The Research Of MIC And Protection Of Copper-nickel Alloy In The Marine Environment

The copper-nickel-tin alloy has drawn increasing attention and research for itsexcellent performance of high strength, good thermal conductivity and excellentresistance to thermal stress relaxation, and has become a promising copper-basedmaterial. Excellent mechanical properties, especially the excellent high temperatureanti-deformation ability, and better anti-seawater corrosion in the copper-basedmaterials make it possible to use this alloy in harsh environment. Consequently，it isvery meaningful to research the corrosion behaviors, mechanisms and targetedprotection of this alloy in sea water and marine microbial environment.In present work, the equilibrium geometries, electronic properties, bondingbehavior, stability and active sites of copper clusters doped with Ni or Sn atom havebeen studied using density functional theory. The results indicate that the impurityatoms change the equilibrium geometries, lower the total energies, and modify theHOMO-LUMO gaps and active sites of copper clusters. Meanwhile impurities preferto be the surface or outer layer of the cluster, may form an oxide film from the activesites on the surface of materials, preventing oxygen and aggressive ions diffusion andprotecting the copper alloy against corrosion. Alloy grain is fine and uniform, nearlyno defects at grain boundary, each ingredient is evenly distributed. After long-termcorrosion experiments in sea water, there is no intergranular corrosion, and thecorrosion product of the outer layer is loose basic cupric carbonate, inner layer is adense oxide layer of nickel and tin. It is the dense oxide layer what improves thecorrosion resistance.In addition to seawater corrosion, microbial corrosion is also an important part formarine materials. Copper and its alloys usually have good antibacterial activity, butmore and more studies have shown that they are more sensitive to sulfate-reducingbacteria (SRB). The SRB in Qingdao sea area is identified as DesulfovibrioCaledonians, and its habit and growth curve have been studied. Using fluorescence microscopy, atomic force microscopy and scanning electron microscopy observedSRB adhesion on the alloy surface and film-forming process, and proposed a dynamicmodel. The corrosion potential, polarization curves and electrochemical impedanceanalysis showed that the passivation make this alloy corrosion resistance rise in SRBculture at initial stage. But with the bacterial growth and the solution environmentdeterioration, bacteria gradually attach to alloy surface, form a colony to the finalbiofilm formation, the alloy comes to the activated state of corrosion stage. Thecorrosion potential shifted negatively, the corrosion current density increased andimpedance values decrease. Loose porous layer of corrosion products formed on thesurface, and there is a higher content of S, so the surface passivation film is destroyed.Therefore, to expand the application of copper-nickel-tin alloys in the ocean,microbial corrosion targeted protection is imperative. First, use PEI anion and silvernitrate to form of PEI/Ag+complexes by coordinate valence, alternating self-assemblywith polyanionic, to successfully gain the polyelectrolyte multilayer films load withsilver ions. Silver ions is reduced to nano-silver in situ, and then deposit low surfaceenergy fluorinated silane layer on the micro-rough structure of polyelectrolytemembrane surface. The final preparation is a low surface energy organic/inorganiccomposite coating system, the inner layer is polyelectrolyte membrane withnano-silver, and the outer layer is a fluoro silane layer. Multilayer are characterizedusing scanning electron microscopy, transmission electron microscopy, UVspectrophotometer, X-ray photoelectron spectroscopy, atomic force microscopy andcontact angle measurements and other experimental apparatus. Silver has beensuccessfully loaded into the multilayer film, and distribute uniformly asnano-spherical particles. On the appropriate layers of multilayer film, thesuper-hydrophobic coating surface is micro-nano composite structure.By using SRB as the experimental bacteria, Short-term antibacterial experimentsand adhesion experiments show polyelectrolyte membrane with silver has a goodantimicrobial, but the duration needs to be improved. For the superhydrophobiccoating system, it shows superior anti-adhesion performance and has long-termeffectiveness. The X-ray photoelectron spectroscopy shows that the silver in polyelectrolyte membrane gradually transformed into silver oxide, and theconcentration of silver ions in the solution increased obviously. While heconcentration of silver ions for the super-hydrophobic coating is always lower. Whythe super hydrophobic coating system possesses good anti-bacterial and resistance tocorrosion. Firstly, super-hydrophobic surface is based on polyelectrolyte film coatingwith nano-silver. Then the fluoroalkylsilane layer with good superhydrophobicproperties effectively inhibits microbial from attaching, simultaneously prevents theexcessive release of silver ion. Last but not the least, the nano-silver contained inpolyelectrolyte layer releases silver ions after oxidation, kills the microbial around,and makes up the outer membrane of the local defects. Under combined effect of themulti-factor, the system demonstrates good antibacterial and anti-adhesion properties.