion process [11].

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To solve these problems, a new generation of intelligent coatings with active

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corrosion inhibitors which contain two functional components has been proposed. One

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part is a primer coating, such as paint or other polymer coating, which acts as a

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physical barrier against a corrosive environment. The other is intelligent

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microcapsules, which have been utilized in a variety of fields, such as drug release,

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flavor preservation, catalyst, slow-release fertilizer and so on [12-16]. They are

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uniformly dispersed in the primer coating and can respond to local environment -3-

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changes associated with corrosion process, such as ionic strength, pH and potential

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changes [17-20], and release corrosion inhibitors to provide continuous protection for

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metal substrates. Recently, there has been growing interest in the use of capsules

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doped with corrosion inhibitors for self-healing ability of intelligent coatings [21, 22].

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In most cases, corrosion reactions are associated with external environment pH

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changes [23]. Therefore, it is ideal to design and establish an intelligent capsule

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containing corrosion inhibitors which responds to pH shifting. To date, in order to find

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an intelligent capsule that suits the application of self-releasing intelligent coatings,

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varieties of pH-sensitive capsule materials have been prepared, and their functions in

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corrosion inhibition have been extensively investigated [24-26]. Firstly, SiO2 materials

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have been widely applied as nanocontainers for their unique properties, such as high

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specific surface area, tunable pore structure, and narrow pore size distribution. For

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instance, mesoporous SiO2 with hexagonally ordered (4~14nm), SiO2 nanoparticles

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based polyelectrolyte were prepared for encapsulation of corrosion inhibitors by

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Falcón [27] and Feng [28], respectively. Furthermore, compared with general

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mesoporous SiO2 materials, functionalized mesoporous SiO2 materials have attracted

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quite an interest owing to their longer-term releasing capability and more sensitive

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response to the environmental changes. For example, the amine-functionalized

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well-ordered mesoporous SiO2 nanocontainer [29], SiO2-imidazoline nanocomposites

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[30] were synthesized and used as pH-sensitive nanocontainer for controlled release of

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corrosion inhibitors.

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Apart from the mesoporous SiO2, polymeric materials such as urea formaldehyde

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[31], polyaniline nanofibers [32], chitosan [33], et al. have also been successfully

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applied in pH-sensitive intelligent coatings because of their versatile network structure

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and functionalized groups. Likewise, as a kind of polymeric materials, hydrogel

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particles are often preferred as microcapsules loaded corrosion inhibitor used for

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intelligent coating system. Chen [34] studied the anti-corrosion capability of a novel

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self-healing system, which contains poly (lactic-co-glycolic) acid hydrogel particles

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loaded with the corrosion inhibitor BTA. Snihirova et al. [35] developed the

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pH-sensitive Eudragit hydrogel particles, which could be able to store a high amount

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of corrosion inhibitors and release the stored inhibitor at acidic environments. In

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addition, an intelligent corrosion inhibitor with pH-sensitive for enhanced corrosion

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protection based on BTA and photo-crosslinking poly (2-dimethylaminoethyl

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methacrylate) hydrogel carrier have been developed by Ren [36].

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However, for most intelligent capsule materials so far, the following issues are

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still unsatisfactory: (1) the compatibility of the inorganic materials with the organic

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coating; (2) the thermal stability of the hydrogel materials; (3) the complicated

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fabrication procedure for scalable production. Fortunately, we can overcome these

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problems by preparing a hybrid hydrogel system based on the skeleton containing

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inorganic and organic parts, in which the inorganic parts stabilize the hybrid system,

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while the organic segments sense the external pH changes. In this work, we have

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addressed the above issues by properly preparing pH-sensitive hybrid hydrogel

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materials through one-pot synthesis method. This well-defined functional hydrogel -5-

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was prepared through the polymerization of HPA, VTEO and AA, and further

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wrapping by PEI. Moreover, BTA is widely used as an effective corrosion inhibitor

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for metal protection [37-39], which was pre-loaded in the synthetic hybrid hydrogel.

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Afterwards, an intelligent coating was developed by the doping of BTA@PHVA/PEI

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in an alkyd primer. SEM, FTIR, TGA/DSC were employed to characterize the

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morphology and composition of the prepared hydrogel. The self-releasing behavior of

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the loaded BTA in response to the change of external pH was investigated by UV-vis

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spectroscopy. In addition, the anti-corrosion performances of the intelligent coatings

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and their anti-corrosion mechanism were investigated by XPS, electrochemical

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measurements, neutral/acid salt spray experiment, EDS and FTIR.

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2. Experimental Section

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2.1 Materials and chemicals

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The chemical composition of the mild steel employed in this work contains (wt

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%) 0.22 C, 1.4 Mn, 0.35 Si, 0.045 S, 0.045 P and Fe for balance. The working

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electrode (WE) for electrochemical measurements was cut into a 1 cm long cylindrical

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rod with a cross-sectional area of 1 cm2, and with only the cross-section exposed as

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the remaining area insulated by epoxy resin. Before each experiment, the WE was

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polished to obtain a mirror finish by a sequence of emery papers of grade No.400, 600,

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1200, 1500 and 2000, and then washed with deionized water and degreased by

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acetone. The steel panels used for accelerated corrosion tests were cut into 5 cm long,

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4 cm wide and 0.2 cm thick, and the surface treatments were the same as the aforesaid

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processes.

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Monomers including hydroxypropyl acrylate (HPA), vinyltriethoxysilane (VTEO)

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and acetic acid (AA) were purchased from Chengdu Chron Chemicals Co., Ltd.

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2,2'-Azoisobutyronitrile (AIBN) obtained from Sinopharm Chemical Reagent Co., Ltd

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was served as oil-soluble initiator. Polyethyleneimine (PEI, molecular weight Mw ~

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600) with a purity of ≥ 99 wt% was purchased from Shanghai Macklin Biochemical

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Co., Ltd. Commercial grade alkyd resin was served by Nanjing Changjiang Paint Co.,

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Ltd. The inhibitor benzotriazole (BTA) and other chemicals with analytical grade were

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all purchased from Chengdu Chron Chemicals Co., Ltd and used as received. All

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solutions were prepared using deionized water (≤ 0.01 mS/m).

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2.2 Synthesis of BTA@PHVA/PEI

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The hybrid hydrogel of poly (hydroxypropyl acrylate – vinyltriethoxysilane –

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acetic acid)/polyethyleneimine pre-loaded corrosion inhibitor benzotriazole

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(BTA@PHVA/PEI) was prepared by one-pot synthesis method. For the whole

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process, the system was mechanically stirred and temperature was maintained by

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thermostatic oil bath at 75 °C. Firstly, the HPA (6.5 g) was dissolved in 60 mL

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2-methoxyethanol in a three-necked flask equipped with allihn condenser,

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thermometer and tap funnel. After HPA completely dissolved, nitrogen (N2) was

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bubbled into the solution to remove the air. Afterwards, AIBN (0.1 g) and BTA (6.0 g)

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were added into the solution successively. Then, the mixture of VTEO (11.4 g) and

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AA (14.4 g) in the tap funnel were dropwise added into the mixed solution

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continuously for two hours. After further 2 hours reaction, the cross-linked polymer

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hybrid hydrogel was obtained. Finally, the resulting viscous liquid was adjusted to pH

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5.0 and mixed with PEI (4.0 g), after being stirred for another 0.5 hours, the hybrid

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hydrogel was wrapped by the PEI. The whole preparation process is shown in Fig. 1a.

