Inhibition of Corrosion-Driven Organic Coating Delamination on Zinc By Graphene Nano-Pigments

Great efforts are being made to replace chromate in organic coatings for use in conjunction with galvanized steel. One area of recent interest has been the development of nano-pigments based on graphene (graphene nano-pigments or GNPs) and their possible application in corrosion-resistant organic films. It has been claimed that lyophilized graphene particles dispersed in organic polymer coatings can effectively inhibit under-coating corrosion and corrosion-driven coating delamination. It has been further proposed that the inhibitory effect of GNPs results from their blocking the through-coating diffusion of reactive species such as oxygen. In this study a commercially available surface-modified GNP based on exfoliated graphite is evaluated as an anti-corrosion additive for organic coatings applied to galvanized steel. The GNP used was supplied by Haydale Ltd. and consists of small secondary particles with a typical planar diameter of 0.3-5 microns and a thickness < 50nm. The GNP secondary particles are in turn composed of varying number of graphene nanoplatelets. The GNP supplied had been subject to plasma processing intended to improve dispersibility in oxygenated solvents and promote compatibility with organic polymers bearing oxygen functionality. An in-situ scanning Kelvin probe technique is used to study the effect of varying GNP pigment volume fractions on the kinetics of corrosion-driven coating (cathodic) delamination. The materials studied consist of GNP pigmented polyvinylbutyral (PVB/GNP) coatings solution cast onto the zinc surface of galvanized steel. A penetrative defect is created in the organic coating and the substrate exposed to aqueous sodium chloride electrolyte in an atmosphere of 95% relative humidity for up to 100 hours. It is shown that at a GNP volume fraction of 2.5% the rate of cathodic delamination becomes reduced by > 90% relative to the unpigmented case. However, it is also shown that increasing the GNP volume fraction from 2.5% to 15% does not result in any significant additional reduction in delamination rate. Typical SKP-derived time-dependent potential (E_corr) versus distance (from the penetrative defect edge) profiles for unpigmented PVB and PVB/5% volume fraction GNP are presented. It is thus shown that GNP addition not only slows the coating disbondment rate but also increases the apparent value of potentials in the intact (undelaminated) coated region by up to 0.5V. The GNP addition also changes the characteristic shape of the E_corr – distance profile in the region of the delamination front. Delamination kinetics are quantified by plotting the position of the delamination front as a function of time for coatings comprising various GNP volume fractions. Analysis of these kinetics suggests that addition of GNP at > 2.5% volume fraction changes the delamination rate law from parabolic (half order) to linear (zero order) with respect to time. This observation is consistent with the rate limiting step changing from under-film cation mass transport to an interfacial processes such as O₂ reduction becoming rate limiting. The mechanism of GNP inhibition is discussed in relation to the electrochemical and mass transfer-blocking properties of graphene.