Made to Measure…Cast Iron

Who are you?: Dr Ron Logan

What is your role?: Water Consultant at Mouchel. Currently on secondment to Thames Water.

What was your PhD about?: Studying the graphitic corrosion of grey cast iron in the buried environment.

I beg your pardon?: Graphitic corrosion is an unpredictable and highly localised form of corrosion specific to grey cast iron, whereby the ferritic component of the cast iron alloy goes into solution leaving behind a soft residue of the original graphite flakes interspersed with iron oxides and other insoluble products.

Why?: Graphitic corrosion on the external surface of the pipes is the principal underlying reason for a pipe to fail and burst in service. Of the 20,000 km of the pipe network that delivers 2.3 billion litres of water a day to London, 44% is constructed from grey cast iron, around half of which have been in the ground for over 100 years. It is therefore imperative to understand the rates and mechanisms by which these pipes are deteriorating as the cast iron network continues to serve London.

And?: The traditional view of graphitic corrosion is that it is a process of dissolution and transport of the iron matrix with the graphite flake structure left behind, undisturbed. My recent doctoral studies showed that this is an incomplete picture. Scanning electron microscopy, along with energy dispersive and wavelength dispersive X-ray spectroscopy (EDS & WDS), was used to characterise graphitic corrosion. The graphite flakes, are in fact deteriorating as part of the corrosion process. It is proposed that the interface between the metal matrix and the continuous graphite flake network acts as transport network for soluble ions. EDS and WDS investigations confirm the presence of chlorides at the corrosion interface of graphitic pits on the external surface of the trunk main.

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Figure 1 : Digital Photomicrograph via Scanning Electron Microscopy: Grey scale image of a cluster of graphite flakes in an iron matrix, with an elemental analysis showing the presence and concentration of Cl- ions, highlighted in red.

So what?: These investigations show that chlorides in the soil are likely to be the major driver of graphitic corrosion. The identification of chlorides as a risk factor has an impact on the overall asset management of the network.

Final Thought: It is not just material scientists and water network engineers who are trying understand how cast iron corrodes. I found that the archaeologists are too, which is helpful as London’s cast iron pipe network can be thought of as an antique that happens to remain in service.