Combating corrosion

Most corrosion protection coatings are multi-layer systems. For bridges and many industrial assets constructed of steel, a zinc-rich epoxy primer is applied first as a sacrificial coating, followed by an epoxy midcoat as the barrier layer and a polyurethane or polysiloxane topcoat to provide UV and chemical resistance.

Proper preparation of the surface is absolutely critical to achieving affective coating performance. For heavy duty stainless steel, that often involves abrasive blasting. Thermal spray – applying molten aluminium, zinc, or zinc/aluminium – is becoming more common for protecting carbon steel. Galvanised metal may or may not need coating.

‘The major issue driving development of corrosion protection coatings today is the need to meet increasingly strict regulatory requirements for volatile organic compound (VOC) content, which has led to the introduction of high solids and 100% solids formulations,’ remarks KTA-Tator’s Jim Machen.

Another trend in the protective coatings segment is the desire of customers to get more for less, according to Trevor Wills, senior technologist with AkzoNobel Marine & Protective Coatings. ‘People want coatings that last longer, are simpler to apply, easier to maintain and cost less,’ he explains. That can be challenging to do while also reformulating to comply with environmental regulations.

The replacement of chromium (VI) inhibiting pigments is a particular challenge for corrosion control coating manufacturers. ‘Hexavalent chromium compounds, strontium chromate in particular, are very effective, but also toxic. Finding alternatives is difficult, partly because we don’t fully understand why they work so well,’ says Lyon. Progress is being made, though, such as with polyphosphates, pyrophosphates, organo-modified metal salts, silica compounds, rare earth metal salts and nano-sized pigments.

For aerospace applications, where aluminium is the common substrate, new magnesium-based technology, initially developed at North Dakota State University in the US, and licensed and further improved by AkzoNobel Aerospace Coatings, looks promising.

The need to find a replacement for hexavalent chromium has also generated interest in developing new corrosion control technologies, such as self-healing (or smart) functional coatings, according to Luz Marina Calle, lead scientist at NASA’s Corrosion Technology Laboratory, US. These coatings can detect the presence of corrosion and respond with the release of encapsulated inhibitors and/or additional resin present in the coating.

‘Such technologies are attractive because they are truly smarter; the inhibitors are released only at the site where they are needed, which reduces exposure to both people and the environment,’ Koch asserts. Smart coatings won’t be widely adopted for some time yet, because asset owners are often risk-averse and require proven solutions. They show promise, however, and will likely become important corrosion control technologies in the future, says Wills. Calle agrees: ‘As we gain a better understanding of the mechanism of corrosion and of corrosion inhibition, we will be able to accelerate progress in the direction of more effective and more environmentally friendly corrosion control.’

Lyon also notes that graphene has significant potential as a protective coating. ‘Graphene has been shown to be a perfect barrier to water, which is a big contributor to corrosion,’ he explains. The challenge: graphene does not adhere to metal. Possible solutions include applying the graphene to a thin polymer (plastic) film that can be applied to metal substrates.

In addition to finding alternative inhibitors, coating manufacturers must develop effective formulations for the variety of materials being used today to address weight issues (aluminium and lightweight steel) and more extreme conditions (temperature, pressure, salinity) that metal substrates are being exposed to (deeper shale and ocean drilling). ‘Selection of the appropriate coating system is therefore more imperative than ever,’ notes Simon Gibbon at AkzoNobel Research, Development & Innovation.

To achieve longer-term cost savings, there has also been growing use of assessment tools to predict maintenance schedules based on past performance. ‘Previously, paint on bridges, for example, was refurbished when the corrosion became very apparent and the entire bridge was coated. Today, owners are looking to paint assets on a schedule that prevents significant corrosion, and if corrosion occurs, to target the repair of only the sites that require repainting,’ explains Trimber.

Increasingly, designers are choosing to use composites, or glass fibre-reinforced polymers (GFRPs), rather than corrosive metals for construction. These lightweight materials offer corrosion resistance with high strength and durability, according to Matt Lieser at glass fibre manufacturer Owens Corning. ‘FRP is now used regularly to replace stainless steel and high-nickel alloys,’ he notes. Lieser adds that the cost of composite materials is comparable to stainless steel, and they are used in chemical plants, flue gas desulfurisation systems, potable and waste water treatments systems, seawater desalination facilities, and other applications. Roughly 600,000t of composite pipe are installed annually.

E-CR glass fibres, manufactured to resist acid and alkali exposure, and alkali-resistant (AR) glass fibres, for use in concrete, produce highly corrosion-resistant composites. GFRP can also be formulated with resins designed to provide enhanced corrosion resistance, such as thermoset vinyl esters and epoxies. The introduction of resin injection, vacuum infusion and other newer impregnation processes has also improved part consistency and reduced the potential for resin voids, improving the durability of composites in corrosive environments.

‘Many engineers that use composites for one application are coming back to find out where else it might be used,’ Lieser comments. He notes, however, that one of the challenges is educating potential users about suitable applications, how to specify these materials, and where to get components. ‘Owens Corning is actively working to educate end users about the advantages of FRP with respect to its corrosion resistance,’ he comments.

In response to the 2002 study funded by the US Department of Transportation, the US Congress mandated the creation of a Corrosion Policy and Oversight Office, responsible for the prevention and mitigation of corrosion of military equipment and infrastructure. Each branch of the US military has adopted programmes to track corrosion costs, identify causes, and implement appropriate solutions. Despite these efforts, in 2010, the US Department of Defence (DOD) estimated that it spent over $22.9bn annually on corrosion-related costs.

One success story involves the use of a new fast-cure epoxy coating system for the jet fuel and ballast water tanks on ships that can be applied in one day rather than three, which is estimated by the Naval Sea Systems (NAVSEA) to result in $6.5 to $7.1m/year cost savings.

The key to addressing corrosion is to increase awareness, according to Eliezer. ‘The main challenges are to increase the awareness of corrosion costs among governments, define international corrosion standards, promote corrosion prevention worldwide, and incentivise corrosion prevention activities by government agencies, corporations and research organisations,’ he argues.

The US Department of Defense, along with NACE International and various corporations, in 2009 funded the first bachelor’s degree programme in corrosion engineering in the US at the University of Akron. ‘It isn’t necessarily technical advances that will have the biggest impact on corrosion prevention and control; it is education of engineers so that they are aware of corrosion issues and the need to design systems such that corrosion is minimised,’ stresses Koch.

Increased training for coating applicators is also critical, according to Lyon. ‘Coatings fail nine times out of ten due to incorrect application. Proper surface preparation and application of corrosion protective coatings requires a complex skill set, and industry must train applicators not only on how to apply coatings properly, but also explain the reasons why, and the consequences that result, when the right procedures and techniques aren’t used,’ he states.

The bidding process for public construction projects also needs to be modified, according to Lyon, as it encourages saving money upfront. ‘It has been clearly demonstrated that spending more initially on proper materials and coating systems can save money over the lifetime of an asset. Thus, corrosion management needs to be brought into the equation,’ he says.

NACE International is now undertaking a new study that will provide a global cost of corrosion estimate and also include practical examples of actions taken to reduce corrosion and the resultant savings. ‘We have observed that when industry and policymakers are shown real numbers that clearly demonstrate the benefit of preventive strategies, they more readily understand the value that is gained,’ says Kevin Garrity, past president of NACE International and senior vp of engineering with the Mears Group.

The two-year, global study will be completed in 2015.

1 G.H. Koch et al, Corrosion cost and preventive strategies in the United States, sponsored by the Office of Infrastructure Research and Development Federal Highway Administration. Report No. FHWA-RD-01-156.