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After the reaction, the product was dried at 60 °C in vacuum to remove redundant

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solvent. For removing the unreacted reagents, the dried solid was immersed in

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deionized water for 5 days, and refreshed the deionized water every 24 hours. Then

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the solid was freezing-dried for 3 days and finely ground to 200 mesh powders by a

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ball grinder. The hybrid hydrogel of poly (hydroxypropyl acrylate –

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vinyltriethoxysilane – acetic acid)/polyethyleneimine without BTA loaded

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(PHVA/PEI) was also prepared as the aforesaid processes while except the addition of

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BTA.

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2.3 Preparation of the intelligent coating

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The serial intelligent composite coatings were prepared via dispersing various

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contents of the as-prepared BTA@PHVA/PEI into the alkyd resin under at least 5

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minutes of mechanical stirring. The formulated mixtures were covered on the polished

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mild steel surface using a brush. The coated mild steel samples were kept in a 40 °C

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oven for five days to obtain a fully curing. The dry-coating thickness was measured by

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a coating thickness meter and it was found to be around 140 μm.

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2.4 Characterization

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The structure and functional groups of the as-prepared hydrogel were

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characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet NEXUS-470,

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USA, attached with ATR). Each sample was scanned in the range of 400-4000cm-1

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wavelength and the IR spectra were recorded. To evaluate the thermal stability of

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BTA@PHVA/PEI and deduce its composition, thermogravimetric

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analyzer/differential scanning calorimetry (TGA/DSC) analysis was carried out on a

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TGA/DSC synchronization analyzer (Mettlertoledo STAR TGA/DSC 851,

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Switzerland). The TGA/DSC analysis was performed under a nitrogen atmosphere in -8-

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the temperature range of 30 ~ 1000 °C with a heating rate of 10 °C min-1. The

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composition of BTA@PHVA/PEI was estimated through thermal decomposition

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process of the samples as the heating temperature increase. Besides, to investigate the

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surface morphology, scanning electron microscope (SEM, JSM-7800F) was used to

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observe PHVA/PEI.

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X-ray photoelectron spectroscopy (XPS, Thermo electron ESCALAB250, USA)

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was employed to reveal the surface composition of the steel sample coated with the

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intelligent coating. To investigate the anti-corrosion mechanism of the intelligent

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coating, after 30 days immersion into 3 wt% NaCl solution at 25°C and the different

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coatings being peeled off, the surfaces of the coated steel panels were analyzed by

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energy-dispersive X-ray spectrum (EDS, JSM-7800F, Japan) and FTIR instruments.

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2.5 Controlled release of BTA

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The UV-vis spectroscopy (Mapada UV-1800, China) was employed to

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characterize the controlled release of the inhibitors BTA from BTA@PHVA/PEI, and

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the BTA-releasing behaviors triggered by the different pH solutions were also

184

investigated. For this purpose, 1.0 g BTA@PHVA/PEI was added into 100 mL NaCl

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solution (3 wt%) to make a suspension. The solution pH values were carefully

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adjusted to 2.0, 5.0, 7.0, 9.0 and 11.0 by adding HCl or NaOH solutions. Release

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profiles were acquired by plotting the absorbance of BTA in the solution at the wave

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length of 259 nm as a function of the time.

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2.6 Electrochemical measurements

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A conventional three-electrode system was used for EIS measurements, in which

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the mild steel samples were served as WE, a large surface area of platinum sheet (2cm

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× 2cm) as the counter electrode and a saturated calomel electrode (SCE) as the -9-

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reference electrode. In order to evaluate the anti-corrosion properties of the

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as-prepared intelligent coatings, the coated WEs were treated with about 100 μm wide

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artificial cross-shaped scratches by a sharp tool. All the measurements were then

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carried out in 3 wt% NaCl solution (pH=6.75) at 25 °C to imitate the seawater

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environment. After the working electrode was immersed in the solution for 1 hours to

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obtain a stable open circuit potential (OCP), the EIS measurements were then

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performed on a CHI660B electrochemical workstation (Shanghai CH Instrument,

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China) with a frequency ranges from 105 Hz to 10-2 Hz and a sinusoidal signal of 5 mV

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in amplitude around the open circuit potential. The results were fitted according to the

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advisable equivalent circuits. Also, the widely used potentiodynamic polarization

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curves were recorded within the potential range of -250 mV to +250 mV (vs. OCP) at

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a scan rate of 1 mV s-1 as well.

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2.7 Accelerated corrosion test

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The accelerated corrosion tests following the recommendation of ISO9227:2006

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standard were carried out by using a salt spray tester (Lanhao Y/Q-250, China). To

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speed up the corrosion process, the artificial cross-shaped scratches were also made on

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the surface of the coated panels before putting into the salt spray tester by a sharp tool.

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3. Results and discussion

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3.1 Characterization results

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3.1.1 FTIR spectra

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The FTIR spectra of BTA@PHVA/PEI and PHVA/PEI are shown in Fig. 2. The

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peaks located at 1047 cm-1 and 927 cm-1 can be assigned to the stretching and bending

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vibrations of Si-O-Si bonds, respectively. While the high-intensity sharp peaks around

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1714cm-1 and the band at around 1269cm-1 can be attributed to C=O stretching - 10 -

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vibration and C-OH stretching vibration from AA and HPA. For the presence of large

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amount of O-H groups in hydrogel molecules, the spectra are dominated by a series of

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stretching vibrations around 3205 cm-1. Meanwhile, the stretching vibrations of -CH,

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-CH2 and -CH3 bonds in the hydrogel samples lead to the high absorption around 2946

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cm-1. On the other aspect, according to the spectrum of BTA@PHVA/PEI, the

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absorption peak around 1103 cm-1 is identified as the vibration of the typical triazole

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rings containing in BTA. Besides, the considerable intensity peaks at 1455 cm-1 and

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1538 cm-1 are attributed to N-H stretching vibration from BTA and/or PEI. Obviously,

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the FTIR results indicate that the inhibitors BTA are successfully loaded in

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PHVA/PEI.

