(Invited) The Mechanisms By Which Graphene Nano-Pigments Inhibit Corrosion-Driven Organic Coating Delamination
Authors: McMurray Hamilton Neil , Williams Geraint , Glover Carol Frances , Richards Calvin 
Published: February 2020

There has been significant recent interest in the development of nano-pigments based on graphene (graphene nano-pigments or GNPs) as possible additives to corrosion-resistant organic coatings. 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 either a) their blocking the through-coating diffusion of reactive species such as oxygen or b) the displacement of electrochemical processes (specifically cathodic oxygen reduction) away from the metal-coating interface and onto the intra-coating graphene surface. In this study a commercially available surface-modified GNP based on exfoliated graphite is used to systematically study the effect of GNPs on electrochemical and mass-transport processes occurring in GNP pigmented polymer films both in a free-standing state and as applied to iron and zinc surfaces.

The GNP used consists of small secondary particles with a typical planar diameter of 0.3-5 microns and a thickness < 50nm. These 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. The GNP pigmented coatings consist of 30 micron thick solution-cast polyvinylbutyral (PVB) composites films. The PVB/GNP ratio is systematically varied to produce a series of composites with GNP volume fractions between zero and 0.2. The O2 permeability of these composites is determined using a photochemical method based on the atmospheric re-oxidation of leucomethylene blue in conjunction with free-standing films to follow the kinetics of through-coating oxygen diffusion. It is thus shown that O2permeability decreases sharply (by 1-2 orders of magnitude) as the GNP volume fraction is increased from zero towards 0.15.

An in-situ scanning Kelvin probe technique is used to study the effect of varying GNP pigment volume fractions on the development of metal-coating contact potentials and on the kinetics of corrosion-driven coating (cathodic) delamination. Under circumstances where the GNP volume fraction is sufficiently high as to allow charge percolation through a continuous network of GNP particles contact potentials are measurable between the metal substrate and PVB/GNP composite coating on both iron and zinc. In both cases these contact potentials are consistent with the work function difference between the relevant metal and graphene/graphite. In the case of zinc this contact potential is significant (>0.5V) and is consistent with the GNP network being the cathode in any galvanic couple involving zinc and the GNPs. In the case of iron the contact potential is relatively small.

When a penetrative defect is created in the PVB/GNP coating and the substrate exposed to aqueous sodium chloride electrolyte in an atmosphere of 95% relative humidity for up to 100 hours. On both iron and zinc it is shown that the rate of cathodic delamination becomes reduced by > 90% relative to the unpigmented case as at a GNP volume fraction increases. Typical SKP-derived time-dependent potential (Ecorr) versus distance (from the penetrative defect edge) profiles for unpigmented PVB and PVB/GNP composite coatings are presented. Delamination kinetics are quantified by plotting the position of the delamination front as a function of time for coatings comprising various GNP volume fractions. Similar dependence of delamination kinetics on GNP volume fraction are observed for composite coatings adherent to both iron and zinc. 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 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. It is concluded that, although a displacement of cathodic O2 reduction away from the metal coating interface is possible (and in fact predicted) the observed inhibition effects are immediately consistent with the measured reduction in O2 permeability at the relevant GNP volume fraction.