Materials Performance

AUG 2018

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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Page 18 of 92

16 AUGUST 2018 W W W.MATERIALSPERFORMANCE.COM MATERIAL MATTERS Continued f rom page 15 of corrosivity for the Cu-Ni samples. From this, the authors assumed that microor- ganisms played an important role in cor- rosion of the metals in seawater. To inspect the sample surfaces in more detail, they were observed using SEM. The oxide morpholog y of the artif i- cial seawater-immersed sample was observed to be completely different from the natural seawater-immersed samples. It was clear that corrosion products were growing over time in seawater. The sur- face oxides were identif ied using EDS. Cu and O were the two main constituents of the oxides, indicating that Cu x O-ty pe oxides developed on the metal surface. Cl was barely detected in the seawater sam- ple. The authors speculate that exfolia- tion of Cu-Cl compounds actively occurred because these compounds were found on the bottom of the bottles. The samples in chlorinated natural seawater were investigated using a simi- lar procedure. The development of corro- sion in the chlorinated natural seawater immersion experiments, the authors note, was somewhat similar to corrosion devel- opment in non-chlorinated artif icial sea- water rather than in non-chlorinated nat- ural seawater. It appeared that the samples barely corroded for the f irst month of immersion, even when polished. According to the authors, residual chlo- rine not only provided the Cu-Ni alloy with fouling protection, but also it reduced the speed of general corrosion of the metal. After three months of immer- sion, the samples showed corroded sur- faces. Green oxides, speculated to be Cu-Cl compounds, mostly remained on the scale surface, although some debris peeled off and was found on the bottom of the bottles. The effect of dosing level did not seem to be strong—up to 5 ppm of residual chlorine. A f ter immersion, the samples were physically cleansed and weighed. It appeared that the solutions in which the samples were immersed (i.e., natural seawater, artif icial seawater, or chlori- nated natural seawater) had a major effect on the samples' weight change. Weight reduction was noticed in the nat- ural seawater immersion tests, whereas the samples gained weight in artif icial seawater. The authors speculate that microorganisms in the natural seawater promoted the corrosion process and ex foliation of the Cu-Cl compounds. Both natural and artif icial seawater pro- v ides Cl for forming Cu-Cl compounds, and Cu-Cl compounds formed in both sea waters during the immersion test. Peel-off of the Cu-Cl compounds occurred on samples immersed in the natural seawater, which resulted in weight loss. A lternatively, in artif icial seawater immersion, the samples gained weight as most of the Cu-Cl compounds remained aff i xed to the metal. From the study, the authors concluded that the corrosion rate was higher for samples immersed in non-chlorinated seawater. A lthough the Cu alloys are known to have antifouling characteristics to a certain degree, they say it seems nec- essary to treat the seawater in applica- tions with CuNi 90/10 alloys. Addition- ally, the corrosion rate was inf luenced by the initial surface condition of the immersed samples. The polished samples corroded most heavily because the bare metal was directly exposed to corrosive seawater when immersed. On the other hand, the as-is and conditioned samples had a lower corrosion rate in natural sea- water. Microorganisms seem to encour- age the exfoliation rate of Cu-Cl com- pounds on the metal surface; and by doing so, they increased the corrosion rate. A lthough Cu-Cl compounds were formed in every test scenario, excessive exfoliation of the Cu-Cl compounds was observed only on samples immersed in natural seawater. The authors add that excessive chlori- nation is undesirable because the corro- sion rate (as indicated by the polarization curves for Cu-Ni alloys in artif icial sea- water) became higher with increasing residual chlorine levels. The study indi- cated that a residual chlorine level up to 5 ppm posed no harm to CuNi 90/10 alloys in terms of corrosion in seawater. It con- trolled the bacterial effect and did not accelerate the metal's corrosion rate. The authors note, however, that care should be taken not to expose Cu-Ni alloys to highly concentrated chlorination, and assert that a f ine line needs to be def ined at which safe use of Cu-Ni alloys is guar- anteed for chlorinated seawater handling service. Therefore, their future work will be focused on determining the maximum limit of residual chlorine for safe applica- tion of CuNi 90/10 alloy in seawater services. —K.R. Larsen Microscopy of sample surfaces after an immersion period of one, three, and six months in non-chlorinated seawater. Samples marked AS were immersed in artificial seawater. Source: CORROSION 2018 paper no. 11418.

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