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3.1.2 Thermal stability analysis

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Fig. 3 compares the TGA/DSC analysis results of PHVA/PEI and

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BTA@PHVA/PEI. In these cases, the TGA curves shown in Fig. 3a exhibit nearly

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three-stage processes of decomposition. In detail, the stage Ⅰ occurs in the

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temperature region of 28 °C ~ 150 °C, presenting the weight losses of 4.66 % and 6.20

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% for the as-prepared PHVA/PEI and BTA@PHVA/PEI, respectively. These weight

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losses correspond not only to the removal of moisture and solvent, but also to the

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degradation of the surface hydroxyl groups [40]. The stage Ⅱ for BTA@PHVA/PEI

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ranges from 150 °C to 320 °C, where a weight loss of 26.51 wt% presents. However,

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the stage Ⅱ for PHVA/PEI exhibits a weight loss of 16.39 wt%, which is 10.12 wt%

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lower than that of BTA@PHVA/PEI. Notably, the melting and boiling points of BTA

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are 98.5 °C and 204 °C, respectively. Based upon the above results, it can be deduced

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that about 10.12 wt% of BTA has been successfully loaded into PHVA/PEI. Upon the

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temperature at 250 °C, the loaded BTA has decomposed completely, and the - 11 -

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maximum decomposition rate of the loaded BTA emerges around 177 °C ~ 213 °C.

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Apart from the decomposition of BTA, the weight loss of PHVA/PEI at stage Ⅱ also

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comes from the degradation of PEI, whose decomposition temperature ranges between

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280 °C and 310 °C. As a result, the approximate weight percentage of PEI wrapped on

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the hybrid hydrogel can be determined to be 16.39 wt%. The major weight losses of

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about 55.66 wt% and 44.16 wt% are observed at the stage Ⅲ within the temperature

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region of 320 °C ~ 900 °C for the as-prepared PHVA/PEI and BTA@PHVA/PEI,

248

respectively. This can be attributed to the degradation of the organic chains and the

249

siloxane part of PHVA/PEI. When the temperature rises to 450 °C, the organic chains

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of the hybrid hydrogel are degraded completely. As a consequence, the weight loss

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ranging from 450 °C to 900 °C is mainly attributed to the degradation of the siloxane

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part in VTEO, which is calculated as 18.68 wt% for PHVA/PEI. Thus, the appearance

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of this stage indicates that VTEO has been introduced into the hybrid hydrogel

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successfully, and the contents of VTEO in the hybrid hydrogel can be estimated to be

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27.30 %. Afterwards, the weight remains stable up to 900 °C. The residual weights are

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mostly the carbon residue and SiO2 salt, which for PHVA/PEI and BTA@PHVA/PEI

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are 20.46 wt% and 18.17 wt%. Obviously, due to the loaded BTA decomposition, the

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residual weight of PHVA/PEI is slightly higher than that of BTA@PHVA/PEI.

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Furthermore, typical DSC curves as shown in Fig. 3b indicate the similar

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behavior as the TGA curves. Firstly, the endothermic peaks around 70 °C were

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observed for both PHVA/PEI and BTA@PHVA/PEI. These peaks not only represent

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the volatilization of the absorbed moisture and solvent in PHVA/PEI, but also the

263

degradation of the surface hydroxyl groups. The loaded BTA in PHVA/PEI is

264

confirmed again by the main thermal decomposition peak around 200 °C.

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Additionally, the endothermic peak was observed around 430 °C, indicating the - 12 -

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degradation of the organic chain backbone of PHVA/PEI. Moreover, the endothermic

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peaks around 800 °C may be related to the degradation of the siloxane part of

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PHVA/PEI.

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In summary, it is concluded that BTA@PHVA/PEI has high thermal stability

270

below 150 °C of the environment temperature from TGA/DSC analysis, indicating

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BTA@PHVA/PEI can remain stable at the coating service temperature. Meanwhile, it

272

is also indirectly calculated that about 10.12 wt% of the inhibitors BTA have been

273

loaded in BTA@PHVA/PEI. The weight percentages of PEI and VETO have been

274

introduced into the hybrid hydrogel, which are estimated to be 16.39 % and 27.30 %,

275

respectively.

276

3.1.3 Morphology of PHVA/PEI

277

To observe the morphology of the PHVA/PEI more intuitively, SEM images

278

were recorded on an electron microscope at 2.00kV. Fig. 4 shows the SEM images of

279

the vacuum-dried as-prepared PHVA/PEI sample and the freezing-dried PHVA/PEI

280

samples. The freezing-dried PHVA/PEI samples had been immersed in the solutions

281

of pH=2.0, pH=7.0 and pH=11.0 for 24 hours before freezing-drying, respectively. It

282

is clear that different conditions cause different morphology. For the vacuum-dried

283

PHVA/PEI in Fig. 4a, the surface is uneven and coarse, which can provide space for

284

water retention and BTA accommodation [41]. A few holes are formed on the hybrid

285

hydrogel surface possibly due to the water evaporation resulting from vacuum dried

286

process [42]. However, after being placed into the basic solution (Fig. 4b), there are

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many holes with varying sizes and wrinkles emerge on the surface of the

288

freezing-dried hybrid hydrogel. Nevertheless, when the solution changes to neutral or - 13 -

289

acidic (as seen in Fig. 4c, d), the holes disappear and the surface of the network is

290

relatively flat and compact. Generally, the holes might be induced into the hydrogel

291

surfaces by the evaporation of the absorbed water, and the wrinkles possibly come

292

from the electrostatic repulsion between -COO﹣and -NH3+ groups in basic conditions

293

[42]. In contrast with the neutral or acidic solutions, the hybrid hydrogel can absorb

294

more water as a result of higher swelling ratio in the basic solutions. After water

295

evaporating, lots of holes can be preserved in dried hydrogel owing to the

296

freezing-dried method (the absorbed water evaporate through sublimation), which

297

keeps the hydrogel dry in its original states. Simultaneously, in view of the Fig. 4c and

298

Fig. 4d, there are some different appearances present as compared with their holes and

299

wrinkles on the hydrogel surfaces. It is implied that the hybrid hydrogel has slightly

300

higher water absorption in neutral solution than in acidic solution.

301

More precisely, this diverse phenomenon may be root from the ionization

302

behavior of the -COOH groups in PHVA/PEI in response to external environment pH

303

changes. In acidic conditions, the amount of absorbed water by PHVA/PEI decreases

304

due to the formation of hydrogen bonds between -COOH groups and -OH groups.

305

However, with the increase of pH values, -COOH groups begin to ionize, the

306

hydrogen bonds between the polymeric chains break, and the electrostatic repulsion

307

force between -COO- and -NH3+ leads to the expanding of the polymeric network [42].

308

Thus, PHVA/PEI absorbs more water and exhibits higher swelling ratio in the basic

309

conditions. As a consequence, PHVA/PEI hydrogel possesses more holes due to the

310

water evaporation under the basic condition than in the acidic. Besides, amounts of

311

water evaporation and strong electrostatic repulsion in PHVA/PEI molecules result the - 14 -

312

evident wrinkles. More importantly, these results also indicate that PHVA/PEI have

313

pH-sensitive properties.

