Mickler-O’Connell Bridge built in 1976
The Matanzas River, located in the northeastern coast of Florida is a narrow salt-water estuary. Sheltered from the Atlantic Ocean by a barrier Island called Anastasia Island. There are two major bridges that connect this island to the mainland, the Bridge of Lions in St. Augustine and the Mickler-O’Connell Bridge located south of St. Augustine.
Unfortunately, the Mickler-O’Connell Bridge is experiencing severe corrosion of its H-piles. The Florida Department of Transportation (FDOT), appointed Florida International University (FIU) to conduct a study of the possibility of microbiologically influenced corrosion (MIC). In 2006, the FDOT’s corrosion group implemented a galvanic cathodic protection (CP) system using bulk zinc anodes, to mitigate the corrosion of the bridge’s H-piles. However, after one year the system proved to be ineffective given that the anodes were depleting faster than anticipated. Further research indicated that corrosion did not cover the entire steel surface. Instead, it appeared as localized sections of pitting and holes; a large percentage of the deficiencies occurred close to the water surface.
Marine environments are known for supporting aggressive corrosion. Generally, the main cause for corrosion is the presence of chlorine ions. This normally affects steel structural elements submerged in seawater, as well as those exposed to the tidal zone and splash zone above high tide. Additionally, the type of corrosion found on this bridge led experts to believe the damage was caused by MIC. FIU assistant professor and graduate program director, Dr. Kingsley Lau, and Ph.D. student Samanbar Permeh, explained how microorganisms adhere to most surfaces in contact with water, and when reproduced, they create exopolymers that influence the chemistry of the surface they are attached (NACE International). This microorganism interaction with the metal surface is known to degrade materials.
To determine if MIC is responsible for the degradation of the bridge’s steel H-piles, Dr. Lau and a team of FIU scientists are working with Duncan and the FDOT corrosion group to investigate the presence of bacteria, nutrient levels, environmental conditions, and other factors at the bridge site that could support MIC.
In 2016, researchers visited the bridge site and gathered water samples from two separate locations at different depths. The samples were analyzed to determine the water’s environmental conditions and the presence of microorganisms to establish a connection to MIC. The results proved there are high concentrations of bacteria present in the area. Researchers explained that the quantity of bacteria found in the water cannot directly correlate to MIC risk. However, such high quantities prove there is a greater possibility for bacteria to multiply and contribute to MIC. “We can put two and two together and say that MIC can happen on this site. Whether those holes are caused by MIC? It can be likely,” Dr. Lau says.
Scientists acknowledged that more research is needed to determine if MIC is causing the severe corrosion of the H-piles. Additionally, the project will also focus on macrofouling – the adherence to marine life – on the steel and its correlation to present bacteria and MIC. Macrofouling attached to the H-piles provided a crevice environment. “What we’re trying to address is the effect from crevice environments that form with macrofoulers, and can they support MIC” Dr. Lau explained, noting that macrofouling along with the manifestation of MIC is thought to cause substantial localized corrosion.
Another reason why the corrosion damage is so extensive might be the inadequate levels of CP. However, it may be affected by the chemical arrangement of the macrofouling organisms. According to the journal NACE International, microorganisms build a layer of calcium carbonate (CaCO₃) that create crevices on the surface of the structure. When applying CP in this environment, it is very difficult to cathodically polarize inside the crevice in a pit. Dr. Lau explains how marine growth is responsible for the crevices formed and that in this case, MIC is abundant in the crevice areas. This explains why mitigation by CP is less effective.
In order to come up with a plausible solution to mitigate MIC in the crevice environment formed by macrofouling, researchers have recreated the setting in a laboratory by building artificial crevices and polarizing them with CP. Also, they plan to bring in samples with existing marine growth from the bridge site. With this, scientists may observe the immediate effects of macrofouling and how it affects the CP system. Therefore, once researchers define the role macrofouling imposes in the corrosion of the H-piles, they will determine whether routine cleaning will decrease its influence on steel. Scientists are also looking into alternative methods to mitigate corrosion, such as coatings on the H-piles to prevent the formation of biofilms, thus cathodic polarization would be improved.
With the presence of various marine environments with similar conditions to those affecting the Mickler-O’Connor Bridge. Researchers have compared the results from the case study tests with environmental databases from Florida’s water management districts, to identify locations with similar settings. As a result, many sites with resembling characteristics were found, confirming microbial activity and the probability of MIC.
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