Biocorrosion Behaviors Of Thermal Sprayed Aluminum-based Coatings In Synthetic Seawater

Corrosion is one of the persistent problems for marine infrastructures,which occurs due to chemical or electrochemical reactions between the marine environment and metals.Presence of microorganisms in corrosive media makes it more complicated.For most metallic materials,such as stainless steel 316L used in the marine environment,their mechanical,physical and chemical properties can be damaged by the activity of microorganisms that attach on their surfaces.In recent years,many researchers are trying to improve marine coatings to prevent biofouling and corrosion.Use of thermal spray techniques has been successful in effectively protecting marine structures from corrosion through providing inorganic coatings for maintenance-free long-term service.Development of biofilms on coating surfaces would certainly influence their anti-corrosion properties.Unfortunately,there are so far few reports available elucidating biocorrosion behaviors of thermal sprayed marine coatings.In this study,stainless steel 316L and thermal sprayed aluminum-based coatings were employed as typical marine materials to investigate colonization behaviors of bacteria on coating surfaces.Furthermore,the effect of different environmental factors,including pH,temperature,culture age,surface topography and hydrodynamic condition,were explored.Four highly related investigations were performed in this work.In the first part,we tried to understand the corrosion behavior of thermal sprayed Al and Al/nano-Al₂O₃ coatings in the presence of bacteria and their biofilms in artificial seawater.The biofilm constructed by Escherichia coli?E.coli?was mainly employed as a simplified model.The microstructure and corrosion behaviors were characterized using electrochemical and surface analysis methods.The biofilm was shown to enhance the corrosion resistance of Al-based coatings in early stages.Simulated results using equivalent circuit models provided direct electrochemical information about liquid/surface interfaces.Results are presented in session 3.1.In the second part of the work,session 3.2,we studied the attachment behaviors of a marine bacterium,namely Bacillus sp.and subsequent formation of bacterial biofilms on stainless steel and thermal sprayed Al coatings in artificial seawater.Colonized bacteria accelerated the corrosion of steel plates,and markedly enhanced the corrosion resistance of Al coatings in early growth stages of bacterial biofilms.After 7 days incubation,the biofilm formed on steel was heterogeneous while exhibited homogeneous features on the Al coating.Atomic force microscopy examination disclosed inception of formation of local pitting on steel plates associated with significantly roughened surfaces.Electrochemical testing suggested that the impact of bacterial biofilms on corrosion behaviors of marine structures was not determined by biofilms alone,but instead attributed to synergistic influence by both biofilms and physicochemical characteristics of the substratum materials.The focus of the third part was to investigate the effect of physical and chemical parameters on Bacillus sp.biofilm formation.In addition,the mechanism of biocorrosion on surfaces of SS 316L and the thermal sprayed Al coatings and how these biofilms sealed the existing pores in the thermal sprayed Al coatings was explored with different electrochemical?electrochemical impedance spectroscopy and cyclic polarization?and surface analysis methods?CLSM,SEM,and AFM?.Results showed maximum bacterial attachment at pH 9 and 30oc,and after 90 days immersion,both SS 316L and the thermal sprayed Al coatings exhibited pitting corrosion.However,the thermal sprayed coatings in the presence of bacteria showed higher resistance against corrosion compared with sterile samples.Results are given in session 3.3.Finally,in session 3.4,the effect of shear stresses?as the major physical force?induced morphological changes on the growth,adhesion and biofilm formation ability of Bacillus sp.under laminar and weakly turbulent flows was investigated.We observed that the early stage of biofilm development was practically unaffected by shear stresses.However,in a mature biofilm,shear stresses determined the disposition of biofilm cells onto the surface.Flows clearly shaped the development of biofilm architecture and community formation,as revealed by CLSM and SEM.Biofilm growth patterns were undirected under laminar flow,while they were clearly directed into ridges and conspicuous streamers under weakly turbulent flow.The microcolonies assumed elongated forms,termed"streamers",possibly due to a pressure drag force.Moreover,we have simulated a model to further describe the shear stress changes in the system.