314

3.1.4 XPS analysis of the intelligent coating

315

The chemical composition of the steel panel coated with the intelligent coating

316

(doped with 10 wt% BTA@PHVA/PEI) was investigated by XPS. Fig. 5a shows the

317

survey spectra, in which four elements of C, O, N and Si are detected on the surface of

318

the coating. The strong N and Si peaks indicate that BTA@PHVA/PEI has been

319

successfully doped into the alkyd coating. In order to further determine the chemical

320

composition of the intelligent coating, the high-resolution spectra of the characteristic

321

elements were analyzed. The C 1s high-resolution spectrum (Fig. 5b) can be fitted by

322

three peaks, the binding energies of 288.5 and 286.0 eV are attributed to C=O, C-O,

323

and the peak at 284.4 eV is ascribed to C-C [43, 44]. The high-resolution N 1s

324

spectrum (Fig. 5c) shows two obvious peaks at 399.2 and 399.8 eV, which are

325

attributed to -N(CH3)2/-NH2 and -N+(CH3)3/-NH3+ in PEI [45], respectively. The O 1s

326

high-resolution spectrum (Fig. 5d) can be fitted by two peaks located at 531.6 and

327

532.5 eV, which can be assigned to -C=O and -C-O, respectively [43, 46]. The Si 2p

328

high-resolution spectrum (Fig. 5e) can be fitted by two peaks, the binding energies of

329

101.4 eV is attributed to -Si-C, and the peak at 102.0 eV is ascribed to -Si-O [47].

330

Therefore, the XPS analysis further proves BTA@PHVA/PEI has dispersed

331

successfully in the intelligent coating.

332

3.2 Releasing characteristics of BTA@PHVA/PEI

333

To investigate the pH-sensitive releasing behavior of BTA@PHVA/PEI, the

334

release characteristics of BTA@PHVA/PEI were studied in neutral, acidic and basic

335

solutions. The release curves of BTA molecules from BTA@PHVA/PEI at different - 15 -

336

pH values are shown in Fig. 6. According to the diagram, the BTA-releasing behavior

337

is related to the pH values of the solutions, and the cumulative release percentage

338

increases with time increasing. In the acidic media, e.g. pH values 2.0 and 5.0, there is

339

only 68.78 wt% and 71.18 wt% of BTA released after 32 hours immersion,

340

respectively. However, when the pH rises to 9.0 and 11.0, the accumulative release

341

percent of BTA is up to 76.34 wt% and 91.02 wt%, respectively, which means most of

342

the loaded BTA has been released, and the accumulative release percentage is

343

obviously higher than that in acidic media. Meanwhile, when the solution pH is 7.0,

344

the accumulative release percentage is between the acidic and basic solutions, whose

345

value reaches 74.51 wt%. As shown in Fig. 1b, this phenomenon may be attributed to

346

the special structure of PEI, whose charge density will decrease as the pH increase,

347

leading the branched polymeric chains of PEI deprotonating and shrinking. The

348

deprotonation of the PEI chains should cause the electrostatic attraction between PEI

349

and the PHVA molecules weakening, thereby the shrinkage of PEI chains and their

350

further weakening of electrostatic attraction with the PHVA molecules result in the

351

fast release of BTA from the hydrogel interior [48]. In addition, when the pH value

352

increases, the enhanced ionization degree of the carboxyl groups gives rise to an

353

increased swelling ratio, which can also result in the releasing rate increasing [49].

354

In order to understand the release model precisely, BTA released from

355

BTA@PHVA/PEI is fitted to the following equation (1) according to Ritger-peppas

356

model [50].

Mt  kt n M

(1)

357

Where M t M  represents the fraction of the released BTA at time t, k is a constant

358

which related to the properties of the BTA delivery system, and n is the diffusion - 16 -

359

exponent which determines the system release mechanism, and it can be calculated by

360

plotting ln M t M  versus lnt. Theoretically, when n<0.5, the release process is

361

dominated by Fickian diffusion model. When 0.5
362

non-Fickian diffusion. When n = 1, the system will be relaxation controlled and

363

exhibits a continuous zero-order release [51]. The values of the diffusion exponent n

364

are calculated following equation (1), and they are found to be 0.22, 0.22, 0.21, 0.18

365

and 0.21 in the solution of pH 2.0, 5.0, 7.0, 9.0 and 11.0, respectively. These values

366

clearly indicate that the release of BTA from BTA@PHVA/PEI follows Fickian

367

diffusion controlled model.

368

3.3 Electrochemical results

369

Fig. 7 represents the Nyquist and Bode plots obtained from the measurements on

370

the mild steel WEs coated with scratched coatings doped with various content of

371

BTA@PHVA/PEI after 0, 2, 4, 6, 8, 24 and 36 hours immersion in 3.0 wt% NaCl

372

solutions. To fit the EIS data measured on various coated WEs, the employed

373

equivalent circuits are shown in Fig. 8, where Rs, Rc, Rct represent the resistance of

374

solution, coating and charge-transfer, respectively. The constant-phase elements CPEc

375

and CPEdl respectively account for the capacitance of the coating and electrical double

376

layer. When the scratched coating immersed in the NaCl solution at the initial time,

377

the coating will be permeable to water but also considered protective. Thus, only the

378

constant phase element CPEc is employed for a more accurate fitting. After 4 hours

379

immersion, the corrosion of WE occurs, and the equivalent circuit with two constant

380

phase elements CPEc and CPEdl are chosen to fit the measured EIS. The impedance of

381

a CPE is given by the following equation (2) [52].

- 17 -

Z CPE  A1 (i )  n

(2)

382

Where A and ω are the CPE constant and angular frequency (in rad s-1), respectively.

383

i2=-1 represents the imaginary number, and n is a CPE exponent which measures the

384

deviation from the ideal capacitive behavior. ZCPE can represent inductance (n = -1, A

385

= L), resistance (n = 0, A = R), Warburg impedance (n = 0.5, A = W) or capacitance (n

386

= 1, A = C). The fitted results of the measured impedance spectra in Fig. 7 are listed in

387

Table 1.

388

As seen in Fig. 7, for the pure alkyd coating, it can be observed a high value of Rc

389

(~ 1391 Ω cm2) and one maximum phase angle when immersion starts. This is the

390

typical capacitive behavior for a coated WE, where the coating shows the barrier

391

properties against corrosion at this time. As for the intelligent coating, there are no

392

significant differences with the pure alkyd coating. Firstly, a capacitive impedance

393

behavior is presented, which means the coating can provide effective corrosion

394

protection for WE. With 2 hours immersion, the value of Rc decreases significantly. In

395

detail, for the coating doped with 5 wt% BTA@PHVA/PEI, Rc decreases to 2469 Ω

396

cm2, which is 586 Ω cm2 lower than immersed initially. However, Rc measured at the

397

4th hour becomes higher than that measured at the 2th hour. When the immersion time

398

reaches 24 hours, Rc shows further increasing. Nevertheless, when the immersion time

399

of the coated WE increase to 36 hours, the value of Rc starts to diminish. This presents

400

some differences from that measured in the pure alkyd coating. Compared with the

401

coating doped with 5 wt% BTA@PHVA/PEI, the coating doped with 10 wt% and 15

402

wt% BTA@PHVA/PEI exhibit the similar variation tendencies, while the value of Rc

403

measured at the 24th hour gradually enlarges from the coating doped with 5 wt% to 15

404

wt% BTA@PHVA/PEI in turns.

