| The attachment of macrofouling organisms can affect the corrosion process of steel structures in service in the marine environment,leading to a loss of structural integrity and service life,as well as significant economic losses and threats to human life.Currently,research on the corrosion behavior of steel under the influence of macrofouling in the marine environment is still in its early stages,both domestically and internationally.Therefore,conducting in-depth research on the marine corrosion behavior and mechanisms of steel structures in service under the influence of macrofouling is of practical significanceEffects of oyster as hard-fouling organisms on corrosion mechanisms of a steel was studied.The results showed that adhered oysters have caused severe localized crevice corrosion.A crevice corrosion model of steel caused by oysters was proposed,showing that oyster secretions can penetrate into the oyster/steel interface and act as a stabilizer of the rust layer,slowing the metal corrosion rate.In contrast,oxygen concentration cells,enriched Cl-,anaerobic bacteria,the presence of organic free radicals and oxidative cross-linking during the curing process of oyster secretions acted as localized corrosion accelerators facilitating the crevice corrosion process in the oyster/steel crevice.The differences in the corrosion mechanism between oysters and ascidians（soft-fouling organisms）were studied.The results showed that the electrochemical processes,oxygen diffusion,ion transport paths,and secretion state differences caused differentiated corrosion forms with fouled steel.Manifested as localized corrosion at oyster/steel interface,while uniform corrosion at ascidian/steel interface from the state of the entire covering interface.A micro-reaction mechanism was finally proposed concerning the effects of Cl-,anaerobic bacteria,enclosed space,and secretions to reasonably explain this phenomenon.Metagenomic techniques were used to analyze the complex microbial communities which promoted the microbiologically influenced corrosion（MIC）process at macrofouling/steel interface.The results demonstrated that the microbial communities at the interface are stimulated in terms of abundance and diversity by the microecological environment provided by macrofouling organism coverage and the nutrients supplied by macrofouling secretions.Noticeable differences exist in the MIC induced by interface microbial communities between hard-fouling and soft-fouling coverage.In the case of oyster/steel interfaces,the unique anaerobic environment stimulated the activity of sulfate-reducing bacteria（SRBs）such as Desulfovibrio and Desulfobulbus,leading to an increase in SRB-related functional genes（dsr）and a decrease in O2-related functional genes（coxC,ccoN,ccoO,ccoP,and ccoQ）.This resulted in severe localized corrosion at the interface accompanied by the formation of FeS.Additionally,the abundance and diversity of microorganisms did not directly correlate with the MIC process on the steel surface.It was believed that only a small number of corrosive microbial communities,represented by SRBs,participated in the MIC process at the interface.The influence of oyster secretions on the corrosion process at the oyster/steel interface was studied.The results showed that the secretions in the oyster shells tightly covered layer,and the seawater in the crevice,formed a potential difference on steel surface and caused the formation of a local corrosion cell,accelerating the further corrosion of the crevice.Meanwhile,the oyster secretions penetrated the oyster/steel interface,caused the rust layer transformed from a loose porous pore structure into a dense layer structure.Oyster secretions acting as rust conversion agents and rust stabilizers,thus protecting the steel substrate.These unique characteristics are due to its composition,molecular weight,and Zeta potential.In addition,it has been proven that amino acids with corrosion inhibiting properties are present in high quantities in the secretions. |
