A coat for all seasons

A protective coating which can adapt to different types of corrosion by modifying its chemical structure has been invented by British materials scientists.

As part of the LINK surface engineering programme, teams at Cranfield and the University of Birmingham developed `Smartcoat’ to protect turbines which encounter a variety of corrosion scenarios.

Consider, for example, a marine gas turbine. Salt spray is sucked in through the air intake and the fuel contains sulphur contaminants. In addition, the turbine blade often heats up in a non-uniform way; one part may be at 900 degrees C, while another is 650 degrees C. In this case, each part of the blade experiences a different type of corrosion.

Smartcoat is able to react to these different circumstances by acting as a single high and low temperature coating. The outer layer is rich in aluminium, which, when the temperature rises, forms aluminium oxide (the best protectant at extremely high temperatures). However, at lower temperatures aluminium oxides do not form quickly enough to confer protection. In addition, pitting can occur on the surface of the turbine due to the presence of salt and sulphur. To combat this kind of corrosion, Smartcoat’s second layer is rich in chromium. When pitting occurs, it is stopped by thermally grown chromium oxide. Thus, parts of the blade operating at high temperatures are protected by aluminium oxide, while those operating at lower temperatures form a layer of chromium oxide.

Smartcoat’s inventors believe that this coating represents a genuine step forward, and the system is currently being evaluated in the field by one of the project’s industrial partners. The team has also received additional funding from the Engineering and Physical Sciences Research Council (EPSRC).