Research On Fungus Corrosion Mechanism Of Aluminum Alloy 7075-T6 In Humid And Hot Marine Environment

Aluminum alloy is widely used in highway construction,automobile,railway vehicles,aircraft,ship and so on due to its low density,good machinability and good recyclability,the excellent performance of aluminum alloy meets the demand for lightweight materials in modern science and technology.In the reports of service failure of aluminum alloy materials,the corrosion induced by fungus is common,especially in the humid and hot marine environment,which has attracted wide attentions of scientists.In this work,marine corrosion environment of high temperature,high salinity and humidity is simulated,the effects of Aspergillus niger(A.niger)on the corrosion behavior and corrosion mechanism of aluminum alloy 7075-T6 were studied,providing theoretical basis for the fungus corrosion protection of aluminum alloy.In this paper,a series of analysis methods,such as high-performance liquid chromatography tandem mass spectrometry(HPLC-MS),gas chromatography(GC),weight loss,electrochemical measurements,including electrochemical impedance spectroscopy(EIS)and potentiodynamic polarization curve,scanning electron microscope(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),3D microscope and so on,were used to systematically study the influences of fungus growth and reproduction process on the early microbial corrosion of aluminum alloy.Researches showed that Aspergillus niger could grow and propagate in corrosive medium with high temperature and high salinity,and formed biofilm on the surface of aluminum alloy 7075-T6.Organic acids and macromolecular biomass were found to enrich in metabolites of A.niger via HPLC-MS detection.Results of gas chromatography(GC)exhibited that the amount of hydrogen generated after inoculation with A.niger was 7.8 times that of the control group,suggesting that hydrogen evolution is one of the main cathodic reactions in the early stage of fungus corrosion of aluminum alloy.The corrosion products and corrosion morphology analyses showed that in the blank contrast test,the corrosion products on the surface of specimens were mainly Al2O3 and Cu Cl2,the corrosion product film was dense,which inhibited the corrosion process of the matrix to some extent;The corrosion products of aluminum alloy in the media inoculated with A.niger were mainly Al(OH)3 and Al2O3,which formed a loose,porous,rod-shaped biofilm enwrapped by some scaly substances,In this case,the corrosion was serious.The electrochemical impedance analysis showed that capacitive loop in high frequency range and inductive loop in low frequency range appeared at the initial stage of soaking in the solution containing sodium chloride no matter whether the fungus existed or not.With the prolongation of soaking time,inductive loop in low frequency range disappeared and the Warburg impedance appeared.The sum of film resistance(Rf)and mass transfer resistance(Rct)in the experimental group inoculated with fungi was less than that in the blank control group,indicating that A.niger promoted the corrosion of the matrix aluminum alloy.The test results of the dynamic potential polarization curve showed that the corrosion current density(icorr)in the blank control group changed little with the increase of soaking time,but the icorr of the experimental group increased significantly.According to in situ characterization on the surface of specimens after removing corrosion products film via the 3D microscope,the maximum depth of the pitting in the presence of A.niger was 22 times that of the blank control group,indicating that A.niger accelerated the local corrosion of the aluminum alloy.The metabolism of A.niger consumed oxygen near the surface of aluminum alloy,and the biofilm also hindered the diffusion of oxygen to the surface of aluminum alloy,therefore,the local oxygen concentration cell was formed under the biofilm.A.niger produced a lot of organic acids,and then the hydrogen ions were produced by the ionization of organic acids,leading to the formation of a locally strong acid environment under the biofilm,which not only promoted the local corrosion of the aluminum alloy,but also promoted the chemical acid corrosion of the aluminum alloy.The macromolecular biomass produced by Aspergillus niger contains functional groups,such as carboxyl,hydroxyl and amines,which can complex metal ions and play the role of anodic depolarization,thus promoting the corrosion of aluminum alloy.