Study On The Effect Of Simulated Fouling Biological Adhesion On Corrosion Of Zn Anode

Sacrificial anodes are widely used for protection of steel structures in marineenvironment due to their high theoretical current efficiency, low active potential and lowcost. Zinc anode outstanding advantage is its self-regulating features. Zinc anode canprotect better for metal facilities, so they are widely used on metal corrosion engineering.Intensive fouling coverage on the anode surface can lead to ultimately failure of sacrificialanodes, which are unable to release effective protection current. The sacrificial anodesfailure caused by fouling organisms community can reduce the service life of platformsand cause security accident which becoming serious. And it can not be ignored in oilextraction process of China. While, there is no report about the mechanism of adhesion offoulings on the sacrificial anode caused failure.Marine corrosion as its extremely high biological activity is quite different from othergeneral corrosive electrolyte solution So, the engineering of cathodic protection, itssacrificial anode surface and underwater steel structures are all covered by biologicalfoulings, which form stable fouling organisms community within several years. Westudied the evolution of fouling organisms through cycle hunging in real sea. It can beconcluded a evolution law of different periods immersed in seawater: from microbesbacteria, to a variety of algae, such as Navicula, Amphora, Nitzschia, and finally to largeoysters and barnacles.Hence, in this paper, the effects of marine fouling organisms on corrosion ofZn-Al-Cd and pure Zn sacrificial anodes have been studied using artificial biofilmsimulation, electrochemical methods (including electrochemical impedance spectroscopy(EIS) and polarization resistance technique), and Surface Analysis though simulating twokinds of algae (Navicula and Amphora) and coating different thickness of calcareoussediments powder to change the surface resistance of anodes. This work might be helpfulon getting a better insight of evaluation of sacrificial anodes, and also well know about theinfluence of marine fouling on the anode.At first, the Navicula and Amphora were enriched and purified from Huanghai and Bohai Sea. Their growth curves were determined by the way of spectrophotometry. Theresults showed that the Amphora growth curve divided into adjustment phase, exponentialgrowth phase and decay phase:1-4days for the adjustment phase, Amphora has slow celldivision when they are inoculated into new nutrient solution;4-7days for the exponentialgrowth phase, the maximum number was reached in seventh day;7-10days for the declinephase, the reproduction of Amphora was getting slower and slower, the number of livingcells declined rapidly because of depletion of limited nutrient. Navicula growth cycle is15days, and has four phase:1-4days for the adjustment phase,4-8days is the exponentialgrowth phase, and grow into maximum number in eighth day;8-12days for the stabilizephase, finally12-15days enter the decline phase.1．The XRD results showed that the ingredient compounds of barnacles and oysterpowder is Calcium carbonate, calcium and magnesium compounds and other traceelements such as Mg、Si、Cl、S.2．The EIS results showed that corrosion rates of zinc samples both exposed to thef/2culture solutions and culture containing algae are increased first and then decreased,and anodes exposed in culture containing algae is greater than samples in pure f/2culturesolutions. The rate of black zinc anodes is increased and then decreased but increasedfinally, while the zinc anodes coated with agar and powder of barnacles and oysters havecompletely opposite rule. The ranking of corrosion rate is black zinc samples> anodeswith agar>> anodes coated with powder of barnacles and oysters.3．The polarization curves results showed that Navicula and Amphora adhering onthe surface of specimens producing O2and metabolite, making the self corrosion potentialdecreased and corrosion current increased, which speed up the process of anodicdissolution. The corrosion potential and corrosion current of black zinc anodes changesmoothly, and that of zinc samples coated with agar and powder of barnacles and oystersshowed a increase trend.4．Self-discharge experiment showed the marine biofilms have some protectiveeffect on anodes in the early attachment stage of fouling organisms. The Zinc sacrificialanode still possesses high current efficiency when subjected biofoulings. But its potentional varys damatically in simulated biofouling environment. This effect is relatedto the much more severe local micro-cell corrosion on the surface of the sacrificial anodecovered with fouling attachment.5．The result of SEM gives the conclusions that local corrosion occurs when surfacewas covered with biofouling layer and large pits emerged in simulated biofoulingenvironment. The same pits phenomenon appeared on surface of samples exposed to thef/2culture solutions with Amphora, and anodes in pure culture solutions showed uniformcorrosion.6．The result of EDS showed that the proportion of carbon and magnesium elementsincreased on the surface of microalgae specimen, indicating that microalgae attached tothe surface growing, metabolism and decay. The corrosion products of anodes coated withagar and powder of barnacles and oysters are zinc hydroxide Zn(OH)2, the element of zinc(Zn) and oxygen (O) on the surface of powder system is greater than the content of theagar system. It is concluded that the film thickening on the metal surface hindered anodedissolution.