Optimization of electrochemical polymerization parameters of polypyrrole on Mg–Al alloy (AZ91D) electrodes and corrosion performance

Abstract

We report on the optimum electropolymerization conditions of polypyrrole (PPy) coatings on Mg alloy AZ91D electrodes from aqueous electrolytes of sodium salicylate via cyclic voltammetry (CV). Results show that initial and end potential values during the electrochemical coating procedure play an important role on the adhesion and corrosion performance of PPy films. Corrosion tests of AZ91D electrodes coated with PPy under optimized conditions show a good corrosion performance during 10 days in Na₂SO_4, without peeling off of these thin films.

Introduction

In recent decades, intrinsically conducting polymers (ICPs) such as polythiophene (PT), polyaniline (PANI) and polypyrrole (PPy) and their derivatives have been explored to be promising nominees in the development of biosensors [1], actuators [2], fuel or bio-fuel cells [3], [4], light emitting diodes (LED) [5] and supercapacitors [6], [7], [8]. These types of coatings on widely used active metals have been also reported to provide promising corrosion protection properties [9], [10]. On the other side, corrosion protection by ICP coatings has been reported to be not favorable under practical conditions [11], [12], as has been discussed in a recent review [13]. However, a good corrosion protection can be achieved in practical applications if these coatings are applied together with other coatings in a sandwich like structure (e.g., conversion coating, second primer layer, and polyurethane topcoat) by release of the dopant anions [14].

One of the most widely studied ICP for corrosion protection is PPy. Because of its relatively easy preparation from aqueous solutions and stability at oxidized state, PPy and its derivatives are reported to be one of the most important candidates for corrosion protection. This biocompatible ICP [15], [16] is also the most widely explored one as a drug delivery material with many advantages as summarized in a recent review [17].

Although there are many studies on the electrochemical polymerization of pyrrole on widely used metals such as zinc [18], aluminum [19], iron [20], copper [21]; and these works report that PPy coatings show good mechanical stability and corrosion protection in corrosive media, only few attempts were aimed to achieve ICP coatings on Mg substrates. Guo et al. [22] showed electropolymerization of aniline on AZ91D alloy surface using an electrochemical pulse method in alkaline solutions, and Jiang et al. [23] reported electropolymerization of pyrrole on a Cu–Ni plated Mg AZ91D surface. Even less information is available on direct electrochemical deposition of PPy on Mg [24] surfaces. This type of surface modification with ICPs may be promising for many applications of Mg alloys, including their usage as drug eluting stents or other types of self-degradable metallic implant materials. In these applications, surface modification techniques are of interests which are able to slow down the dissolution rate of Mg, but not completely stop corrosion.

Due to its fast dissolution rate and very negative corrosion potential of Mg, direct polymerization of conducting polymers on the Mg surface is difficult. To overcome this problem, surface passivation is required [21] prior to initiation and attachment of conductive polymer coating. Different to this approach, Aeiyach et al. [25] and Hermelin et al. [26] recently showed electropolymerization of PPy coatings from sodium salicylate solutions on zinc without any pre-treatment. Taking the advantage of sodium salicylate, film formation occurs in a one-step process, and does not need any preliminary treatment of the metal.

Electropolymerization of pyrrole in presence of salicylate ions has been studied previously. Petitjean et al. [27] claimed that in aqueous media, sodium salicylate is the most suitable candidate that allows the formation of an adherent and homogeneous PPy film on active metals without pre-treatment. Moreover, Cascalheira et al. [28] determined the oxidation peak of salicylate at around 1 V and showed the formation of a Cu(II)-salicylate film is favoured at the electrode surface and contributes to its passivation.

We have recently demonstrated for the first time electrochemical polymerization of PPy on Mg alloy AZ91D electrode in 0.5 M sodium salicylate solution in the potential region between −0.5 V and 1.0 V [29]. Successful coating of AZ91D with PPy was proved by Fourier-transform infrared spectroscopy (FT-IR), X-ray induced photoelectron spectroscopy (XPS) and time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS) measurements. In the present study, we explore the coating parameters in detail, in order to optimize electropolymerization conditions from the same electrolyte solution. Moreover, the effect of electrochemical parameters used on the corrosion performance of PPy/AZ91D system is studied during 10 days, to determine if the system has potential for tailoring the corrosion rate of Mg alloys in view of biomedical applications, for instance to be further developed into drug-releasing coating.