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New perspective on microbiological-influenced corrosion of steel in seawater

NACE International, August 12, 2014

A study by Australian researchers throws new light on microbiological-influenced corrosion (MIC) of steel in seawater by showing that pitting plays a key role. 

Bacteria and other microorganisms are known to affect the corrosion of steel. Laboratory tests carried out by many researchers have shown this phenomenon, a.k.a. “microbiological-influenced corrosion” or “MIC,” exists.

One downright odd and puzzling aspect of MIC that two researchers at The University of Newcastle in Australia noticed and decided to explore further was that while their fieldwork samples seemed to show no effect of microorganisms for the first year, it mysteriously showed up later.

In a paper published in the August 2014 issue of CORROSION, researchers Robert E. Melchers and Robert J. Jeffrey describe their work exploring MIC and the long-term corrosion of mild steel in natural and ultraviolet-treated coastal seawater.

For corrosion research, small metal test strips called “coupons” are frequently used to assess and predict the speeds at which metals used in outdoor environments will succumb to corrosion. Results based on the loss of mass coupons, a.k.a. “general corrosion,” provide an average type of result.

“We began to suspect a local effect—pitting—was involved that mass loss wasn’t registering,” explains Melchers, a professor of civil engineering at The University of Newcastle. “So that’s what we decided to test. If correct, it would change the perception of how microbiological-influenced corrosion works.”

Putting their theory to the test, they simply compared corrosion in local natural seawater with the corrosion measured after filtering and “sterilizing” seawater from exactly the same source—as well as it could be sterilized using a series of filters and ultraviolet radiation. “There was little change to the seawater, except that it killed off nearly all of the living organisms,” Melchers notes. 

The researchers’ work “throws new light on exactly what MIC does to steel in seawater. Mass loss is only part of the picture,” Melchers says. “For applications such as a pipelines, pitting is much more significant. Our findings tie in nicely with

other work we’ve done recently for the offshore oil and gas industry that showed very severe corrosion in polluted waters is almost entirely pitting.”

In terms of applications, the work will help to predict what’s likely to happen for existing major infrastructure—including water injection pipelines, mooring chain systems for floating storage and production units, holds in bulk carrier ships, ballast tanks in older ships, and cast iron pipe external corrosion.

“The really interesting question now is how to reduce or prevent MIC,” says Melchers. “The usual approach is to hit them with biocides, but there are increasing pressures to disallow this because it causes other problems. While cathodic protection—using sacrificial anodes to protect steel—can be effective for some simpler industrial systems, it’s certainly not ideal for all. Cathodic protection itself is expensive and needs constant maintenance, so we’re beginning to look at microbial systems to reduce microbial systems to reduce microbial metabolism—as are many other researchers.” 

More Information: The paper, “Long-Term Corrosion of Mild Steel and UV-Treated Coastal Seawater,” written by Robert E. Melchers and Robert J. Jeffrey, appears in NACE International’s journal, CORROSION, August 2014, Vol. 70, No. 8, pp. 804-818. See: http://dx.doi.org/10.5006/1122

About NACE International: Founded in 1943, NACE International, The Corrosion Society, serves 33,000 members in 130 countries. Based in Houston, Texas, with offices in the U.S., China, Malaysia, Brazil and Saudi Arabia, the organization serves all industries impacted by corrosion and provides the most specified technical training and certification programs, conferences, industry standards, reports, and publications focused on corrosion prevention and mitigation. 

CORROSION is a technical research journal devoted to furthering the knowledge of corrosion science and engineering. The technical articles selected for publication in CORROSION provide a permanent record of the latest progress in the science and technology of corrosion control. The journal is directed at scientists and engineers concerned with the phenomena of corrosion processes and the protection of materials in corrosive environments. For more information, please visit http://corrosionjournal.org. 


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