Microbiologically influenced corrosion Deciphering steel-eating bacteria

Every year, corrosion by microorganisms causes billions of dollars of damage worldwide. So far, no remedy has worked against them permanently. Scientists at BAM have found a new way to combat the issue by simulating actual environmental conditions in the laboratory in order to decipher the mechanisms.

Andrea Koerdt became suspicious about a pipeline in Nigeria: the BAM biologist read a case study about an oil pipeline in the West African country that was heavily corroded. However, there was hardly any sulfate in the soil around the pipe. The sulphur compound is the most important nutrient for certain microorganisms, called sulfate reducing bacteria, which are usually responsible for this kind of destruction.

Solving the mystery

Many people do not know that microbes can cause corrosion, even though this form of damage is widespread. Several BAM teams are researching microbiologically influenced corrosion or MIC for short. Microbes are unicellular organisms that cannot be seen with the naked eye. There are millions and millions of different species, including bacteria, archaea and certain fungi. Microbes can damage surfaces in two ways: either they can break down the material, causing craters and holes by removing electrons or entire atoms from it. Or their metabolism produces chemical compounds that can interact with other substances to attack the surfaces on which they settle. They unfold their subtle destructive works on a small scale almost everywhere – with fatal consequences for the entire infrastructure: they attack concrete, sewage pipes, create holes in diesel tanks and decompose the foundations of wind turbines. No material, be it metal, plastic or glass, is safe from them.

Year after year, unicellular organisms cause enormous economic damages. According to experts at the World Corrosion Organization in New York, the damage is estimated at up to three percent of the gross national product of industrialised countries. This would equate to several billion euros for Germany alone. The worldwide costs may be around 3.3 trillion dollars. Researchers use toxic biocides to fight microbes: they develop hydrophobic surfaces which repel the water that unicellular organisms urgently need to exist. "But microbes are survival artists," says Andrea Koerdt. "They quickly adapt to more difficult environmental conditions.”

The biologist noticed something else when she read the case study about the pipeline in Nigeria's oil fields: the researchers did not find any sulfate reducing bacteria in the earth layers around the pipeline that could so alarmingly destroy steel. However, they detected methanogens – anaerobic unicellular organisms that are at best considered moderately corrosive. Might these underdogs from the realm of microbes be responsible for the disastrous pitting corrosion?

Understanding microbes

Andrea Koerdt decided to choose a completely new way in the fight against corrosion: she first wanted to thoroughly understand how microbes work. Then, in a second step, she aims to develop countermeasures against them. "My strategy is to find out how and why microorganisms do something," she says. In order to classify the corrosion rate, which is the damage caused by unicellular organisms, researchers typically enclose bacterium cultures together with a metal plate in a serum bottle. After a few weeks, they open the container again and weigh the metal piece. They determine the corrosion rate of the microorganism from the weight loss.

Sulfate reducing bacteria, for example, reach a rate of 0.7 millimetres per year – that means they would completely eat through a pipe with a wall thickness of one centimetre in about 14 years. Andrea Koerdt believes that this procedure is based on two false assumptions. Firstly, microbes do not carry out their destructive work uniformly. They can erode deep craters into the metal in some places and leave other parts completely untouched. This means that the corrosion rate does not realistically reflect their destructive potential.
Second, nature is not a closed system: everything interacts with each other.

The environment in a test tube

Andrea Koerdt therefore chose a different experimental set-up in order to simulate the real environmental conditions in the laboratory. She filled a glass column about 20 centimetres high layer by layer with metal, sand or glass beads. Then she added cultures of a methanogen, those outsiders from the world of microbes who are said to have a low corrosion rate of only 0.06 millimetres per year and whom the researchers have found around the Nigerian pipeline. A supply line and a drain at the glass column enabled Andrea Koerdt to change the pH value in the small environmental system. Moreover, nutrients and metabolic products were no longer able to accumulate in it but could be washed away – just like in nature. The average corrosion rate of the methanogen skyrocketed to about 0.16 millimetres – almost tripling. The peak value was even around 0.34 millimetres.

This piqued the curiosity of the BAM biologist: she tried to further increase the corrosion rate, changed the pH value again, modified the salt concentration and the temperature. She added trace elements in various concentrations and compositions to the liquid medium that she passed through the column. In doing so she was able to increase the rate of another methanogen by almost eightfold. "We didn’t expect the unicellular organism to do this," Koerdt says today. "And we are far from being done with our experiments. The more we know about how microbes work, the easier it will be to find a remedy against them in the future," says the BAM biologist. Perhaps one day, the findings from her experiments will enable us to develop metal alloys that are better protected against pitting corrosion.

Andrea Koerdt also wants to test materials from the 3D printer for their susceptibility to corrosion in her glass columns – a field still largely unexplored. She wants to establish international cooperation and develop a network with industry in order to offer more realistic test methods in the future. The aim is to provide effective protection against microbes for pipelines such as those in Nigeria at some point in the future.