Pipelines constitute a major infrastructure investment frequently carrying materials which in the event of failure can cause significant loss to the owner and serious potential for environmental damage. To fulfil their role pipelines often run long distances either underwater or underground. This physical challenge is often further complicated by the crossing of international borders introducing complex codes and standards of management. Coatings are essential to the protection of pipelines from corrosion and subsequent failure but are themselves subject to degradation by severe abrasion, hydrothermal aging and chemical degradation. These coating systems are typically considered to be passive or active. Passive systems prevent corrosion by blocking key elements of water, oxygen and salts from reaching the pipe surface. Cathodic protection systems (CP) are reactive systems designed to protect pipelines in the event of failure.
Graphene was first produced and identified in 2004 by the group of Andre Geim and Konstantin Novoselev at the University of Manchester, an event which was followed by the Nobel prize for Physics in 2010. One of the remarkable properties of graphene is its impermeability to gases. Graphene manufactured as a single monolayer is time consuming, expensive and difficult to scale. Graphene nanoplatelets (GNPs) offer a cheap and scalable alternative for use in barrier systems. Much research has been carried out on the implementation and use of graphene in coatings including those for pipelines. Direct application of GNP into epoxy has been discussed by Battocchi et al (1) who observed that low level additions of GNP offered improved barrier properties and corrosion mitigation together with improved abrasion resistance. Budd et al(2) applied GNP in laminate structures for flexible risers demonstrating the potential barrier properties of graphene in aggressive conditions. Applied Graphene Materials (AGM) GNPs are manufactured using the company’s patented proprietary “bottom up” process, yielding high specification graphene materials. AGM produce a range of GNP dispersions capable of easy addition into coating systems and have undertaken significant development activity to demonstrate their use in coating systems enabling improved in barrier performance and corrosion resistance.
Current organic coating systems designed for protective coatings applied in harsh environments, such as bridges, are typically comprised of a number of different coating layer, each providing a different set of properties. A basic system usually consists of three layers, which may include a zinc rich primer coat offering sacrificial protection, an intermediate coat and a final topcoat for environmental protection. Typical dry film thicknesses of these coats is around 50 to 150 µm for the primer and intermediate coat and 50 µm for the top coat. Recently it has been demonstrated that GNPs, both as prepared and chemically functionalised, when incorporated into an organic coating system or host matrix, provide via a highly tortuous path which acts to impede the movement of corrosive species towards the metal surface (Okafor et al[3) ) creating a passive corrosion protection mechanism. In support of this, previous work by Choi et al (4) has also shown that very small additions of GNPs decreased water vapour transmission rates indicating a barrier type property, while some authors Aneja et al(5) also report an electrochemical activity provided by graphene within coatings. The introduction of GNPsinto the intermediate coat has recently been demonstrated by AGM(6) to increase significantly the impedance of a protective coating system as measured by EIS when studied in conjunction with Neutral Salt Spray testing (ASTM B117). The intermediate epoxy was formulated as shown below in Table 1.
Three different GNP-containing variants of the control were prepared (D1-D3) using the same initial preparation route as for the epoxy prototype base, by substituting commercially available GNPcontaining dispersion additives (formulation component 10) for epoxy in the final step (formulation component 9). The GNP dispersion additives were effectively treated as masterbatches, and were added in varying amounts according to their graphene content and the final GNP content specified in the end coating (Table 1). The dispersion used in the preparation of D1 and D3 contained a reduced graphene oxide type GNPs (A-GNP10). The dispersions used in the preparation of D2 contained GNPs of a ‘crumpled sheet’ type morphology with a relatively low density and high surface area (A-GNP35). In addition, dispersion D3 based on A-GNP10 contained an active corrosion inhibitor.
Prior to coating application, all substrates were degreased using acetone. Each first coat was applied to grit blasted mild steel CR4 grade panels (Impress North East Ltd.), of dimensions 150 x 100 x 2mm, by means of a gravity fed conventional spray gun. The over coating interval was 3 hours with all panels permitted a final curing period of 7 days at 23°C (+/-2°C). Dry film thickness of the prepared coatings were in the range of 50-60 microns for single coat samples and 150-160 microns for multi coat samples. Full details of the coating systems prepared can be seen in Table 2. All substrates were backed and edged prior to testing.
The panels were placed in a Neutral Salt Spray corrosion chamber, running ISO 9227 for a period of up to 1440 hours. This test method consists of a continuous salt spray mist at a temperature of 35°C. Panels were assessed at 10 day (240 hour intervals) for signs of blistering, corrosion, and corrosion creep in accordance with ISO4628. These assessments were complimented with electrochemical measurements, carried out at the same intervals. All electrochemical measurements were recorded using a Gamry 1000E potentiostat in conjunction with a Gamry ECM8 multiplexer to permit the concurrent testing of up to 8 samples per run. Each individual channel was connected to a Gamry PCT1 paint test cell, specifically designed for the electrochemical testing of coated metal substrates.
Figure 1 shows the progression of impedance modulus for the three coat system samples, measured at 0.1 Hz, over the time period during which the samples were subjected to NSS conditions. Initial impedance values (recorded at t=0) range from the orders of 108 to 1010 Ω.cm2 . The control sample, consisting of a zinc rich primer coat, a layer of commercial equivalent epoxy and polyurethane topcoat, displays the lowest overall impedance values in addition to one of the higher rates of decrease of impedance from the t=0 point. When GNPs are introduced to the intermediate layer, the impedance modulus is increased suggesting that the inclusion of GNPs is acting to increase the barrier performance properties of the system as a whole. The incorporation of A-GNP35 into D2 gave a final system uplift of 5 orders of magnitude above the control. Throughout the testing the D2 formulation showed little change in impedance, compared to the other samples. The achievement of >109 Ohm.cm2 @ 0.1Hz over a period of 1440 hours in neutral salt spray outperformed existing technology in barrier performance equating to a C5 high rating for salt spray performance according to ISO12944-1.
The choice of coating system for pipelines is typically influenced by the geographical region and is often made between thick or thin film build. Critical requirements of coatings in either case are:
• Excellent adhesion
• Low permeability
• Resistance to cathodic disbondment
• High electrical resistance
Thin build coating systems are typically based on Fusion Bonded Epoxy (FBE) either single or double layer being the preferred approach in the North American market. Alternatives might also include high build epoxy or polyurethane. Typically such thin build systems utilise an active CP system to provide additional corrosion protection. Graphene modification as shown by Battochi(1) and by AGM(6) might easily be incorporated into such epoxy or polyurethane systems through the use of AGM’s dispersions. The known electrical conductivity of Graphene might give cause for concern if the incorporation changes the insulating characteristics of the film. The GNP modification demonstrated by AGM is however substantially below the percolation threshold required for conductivity and the net impact on epoxy conductivity is considered negligible (Figure 2).
Thick build coating systems used in other parts of the world are typically 3 layer polyolefin (3LPO and might be polyethylene or polypropylene). AGM has experience in master-batching Graphene into thermoplastics and as such there is no obstacle to the introduction of GNPs into of the main body of the coating. GNP might also be introduced into the adhesive copolymer layer applied to the FBE typically used as a base for the 3LPO coating system.