Plant Maintenance: Efforts Pin Down Corrosion

Diverse efforts seek insights for understanding and combating the pervasive problem.

By Seán Ottewell, Editor at Large

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Corrosion eats up 3–4% of global gross domestic product each year, according to a 2016 study by NACE International. That translates to an annual cost of about $2.5 trillion, says the Houston-based organization that focuses on corrosion prevention and control. However, corrosion remains an elusive as well as an expensive problem to pin down.

“Chemically we understand what corrosion is — but unfortunately it doesn’t occur uniformly at all. If it did, it would be easy enough to predict the rate of corrosion. What we need is well-controlled corrosion films to protect metals. It’s understanding why corrosion accelerates suddenly and takes place in a particular location that is crucial here,” explains Philip Withers, a professor of materials at the Royce Institute, University of Manchester, U.K.

“What is less well understood are specific features of corrosion. For example, corrosion at the atomistic level is a non-deterministic, stochastic process. So, the corrosion rate on a piece of equipment that is being used in exactly the same way from day to day will vary from day to day. Engineers don’t understand this way of thinking,” adds Stuart Lyon, AkzoNobel professor of corrosion control at the university.

In an effort to tackle this problem, last July BP, London, teamed up with the University of Manchester in a £5-million (≈$7-million) collaborative project. The funding is coming jointly from BP and the U.K.’s Engineering and Physical Sciences Research Council (EPSRC).

The project will bring together top researchers from the company, Imperial College London and the University of Cambridge — many of whom already work together on corrosion research through the six-year-old BP International Centre for Advanced Materials (BP-ICAM) at Manchester University — along with additional experts from the University of Leeds and University of Edinburgh.

This project stems from an earlier BP-ICAM effort which studied the fundamental processes that initiate corrosion.

“Manchester, Cambridge and Imperial have been working together for more than five years with BP looking at a range of advanced materials problems. But to solve these, we needed to bring in new skills. So, we recruited expertise from Leeds on tribocorrosion [material degradation due to the combined effect of wear and corrosion] and expertise from Edinburgh on how high pressure can affect the behavior of interfaces,” says Withers, who serves as principal investigator on the new project.

By combining this expertise with different skills in modelling and imaging as well as performing experiments under real-life conditions, the team hopes to answer three fundamental questions: What happens at the start of corrosion? How does it then propagate? And what occurs in tribocorrosion? Some of the basic understanding gained should enable improving current materials; the team will focus particularly on developing better coatings and inhibitors as well as wear-side lubricants and additives that can be used with them to extend equipment life (Figure 1).

By applying synchrotron radiation, among other techniques, the researchers hope to understand the very early stages of oxidation. Such radiation penetrates the surface of corrosion films and helps to show the importance of material stresses and densities on how protective layers break up in localized areas.

“Imaging is very important and we are now able to cheat the fundamental limits of the accuracy to get amazing resolutions. Fifteen-to-twenty years ago, for example, 20–30 microns was high resolution with X-ray imaging. Today, we are [at] the 50-nm scale. The great thing about using X-rays is that you can look through materials, so you get to see pits and other features and understand them at the sub-micron scale,” Withers explains.

Other imaging techniques used include atomic force microscopy, scanning electron microscopy and transmission electron microscopy (Figure 2).

Although the collaboration was announced in July, research really began in November. The team already has made progress: “What we have done is the basic modelling of the early stages of corrosion, looking at how structures change because, for example, the film gets thicker and this, in turn, affects diffusion and diffusion pathways. We have seen how corrosion films build up and this is very similar to the films that prevent wear. Further, when corrosion and wear occur together, the degradation accelerates and, so, we are looking at the interaction of the two. The interesting thing here is that one plus one can equal 1,000. This is because we can study the structure of corrosion and structure of wear individually — but acting together, their effect can be multiplied 1,000-fold.”

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