Synthesis and evaluating corrosion protection effects of emeraldine base PAni/clay nanocomposite as a barrier pigment in zinc-rich ethyl silicate primer

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Abstract

In the present work, polyaniline/clay nanocomposite (PAniCN) was synthesized by chemical oxidative polymerization of aniline monomers in the presence of Closite30B powders. XRD and SEM examinations were used to examine the intercalation and morphologies of PAniCN, respectively. Electrical conductivity test showed that the conductivity of final PAniCN was higher than pristine PAni by one order of magnitude. Synthesized nanocomposite was added to the zinc rich ethyl silicate primer to modify its barrier properties. The corrosion protection performances of modified and unmodified primers were evaluated using open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) in 3.5% sodium chloride solution for a period of 120 days. It was found that the modified primer had higher barrier properties than original primer. After 120 days of immersion, resistance of modified and unmodified primers reaches 5.565 × 103 Ω cm2 and 6.056 × 102 Ω cm2 respectively. The OCP of both primers were lower than −800 mV/SCE during the immersion. Besides, the OCP of modified primer was higher than the original primer due to the passivation and barrier effects of PAniCN. Results revealed that the performance of modified primer improved strongly.

Introduction

For many years organic coatings have widely been used to protect the metallic parts against corrosion and harsh environments. Selection of appropriate coating is one of the most important tools in protecting sensitive metals. There are three types of organic paint coatings used in corrosion protection [1]: Electrochemically active coatings, Sacrificial coatings and Barrier coatings. Electrochemically active coatings contain inhibitive pigments like the chromate compounds that produce non-porous strong oxide film on the surface of metallic parts and passivate it even at the absence of oxygen [2].

However, chromate containing coatings have become a highly controlled subject by the governmental agencies due to their adverse effects on environment as well as human safety. Organic coatings containing conductive polymers are believed to be the promising alternatives to the environmentally hazardous chromate containing coatings [3], [4]. In recent years, there has been a great interest in using electrically conductive polymers to protect steels, other metals and alloys [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Among these polymers, polyaniline (PAni) is one of the most widely used conductive polymers because of its environmental stability, good process ability and relatively low cost [16]. Many studies have provided strong indications that polyaniline offers corrosion protection for ferrous and non-ferrous alloys and can be used as the primer [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30].

Sacrificial coatings, such as zinc rich primers (ZRPs) act as a cathodic protective agent. In these coatings, zinc powder acts as a sacrificial anode and the corrosion will be retarded [1]. ZRPs can only protect steel parts cathodically when zinc particles are electrically connected to the substrate, Therefore, these coatings should contain high levels of zinc powder (over than 80% by weight). But high amount of zinc powders causes losses in adherence and increases the film porosity [1], [7]. Barrier coatings that block the passage of oxygen and moisture to metal surface have numerous advantageous [7].

Corrosion protection mechanism of PAni is still not fully understood, and there is no universally accepted mechanism for the protection. Some researchers believe that it protects aluminum alloys and steels via a barrier mechanism, but others say that forms a passive oxide film on metallic surface through an oxidation–reduction process [31]. Wessling reported formation a passive layer consist of Fe3O4 and Fe2O3 when PAni directly used as a primer on steel substrates [10], [11]. It seems that PAni is acting as a barrier and also has an electrochemical protection effect [12]. One of the advantages of a PAni containing coating is that it has more tolerance to the pin holes due to its passivation ability. A number of authors [9], [32], [33], [34] have reported the use of PAni with other additives for corrosion protection. Sathiyanarayanan et al. [9] showed that the polyaniline–TiO2 composite containing coating is able to protect steel materials more efficiently than that of the PAni. Radhakrishnan et al. [32] reported the formulation of smart corrosion resistant coatings based on conducting polyaniline–nano-TiO2 composites. They claimed the exceptional improvement in performances for these coatings are associated with increasing of barrier properties, prevention of charge transportation by the nanosize TiO2, redox properties of PAni, as well as very large surface area availability for the liberation of dopant due to the presence of nano-size TiO2. Recently Chang et al. [33] found that introduction of organo-MMT into emeraldine base of PAni may increase the length of diffusion pathways for oxygen and water as well as decreasing permeability of the coating which are reflecting a significant enhancement in corrosion protection on metallic surface. Meroufel et al. [34] have recently reported the addition of polyaniline into zinc rich powder coatings in order to enhance the electrical conductive paths between zinc particles inside the coating and the steel substrate. They found that the corrosion potential remained stable in cathodic region during the 100 days of immersing in 3.5% sodium chloride solution.

This paper presents the experimental results for synthesizing PAniCN and determining the electrochemical analyzing of corrosion protection performance for zinc rich ethyl silicate primer modified with PAniCN in comparison to unmodified primer. PAniCN powder can acts as a barrier and as electrochemically active inhibitor. Therefore, all three mechanisms of protection are collected in a modified ZRP and it can be predicted that this primer have a very good protecting efficiency.

Section snippets

Synthesize of PAni/clay nanocomposite

Aniline monomers and ammonium persulfate (APS) were purchased from Merck Company and used as received. Cloisite 30B nano-clay with a cation exchange capacity (CEC) of 90 meq./100 g was purchased from Southern Clay Products Company. Undoped PAni/clay nanocomposite was synthesized by chemical oxidative polymerization of aniline with APS as an oxidizing agent. An aqueous suspension containing 0.5 g nanoclay in 100 ml was prepared and sonicated in ultrasonic bath for 2 h, then 5 g of aniline was added to

Characterization of nanocomposite

The scanning electron microscope (SEM) images of synthesized PAniCN and as received Cloisite 30B were exhibited in Fig. 2. SEM images clearly show the presence of tightly packed particles of nano-clay especially in the case of as received Cloisite 30B. The size of PAni particles is in a range from about 70 to 150 nm. The FTIR spectrum of synthesized PAniCN is shown in Fig. 3. Important characteristic peaks of PAniCN are assigned as follows: the peaks at 1578 and 1487 cm−1 are due to C–N

Conclusion

Polyaniline clay nanocomposite (PAniCN) was synthesized using the chemical oxidation of aniline with ammonium persulfate in the presence of Cloisite 30B nanoclay. SEM and XRD studies revealed the formation of an immiscible composite of nano-clay in PAni matrix. Conductivity measurements indicate that the synthesized nanocomposite show higher conductivity compared to the original undoped PAni. Shift in open circuit potential for modified ZRP with time is an indication of formation a passive

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