ABSTRACT

The main purpose of this study was to monitor the growth of the marine bio-film, of micro/macro-organisms, on the surface of different metallic alloys in seawater by electrochemical impedance spectroscopy (EIS). The alloys used in this study were; UNS 1020 carbon steel, stainless steel 304, stainless steel 316L, Sanicro 28, Cu-Ni 70-30, Hastelloy G-30, and titanium. The EIS was used to measure the A.C. Impedance (Z) and the double layer capacitance (Cdl) of the formed bio-film in seawater on a frequent basis. The total exposure time of the tests was ranged between 90 days to 180 days. The visual inspection of the tested samples showed a bio-film formation on the surface of these samples. The microbiologically induced corrosion(MIC) was observed only on the carbon steel. Monitoring the growth of the bio-film formation was accomplished by the EIS during the 90-180 days exposure of the tested samples. A gradual monitoring of the growth of the bio-film formation was achieved by mathematically correlating the obtained the A.C. Impedance (Z) and the double layer capacitance (Cdl) of the bio-film to the thickness of the bio-film formation. The advantage of EIS is a non-invasive technique with a sensing (spatial) resolution in a nanometer scale in a comparison to other techniques of monitoring the growth of bio-films on metallic alloys in aqueous solutions.

INTRODUCTION

It has been widely known that marine bio-fouling of metal surfaces in contact with seawater is the main cause of several technical problems and economic loss in industry. Marine bio-fouling is defined as the community of organisms found growing on permanently submerged surfaces of objects in seawater. It has been reported that a total of 84 species of fouling organisms belonging to 69 genera, 49 families, and 10 phyla were recorded over the entire year dependent on the temperature of the seawater1. The growth of this community usually interferes with the efficient use of the surfaces. For example, the organism is known to reduce the flowing of the fluids through the pipelines, to reduce the heat transfer in the heat exchanger systems, to reduce the ship’s speed and encrusting of the support legs of oil rigs2-3. Marine micro fouling is defined as the growth of micro-organisms at metal-solution interfaces. Also, marine micro fouling can be even up to a few micrometers thick. It is known that marine micro fouling reduces heat transfer coefficient in heat exchange systems by as much as 40% besides causing what is known as microbiologically induced corrosion (MIC)4-5. Different types of bacteria cause corrosion by various mechanisms ranging from formation of differential aeration cells to production of aggressive environments through chemical changes6. It is expected that the seasonal changes in the Gulf seawater will have an effect on the MIC, due to the growth of the marine biofouling film at metal-solution interfaces. In other words, the growth of the marine biofouling film at metal-solution interfaces would control the corrosivity of the submerged metals in seawater.

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