Corrosion Protective Graphene-Oxide Composite Coating for Metal Alloys

Abstract

Corrosion is still damaging a sizable part of production, especially made of different metal alloys. Thus, more effective protective coatings than today’s dominating thick ones, reaching up to several hundred microns in thickness, must be developed. In this work we introduce a submicron nanocomposite protective coating based on a layer of graphene nanoplatelets (NPs) and deposited onto it laminate of metal oxides nanolayers ‒ the graphene layer was prepared by spinning and the laminate by atomic layer deposition (ALD) method.1 Mainly the coating is tested on AISI 304 alloy (SS) but in the presentation also the first results of its use on some lightweight alloys will be given. Previously was shown that prepared by chemical vapor deposition (CVD) graphene layer cannot work as a long-term corrosion protective coating even on the catalyst metals, copper or nickel, on which it was synthesized because of multiple defective sites on it.2,3 We also showed one possible way to better the CVD graphene coating performance by using a polypyrrole toplayer that covers its defects and was deposited onto it by electrochemical method.4 But still the coating failed during an immersion test over 100 h in salt solution. For our new coating we prepared graphene oxide (GO) nanoplatelets by a modified Hummers method and reduced the material chemically getting the reduced graphene oxide (rGO).5 The rGO has mainly submicron lateral dimensions and nanometric thickness, thus consisting of one to some graphene layers, see Fig. 1a. NPs of the rGO have still remarkable number of OH- groups and other oxygen consisting radicals, and they are even more defective than NPs of GO, compare the D and G band intensities in Raman spectra in Fig. 1b. This is probably one reason why the interface layer of rGO NPs is not accelerating corrosion as the most passive graphite layer must do on metal surface, but inversely, bettering the corrosion protection of SS by our new composite coating, see the polarization curves in Fig. 1c. Here must be stressed that the only rGO or laminate coatings withstand much shorter time in salt solution immersion test than the rGO-laminate composite coating showing no change after 30 days’ immersion. The additional functions of the graphene NPs’ interface layer that bettered the corrosion protection of the composite coatings are improved ion barrier character of the coating and shielding of the local electrical fields on the oxidized metal surfaces, making the interface below the oxide laminate more homogeneous. The last effect is enhanced by the enhanched electrical conductivity of the rGO if compared with GO. The perspective of the introduction of the new coating into industry will be discussed in the presentation. L.A. and J.K. thank the European Social Funds of Doctoral Studies and J.M. acknowledges the Internationalization Program DoRa for financial support. This work was also supported by the Estonian ME&S by the projects IUT2-24, TK117 and TK141. The Portuguese FCT is also acknowledged for PhD grant SFRH/BD/72161/2010 (A.M.) and for financial support under contract UID/QUI/00100/2013.