- 18 -

405

In addition, as shown in Table 1, it can be seen that the value of Rct and CPEc-T

406

for the pure alkyd coating decreases and increases with time goes, respectively,

407

proving the continuous degradation of the coating in the solution. For instance, the

408

value of Rct decreases to 399  cm2 when the coated WE after being placed into the

409

solution for 36 hours, which is lower than the original value. However, for the

410

intelligent coatings, Rct decreases and CPEc-T increases during the first 2 hours

411

immersion. Then they inversely turn to increase and decrease until the immersion time

412

reaches 24 hours, respectively. For example, at the 24th hour immersion, the values of

413

Rct are 3643  cm2, 5202  cm2, 5469  cm2 for 5 wt%, 10 wt% and 15 wt%

414

BTA@PHVA/PEI doped coatings, respectively. However, when the immersion time

415

is up to 36 hours, the values of Rct reduce to 2157  cm2, 2918  cm2 and 3100 

416

cm2, respectively. This is mainly owing to the reduction of the releasing BTA.

417

Generally, Rc is regarded as the resistance of a coating in a corrosive environment. Rct

418

is a parameter that negatively correlates with the corrosion rate, and it has been widely

419

used to describe the anti-corrosion capability of the material in the corrosive

420

environment. During the immersion period, the decreased Rct (Rc) and increased

421

CPEc-T (CPEdl-T) of the pure alkyd coating reveal that the anti-corrosion capability of

422

the coating is deteriorated gradually. However, although Rct (Rc) and CPEc-T

423

(CPEdl-T) of the intelligent coating show the same variation tendencies as the pure

424

alkyd coating within the first 2 hours immersion, the intelligent coating exhibits a

425

continuous increase of Rct (Rc), decrease in CPEc-T (CPEdl-T) during the period of 2 ~

426

24 hours immersion. This can be attributed to the releasing of BTA inhibitors stored

427

inside BTA@PHVA/PEI, which has been confirmed by the releasing behaviors of

428

BTA@PHVA/PEI. When mild steel suffers corrosion, the increasing pH value [53] of

429

the corrosion area will accelerate the releasing rate of the stored BTA inhibitors. Then, - 19 -

430

the released BTA can form a lower dielectric adsorptive film on the steel surface，

431

which will hinder the further corrosion reaction and lead to CPEc-T (CPEdl-T)

432

decrease and Rct (Rc) increase. Once the content of the doped BTA@PHVA/PEI in the

433

intelligent coating is up to 10 wt% and 15 wt%, there are greatly increasing of Rct (Rc)

434

and dropping in CPEc-T (CPEdl-T). However, when the immersion time over 24 hours,

435

Rct (Rc) values begin to decrease, indicating the reducing anti-corrosion capability of

436

the coating. This is because the releasing amount of the pre-loaded BTA is reduced as

437

mentioned above.

438

Besides, potentiodynamic polarization was employed to further investigate the

439

anti-corrosion performance of the intelligent coating. Fig. 9 shows the

440

potentiodynamic polarization curves of the mild steel WE coated with scratched

441

intelligent coatings after 0 and 24 hours immersion in 3.0 wt% NaCl solutions,

442

respectively. The associated electrochemical parameters such as corrosion potential

443

(Ecorr), corrosion current density (icorr), anodic and cathodic Tafel slope (βa, βc) are

444

deduced from the polarization curves and listed in Table 2. It can be observed that the

445

values of icorr slightly increase at the initial immersion, and Ecorr slightly shift towards

446

to positive direction as the incremental content of the doped BTA@PHVA/PEI,

447

indicating the intelligent coating shows better anti-corrosion abilities than the pure

448

alkyd coating. After 24 hours immersion, icorr value of WE coated with the pure alkyd

449

coating sharply increases, suggesting the corrosive species permeate into the WE

450

surface and the corrosion occurs. Meanwhile, the icorr values of WE coated with the

451

intelligent coatings inversely decrease with the content of the doped

452

BTA@PHVA/PEI increasing. According to the above mentioned results, it can be

453

deduced that the improved corrosion protection abilities account for the inhibitor BTA

454

molecules have released from BTA@PHVA/PEI when the corrosion occurs, and then - 20 -

455

adsorbed on the WE surface for retarding corrosion process. Thus, it can be

456

pronounced that the intelligent coating presents remarkable active anti-corrosion

457

performance. In addition, as shown in Fig. 9b, after 24 h immersion, the Ecorr values of

458

the coated WE move towards to positive direction, and Tafel constants βc change with

459

the content of the doped BTA@PHVA/PEI increasing, suggesting that the intelligent

460

coating retards the corrosion process of mild steel mainly through affecting the

461

cathodic hydrogen evolution mechanism [54].

462

Hence, BTA@PHVA/PEI plays a vital role in improving the active anti-corrosion

463

capability of the developed coatings. The effect mainly attributed to two aspects.

464

Firstly, the as-prepared BTA@PHVA/PEI has a good compatibility with the alkyd

465

primer coating, and would not affect the coating properties. Secondly, once a corrosive

466

environment associated with high pH conditions or the corrosion leads to the corrosive

467

area pH value increases, BTA@PHVA/PEI can sense this basic environment and start

468

to release BTA on demand for corrosion inhibition.

469

3.4 Accelerated corrosion test results

470

Fig. 10a ~ d show the results obtained from the neutral salt spray tests of the

471

scratched steel panels with different coatings after 240 hours exposure. In these

472

images, it is possible to see that the coated panel with the pure alkyd coating present

473

serious corrosion in different zones, especially around artificial scratches. Besides,

474

there are intensely blistering under the coating, indicating a permeation of the NaCl

475

solution towards the metal surface through the scratches and the edges of the steel

476

panels. As expected, the steel panels coated with the intelligent coatings have less

477

corrosion products and blisters around the scratched area. Notably, the intelligent

478

coating containing 10 wt% BTA@PHVA/PEI exhibits the best anti-corrosion - 21 -

479

performance, with which panel shows the least corrosion products and blisters around

480

the scratched area. Generally speaking, with the content of the doped

481

BTA@PHVA/PEI increasing, the anti-corrosion performance of the intelligent coated

482

steel panels will be enhanced because of the releasing amount of the inhibitor BTA

483

increase. However, the adhesion force between the intelligent coating and steel

484

substrate will gradually weaken with the content of the doped BTA@PHVA/PEI

485

increasing. Accordingly, the intelligent coating doped with 15 wt% BTA@PHVA/PEI

486

shows poorer adhesion performance than the coating doped with 10 wt%

487

BTA@PHVA/PEI, resulting in the blisters or delamination at the interface of the

488

coating. Consequently, the corrosion species will invade into the surface of the steel

489

panel and then render the corrosion occurrence. Thereby, the anti-corrosion

490

performance inversely becomes poor when the content of the doped

491

BTA@PHVA/PEI is up to 15 wt%.

