Most of us have seen corrosion, or rust, on metal that has been exposed to the elements, especially in coastal environments, and this can be a big and expensive problem to fix for ship owners and those who maintain port infrastructure.
Swinburne University of Technology researchers, led by the Faculty of Science, Engineering and Technology’s Dr Scott Wade, are examining a particularly dangerous and poorly understood form of corrosion, known as Microbiologically Influenced Corrosion (MIC).
In MIC certain microscopic bacteria, 100 times smaller than the average diameter of human hair, can dramatically increase corrosion rates.
“The rapid attack of MIC means that instead of the corrosion taking 100 years to compromise a structure, it may take only a few years,” Dr Wade said. “Then you can end up with a structure failing much earlier and that makes a big difference in terms of safety, cost and maintenance.
“In the marine environment these structures are metal areas, like the inside of ship hulls and the walls of ports or harbours.
“Think of, for example, a navy ship or a cargo loading dock. If it is affected by MIC, then not only do you have the obvious repair bill but also the costs of lost productivity as a result of not having that asset available which can be just as much of a problem.”
MIC is caused by different types and combinations of microbes, which are typically categorised by characteristics, such as the by-products they create (for example acid producing bacteria) or the processes they perform as part of their life cycle (for example sulphate reducing bacteria). The microbes tend to fall into two groups based upon their oxygen requirements- one being aerobic or requiring oxygen, and the other being anaerobic which requires little or no oxygen.
MIC involves a range of different processes that can attack a material. In general, MIC starts by bacteria attaching to a surface and forming what is called a biofilm. Within this biofilm microbes of differing types and groups interact allowing them to survive in environments that are usually hostile to them. For example, in an aerobic environment, anaerobic bacteria would usually be inhibited or die. However, if the aerobic bacteria reside in the outer layer of the biofilm consuming the oxygen in the water, the inner portion of the biofilm experiences a reduced oxygen level allowing anaerobic bacteria to thrive. It is the combination of numerous microbes that leads to attack of the material underneath them.
The researchers, from across various disciplines, have been working with industry to better understand this process so that they can develop strategies to minimise the dangers that MIC presents.
“We have experts in a range of key areas of science and engineering at Swinburne collaborating on MIC, which is critical to understanding this complex problem,” Dr Wade said.
“The difficulty with MIC is that you are never sure when it is going to start, but if we can detect risk factors we can act on it before it becomes a problem.
“Another challenge we face is that there are no standard tests for MIC. What we are trying to do is work out how you should test for MIC properly in the lab and in the field. We’ve shown, for example, that depending upon the way you set up your lab test you can force exactly the same type of bacteria to either stop corrosion or to rapidly increase it.”
MIC doesn’t just occur in structures exposed to elements in coastal areas, it can also happen in other sites, such as fire sprinkler systems, where water may stagnate. In addition to steel MIC can occur with a range of materials including copper, stainless steel, brass and aluminium.