Research On Seawater Corrosion Of Nickel Aluminum Bronze And Graphene Composite Protective Coatings For Ship Propeller

The propellers of marine ships are large castings,on which it is difficult to avoid casting defects.Large defects often need to be repaired by welding,but for small and diffuse defects,they are difficult to repair,so they can only be serviced without repairing.At present,there are few research reports on the influence of casting defects and welding repair structures on the corrosion behavior and mechanism of nickel-aluminum bronze in seawater.Nickel-aluminum-bronze propellers are directly exposed to service in seawater in our country.They not only be subject to the interaction of seawater corrosion and cavitation but also be attached and fouled by marine organisms,which lead to a significant drop in propulsion efficiency.Therefore,it is necessary to protect the propellers against corrosion,fouling and cavitation.The most effective technical way to achieve anticorrosion,antifouling and cavitation resistance at the same time is to apply protective coatings.At present,foreign countries have developed a composite coating composed of epoxy primer,elastic connecting paint and polyurethane or fluoropolymer topcoat and put it into commercial promotion and application.However,it is still in the stage of technology tracking research and development in China and there are no availab le commercialized products.In order to get rid of the dependence on foreign coating products and the passive state of tracking research and development,this paper proposes a new waterbore composite coating protection system.As a result,waterbore silicate graphene zinc-contai ning anticorrosive coatings,waterbore polyurethane graphene elastic coatings and waterbore silicone coatings are designed,prepared and investigated.Non-defective specimen,welding repair specimen and defective specimen are selected on the propeller.The influence of welding repair structures and casting defects on the corrosion behavior and mechanism of nickel-aluminum bronze is investigated by corrosion weight loss,scanning electron microscope（SEM）,X-ray diffraction,electrochemical impedance spectroscopy（EIS）and polarization curve（PC）.The results show that casting defects and welding repair structures cause galvanic corrosion of nickel-aluminum bronze,which accelerates the corrosion rate.The galvanic corrosion caused by welding repair make th e corrosion product film loose and porous in the early stage and thicker in the later stage,but it does not affect the multilayer structure of the corrosion product film.The passivation film resistance（R_f）and the charge transfer resistance（R_t）in the equivalent circuit are smaller in the early stage of corrosion but larger in the later stage.The galvanic corrosion caused by defects prompts the base metal to quickly form corrosion holes and causes the local surface to always be in an accelerated dissolution state,resulting in the inability to form a continuous corrosion product film.Therefore,in the equivalent circuit,there are no R_f and the R_t value is minimi zed.K⁺modified reduced graphene oxide（r GOK⁺）is prepared by microwave-assisted KOH reduction graphene oxide process.The difference in sedimentation rate of zinc powder and r GOK⁺is used to prepare waterborne silicate graphene zinc-containing anticorrosive gradient coatings of the upper enriched r GOK⁺and the lower r GOK⁺and zinc powder alternating.SEM,X-ray photoelectron spectroscopy（XPS）,EIS and PC are used to study the effect of r GOK⁺content on the anticorrosion performance and mechanism of the coatings.The results showed that the corrosion potential and open circuit potential of the coatings are always between the zinc powder and the substrate when the coatings are immersed in 3.5%Na Cl solution for 32days.Therefore,the sacrificial anode effect of the zinc powder is satisfied and the coatings have good corrosion resistance.The higher the content of r GOK⁺,the smaller R_t which characterizes the interface reaction in the equivalent circuit.It means that the more zinc powders participate in the sacrificial anode reaction,resulting in more Zn Cl₂,Zn（OH）₂ and Zn₅（OH）₈Cl₂,leading to a larger coating resistance（R_c）in the equivalent circuit.The graphene zinc coating 1050KGS with the highest content of r GOK⁺relies on the shielding effect and the enhanced sacrific ia l anode effect of reduced graphene oxide to achieve effective corrosion protection for nickel aluminum bronze under the condition that the amount of zinc powder is reduced by 30 wt.%.The ultrasonic dispersion process is used to prepare KOH andγ-aminopropyltriethoxysilane（LT550）modified graphene（KLGO）with graphene oxide,dicyclohexylcarbodiimide and KOH as raw materials.The solution polymerization process is adopted to prepare silane modified polyurethane prepolymer（MPP）with polyethylene glycol,polyether polyol,isophorone diisocyanate and LT550 as raw materials.The dispersion of KLGO with different contents in 50%ethanol aqueous solution is prepared by ultrasonic dispersion process.The dispersion is mixed with MPP by mechanical stirring to prepare the waterborne polyurethane paint.The waterborne polyurethane elastic coatings with a honeycomb micro-nano structure are obtained after moisture curing.The influence of KLGO content on the coating structures,surface morphology,elongation,elastic modulus and antifouling properties is investigated by XPS,nuclear magnetic resonance,infrared spectroscopy,SEM,confocal laser scanning microscopy,contact angle,tensile test,benthic diatom and bacterial adhesion evaluation methods,etc.The results show that the surface siloxane content,structure effect of honeycomb,elongation,elastic modulus,amphiphilic it y and antifouling properties of the coatings increase first and then decrease with the content of KLGO.Among them,the best structure and performance coating is the graphene-enha nced coating 50KGWPU.The coating has the most siloxane content on the surface.Its honeycomb size,honeycomb depth,elastic modulus,elongation at breakis,water contact angle,diiodomethane contact angle,concentration of chlorophyll of benthic diatoms and colony density attached after washing are about 600 nm,100-200 nm,1.32 MPa,261%,19°,12°,4.95mg/m² and 0.18×10⁶ CFU/m L,respectively.Compared with ordinary polyurethane coatings and waterborne silicone coatings,the amount of adhesion of diatoms and bacteria is significantly reduced.The coating has multiple synergistic antifouling mechanisms of fouling resistant,fouling release and fouling degrading.Finally,on the basis of the aforementioned research screening out the waterborne silicate graphene zinc-containing coating（bottom layer）and the waterborne polyurethane elastic coating（middle layer）,waterborne silicone rubber coatings（surface layer）and composite coatings（bottom layer+middle layer+surface layer）for propeller protection are prepared.The corrosion and cavitation performance and the interaction between cavitation and corrosion in 3.5%Na Cl solution are investigated.The results show that in the 3.5%Na Cl solution,the damage of the composite coatings caused by the interaction of cavitation and corrosion is much greater than that of corrosion.KLGO significantly improves the elongation,elastic modulus and hardness of the waterborne polyurethane elastic coating,thereby improving the cavitatio n resistance of the composite coatings.The cavitation resistance of the composite coatings is closely related to the surface roughness.The cavitation damage of the composite coatings mainly occurs in the matrix of the silicone rubber coating.