Enhancing the corrosion resistance of a commercial magnesium alloy using polyaniline coating

Baloch, Asif A. (2018) Enhancing the corrosion resistance of a commercial magnesium alloy using polyaniline coating. Masters (Research) thesis, James Cook University.

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Magnesium alloys are attractive materials for structural engineering applications due to their high strength-to-weight ratios and formability. However, the widespread use of magnesium alloys has been hindered by their poor corrosion resistance. Generally, magnesium alloys containing low amounts of impurities (iron and silicon) have acceptable atmospheric corrosion resistance, but the corrosion resistance deteriorates in chloride-containing environments.

Many surface engineering/coating techniques, such as electroplating, anodisation, microarc oxidation, chemical conversion and polymer coatings, have been studied to enhance the corrosion resistance of magnesium and magnesium-based alloys. However, in recent years, conducting polymers have gained high interest for use as corrosion protection coatings on engineering materials such as steel, copper and magnesium due to their unique electrical properties. Furthermore, these conducting polymers can be easily processed and are economically viable for use as metal coatings. Polyaniline and polypyrrole are the conducting polymers most commonly studied for use as coating materials for the corrosion protection of metals. In recent years, electrochemical polymerisation techniques have been widely used due to their characteristics of being environmentally friendly, easy to process, rapid and requiring few chemical additives. However, synthesis of polymer coatings is challenging when forming coatings on active metals, due to oxidation or prior dissolution of the metal substrate.

In this study, following cyclic voltammetry coating, the voltammetric curves for the alloy in the polyaniline sodium salicylate electrolyte (PASS) showed that the first-forward cycle had strong magnesium oxidation until ~0.75 V and, above that, oxidation of aniline started to occur. However, in subsequent forward cycles, magnesium oxidation was insignificant due to the formation of polyaniline, which occurred during the reverse cycles. In contrast, in the polyaniline potassium hydroxide electrolyte (PAPH), which contained strongly alkaline potassium hydroxide, there was little dissolution of magnesium during the first-forward cycle. Based on the anodic peak current density, it can be said that the oxidation of aniline in PAPH electrolyte was significantly lower than in the PASS electrolyte. Further, it was noted that the peak current increased as the number of cycles increased in the PAPH electrolyte, which is in contrast to that observed in the PASS electrolyte. The corrosion rates of the bare metal, and the PASS- and PAPHcoated alloy samples were 2.02, 0.34 and 4.92 mpy, respectively. The degree of protection (DP) provided by the PASS coating was ~83%.

The second phase of coating testing was performed using the galvanostatic technique at current densities of 14, 18 and 20 mA/ cm2. Potentiodynamic polymerisation tests of bare samples and samples coated with aniline and sodium salicylate were carried out in a chloride-containing solution. The corrosion tests showed substantial decreases in the corrosion currents of samples coated in aniline and sodium salicylate electrolyte. The systematic decrease in corrosion current revealed that the sample coated at 20 mA/cm2 had better corrosion resistance than those coated at 18 and 14 mA/cm2. The DPs of the samples coated at 20, 18 and 14 mA/ cm2 were ~97%, ~48% and ~5%, respectively. The corrosion rates for the bare alloy and alloys coated at 14, 18 and 20 mA/cm2 were 6.27, 5.96, 3.27 and 0.113 mpy, respectively. For all the samples, coating was carried out for 600 seconds. However, at high current densities and high coating times, samples started substantial hydrogen evolution. Severe bubble formation hindered the coating process, causing blisters in the coating.

Alkaline pre-treatment of the magnesium alloy prior to electropolymerisation of aniline was carried out to enhance corrosion resistance. Pre-treatment of the alloy was done in 3 M sodium hydroxide solution at constant potentials o f − 750mV and −195mV. Electropolymerisation was performed using a cyclic voltammetry technique in an acidic electrolyte containing aniline and sodium salicylate. The corrosion protection provided by the polyaniline coating on the alloy was evaluated using potentiodynamic polarisation in a chloride-containing solution. The electrochemical results showed that the polyaniline coating reduced the alloy's corrosion rate by almost an order of magnitude. However, post-corrosion analysis revealed delamination of the coating. It is suggested that polyaniline did not adhere well to the hydroxide/oxide layer that formed on alloys during pre-treatment.

This study found that aniline, as a conducting polymer, formed better coatings in acidic electrolytes than in alkaline ones. The propagation of aniline in the emeraldine state (dark green colour coating) produced good coatings on the surfaces of magnesium alloy samples. The coating obtained at 20 mA/cm2 by the galvanostatic technique provided the highest corrosion protection. However, at current densities higher than 20 mA/cm2, hydrogen evolution hindered coating formation. The effect of coating time is also important in the galvanostatic coating technique; hydrogen evolution increased with coating time. Alkaline pre-treatment of magnesium alloy did not enhance the coating morphology. During potentiodynamic polarisation tests, polyaniline coatings tended to peel off due to weak adhesion with the metal substrate. The outcomes of this study may help improve the coating quality of magnesium alloy used in small gadgets and automotive industry. However, there is still more work needs to be done to form pore free coating on the metal for commercial use.

Item ID: 56015
Item Type: Thesis (Masters (Research))
Keywords: magnesium alloy, polyaniline, corrosion
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Copyright Information: Copyright © 2018 Asif A. Baloch
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 4: Baloch, Asif, and Kannan, M. Bobby (2017) Electropolymerisation of aniline on AZ91 magnesium alloy: the effect of coating electrolyte corrosiveness. Metals, 7 (12)

Date Deposited: 02 Nov 2018 05:15
FoR Codes: 09 ENGINEERING > 0912 Materials Engineering > 091207 Metals and Alloy Materials @ 100%
SEO Codes: 86 MANUFACTURING > 8612 Fabricated Metal Products > 861201 Coated Metal and Metal-Coated Products @ 100%
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