492

Additionally, the results obtained from the acid salt spray tests of the scratched

493

steel panels with different coatings after 240 h exposure are shown in Fig. 10e ~ f. In

494

these images, beside the panel coated with pure alkyd coating suffers slightly more

495

serious corrosion than in the neutral salt spray tests, it is not found obvious differences

496

between the results obtained from the neutral and acid salt spray tests. Likewise, rare

497

corrosion occurs in the panels coated with the intelligent coatings, especially for the

498

coated panel doped with 10 wt% BTA@PHVA/PEI, further indicating the remarkable

499

anti-corrosion performance of the intelligent coating.

500

Therefore, it is demonstrated that the addition of BTA@PHVA/PEI into the

501

coating can effectively improve the anti-corrosion capability for mild steel, and the

502

optimum content of the doped BTA@PHVA/PEI is 10 wt%.

- 22 -

503

3.5 Anti-corrosion performance study

504

After releasing from BTA@PHVA/PEI, the BTA molecules would effectively

505

prevent corrosion of mild steel because they can be absorbed on the steel surface to

506

form a protective adsorptive film [55]. The three heterocyclic nitrogen atoms polar

507

units in BTA molecules are served as the reaction center for chemisorption with mild

508

steel. Strongly covalent bonds can form between N-atoms in the triazole ring and iron

509

atoms therefore displacing water molecules from the steel surface, sharing electrons

510

between the BTA molecules and iron through partial transference of electrons from

511

the polar N-atoms to the steel [56]. The formed adsorptive film has been confirmed by

512

the EIS results in Table 1.

513

Fig. 11 shows the SEM morphology and EDS spectra of the steel panel surface,

514

of which the coating was peeled off after 30 days immersion into 3 wt% NaCl

515

solution. Table 3 shows the EDS elemental composition results. As shown in Fig. 11,

516

with the doped BTA@PHVA/PEI increasing in the alkyd coating, the artificial

517

scratches on the panel surface become more apparent and the corrosion holes reduce

518

gradually. On the contrary, for the steel panels coated with the alkyd coating doped

519

with 10 wt% PHVA/PEI, the artificial scratches become blurry and multiple

520

corrosion holes emerge, which demonstrate that the substrate surface has suffered

521

from corrosion. Thus, it is verified that the coating doped with BTA@PHVA/PEI

522

exhibits better anti-corrosion performance than that only containing PHVA/PEI, and

523

a higher content of BTA@PHVA/PEI can prevent mild steel from corrosion more

524

effectively. From another aspect, the results of the elemental composition show an

525

increasing N content with the increasing content of BTA@PHVA/PEI doped in the

526

coating. As stated above, the element N is mainly from the adsorption inhibitors

527

BTA on the panel surface that released from BTA@PHVA/PEI, indicating BTA can - 23 -

528

self-release from BTA@PHVA/PEI and be adsorbed on the steel panel surface while

529

the corrosion occurs, this is in agreement with the results discussed above.

530

Furthermore, FTIR is used for confirming the adsorptive inhibitors on the steel

531

panel surfaces. As shown in Fig. 12, a new peak is found at 1248cm-1 in all the spectra

532

obtained from the surfaces of the steel substrates, which have been coated with the

533

intelligent coating and then immersed into 3 wt% NaCl solution for 30 days. This peak

534

can be assigned to the –N=N–N– stretching vibration, which roots from the BTA

535

inhibitors only, and the intensity of this peak becomes higher with the content of

536

BTA@PHVA/PEI increasing. Notably, this peak is not presented on the spectrum

537

measured on the substrate surface which has been coated with the coating doped with

538

10 wt% PHVA/PEI. Therefore, it is demonstrated that the preloaded inhibitors BTA

539

have been released from BTA@PHVA/PEI, and have been adsorbed on the steel

540

substrate surface to form a lower dielectric film for preventing further corrosion,

541

which has been well proved by EIS measurements.

542

4. Conclusions

543

(1) A novel hybrid hydrogel of BTA@PHVA/PEI was prepared through one-pot

544

synthesis method, which is more applicable for scalable production. The amount

545

percentage of the loaded BTA inhibitors is about 10.12 wt%. The self-releasing

546

behavior of BTA from BTA@PHVA/PEI follows Fickian diffusion model and

547

depends on the external environment pH values.

548

(2) An intelligent coating with the satisfactory compatibility has been fabricated

549

based on the alkyd primer doped with BTA@PHVA/PEI powder (the optimal content

550

of BTA@PHVA/PEI is 10 wt%). The intelligent coating shows remarkable

551

anti-corrosion performance for mild steel through forming an adsorptive film. - 24 -

552

(3) The as-prepared intelligent coating can be served as promising coating for

553

corrosion protection of mild steel, which can be also used for corrosion protection of

554

other metallic materials.

555

Acknowledgements

556

This work was supported by the Research Program of Chongqing Industry

557

Polytechnic College (GZY201708-ZA), the National Natural Science Foundation of

558

China (21573028, 21773019) and the Graduate Research and Innovation Foundation

559

of Chongqing (CYB18044).

560

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[43] L. Liu, J. Lei, L. Li, J. Zhang, B. Shang, J. He, N. Li, F. Pan, Robust Rare-Earth-Containing

669

Superhydrophobic Coatings for Strong Protection of Magnesium and Aluminum Alloys, Advanced

670

Materials Interfaces 5 (2018) 1800213-1800219.

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[44] L. Zhao, Q. Liu, R. Gao, J. Wang, W. Yang, L. Liu, One-step method for the fabrication of

672

superhydrophobic surface on magnesium alloy and its corrosion protection, antifouling performance,

673

Corrosion Sci. 80 (2014) 177-183.

674

[45] X. Hu, Y. Wang, L. Zhang, M. Xu, Construction of self-assembled polyelectrolyte complex hydrogel

675

based on oppositely charged polysaccharides for sustained delivery of green tea polyphenols, Food

676

Chem 306 (2020) 125632-125639.

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[46] J. Deng, X. Li, X. Wei, Y. Liu, J. Liang, N. Tang, B. Song, X. Chen, X. Cheng, Sulfamic acid modified

678

hydrochar derived from sawdust for removal of benzotriazole and Cu(II) from aqueous solution:

679

Adsorption behavior and mechanism, Bioresour Technol 290 (2019) 121765-121771.

680

[47] K. Sokolowski, M. Zambrzycki, A. Fraczek-Szczypta, S. Blazewicz, Ceramic coating formation during

681

carbothermic reaction of polysiloxanes with carbon and graphite materials, Materials Chemistry and

682

Physics 238 (2019) 121908-121918.

683

[48] G.L. Li, M. Schenderlein, Y. Men, H. Möhwald, D.G. Shchukin, Monodisperse Polymeric Core-Shell

684

Nanocontainers for Organic Self-Healing Anticorrosion Coatings, Advanced Materials Interfaces 1

685

(2014) 1300019-1300025.

- 28 -

686

[49] H. Zhao, J. Gao, R. Liu, S. Zhao, Stimulus-responsiveness and methyl violet release behaviors of

687

poly(NIPAAm-co-AA) hydrogels chemically crosslinked with beta-cyclodextrin polymer bearing

688

methacrylates, Carbohydr Res 428 (2016) 79-86.

