(27i) Improving Biomass-Derived Graphene Coating on Transitional Metals

After the discovery of graphene in 2004, its anti-corrosive property to prevent metal corrosion has been explored as one of its potential applications. Because the bond length of the carbon atoms in graphene (0.142 nm) is very narrow, graphene acts as a cathodic coating that offers corrosion resistance for a longer period while the property of the underlying metal is not altered by the graphene coating, allowing it to be utilized as an anti-corrosive coating. In this study, a novel graphene synthesis method was used to grow graphene on transition metals at atmospheric pressure, without the use of flammable gases (methane, hydrogen). Furthermore, this method affords the advantage of allowing graphene growth on both sides of the transition metal foils. A renewable solid carbon source, biochar, was used as the graphene precursor and was prepared by hydrothermal carbonization of cellulose. Biochar was placed on the metal and heated to 1000 ^oC using a quartz tube furnace and upon cooling graphene precipitates on both sides of the metal foil. Iron and cobalt are used in this study as their carbon solubility at 1000 ^oC are 1.4 wt % and 0.3 wt% respectively. At high temperature, carbon atoms from biochar dissolve into the metal foils and upon cooling, form metal carbides which, depending on the metal, may decompose and precipitate to form graphene over the foils. XRD and Raman analysis were used to confirm graphene formation as well as the numbers of layers precipitated. Graphene had a good coverage on cobalt with very low defect due to the absence of D peak and both monolayer (2D/G =3.11) and few-layer graphene (2D/G =0.356) were detected. On the other hand, for iron, Raman analysis revealed that only few-layer graphene (2D/G =0.312) was present with low coverage due to presence of uncovered iron as well as the metastable iron carbide as detected by XRD. The anti-corrosive properties of the graphene coatings were tested using 3.5 wt% NaCl solution using an electrochemical cell and linear polarization resistance (LPR) techniques were employed to calculate the corrosion rates. Pure iron and cobalt exhibited corrosion rates of 0.195 and 0.047 mmpy (millimeter per year), respectively. Both graphene coated iron and cobalt showed an increased corrosion rate when compared to the pure metals. In case of cobalt, the presence of monolayer did not improve the corrosion resistance as monolayer graphene promotes galvanic corrosion. In case of iron, the uncovered regions promoted pitting, followed by galvanic corrosion. To enhance the coverage of graphene, increased biochar- metal contact using a weight, performing more than one run with fresh biochar on same sample and optimizing the heating rate were all strategies adopted to improve the corrosion resistance. For example, when a stainless-steel plate was placed as a weight on top of the biochar for enhanced solid-solid contact, the corrosion rate of graphene coated iron reduced to 0.017 mmpy due to better improved coverage.