Materials Performance

JUN 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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MP Matters" department. Contact MP Editor Fax: E-mail: 16 JUNE 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 6 would kill them off before they could do any thing to our metal," she adds. After the OCP measurements were taken, cyclic potentiodynamic scans were done between –0.4 V and 0.4 V vs. the OCP at a scan rate of 0.167 mV/s to determine how the presence of bacteria contributed to corrosion. Tafel plots based on these scans were used to determine the corro- sion potential and corrosion currents for each experiment. The samples were then removed from the polarization cell, treated, dried, and made ready for viewing using scanning electron microscopy (SEM). These initial results suggest that under the test conditions, P. f luorescens increased the corrosion of the steel. For samples with and without bacteria, a pos- itive shift in the OCP measurements was seen over the 24 h test time, which sug- gests the buildup of a protective f ilm on the steel due to the peptide-based nutri- ents in the agar electroly te mixture. According to the authors, however, the bacteria had two key effects on the OCP. They made the overall potential values more negative than those for samples without bacteria, and they also increased the variability of the OCP measurements from one point to another. The more neg- ative OCP measurements with the addi- tion of bacteria to the system imply that the bacteria were potentially interfering with the development of the protective f ilm. The samples with bacteria also had more negative corrosion potential read- ings and higher corrosion currents. SEM imaging of the surfaces of sam- ples with bacteria illustrated different protective f ilm morphologies between the samples that underwent a 3-h relaxed potential hold period and those that underwent a 24-h relaxed potential hold period. The 3-h samples showed very few obviously present bacteria cells on the surface, with only one being clearly iden- tif ied on the surface area examined with SEM. The corrosion product on the sur- face also appeared to be less dense. On the 24-h samples, more bacterial cells were visible on the surface, and the oxide f ilm itself was denser and not dominated by corrosion products. The authors explain that the longer free potential hold time led to a decrease in the electrochemical drive that causes corrosion to occur, and propose that with the increased hold time, a greater buildup of nutrients was possible, and that these nutrients formed a protective layer on the steel surface that had an inhibitive effect. The addition of bacteria to the system, however, decreased electrochemical sta- bility that was intensif ied with the longer free potential hold. This suggested that the longer a biof ilm has to form, the more it competitively interacts with the oxide/ nutrient f ilm forming on the steel surface. "Going in we weren't sure what effect the bacteria would have, as there were very few studies conducted with our spe- cies of choice," Spark comments. "In some earlier work we had shown that the pep- tide nutrients we were using had an inhibitive effect on corrosion, so we weren't surprised that with extended time for a f ilm to form before accelerated corrosion testing, there were indications of this with and without bacteria. This also helped us to understand why the effect of the bacteria was not as large as we might have expected looking at stud- ies with other bacterial species." With adaptation, the authors say, this system could be used to accurately repli- cate corrosion in a soil environment with the introduction of nutrients to match the soil environment in question and cul- tured biof ilms found along buried pipe- lines. "We are hoping this research will enable more information to be gathered about the mechanisms of localized corro- sion and how it occurs and how bacteria and other microbes interact with the environment and the metals buried in soil. Now that we have a suitable analogue for soil in the laboratory, we can investi- gate many different parameters for this that can then be used to inform policies for coatings, materials, and the initial sites chosen for pipelines as well as policy around the methods for monitoring, repairing, and replacing pipelines that have been installed," Spark says. References 1 G. Moore, "Corrosion Challenges—Urban Water Industry," Corrosion Challenges Proj- ect 2010, Australasian Corrosion Associa- tion, 2010. 2 A. Spark, et al., "MIC Studies of Buried Pota- ble Water Pipelines Using Semi-solid Agar as an Analogue for Soil ," CORROSION 2016 paper no. 7850 (Houston, TX: NACE Interna- tional, 2016). (a) (b) MATERIAL MATTERS SEM images of the flm formed on the CS sample surfaces with bacteria after (a) a 3-h relaxed potential hold, and (b) a 24-h relaxed potential hold. The arrows point to bacterial cells. Scale bar = 20 μm. Continued f rom page 15

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