689

[50] C. Liu, Y. Chen, J. Chen, Y. Chen, J. Chen, Synthesis and characteristics of pH-sensitive

690

semi-interpenetrating polymer network hydrogels based on konjac glucomannan and poly(aspartic

691

acid) for in vitro drug delivery, Carbohydr. Polym. 79 (2010) 500-506.

692

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693

microgel particles randomly dispersed in a gel matrix, J. Phys. Chem. B 108 (2004) 10893-10898.

694

[52] A. Yabuki, T. Shiraiwa, I.W. Fathona, pH-controlled self-healing polymer coatings with cellulose

695

nanofibers providing an effective release of corrosion inhibitor, Corrosion Sci. 103 (2016) 117-123.

696

[53] J.Y. Chen, X.B. Chen, J.L. Li, B. Tang, N. Birbilis, X.G. Wang, Electrosprayed PLGA smart containers

697

for active anti-corrosion coating on magnesium alloy AMlite, Journal of Materials Chemistry A 2 (2014)

698

5738-5743.

699

[54] P.M. Dasami, K. Parameswari, S. Chitra, Corrosion inhibition of mild steel in 1M H2SO4 by

700

thiadiazole Schiff bases, Measurement 69 (2015) 195-201.

701

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702

Electrochemical and AFM Characterization, Journal of Materials Engineering and Performance 24

703

(2015) 4997-5001.

704

[56] K. Ramya, R. Mohan, A. Joseph, Interaction of Benzimidazoles and Benzotriazole:Its Corrosion

705

Protection Properties on Mild Steel in Hydrochloric Acid, Journal of Materials Engineering and

706

Performance 23 (2014) 4089-4101.

- 29 -

708 709

Fig. 1. (a) Schematic representation of the BTA@PHVA/PEI synthesis, and (b) the

710

self-releasing of BTA@PHVA/PEI responses to pH changes.

30

713

3205

2946

927 992 1047 1103 1214 1269 1390 1455 1538 1714

1000

1500

3205

2946

1269 1390 1455 1538 1714

1047

751

927

751

Transmittance (%) 500

712

BTA@PHVA/PEI

PHVA/PEI 2000

2500

3000

3500

4000

-1

Wavenumber (cm )

Fig. 2. FTIR spectra of the as-prepared PHVA/PEI and BTA@ PHVA/PEI.

31

a

b

BTA@PHVA/PEI PHVA/PEI

100

60

Ⅱ

Ⅰ

Ⅲ

40

-60 -80

-100 -120

20 0

BTA@PHVA/PEI PHVA/PEI

-20 -40

Heat flow

Weight (%)

80

0

-140 0

200

715

400 600 Temperature (C)

800

1000

0

200

400 600 800 Temperature (C)

1000

716

Fig. 3. (a) TGA curves of PHVA/PEI and BTA@PHVA/PEI, and (b) corresponding

717

DSC spectra.

32

719 720

Fig. 4. SEM micrographs of the as-prepared PHVA/PEI after vacuum-dried (a) and

721

the freezing-dried PHVA/PEI after immersion in the solutions of pH=11.0 (b),

722

pH=7.0 (c) and pH=2.0 (d), respectively.

33

a

Survey

C 1s

b

C 1s

C-C

c

N 1s

C-O

C=O

N 1s

-N(CH3)2/-NH2

Intensity (a.u)

Intensity (a.u)

Intensity (a.u)

O 1s

+

+

-N (CH3)3/-NH3

Si 2p

0

200

d

600 800 1000 Binding Energy (eV)

526

530 532 534 Binding Energy (eV)

284

e

286 288 Binding Energy (eV)

536

538

98

290

394

396

398 400 402 Binding Energy (eV)

404

406

Si 2p

Si-C Si-O

Intensity (a.u)

C-O

528

282

1200

O 1s

C=O

Intensity (a.u)

724

400

100

102 104 Binding Energy (eV)

106

725

Fig. 5 (a) XPS survey spectra of the coating doped with 10 wt% BTA@PHVA/PEI.

726

XPS high-resolution spectra of the coating doped with 10 wt% BTA@PHVA/PEI: (b)

727

C 1s, (c) N 1s, (d) O 1s and (e) Si 2p.

34

Cumulative release (%)

0.9 0.8 0.7 0.6

pH=2.0 pH=5.0

0.5

pH=7.0 pH=9.0

0.4 0.3

729 730

pH=11.0 0

5

10

15 20 25 Time (h)

30

35

40

Fig. 6. Release profiles of BTA from BTA@PHVA/PEI at different pH values.

35

2

0.8 0.6

1.0 Hz

0.4

0.8

1.0

2

6 4

1.4

-2

d

0h 2h 4h 6h 8h 28 h 36 h Fitted line

8

1.2

0.01 Hz

1.0 Hz 4 6 2 Z/k cm

733

6

0

2

6 8 2 Z/k cm

10

1

2

3

4

5

8 6

2

Z/k cm

5

45 30 15 0

-1

0

1

2

log(Freq)/Hz

20

3

4

5

75

0h 2h 4h 6h 8h 24 h 36 h Fitted line

16

0.1 Hz

60

4

-2

0.01 Hz

75

0h 2h 4h 6h 8h 24 h 36 h Fitted line

10

12

h

0h 2h 4h 6h 8h 28 h 36 h Fitted line

0

12

2

0.1 Hz

4

-1

log(Freq)/Hz

0.01 Hz

734

2

45

15

0

10

60

2

0

15

75

30

2

20

5

4

2

0

4

0h 2h 4h 6h 8h 24 h 36 h Fitted line

-2

1.0 Hz

4

3

6

10

Z/k cm

8

8

f

0h 2h 4h 6h 8h 24 h 36 h Fitted line

10

2

0

2

e 12

1

log(Freq)/Hz

10

2

0

0

8

1.0 Hz

0

-1

12

60 45 30

8 15 4

0

1.0 Hz 0

735

0

0 5

10 2 Z/k cm

15

-Phase/ 

0.6 0.8 2 Z/k cm

-2

20

-1

0

1

2

log(Freq)/Hz

3

4

5

-15

736

Fig. 7. Impedance diagrams, i.e., both Nyquist (a, c, e, g) and Bode (b, d, f, h) plots

737

measured on the steel WEs coated with the alkyd coatings (with artificial 36

-Phase/ 

0.4

10

0.0

-Phase/ 

c 10

2

20

2

0.2

732

-Z/k cm

30

0.6

Z /k cm

0.0

-Z/k cm

40

0.2

0.1 Hz

0.0

g

50

0.4

0.2

-Z/k cm

1.2 1.0

0.01 Hz

60

0h 2h 4h 6h 8h 24 h 36 h Fitted line

1.4

-Phase/ 

1.2 1.0

-Z/k cm

b

0h 2h 4h 6h 8h 24 h 36 h Fitted line

2

1.4

Z/k cm

a

738

cross-shaped scratches) doped with various contents of BTA@PHVA/PEI in 3 wt%

739

NaCl solution. (a, b) no doped, (c, d) 5 wt%, (e, f) 10 wt%, (g, h) 15 wt%.

