| Today in the 21st century, all of the coastal countries come to pay their attention to the development of marine. Our government has regarded the construction of marine powerful nation as a great task. However, the strong corrosive marine environment with serious bio-fouling has been as a barrier to the development and utilization of marine. So it is a valuable topic to study the anti-corrosion and anti-fouling.Cathodic protection, an effective means of anticorrosion is widely used to prevent metal corrosion immersed in seawater and mud. As one of the results of impressing cathodic protection, there is calcareous deposit with thickness of 50～100μm in the surface of metal. Calcareous deposit constitutes a diffusion barrier towards dissolved oxygen and decreases the energy needed to satisfy abundant protection as well as weaken the corrosion fatigue and hydrogen penetration, so calcareous deposits has attracted the attention of many researches and designers.Fouling organisms widely live in marine, biofilm could develop in samples immersing in marine within 2 hours and micro-organisms follow to appear in it. It is authentic that bio-fouling affect the cathodic protection negatively. But the relation between bio-fouling and calcareous deposit was ignored.Selection of the subject was based on the objective fact occurred in project case. Explore the pre-existing calcareous deposit on the corrosion behavior ofQ235 carbon steel in the seawater with fouling organisms. The test was conducted in laboratory and the field respectively. In the laboratory, Calcareous deposit formed on carbon steel was acquired by applying galvanostatic polarization in nature seawater. The quality of calcareous deposit was investigated by XRD and SEM characterization and revealed the formation of calcareous deposit at a current density of-30μA/cm2 was uniform with fine crystalline phase.Study the influence of pre-existing calcareous deposit on the corrosion behavior of Q235 carbon steel in PGC culture inoculated SRB. Fluorescence microscope was used to the attachment of bacteria in samples immersing in inoculated PGC culture with different exposure time, result showed that calcareous deposit benefit the attachment of bacteria in early region. The electrochemical results revealed that the open circuit potential (OCP) of bare carbon steel specimen changes are obviously influenced by the bacteria metabolic activity. Moreover, the Rct value of pre-existing calcareous deposit carbon steel was much greater than that of the bare. Which means the former suffered a low corrosion rate. Surface analysis by SEM and EDS characterization revealed that there was damage in calcareous deposit after immersing in PGC for 11 days, which could be proved by SEM images and the variety of component. The circular particles and a high content of iron and sulfur mean the corrosion product in bare steel was FeS. It revealed that calcareous deposit could inhabit microbial corrosion of metals.The corrosion behavior ofQ235 carbon steel with and without calcareous deposit was studied in f/2 culture medium containing Amphora by SEM, FT-IR and electrochemical measurement technique. The result showed calcareous deposit decreased the corrosion rate. The biofilm was formed on surface of steel with the attachment of Amphora and its metabolite, which could suppress the mass transfer process. However, it is not obvious to hinder the diffusion of oxygen in solution. Calcareous deposit is apt to combine with Amphora forming composite and protective film on surface of steel. Moreover, this film could partly suppress the diffusion of oxygen in solution to surface of steel. Then, the result also revealed the corrosion of steel under the composite film met the following phases:solution run through the film and cause corrosion of steel, then the corrosion rate decreased with the accumulation of mixed film (consists of corrosion products and biofilm). After that, with the damage of the mixed film, the corrosion rate increased again.A 5-months test was conducted at the real ocean circumstance about the calcareous deposit formed by sacrificial anode on the anticorrosion and electrochemical behavior of carbon steel, alteration of bio-fouling attachment during the whole fouling stage. The result showed calcareous deposit obtained by cathodic protection was favorable to attachment of marine micro-organisms. However, macro-organisms would like to adhere directly on substratum. The main composition of calcareous deposit formation was calcium carbonate in the field, doping with a small quantity of clay, the growth of deposit overwhelm biofilm.. In addition, interrelationships between bio-fouling, Electrochemical measure and SEM were used to investigate calcareous deposits and corrosion behavior of carbon steel under cathodic protection, which showed calcareous deposit could control corrosion of carbon steel lasting 4 weeks without cathodic protection in the field and biofilm could influence this function positively. Carbon steel specimens coupled to anode suffered a weak pitting and crevice corrosion, accumulating along with time. |
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