Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/818289
18 MAY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 5 MATERIAL MATTERS Continued f rom page 17 the appearance of black rust spots—with increased graphene loading. The best performance was achieved with 5.0% of Sample 1 and 0.5% of Sample 2. Following the positive results in the cyclic salt fog testing, the graphene- enhanced epoxy coatings were subjected to immersion testing. Steel panels, hand- abraded and cleaned with xylene, were fully immersed in synthetic seawater (prepared according to ASTM D1141 7 ) at ambient temperature (20 to 30 °C) for 30 days. Upon completion of the immer- sion testing, samples were cross- sectioned and imaged by scanning electron microscopy (SEM). Figure 2 shows photographs of the epoxy-coated steel panels before and after the 30-day immersion in synthetic sea- water. The panel with the graphene-free control epoxy suffered severe corrosion and rusting, while the panels with the graphene-loaded epoxy were virtually cor- rosion free. The graphene significantly enhanced the corrosion mitigation of the epoxy coating even at loading levels as low as 0.1 wt%. Typically, corrosion mitigation improved as the loading level of graphene increased. SEM images for Sample 2 (Figure 3) show the coating remained intact with no rusting or corrosion visible at the surface of the substrate. The diffu- sion of water and salts to the surface was delayed by the graphene in the coating. Because corrosion is an electrochemi- cal process—an electrical current is pro- duced as the metal in the substrate is oxi- dized—electrochemical monitoring of a substrate during immersion testing pro- vides useful information about how well the coating is protecting the steel panel. Monitoring this electrical current can quantif y the amount and rate of corro- sion. In experiments with electrochemi- cal testing, panels coated with graphene- loaded epoxy were immersed in synthetic seawater and measurements were taken using a three-electrode system. The cor- rosion current for each sample was moni- tored over the 30 days of immersion. The corrosion current recorded for the samples coated with graphene-loaded epoxy was roughly 1,000 times smaller than for the control sample coated with graphene-free epoxy. This very low corro- sion current correlates with the conclu- sions from the visual assessment and the SEM analysis, and conf irms that the addi- tion of graphene to the epoxy improves the corrosion protection provided by the epoxy coating. The proposed explanation for this is that the graphene nanoplatelets act as a barrier to the diffusion of water and cor- rosive salts through the epoxy coating. The water vapor transmission rate (W V TR) through the epoxy coatings was measured following ASTM D1653-03 8 using Test Method B (wet cup method), Condition A (23 °C, 50% relative humid- ity). For this test, a paper substrate was coated with samples of graphene-free epoxy and epoxy loaded with Sample 1 and Sample 2. The results of the W V TR test showed a clear reduction in the diffusion rate of moisture through the graphene-loaded epoxy samples. The data show that the graphene-free control epoxy had a W V TR of ~200 g/m 2 /day. The addition of the smallest amounts of graphene tested, 0.1 w t%, reduced this diffusion rate by a factor of almost 100. A W V TR of <10 g/m 2 / day was recorded for all graphene-loaded epoxy samples. The data support the proposed mecha- nism that the graphene nanoplatelets form a very effective diffusion barrier by form- ing a tortuous path to the substrate and greatly increasing the time it would take for corrosive elements to migrate through the coating. The effective barrier proper- ties of the graphene nanoplatelets in the epoxy coatings may explain the corrosion mitigation observed on the steel panels in both immersion testing and cyclic salt fog testing. Source: Lynn Chikosha, Adrian Potts, and William Weaver, Applied Graphene Materials, appliedgraphenematerials.com. Contact Marie Robinson—email: info@ appliedgraphenematerials.com. References 1 S. Bohm, "Graphene against Corrosion," Nat. Nanotechnology 9, 10 (2014): pp. 741-742. 2 R .S. Raman, et al., "Protecting Copper from Electrochemical Degradation by Graphene Coatings," Carbon 50, 11 (2012): pp. 4,040- 4,045. 3 R.V. Dennis, et al., "Graphene Nano-Compos- ite Coatings for Protecting Low Alloy Steels from Corrosion," Am . Ceram . Soc. Bull . 92 (2013): pp. 18-24. 4 ISO 1514:2016, "Paints and varnishes—Stan- dard panels for testing" (Geneva, Switzerland: ISO, 2016). 5 BS EN ISO 11997-2:2013, "Paints and var- nishes. Determination of resistance to cyclic corrosion conditions. Wet (salt fog )/dr y/ humidity/UV light" (London, U.K.: BSI, 2013). 6 ISO 4628-2:2016, "Paints and varnishes—Eval- uation of degradation of coatings—Designa- tion of quantity and size of defects, and of in- tensity of uniform changes in appearance— Part 2: Assessment of degree of blistering " (Geneva, Switzerland: ISO, 2016). 7 ASTM D1141-98 (2013), "Standard Practice for the Preparation of Substitute Ocean Water" (West Conshohocken, PA: ASTM, 2013). 8 ASTM D1653-03, "Standard Test Methods for Water Vapor Transmission of Organic Coating Films" (West Conshohocken, PA: ASTM, 2003). —K.R. Larsen