By Alex Sibley
While rust, the corrosion product of iron, is the most common and obvious symbol of corrosion, almost all metals that have been used from ancient times to modern alloys are damaged by exposure to oxidising agents (like air), with costs calculated to be approximately $2.5 trillion USD. Especially vulnerable is the metal magnesium, a lightweight and strong material with properties that make it ideally suited to the automotive and aeronautical industries. Unfortunately, another property it possesses is high reactivity – magnesium is one of the main ingredients in flares that can burn underwater – a significant problem! The group of chemicals known as organosilanes (similar to what you would find in tube of silicone gap filler) can be used to create protective coatings that are environmentally benign, compared to toxic and carcinogenic coatings containing chromium – perfect! Problem solved, we can all go home.
Not quite. The chemistry needed to produce organosilane films involves acidic conditions, which are a minor concern for most materials, but how will this affect highly reactive magnesium? By using state of the art electron spectroscopy and microscopy facilities, we are able to study the structure and composition of organosilane films on the micro and nanoscales. By combining two techniques: Secondary Electron Microscopy and Scanning Auger Microscopy, we can see images of surfaces, as well as their elemental compositions. This information reveals that the acidic conditions used to produce organosilane films damages the magnesium and leads to incomplete coatings (Figure 1).
So, how can we create thin, protective films on magnesium where conventional methods have failed? The answer is by using the state of matter known as plasma. The term might seem more at home in science fiction but plasma is more common in the natural world than most people realise. In fact, given that the sun is primarily composed of plasma, it is hard to go a day without seeing one. More terrestrial examples include the bright flash of a lightning strike and the gas glowing inside a fluorescent light. These examples have little to do with the creation of thin films though, so how do we get from liquid chemistry to advanced plasma processing?
By completely sealing a steel chamber and pumping it to an extremely low pressure, similar to that of outer space (the science fiction influence continues), we create conditions whereby liquid organosilane is turned into a vapour. Radio waves (at a similar frequency to the one that is used in RFID devices) are used to ignite the gas into a charged plasma state (Figure 2) and the complex mixture of high-energy species within the plasma allows a film to be produced.
All this occurs in a clean environment without any organic solvents or other contamination. Not only that, but by adjusting the parameters and mixture of gasses in the plasma, we can control the composition and thickness to almost nanometre precision! This is surely a 21st century solution to a very ancient (and expensive) problem.
- Koch, G., et al., International Measures of Prevention, Application, and Economics of Corrosion Technologies Study, G. Jacobson, Editor. 2016, NACE International: Houston, Texas, USA. p. 1-216.
- van Ooij, W.J., et al., Corrosion Protection Properties of Organofunctional Silanes–An Overview. Tsinghua Science & Technology, 2005. 10(6): p. 639-664.