740 741

Fig. 8. Electrochemical equivalent circuits for fitting the impedance data. (a) for one

742

time constant impedance plots, and (b) two time constant impedance plots.

37

a

-2

-6

Blank 5 wt % 10 wt% 15 wt%

-6

-8

-8 -0.9

744

-2

-4 Log i / A cm -2

-4 Log i / A cm-2

b

Blank 5 wt % 10 wt% 15 wt%

-0.8

-0.7

-0.6 -0.5 E/V

-0.4

-1.0

-0.3

-0.9

-0.8

-0.6 -0.7 E/V

-0.5

-0.4

-0.3

745

Fig. 9. Potentiodynamic polarization curves for WEs coated with the alkyd coating

746

(with artificial cross-shaped scratches) doped with various contents of BTA@

747

PHVA/PEI after immersion in 3 wt% NaCl solution. (a) 0 hours of immersion time,

748

(b) 24 hours of immersion time.

38

750 751

Fig. 10. Images of the steel panels with the coating undoped and doped

752

BTA@PHVA/PEI after 240 h exposure of the neutral (a ~ d) / acid (e ~ h) salt spray

753

tests. (a, e) pure alkyd coating, (b, f) 5wt%, (c, g) 10wt%, (d, h) 15wt%

754

BTA@PHVA/PEI doped.

755

39

756 757

Fig. 11. The SEM morphology and corresponding EDS spectra of the steel panel

758

surfaces, which have been coated with different coatings and then immersed into 3

759

wt% NaCl solution for 30 days. (a) The coating doped with 10 wt% PHVA/PEI, the

760

coating doped with (b) 5 wt%, (c) 10 wt%, (d) 15 wt% BTA@PHVA/PEI.

761 40

1719

5 wt % BTA@PHVA/PEI doped

1719

10 wt % BTA@PHVA/PEI doped

2926

1248

2926

10 wt% PHVA/PEI doped

1248

728

2926

728

Transmittance (%)

728

1719

2926

762

1000

763

1719

728

1248

15 wt % BTA@PHVA/PEI doped

2000

-1

3000

4000

Wavenumber (cm )

764

Fig. 12. FTIR spectra of the steel panel surfaces, which have been coated with

765

different coatings and then immersed into 3 wt% NaCl solution for 30 days.

766

41

Table 1. Parameters determined from fitting EIS data in Fig. 7

768

Samples

Alkyd coating

Coating doped with 5 wt% BTA@ PHVA/PEI

Coating doped with 10 wt% BTA@ PHVA/PEI

Coating doped with 15 wt% BTA@ PHVA/PEI

Time

Rs

Rc

CPEc CPEf-T

CPEdl

Rct

n

CPEdl-T (F cm2 sn-1)

n

( cm2)

(h)

( cm2)

( cm2)

0

12.6

1391

7.11×10-4

0.6340

―

―

―

2

11.6

1271

5.42×10-4

0.6079

―

―

―

4

10.2

1134

1.13×10-3

0.4743

3.10×10-3

0.7556

696

6

12.5

1096

1.63×10-3

0.4576

3.60×10-3

0.7526

573

8

16.2

996

1.95×10-3

0.4676

4.02×10-3

0.6826

513

24

16.4

833

2.15×10-3

0.5027

4.61×10-3

0.7068

459

36

15.9

456

2.55×10-3

0.5519

4.91×10-3

0.6851

399

0

28.0

3055

2.48×10-4

0.7265

―

―

―

2

24.7

2469

1.98×10-4

0.7317

―

―

―

4

24.8

3369

2.08×10-4

0.7196

3.82×10-4

0.7913

970

6

22.4

4565

1.62×10-4

0.6756

1.22×10-4

0.6929

1049

8

23.9

5496

1.58×10-4

0.7713

8.97×10-5

0.6733

1676

24

24.5

9632

1.20×10-4

0.7684

6.31×10-5

0.6975

3643

36

19.4

6425

1.91×10-4

0.7700

8.46×10-5

0.6954

2157

0

33.5

7509

1.47×10-4

0.8087

―

―

―

2

32.5

4146

1.72×10-4

0.7476

―

―

―

4

31.8

8438

1.89×10-4

0.7564

3.18×10-4

0.7574

1791

6

33.6

9226

1.53×10-4

0.8090

2.21×10-4

0.7285

2244

8

33.1

9767

1.49×10-4

0.8094

9.16×10-5

0.7570

2332

24

33.4

11942

1.18×10-4

0.8008

6.17×10-5

0.7586

5202

36

34.2

10872

1.39×10-4

0.8112

8.61×10-5

0.7669

2918

0

33.1

7755

1.56×10-4

0.8368

―

―

―

2

25.4

5706

2.95×10-4

0.8093

―

―

―

4

34.6

8132

1.69×10-4

0.8395

3.36×10-4

0.7678

1702

6

33.0

9159

1.47×10-4

0.8291

1.31×10-4

0.7805

1986

8

31.4

10284

1.37×10-4

0.8039

7.27×10-5

0.7912

3050

24

28.8

11976

1.30×10-4

0.7322

5.18×10-5

0.6179

5469

36

31.2

9666

1.76×10-4

0.8143

6.53×10-5

0.6305

3105

(F cm2 sn-1)

42

Table 2. Potentiodynamic polarization parameters from Fig. 9

770

The doped content of the BTA@ PHVA/PEI

Immersion time (h)

Ecorr

-βc

βa

icorr

(mV)

(mV/dec)

(mV/dec)

(A cm-2)

0 wt%

0 24 0 24 0 24 0 24

-0.709 -0.713 -0.651 -0.690 -0.624 -0.652 -0.592 -0.645

78.61 90.38 93.94 72.96 77.83 76.33 55.10 50.42

31.25 30.02 30.35 28.95 28.44 27.38 24.68 34.12

3.28×10-6 7.32×10-6 3.19×10-6 1.20×10-6 2.92×10-6 9.45×10-7 2.80×10-6 3.42×10-7

5 wt% 10 wt% 15 wt%

43

772

Table 3. EDS results of the steel panel surfaces, which have been coated with

773

different coatings and then immersed into 3 wt% NaCl solution for 30 days. Element (wt%)

C

N

Si

Fe

Steel coated with 10 wt% PHVA/PEI doped alkyd coating

9.15

0.12

0.12

90.61

Steel coated with 5 wt% BTA@PHVA/PEI doped alkyd coating Steel coated with 10 wt%

5.08

0.21

0.17

94.54

6.78

0.32

0.14

92.76

5.12

0.41

0.25

94.22

BTA@PHVA/PEI doped alkyd coating Steel coated with 15 wt% 774 775 776 777 778 779 780

BTA@PHVA/PEI doped alkyd coating

 A novel hybrid hydrogel of BTA@PHVA/PEI was synthesized by one-pot synthesis method.  The releasing rate of BTA@PHVA/PEI rises upon external pH values due to its pH-sensitivity.  The smart coating based on BTA@PHVA/PEI can provide strong corrosion protection.

781 782 783 784 785

44